Basics of Conveyor

Design of belt conveyor

1. Introduction to material handling equipments

In any industrial process, the product being manufactured passes through various phases and it needs to be transported from place to place. This could involve processes such as transporting of raw material to the machines and then shifting the machines from one station to another station and finally to the store or warehouse. This involves the use of material handling equipment. Simplest form of material handling is to take material from one place to another place manually or with the help of worker. In large production setups, where the production rates are high and the product to be handled is such that manual transportation is not possible, sophisticated material handling systems would be required. Material handling system does not contribute directly to the product value, but it adds to the cost of the product and is therefore sometimes is referred to as a necessary evil. In fact, least handling is the best handling.

1.1 Basic objectives

These basic objectives that a material handling system should fulfill are:

1. Quick and and precise pick-up of of loads. 2. Quick and efficient efficient transfer of load load with planned planned time interval. interval. 3. Transport of loads in planned planned quantity. quantity. 4. Safe transport without any damage. 5. Accuracy in delivering delivering at the destination. destination. 6. Automation with minimum human element. 7. Low initial and operational operational costs. 8. Simple and easy to maintain. maintain. 9. Safe operation. operation.

1


1.2

classifications

The

material

handling

system,

based

on

design

and

operational

characteristics can be broadly classified in to three groups as shown below:

Material handling equipment

I. Hoisting equipment a) Pure hoisting hoisting equipment: equipment: jacks, winches, winches, pulley blocks blocks etc. b) Cranes: EOT cranes, jib cranes cranes etc. c) Elevators: lift elevator, elevator, bucket bucket elevator elevator etc. II. Conveying equipment: Belt conveyors, Chain conveyors, Screw conveyors, Apron conveyors. III. Surface and overhead overhead equipm equipment: ent: Fork lifts, Trucks, Railway Railway cars, Overhead Overhead monorails.

1.3 Basic principles of selecting selecting material material handling handling system system

1. Direction of load travel. 2. Length of lload oad travel. 3. Properties and characteristics characteri stics of the material being handled. 4. The rate of flow of mater material. ial. 5. Kind of of the production process. 6. Method of loading and unloading. 7. Existing layout and conditions of the work space. 8. Initial and operational operational costs. costs.

2


1.4 Some important material handling system

1.4.1 Conveyor

• Belt Conveyor • Apron Feeder Feeder • Screw Conveyor • Deep Pan Conveyor • Drag Chain Conveyor • Flexowell Conveyor • Rope way Trolley • Skip Charging System 1.4.2 Stacker Reclaimer

• Linear stacker Reclaimer • Bridge type reclaimer • Circular stacker cum reclaimer • Bucket wheel stacker cum reclaimer 1.4.3 Wagon tippler

• Side discharge • Central discharge 1.4.4 Vibro Screen

• Linear movement • Circular movement

3


2. Belt conveyors

A belt conveyor consists of an endless belt of a resilient material connected between two pulleys and moved by rotating one of the pulleys through a drive unit gearbox, which is connected to an electric motor. The driving pulley end is called as head end, and the pulley is called as head pulley . Conversely, the other pulley is at the tail end

and is referred to as the tail pulley as shown in figure 2. Material is conveyed by placing it on the belt, through a feeder. As the belt rotates, the material is carried with it on the other end, where it is then dropped in the discharge chute. It should be noted that discharge can be arranged at any point along the run by means of special discharge devices. As the belt rotates, due to the weight of the belt and the conveyed material, the belt will sag. To support this sag, rollers called as idlers or idler pulleys are placed on both sides (carrying side and the return side). Closely spaced idlers are placed at the loading point, as there is some impact due to the falling material and overcrowding of the material in this region. The belt is subjected to tension and it being from a resilient material is prone to elongation. This reduces the tension in the belts. Reduction in tension causes slackness of the belt on the pulleys resulting in slippage and loss in power. To compensate for this, a tensioning device called as take-up arrangement is used.

Figure 2 Belt conveyor

4


2.1 Types of belt conveyors

I. Channel Stringer Belt Conveyors II. Truss Frame Conveyors Conveyors III. Slider Bed Belt Conveyors IV. U-Trough Belt Conveyors V. Flat Slide Belt Conveyors VI. Totally Enclosed Belt Conveyors VII. Custom engineering conveyors

2.2 Advantages of belt conveyor over other system

1. Can be operated over long distances over any kind of terrain. 2. Having high load carrying capacity and carry all kinds of loads. 3. Noiseless as compared to chain conveyors. 4. Much simpler to maintain maintain and don’t require require any major lubrication lubrication system like chain chain conveyors. 5. Their reliability reliability has been proved over over a long period by its use in the industry. industry. 6. Environmentally Environmental ly more acceptable. 7. Low labor and low energy requirements. 8. Unlike screw conveyors, conveyors, belt conveyors conveyors can be easily used used for performing processes functions in a production line.

5


2.3 Types of conveyor layout

(A) Horizontal

(B) Inclined upwards

(C) Inclined upwards – Horizontal

(D) Horizontal- Inclined upwards

(E) Horizontal Inclined Horizontal

(F) Inclined Horizontal Inclined Inclined

6


2.4 Methods of loading and discharging

Consideration of assumption:

1) The material should be placed centrally on the belt. 2) The material should be fed in the direction of belt travel and at a speed as near as possible to that of the belt. A) Hopper based based loading. loading. B) Processing unit based loading. C) Loading from a preceding conveyor i) Head and discharge ii) Both end discharge iii) Plow discharge D) Tripper discharge

2.5 Major equipments of belt conveyor

i.

Conveyor Belt

ii.

Pulleys

iii.

Idlers

iv.

Coupling

v.

Bearing

vi.

Drive unit

vii.

Electric motor

viii.

Cleaning device

7


2.6 Requirement of belt which is to to be used used in belt conveyor

2.6.1

High strength: The belt is subjected to tensile loads. It is also subjected to other

loads due to scrapers, plows. The material fed also creates an impact load on the belt. All these conditions require the belt to have high strength.

2.6.2

Low self weight: The belt is continuously driven on the pulleys. The power

requirement to drive this belt is dependent on its weight.

2.6.3 High wear resistance: resistance: The belts are subjected to rough working conditions over a

long period of time. Besides this, scrapers, plows, and other cleaners further create wear as they rub over the belt surface. The belt should thus have a high wear resistance to survive in tough conditions.

2.6.4

Low elastic and permanent elongation: Any elongation in the belt reduces the

tension created in the belt. This would reduce the power transmitting capacity of the belt should have a low elastic and permanent elongation.

2.6.5 Flexibility: They should have a good flexibility in the longitudinal and lateral planes.

In many cases, belts are made to run over many pulleys. The belt material should have the necessary flexibility to mould over the idlers.

2.6.6

High resistance to ply separation: Belts are made from plies, which are bonded

with a rubber element. The bonding of the plies should be such that it doesn’t separate out due to the repeated bending bending of the belt over the pulleys.

2.6.7

Low water absorption capability: Water if it gets absorbed by the belt increases

the weight of the belt. This would result in increased power consumption and reduced conveying capability. capability. It also gives more dimensional stability of the belt.

8


2.6.8 Suitable working environmental environmental conditions: Humidity, extreme heat or cold. The

belt material should be good enough to ensure that it works with optimum results under such working environmental conditions.

2.7 Introduction to Troughed Belt Conveyor:

There are many possible variations in the design of a troughed belt conveyor depending on the purpose and duty for which the conveyor is being designed. Similarly the choice of individual components, features and accessories found on a conveyor should be selected on the basis of the functions which have to be performed by the conveyor. Troughed belt conveyors offer an efficient means of transporting materials in large quantities (bulk), over distances ranging from a few meters to several kilometers, continuously. As will be seen below, troughed belt conveyors are only one of the types of belt conveyors available in the market today however, the troughed belt conveyor takes numerous forms and is used in many different applications with tremendous success. It is important to draw a distinction between bulk handling of materials and unit handling. The former refers to the transportation of particulate product(s) on a continuous basis for example, the conveying of lumpy ore from a mine to a processing plant or for transporting coal from a stockyard to a bunker above a crusher. 'Unit handling' on the other hand is generally described as discontinuous as this involves the transportation of for example, packed boxes, filled bags of cement and so forth. A troughed t roughed belt conveyor as described in this refers to conveyors which are used to convey product in bulk.

9


2.7.1 Types of Troughed Belt Conveyors

The term 'troughed' belt conveyor originates from the form of the carrying belt within the supporting idler sets and differentiates this conveyor from alternative bulk handling belt conveyor types which include 'Pipe', 'Sicon', 'Sandwich', 'Pocket or Sidewall', 'Cablebelt', 'Square', 'U-con' conveyors, etc. Examples of these different types of conveyors can be seen below.

The type of conveyor to be used in any particular application depends on a number of factors including the conveying route, properties of the material to be transported, environmental considerations etc.

10


3. Conveyor belt Conveyor belt is made up of compounds comprised of natural rubbers, styrene-butadiene rubber blends of natural and other synthetics, nitriles, butyl, ethylene propylene-based polymer, polychloroprene, polybutadiene, polyvinyl chloride, urethanes and silicones, etc., Each of those elastomers has specific usefulness for various ranges of properties and operating conditions from which manufacturers and end-users can choose.

Nylon or EP Belt

Steel cord

Conveyor belting and its corresponding cover composition can be designated as either (1) General Purpose Belting, or (2) Special Purpose Belting. Each of these two broadly classified groups should be further defined depending upon the specific end use.

3.1 Constructional details

Conveyor belts generally are composed of three main components: 1) Carcass 2) Skims 3) Covers

11


3.1.1 Carcass:

The reinforcement usually found on the inside of a conveyor belt is referred to as the carcass. The functions of a carcass include the following: •

Provide the tensile strength necessary to move the loaded belt.

•

Absorbs the impact of the impinging material being loaded on to the conveyor belt.

•

Provide the bulk and lateral stiffness required for the load support.

•

Belts are connected at the ends by splicing them with belt fasteners. The carcass should provide the necessary strength to hold fasteners. The carcass is normally rated by the manufacturer in terms of maximum

permissible operating operating tension. The carcass can of two major types: 1. Fabric ply type 2. Steel cord type

3.1.2 Skims:

The rubber, PVC or urethane between the plies is called as skim. Skims are important contributors to internal belt adhesions, impact resistance and play a significant role in determining the belt load support and trough ability. Improper skims can give reverse effect too. It can lead to ply separation failure. failure.

12


3.1.3 Covers:

They are used in conveyor belt construction to protect the conveyor belt carcass and also to extend its service life. Its desirable properties such as:

1) Textures. 2) Clean ability. 3) A specific specific co-efficient co-efficient of friction. friction. 4) A specific color. 5) Cut resistance. 6) Enhanced impact resistance. 7) Hardness. 8) Fire, oil and chemical chemical resistance. resistance.

13


4. Conveyor pulley The most commonly used conveyor pulley is the standard steel pulley. They are manufactured in a wide range of sizes and consist of a continuous rim and two end discs fitted with compression type hubs. In most wide faced conveyors pulleys, intermediated stiffening discs are welded inside the rim. Outer pulleys available are self cleaning wing types, which are used at the tail, take up or snub locations where materials tends to built up on the pulley face, and magnetic types which are used to remove tramp iron from the material being conveyed.

4.1 Conveyor Pulley Assemblies

Conveyor pulley construction has progressed from fabricated wood, through cast iron, to present welded steel fabrication. Increased use of belt conveyor has led industry away from custom-made pulleys to the development of standard steel pulleys with universally accepted size range, construction similarities, and substantially uniform load carrying capacity for use with belts having a carcass composed of plies or layers of fabric. “Standard” drum and wing pulleys are suitable for these applications. The present trend, however, is to use higher tonnage conveyor system with wider, stronger belts that incorporate a carcass of either steel cables or high strength tensile members. In these applications, where high tensions are encountered, the use of custom made “engineered” welded steel pulleys is dictated.

4.2 Type of pulley based on fabrication

1. Typical welded steel pulley. 2. Fabricated curve crown pulley. 3. Spun end curve crown pulley. 4. Lagged welded steel pulley. 5. Welded steel pulley with grooved lagging. 6. Slide-lagged pulley. 7. Lagged wing pulley. 8. Fabricated wing type pulley. pulley.

14


4.3 Type of pulley based on Function

1) Driving pulleys (Head and Tail pulleys) 2) Snub pulleys 3) Idlers a) Carrying idlers. b) Return idlers. 4.3.1 Head pulley

Normally the discharge end of the conveyor where the material is transferred to another conveyor is called as the Head end and the pulley in this end is called the head pulley. Most of the cases the drive is attached to the Head end of the pulley and so head pulley will designed stronger and bigger when compared to others. Head pulley is rubber lagged to increase the grip or friction between belt and Pulley.

Figure: Head Pulley

15


4.3.2 Tail pulley

The pulley which is situated in the receiving end of the conveyor is called as tail pulley. Some times Screw take-up will be situated in this pulley. This pulley is movable when take up is kept in this. When belt takes a turn for take-up arrangement or for any other drive arrangement this term comes. This acts as a support when belt takes a turn. 4.3.3 Snub pulley

Snub pulleys are incorporated into the design of a conveyor in order to increase the angle of wrap of the belt on the drive pulley. The greater wrap angle on the pulley allows more power to be introduced into the belt as is passes around the drive pulley without slip occurring. In this way, fewer drives are needed on longer conveyors or conveyors with high conveying loads.

16


5. Idlers The needs for Idlers are to give proper support to conveyor and also to the Material to conveyor. An endless conveyor belt in a conveyor structure is dragged from the tail pulley where material is loaded onto the conveyor, to the head pulley or drive pulley where the material is discharged. Between a conveyors' tail and head pulleys, whether the distance is a number of kilometers or merely a few meters, the carrying and return strand belting is supported on idler sets. The rolls are fitted with antifriction bearings with seals and with adequate lubrication packed into it. The friction f riction between the roller surface and the belt makes the rollers to rotate and thus material is transferred from one point to another through belt conveyor.

Figure 5 The arrangement of Idlers in a Belt Conveyor

17


5.1 Idlers serve following functions

1)

Support the the belt and the conveyed material on the upper run run and the belt in the lower run with minimum frictional resistance.

2)

Spacing of the idlers is is reduced near the loading point, so as to support support the belt due to impact of material in that region. This would prevent the belt from wearing quickly

3)

I dle dlers help in in cente centering ri ng the the belt belt and gui guidi ding ng it to the drive drive and snub snub pul pullleys.

5.2 Type of Idlers 5.2.1 Carrying idler sets

These idler sets support the carrying-side (top) conveyor belt onto which the material is loaded and and transported. transported.

In the loaded loaded zone we have Impact Impact carrying Idler Idler which is

covered by rubber material to absorb the loads as the loading or transferring points. Also we have Self-aligning carrying idlers idlers to avoid the belt off tracking.

Figure 5.2.1 carrying idlers

18


Carrying or troughing idler sets usually comprise between two and five individual idler rolls mounted into a common base, which is attached to the conveyor structure. Each idler roll in a ‘set’ comprises its own set of bearings, seals, shaft and outer shell.

5.2.2 Return idler sets

These idler sets support the return-side (bottom) conveyor belt which returns to the tail pulley after having discharged product over the head pulley. The diagram shown above is flat return type of Idler where only one flat roller is used. The return idler may also have more than one idler arrangement which is called as Garland type idlers.

Figure 5.2.2 return idlers

19


Once the material has been discharged from the carrying belt, the return belt is guided back to the tail pulley on return idlers. The impact, carrying and return idlers are spaced at different intervals. On the carrying-side, the mass of the belt plus the load conveyed is greater than the mass to be supported on the return-side and thus, for the tension in the conveyor belt (by the take-up and induced by the drive unit), the idler spacing is selected accordingly. This 'sag' in the belt between the carrying and return idler sets must therefore be designed on the basis of the heaviest load that the conveyor is to transport.

20


6. Coupling 6.1 Function

As the name indicates Couplings Couplings are the device used to couple or connect two shafts, this is one of the most important component of any drive system. Since it is impossible to maintain co linearity between two shafts couplings are designed to provide better flexibility to allow initial or running shaft misalignment. Following are the type of flexible couplings.

• Fluid Coupling • Chain Coupling • Geared Coupling • Grid coupling • Universal coupling

6.1.1 Fluid Coupling

There are three essential parts to a fluid coupling: the driving (input) section known as the impeller the driven (output) section known as the runner and the casing which bolts to the impeller enclosing the runner providing an oil tight reservoir. Both impeller and runner each represent represent half of a hollow torus with flat radial vanes. At the inner circumference circumference a conical baffle is attached to the impeller and a flat baffle is attached to the runner.

21


These components comprise the working circuit. The operation of the fluid coupling requires mechanical input energy, normally provided by a standard NEMA B electric motor which is connected to the impeller and casing. The runner, which has no mechanical connection with the impeller, is directly connected to the driven load. A variety of mechanical connections; couplings, sheaves, and hollow shaft mountings, are available to provide the mounting configuration best suited to the application. Finally the fluid coupling must be initially charged by removing the fill (fusible) plug and adding the recommended amount of oil based on the required torque. Starting

Standard NEMA B motors are recommended when using fluid couplings and will start virtually unloaded. Since the motor is mechanically connected to the impeller and casing, the low inertia of these components and the oil are the only loads imposed. As the electric motor accelerates to running speed, the impeller begins to centrifugally pump oil to the stationary runner. Transmission of oil is diffused by the conical impeller baffle, producing a gradual increase in torque, allowing the motor to accelerate rapidly to full running speed. When all the oil is pumped into the working circuit, continuous circulation of oil will occur between the impeller and runner forming a flow path like a helical spring formed.

22


As soon as the transmitted torque torque reaches the the value of the resisting torque, torque, the runner starts rotating and accelerates the driven load. The time required to reach full speed is dependent on the inertia of the driven load, the resistive torque, and the torque being transmitted by the fluid coupling. Running

The operation of a fluid coupling is based on the hydrokinetic principles and requires that the output speed be less than the input. This difference in speed is called slip. Further this principle provides that the output torque is equivalent to the input torque, since windage and oil circulation losses are negligible. Therefore, efficiency equals 100% minus the percent of slip. At full running speed fluid couplings will normally slip between 1% and 4%. The oil circulation between the impeller and runner has formed a vortex at the outside circumference of the working circuit and is no longer deflected by the conical baffle. Overload – Stall

Should the load torque increase, the slip will increase, which causes the runner to drop in speed. The vortex of oil circulating between the impeller and runner will expand to provide additional torque. The extent to which this vortex can expand is limited by the flat baffle on the runner. Consequently fluid couplings provide inherent overload protection.

If the increase in torque causes the oil in the working circuit to expand to the point of contacting the baffle, the coupling will stall and slip will be 100%. This continuous high slip generates heat and the oil temperature will rise unless the overload is removed. When the temperature rises to the temperature limit of the fusible plug, the core of the plug will melt, release oil from the coupling and effectively disconnect power to the output shaft.

23


To prevent the loss of oil the use of a proximity cutout switch or thermal trip plug and limit switch is recommended. Coupling guards must be designed to permit air circulation for cooling and to protect when oil is released from fusible plug due to overload. 6.1.2 Chain Couplings

Chain couplings operate similarly to gear couplings. Sprockets on each shaft end are connected by a roller chain.

Figure 6.1.2 chain coupling

The clearance between its components as well as the clearance in mating the chain to the sprockets compensate for the misalignment. Loading is similar to that of geared couplings. Packed Grease Lubricants is primarily used with this types of construction, necessitating necessitating a sealed sprocket cover. A detachable pin or master link allows removal of the chain.

24


6.1.3 Gear Coupling

Gear couplings compensate for misalignment via the clearance between gear teeth.

Figure 6.1.3 Gear Couplings

Shaft-mounted external gear teeth on both shafts mate with internal gear teeth on a housing that contains a lubricant. Other designs mount external teeth on only one shaft, mating with internal teeth mounted to the other shaft. Acceleration or deceleration can result in impacts between gear teeth due to backlash from the clearance being taken up on opposite sides of gear teeth. Misalignment will result in sliding relative motion across mating teeth as they pass through each revolution. revolution.

25


6.1.4 Grid couplings

External grid couplings use a corrugated steel grid that bends to compensate for loading induced by misalignment.

Figure 6.1.4 Grid Coupling Coupling

Grooved discs attached to the ends of each shaft house the grid, which transmits torque between them. Low amplitude sliding motion develops between the grid and grooves as the grid deforms under load, widening in some locations and narrowing in others over each revolution.

26


6.1.5 Universal Coupling

Universal joints are used for maximum allowable misalignment up to 20 to 30 degrees, depending upon the specific design. They are used extensively for the drive shafts of vehicles to allow the wheels to move with the suspension system. Universal joints use a four-spindled component called the spider to connect two shafts terminating in yokes or knuckles at right angles.

Figure 6.1.5

Universal Joint

Each of the four spider journals is supported by a bearing or bushing contained in one of the knuckles, which allow articulation. In some cases, greater articulation can decrease wear rates by allowing more complete development of a lubricating film.

27


7. Bearing As we all know, Bearings are used to give support the shaft of the roller or idle pulley at both ends. They give also rotational motion of shaft by giving it support with very less friction. Though some friction is taken place due to the metal to metal contact inside the bearing between metal balls and metal casings, it is very negligible as compare with the direct contact of rotating shaft and main frame of the conveyor. Types of Bearings: There are many types of bearings, each used for different purposes. These include ball bearings, roller bearings, ball thrust bearings, roller thrust bearings and tapered roller thrust bearings. 7.1 Ball Bearings

Ball bearings, as shown below, are probably the most common type of bearing. They are found in everything from inline skates to hard drives. These bearings can handle both radial and thrust loads, and are usually found in applications where the load is relatively small.

Figure 7.1 Exploded view

28


In a ball bearing, the load is transmitted from the outer race to the ball, and from the ball to the inner race. Since the ball is a sphere, it only contacts the inner and outer race at a very small point, which helps it spin very smoothly. But it also means that there is not very much contact area holding that load, so if the bearing is overloaded, the balls can deform or squish, ruining the bearing. 7.2 Roller Bearings

Roller bearings like the one illustrated below are used in applications like conveyer belt rollers, where they must hold heavy radial loads. In these bearings, the roller is a cylinder, so the contact between the inner and outer race is not a point but a line. This spreads the load out over a larger area, allowing the bearing to handle much greater loads than a ball bearing. However, this type of bearing is not designed to handle much thrust loading.

A

variation of this type of bearing, called a needle bearing, uses cylinders with a very small diameter. This allows the bearing to fit into tight places.

Figure 7.2 Cutaway view of a roller bearing

29


7.3 Ball Thrust Bearing

Ball thrust bearings like the one shown below are mostly used for low-speed applications and cannot handle much radial load. Barstools and Lazy Susan turntables use this type of bearing.

8. Drive unit for belt conveyor A) Direct gear motor drive B) Drive through parallel shaft gear box C) Drive through primary reduction by v belt and secondary by gear box D) Drive through spiral bevel or worm gear box

9. Motor Motor is a prime source of the energy to run the whole belt conveyor system. By taking current, it produces the mechanical work and this mechanical work is given to head pulley or tail pulley of the conveyor by means of gear box drive as discussed above. We can also use an induction motor with variable speed drive by changing its frequency.

10. Cleaning device An important property of the rubber covered conveyor belts is the high coefficient of friction of rubber. This reduces the tendency of material to slip on inclines. However it also increases the difficulty of cleaning the belt.

Some of the devices for belt cleaning are discussed below: a) Belt scraper b) Rotating belt cleaners c) Water spray and wiper

30


11. Problem Design a belt conveyor to transfer 200 t/hour of foundry sand through a horizontal distance of 20 meter. Foundry sand has density of 1.25-1.3 t/hour. Assume all the related data for belt speed and angles. Co-efficient of friction between belt drive roller and belt is 0.3. What happens when the transfer of same material at some angle for the same condition? Conclude from results.

Given data

Material is to be conveyed = foundry sand Length of the conveyor = 20 m Capacity of the conveyor = 200 tonnes Type of the conveyor = horizontal

31


Solution:

11.1 Design of belt

Important factors are to be considered: considered: a) b) c) d) e) f)

Angle of repose and angle of surcharge Flow ability Effective belt width for material material Volume capacity of bel belt, t, Q Mass capacity of belt belt Belt speed

11.1.1 Selection of belt width

•

angle of repose of the material to be conveyed = 45 degree

•

therefore surcharge angle = 30 degree

32


11.1.2 Density of the material material which which is to be conveyed conveyed •

density = 1.25 – 1.3 tonnes/m3 (from following table) Density and angle of repose of commonly conveyed conveyed materials

Material

Recommended max. angle of belt to horizontal, deg

Density tones/m3

Angle of repose, deg

Anthracite, fine, dry dry

0.8-0.95

45

Gypsum, small lump

1.2-1.4

40

Clay, dry, lump

1.0-1.5

50

1.5-1.9

45

1.2

45

Gravel

12

Earth, dry Foundry sand

24-26

1.25-1.3

45

Ash, dry

23

0.4-0.6

50

Lime stone, Lump

20

1.2-1.5

45

Coke

17

0.36-0.53

50

Wheat flour

23

0.45-0.66

55

Oat

18

0.4-0.5

35

Saw dust

27

0.16-0.32

39

Dry sand

18

1.4-1.65

45

Wheat

18

0.65-0.83

35

Iron ore

18-25

2.1-2.5

50

Peat

18

0.33-0.41

45

Coal(from mine)

18

0.65-0.78

50

Dry cement

20

1.0-1.3

50

Slag, anthracite

22

0.6-0.9

45

33


11.1.3 Angle of throughing belt

Angle of throughing throughing, β = 36 degree

The cross section of the lump on throughed belt of width ‘w’ is shown in figure

Area of c/s of lump, lump,

A =1 / 2 ( .6 W +.6 W +2 × 2 W co c o sθ ) × .2 W si s i n θ 1 / 2 ( .6W

+

+

..4W c o sθ )1 / 2 ( .6W

+

.4W c o s θ ) c o tφ

Where,

θ = throughed angle φ = angle of repose

34


Volume/meter Volume/meter length of the belt = A x l This must be equal to volume of material to be conveyed/meter length of belt,

M ass/me ss/meter ter le l ength of the bel belt A×

L =________________________ ___________ ________________ ___ density of the material

∴

mc/ρ =1/2(.6w +.6w+2

×

= mc/l

.2w cosθ ) × .2w sinθ

+1/2(.6 /2(.6w+ w+.4w .4w cos cosθ )1/2 )1/2(.6 (.6w+ w+.4w .4w cos cosθ )cot )cotφ ------ 1 Where, mc = mass/meter length of belt Ρ

= density of material

If we know the values of m c, Ρ, θ and Ø then we can find width of the belt from standard belt size data.

Finding belt size width,

Assume, travel speed speed initially initially = 2.5 meter/s, Mass rate = 200 tonnes/hour Ø = 36 degree Θ

= 45 degree

35


Therefore, mc = mass rate to be conveyed / speed of travel --------------------------------------------- 2 = (200 x 1000) / (3600 x 2.5) = 22.22 kg And the density density of the foundry sand = 1.3 tonnes/m tonnes/m 3 = 1300 kg/m3

Now putting this value in equation 1,

Therefore 22.22/1300 22.22/1300 = 1/2 (.6w+.6w+2 x.2w cos36) x .2w sin36 +1/2(.6w+.4w cos36)1/2(.6w+.4w cos36)1/2(.6w+.4w cos36) cot45

= (.7618w)(.1175w) + (.4618w)(.4618w) = .3027w2 Therefore, 22.22 / 1300 = 0.3027w2

W min = 0.2376 m or 237.6 mm

This is minimum width required of belt, to avoid spillover select 400 mm width of belt which is a standard one.

Now, let recalculate the equation 1, because according to belt width in mm belt speed may vary.

So, for 400 mm belt width, maximum recommended speed, speed, v = 2 m/s (from the table)

36


Allowable belt conveyor speed in m/s

Type of material conveyed

Belt width, mm 400

500

650

800

1000

Gravel, stone, coal, ash, ores

1.5

1.75

2

2.75

2.75

Coke, friable materials

1.25

1.5

1.5

1.75

2.0

Dry and wet sand, grains and

2

3

3.5

4

4

1.25

2

2

2.5

2.5

-

-

1.75

1.75

2

light materials Abrasive materials, materials, fine coke, coke, slag, crushed ore Abrasive materials: materials: In large lumps, slaggy rock, ores

Now put this value in the equation 2,

mc = (200 x 1000) / (3600 x 2) = 27.8 kg

Now put this value of mc in equation 1,

27.8/1300 = 1/2 (.6w+.6w+2 x.2w cos36) x .2w sin36 +1/2(.6w+.4w cos36)1/2(.6w+.4w cos36)1/2(.6w+.4w cos36) cot45 Therefore, 27.8/1300 = 0.3027w 2

And so, w min = 0.265 m = 265 mm

37


Here, we conclude that belt must require minimum 0.265 m width to avoid spill over select 400 mm belt width.

11.1.4 Mass of the belt/meter length of the belt mb = 5 kgf /meter (from table)

Weight of conveyor belts, kgf/meter

Belt width, mm

400

500

650

800

1000

Average weight weight

5

6.5

9

12

16

per meter run, kgf

11.1.5 Length of belt for one passes,

Here conveyor is having horizontal layout and so the angle of elevation = 0 degree

Angle of elevation elevation

ά

= tan -1 0/20 = 0

So, length of belt l = 20/cos 0 = 20 m

38


11.1.6 Total length of belt passes,

(We can calculate this after the calculation of pulley diameter and the thickness of belt,)

We are getting,

Diameter of the pulley D = 500 mm =0.5 m Thickness of the belt t = 8 mm = 0.008 m

So, Total length of the belt = D + 2 (t/2) + 20(2) = 0.500 + 2(0.008/2) + 40 = 0.504 + 40 = 40.504 m

39


11.1.7 To check the the no. of plies of belt,

Here, our belt width = 400 mm And the density density of conveyed material material = 1300 1300 kg/m 3 So, from table

Table to check the number of plies of belt

Density of material conveyed, t/m3

Belt

over

0.4

0.85

1.25

1.75

width,

Up to

0.85

1.25

1.75

2.5

mm Designation Designation of belt fabric, oz No. of

28

32

28

32

36

42

32

36

42

48

32 36

42

48

Min

3

3

3

3

-

-

4

-

-

-

-

-

-

-

Max

5

4

5

4

-

-

4

-

-

-

-

-

-

-

Min

3

3

4

4

3

-

4

4

-

-

-

-

-

-

Max

6

5

6

5

5

-

5

5

-

-

-

-

-

-

Min

4

4

5

4

4

3

5

5

4

-

5

5

4

-

Max

7

6

7

6

5

5

6

5

5

-

6

5

5

-

Min

4

4

5

5

4

4

5

5

4

3

6

6

4

4

Max

8

7

8

7

6

6

7

6

6

5

7

6

6

plies, i 400

500

650

800

5

40


1000

Min

4

4

6

5

5

5

7

6

5

4

8

7

6

5

Max

10

9

10

9

8

8

9

8

8

7

9

8

8

7

Designation Designation of belt fabric, oz 32 No. of plies i min = 4 i max = 4

41


11.1.8 Working tension in conveyor belt,

For oz 32, belt & mechanical joint and gravity take up joint belt,

Working tension per mm width of ply is taken from the table

Working tension in conveyor belts

28 oz

32 oz

36 oz

42 oz

48 oz

0.54

0.62

0.71

0.82

1.07

0.48

0.57

0.66

0.77

0.98

Designation

Working tension,

Vulcanized

kgf/mm

joint &

width per

gravity take

ply

up

Mechanical joint and gravity take up

Working tension is 0.57 kgf/mm width per ply.

42


11.2 Design of end pulleys 11.2.1 Diameter of pulley

The pulley dimension can be decided on the basis of the number of plies in the belt, the minimum pulley diameter ‘D

D min = k x i

min’

can be roughly approximated by this formula:

where, k = multiplying multiply ing factor which depends on the number of plies = 125 (for 2 to 6 plies) = 150 (for 6 to 12 plies) i = number of plies

So, our calculation k = 125, because belt plies in our case are 4, i = 4. Now put these values in the main formula: D min = 4 x 125 = 500 mm.

11.2.2 Width of the pulley

The pulley width can be taken as ‘W’

Formula: W = b + 2 x s

where, b = belt width in mm S = side margin (taken as 60 to 75 mm)

Assume, side margin margin s = 65 mm mm & Calculated belt width b = 400 mm So, W = 400 + 2 x 65 = 400 +130 W = 530 mm.

43


The calculated value should then be rounded off to the nearest standard size.

Some of the standard sized pulleys are listed in table, most of the preferred sizes are based on the R 10 series of preferred numbers.

So, pulley width = 600 mm.

belt type/width/pulley diameter(mm)/piy CC be belt lt

BE/B BE /BR R

5

6

7

8

9

10

11

12

600 60 0

800 80 0 10 1000 00 12 1250 50 12 1250 50 14 1400 00 16 1600 00 16 1600 00

250 315

400

500

630

800 100 1000 0 125 1250 0

NN150

250 315

400

500

630

800 1000 1250

NN20 NN200 0

31 315 5 40 400 0

500 500

630 630

800 800 1000 1000 1250 1250 1400 1400

NN25 NN250 0

40 400 0 50 500 0

630 630

800 800 1000 1000 1250 1250 1250 1250 1400 1400

NN300 N300

500 630

800 1000 1000 1250 1250 1400 1400 1400 400 1600 1600

NN400

630 800 1000 1250 1400 1600 1600 1800

EP100

EP belt

4

400 40 0 50 500 0

NN100

NN belt

3

250 315

400 400

500 500

630

800 800

800 100 1000 0

EP150

250 315

400

500

630

800

800 1000

EP20 EP200 0

500 630

800 1000 1000 1250 1250 1400 1400 1400 400 1600 1600

EP250

630 800 1000 1250 1400 1600 1600 1800

EP300

630 800 1000 1250 1400 1600 1600 1800

EP400

800 1000 1250 1400 1600 1800

We are selecting here NN type of belt for 500 mm diameter pulley and width of pulley is 600 mm. So, belt type is NN 200.

3

Rubber cover property

Code

Tensile strength(Mpa)

Elongation rate(%)

Abrasion(mm )

Scra Scratc tch h spli splitt

H

≥24

≥450

≤120

Abrasion

D

≥18

≥400

≤100

Common

L

≥15

≥350

≤200

44


11.3 Design of idlers: 11.3.1 Weight of revolving parts of idlers in kgf/ idler assembly, assembly,

Our belt width = 400 mm so, assume that tube diameter of idlers = 125 mm and diameter of antifriction bearing = 25 mm. From table, Weight of revolving parts of idlers in kgf/idler assembly

Belt

Tube diameter, mm

100

125

140

150

width, mm

Bearing

20

25

20

25

20

25

20

25

16.4

20

-

-

-

-

diameter(antifriction), mm

Troughing idles

400

13.6 14.1

500 650

-

-

17.9

21.6 24.4 24.6

-

-

800

-

-

19.4

23.1 26.2 27.4

-

34.7

1000 1000

-

-

-

-

-

37.8 37.8

7.6

7.8

16.4

20

-

-

650

-

-

11

12.2 15.8 15.9

-

-

800

-

-

12.65 13.7 18.2 18.8

-

22.5

1000 1000

-

-

-

25.1 25.1

29.9 29.9 30.2 30.2

Straight idlers

400

-

-

500

-

-

21.3 21.3 21.4 21.4

Then, the weight of throughing idler = 20 kgf/idler assembly, and The weight of the straight idler = 10.7 kgf/idler assembly

45


11.3.2 Maximum Maximum spacing for idlers in meter,

Belt width = 400 mm and Density of conveyed material = 1300 kg/m 3

From table,

Maximum spacing between idlers, m

Density of material conveyed,tonne/m3 conveyed,tonne/m3 Belt width,

Over

0.4

1.2

2.0

Return

mm

Up to

1.2

2

2.8

idler

400

1.6

1.5

1.4

3.0

500

1.6

1.5

1.4

3.0

650

1.6

1.5

1.4

3.0

800

1.5

1.3

1.2

3.0

1000

1.5

1.3

1.2

3.0

So, max. Spacing for carrying idlers = 1.5 m And for return idlers max. Spacing = 3 m

46


11.3.3 For finding no. of both idler

a. carrying b. return

Here, we have found max. Spacing between two idler assemblies for both carrying and return idlers are a.5 m and 3 m respectively.

Our total length of conveyor is equal to 20 meters. So, we can find out the total no. of carrying idlers and return idlers.

For carrying idlers = 20/1.5 = 13.33 = 14 idlers assemblies. For return idlers = 20/3 = 6.66 = 6 idlers assemblies.

47


11.4 Selecting the drive (head or both):

In this case, we have to select that drive which consists maximum intensity of pressure. Higher the intensity of pressure more friction between the belt and the pulley can be achieved. Because of that, better gripping of belt with the drive pulley can be achieved and negligible chance of slipping of belt. Better power transmission by belt can be achieved.

Pushing is better phenomenon than the pulling one and by calculations we could find that the intensity of pressure per square mm of belt is higher on head pulley so that we select head drive.

48


11.5 Estimation of power:

Work is required to raise the material against the gravity and to overcome the rolling resistance resistance between idler rollers and belt. It is very easy to estimate the work to raise the material against the gravity. To estimate the rolling resistance, resistance, we have to calculate the normal reaction between rollers of top run and bottom run. Normal reaction between top run and idlers depends upon total mass of the material and belt on top run. While, for bottom run it will depend upon mass of belt on roller. The co-efficient of rolling resistance kr for such application lies between 0.15 and 0.30.

Normal reaction for top run R t = g x m1x l x cos ά Normal reaction for bottom run R b = g x m2 x l x cos ά

Where m1 = mb + mc m2 = mb mb = mass of belt/unit length of belt mc = mass of material conveyed/unit conveyed/unit length of belt l = length of belt

11.5.1 Mass of material conveyed /unit length of belt m c = mass rate to be conveyed / speed of belt travel = 200 x 1000 / (3600 x 2.0) mc = 27.8 kg / meter and m b = 5 kg / meter Therefore,

m1 = 27.8 + 5 = 32.8 kg / meter

49


Resistance to motion of top run F t = Kr x Rt = Kr x g x m1x l x cos

ά

= 0.2 x 9.81 x 32.8 x 20 x cos 0 = 1287.07 N

Resistance to motion of bottom run F b = Kr x Rb = Kr x g x m2 x l x cos ά = 0.2 x 9.81 x 5 x 20 x cos 0 = 196.2 N

Force to raise the material F r = g x mc x l x sin

ά

= 9.81 x 27.8 x 20 x sin 0 =0N Therefore, Power required = total force x speed = (Ft + Fb + Fr ) x v = (1287.07+196.2+0) x 2 = 2996.54 W

Power of motor = power required / transmission efficiency efficiency = 2996.54 / .75 = 3955.386 W = 3.955 KW Power of motor = 4 KW

50


Case 2: when the transfer of the given material is at 20 degree from the

horizontal surface, Resistance to motion of top run F t = Kr x Rt = Kr x g x m1x l x cos

ά

= 0.2 x 9.81 x 32.8 x 20 x cos 20 = 1209.45 N

Resistance to motion of bottom run F b = Kr x Rb = Kr x g x m2 x l x cos ά = 0.2 x 9.81 x 5 x 20 x cos 20 = 184.36 N

Force to raise the material F r = g x mc x l x sin

ά

= 9.81 x 27.8 x 20 x sin 20 = 1865.50 N

Therefore, Power required = total force x speed = (Ft + Fb + Fr ) x v = (1209.45+184.36+1865.50) (1209.45+184.36+1865.50) x 2 = 6518.62 W

Power of motor = power required / transmission efficiency efficiency = 6518.62 / .75 = 8691.49 W = 8.61 KW Power of motor = 9 KW

51


12. Problem Conclusion

From this example of designing belt conveyor, we can conclude that at transportation of material at some angle through belt conveyor system, consumes more power than the transportation with zero angle to the horizontal plane. plane. Having some angle of inclination, increases the gravitational force with each instance and thus the force to transfer the material from lower side to upper side is quite power consuming.

52


13. Software for analysis

13.1 Introduction of pro-belt

Pro-belt computer software has been proven in the field to be highly accurate for the design of all belt conveyors of any size or configuration. We can our pro-belt calculations for a very long overland conveyor with a length of 12,345 meters. The conveyor had a very small decline of 41 meters. This was an excellent example to use to certify the accuracy of pro-belt because most of the belt tension and power requirement came from friction calculations and very little came from lift requirements. High incline conveyors are not a good example to use because lift power is a scientific absolute. The calculations were made with our metric version, but the English version gives identical results after units conversion. The owner field tested this conveyor in 1995. The testing was extensive and was considered to be highly accurate. Pro-belt gave a "Recommended Minimum Motor Power" of 1746 kW versus the field test result of 1720 kW. The pro-belt analysis was 101.5% of the actual power requirement. This accuracy within 1.5% is considered to be outstanding because this very long overland conveyor did not have much lift. The most important calculation calculation in belt conveyor design is to estimate the idler friction and belt flexure friction accurately, since the lift calculation is a scientific absolute. Pro-belt goes beyond CEMA to provide extremely accuracy results. It allows belt conveyors to be divided into multiple sections with different loading conditions conditions in each section. The software gives identical accuracy whether the conveyor is divided into a few sections or many sections. The designs are accurate for conveyors of any length including extremely long overland belt conveyors.

53


The pro-belt pulley shaft design program was certified accurate by one of the largest independent engineering firms in the world. The program calculates the deflection at the pulley hub based on a free shaft analysis. The bending moment between the shaft and hub is neglected. This gives a conservative design that eliminated many of the common pulley failures. The program also performs a combined bending and torsional fatigue analysis throughout the shaft in accordance with ANSI / ASME Standard B106.M. The drive pulley extension is also analyzed for either directly connected drives or shaft mounted drives. A warning message is displayed when the deflection or stress is outside of allowable limits. The pro-belt feeder program was tested by a world renowned bulk materials handling equipment manufacturing company. They tested a very large belt feeder at Kennecott Copper in Magna, Utah. The results were highly accurate and slightly on the conservative side. They proceeded to use the pro-belt feeder program to design fifteen (15) very large belt feeders for a large ore processing project project in South America. Our feeder program formulas are proprietary. Highly accurate results are produced for belt feeders of any size operating with any bulk material. We will not publish the formula used to obtaining this high degree of accuracy. The feeder program can also be used to model apron feeders and drag chain conveyors. Belt feeders can be designed with either slider bed or idler supported belts. The feeder and have a conveyor section as well as a feed hopper section. The three page report gives the detailed results for all friction loads within the feeder. 13.2 working

Pro-belt can be used to design horizontal curves in any troughing belt conveyor of any length and belt sizes in either English or Metric units. The troughing belt system and the return belt system are both included in the design. The troughing idlers must be the 3-roller design of equal troughing angle on each lateral roll. The lateral rolls can be longer than the middle roll to allow more drift, if desired. The idler troughing angle is generally between 30 and 45 degrees. The return idlers must be a 2-roller "V" design.

54


Return idlers generally have a troughing angle between 10 and 30 degrees but more commonly in the 15 to 20 degree range. Flat idlers cannot be used in horizontal curve conveyors neither on the troughing belt nor on the return belt. Pro-belt calculates the drift to the inner curve and drift to the outer curve under any belt tension or loading condition. Maximum drift to the inner curve occurs at maximum belt tension when the belt in the curve area is empty. The conveyor requires a load case analysis with the conveyor loaded to the curve but empty thereafter. The acceleration belt tensions should be used to check the curve design for maximum drift to the inner circle. Tail drive regenerative conveyors would normally have a maximum inner circle drift with the opposite conditions; i.e., stopping with the belt empty in the curve and full thereafter. This program will design the curve for both loaded and empty conditions on the same report. The return belt can also be checked for drift to the inner circle and the outer circle. Maximum drift to the outer curve occurs at minimum belt tension when the belt in the curve area is loaded. The conveyor requires a load case analysis with the conveyor loaded in the curve or throughout. The stopping belt tensions should be used to check the curve design for maximum drift to the outer curve. Tail drive regenerative conveyors would normally have maximum outer curve drift with the opposite conditions; i.e., accelerating with the belt full throughout. Many other load case possibilities should be considered. Pro-belt allows faultless belt tracking of the conveyor belt by setting idler banking, tilt and tracking. 13.3 nomenclatures

The Idler banking angle or super-elevation is the primary conveyor design parameter used to provide a faultless tracking of the conveyor belt. The idlers are raised on the inner side of the curve to provide a slope to the idler base. Banking angles of 8 degrees are common. A high banking angle will succeed in reducing the empty belt drift to the inner curve but will also cause an increase in drift of the full belt to the outer curve.

55


Banking with an idler tilt up to 2 degrees has been a successful approach to curve design. Provisions in the design should allow an adjustment of the idler banking, tilt and tracking in the field, if necessary. The Idler tilt angle is the secondary conveyor design parameter used to provide a faultless tracking of the conveyor belt. The idlers are tilted forward at the top in the direction of belt travel to provide up to 3 degrees of tilt to the idler base. The belt tracking will probably not be as effective as indicated by the program when set above 3 degrees. A tilt angle of 2 degrees has been shown to be a successful approach to curve design. The Idler tracking angle is a tertiary conveyor design parameter used to provide a faultless tracking of the conveyor belt. The idlers are moved forward in the direction of belt travel on the inner side of the curve to provide up to 2 degrees of tracking to the idler base. Tracking utilizes belt friction on the idlers to effect a movement in the belt and is not the most reliable method for belt guidance. However, tracking can be used in the field to make minor adjustments in the belt drift. A small tracking angle can make a large change in the drift. The sum of tracking plus tilt should not exceed 3 degrees. The Throughout belt factors are used to analyzes the effect in belt drift as a result of belt trough ability. A belt which is stiff transversely has a reduced trough ability and applies a greater force on the lateral idler rolls and a lower force on the central idler roll. A belt with greater stiffness and lower trough ability actually improves belt tracking. The Material follow factors represent the ability of the material to follow the drift of the belt without moving toward the lower edge. A value of 1.0 represents a material that has no movement when the belt drifts on the idlers. The value will be zero for very free flowing materials that flow like water. The Belt friction adjustment uses a friction multiplier to adjust the friction between the belt and the idlers. This friction is very important in layouts which include tilt and/or tracking of the idlers. The belt will not track as intended when the actual friction is

56


less that assumed. The friction multiplier can be changed to see the sensitivity of the final horizontal curve design to friction.

The Allowable belt friction is calculated based on the input parameters. The allowable belt drift is dependent on the unused space on the lateral idlers, belt edge distance with the given material loading and material follow factors. The belt is allowed to drift slightly beyond the edge of the lateral rollers by an amount of 1 inch or 25 mm. The actual drift results is compared to the allowable belt drift and an "OK" or "FAIL" indicator is given for each section point on the curve. These results are given for all three conditions: 1) troughing belt empty, 2) troughing belt loaded and 3) return belt.

13.3 advantages of analysis software (pro-belt)

a) It is used worldwide. b) It is easy to use. use. c) Will design any belt conveyor. d) Allows any length without limit. e) It is flexible. f) Designs belt feeders, feeders, apron feeders & chain drag conveyors. conveyors. g) Designs pulley shafts. h) Provides unlimited technical assistance. i) It is very easily justified. j) It is DOS based but easier and faster than Windows Windows programs. programs. k) It is unquestioned unquestioned in accuracy.

57


14.

Dynamic analysis For simple conventional belt conveying systems there are many well

documented procedures procedures to calculate required powers, tensions and other factors. With the assistance of computers, this rigid body analysis has been successfully streamlined, allowing the designer to concentrate on the problems of chute design, drive house layouts and components of manufacture. However, with the growing need for larger, longer conveying systems, there is a need to refine the analysis procedures. Generally, when conveying systems are analysed as rigid bodies in a stationary state, the effects of boundary conditions conditions - starting and stopping - are ignored. Dynamic Analysis is a computer simulation of the properties and performance of the system in motion - an analysis of the starting and stopping characteristics characteristics of a belt conveyor. Explaining dynamic analysis

During steady state running.. Figure 1 shows a simple conveyor system during steady state running, modeled as a series of masses and springs. In a steady state condition, the spring extension before the drive (L1) and after the drive (L2) remains constant. In the steady state condition, the torque due to the effective belt tension on the pulley is matched by the torque produced by the drive.

58


FIG. 1 Mass-spring representation representation of steady state running

The tension in the springs between the masses is determined by their stiffness and extension. Since during steady state running, the distance between the masses (and the spring extension) remains constant, the tension in the springs also remains constant. After a coasting stop...

Figure 2 shows the same system, after the drive has been turned off. 1. The removal of the drive drive torque from the pulley pulley leaves the torque torque unbalan unbalanced. ced. This results in a rapid deceleration of the drive pulley from v to (v._v), so that the rim of the pulley moves a shorter distance than do the masses adjacent to it.

FIG. 2 Mass-spring representation representation of Coasting stops

59


2. This results in the shortening shortening of the spring upstream upstream of the pulley (with a resulting resulting decrease in tension) and a lengthening of the spring downstream and an increasing in tension. 3. The change in tension on only one side of the masses adjacent to the pulley, subsequently produces a force imbalance on these masses which causes them to decelerate. 4. The deceleration causes changes in the extension and hence tension of the springs on the other side of the masses. The resulting force imbalance causes the disturbance to propagate further along the conveyor. The resulting wave of decreased tension propagates down the carry side of the conveyor and a wave of increased tensions propagates down the return side. For simplicity, the variations in tension will be referred to as "compression" and "tension" waves. These labels are not entirely accurate, since it is not possible to get true compression on a conveyor belt, but the terms are widely accepted. If the magnitude of the "compression" wave is greater than the actual steadystate tension of a region of the conveyor through which the wave passes, highly non-linear behavior will result. The belt tension in the region will not become negative but extremely low tensions and large belt sag between idlers will occur. Destructive dynamic effects frequently result from this type of occurrence. occurrence.

60


FIG. 3 Mass-spring representation representation of coasting stop with gravity take-up Coasting stop with gravity take up Figure 3 is the same as Figure 2, except that a take-up is located immediately downstream of the drive. The increase in tension downstream of the pulley, instead of inducing a tension wave, produces a force imbalance on the take-up, causing it to accelerate upwards. This upward movement of the take up, absorbs the "tension" wave. As a result, the return side of the belt is unaffected by the drive stopping until the "compression" wave from the carry side has traveled completely around the conveyor. The speed with which the initial waves propagate is a function of the system mass and the belt axial stiffness. The loaded side of the belt will be heavier and consequently waves on the carry side will propagate more slowly than waves on the return side. Other important factors are the drive inertia and the belt stiffness. The steady state velocity of the belt does not influence the magnitude magnitude of the stress wave.

61


14.2 When we can use it?

There are no cut off points to dynamically analyzing a conveying system. At present it is usual to analyze large systems with complex profiles. Dynamic analysis tends to be introduced for conveyors in excess of 1000m with capacities about 100 tph. This may be the norm, however any conveying system experiencing large take-up movements or adverse shock wave propagation resulting in premature pulley or drive failure, should fall into the spectrum. It is often possible to identify dynamic problems in a conveyor when large movements of the take-up occur. These movements are related either to elastic stretch in the belt, thus certain sections of the belt moving at different speeds to other sections, or large quantities of belt being dumped between the idlers causing a loss of tension, or a combination of these symptoms. It has unfortunately become common practice to eliminate the symptom of take-up movement by "fixing it" with a winch. This has the effect of making the design perform as a rigid body, and it must therefore be over-designed to cater for the effects of wave propagation. Failure to over-design, results in the shock wave destroying pulley or drive components. Since this normally occurs in a progressive way (fatigue), it does not always show itself as a dynamic problem.

62


benefiting from dynamic analysis

The advantages are generally seen in reduced costs and downtime. A dynamic analysis can give the designer the confidence to reduce safety factors, thereby lower the specifications for belting and allowing an increase in idlers spacing, thus reducing power use and component spares holding. It should be noted that conveyors set up with dynamic analysis techniques are operating with a safety factor for the belt of less than five.

FIG. 4 A typical examples of theoretical and actual readings. The take-up movement is an ideal test for model accuracy as it can be measured in the field, giving substantial justification for confidence in the modeling.

63


14.3 Conclusion

Setting up a basic model although very time consuming and highly complex, allows the designer greater freedom in designing the system. A full analysis needs upward of 100 runs to satisfy all the possible combinations of events. Today's computers require two hours to complete a single analysis, and a full analysis would therefore take in the region of 200 working hours. When the basic analysis has been concluded, and problems like shock wave propagation, excessive take up or negative belt tensions have been identified, it is then possible to test various solutions. What if a brake were installed. Where is the best position for a brake? Should we increase system inertia? All these questions can be investigated. investigated. Once this exercise has been completed, it is then possible to show the results of the full analysis using computer simulated models which, through a time base, can display the operating parameters allowing for the correct formulation of the control philosophy. By using the simulation approach right from the design stage, the designer can advance the roll of the conveyer to greater heights and lengths. Eliminating transfer points and pushing the conveyor to higher speeds, knowing that he is not sacrificing safety, will ensure that the final system will provide cost-effective bulk transportation for many years to come.

64


15. Bibliography:

¾

www.nbelts.com

¾

www.conveyorkit.com

¾

www.pro-belt.com

¾

Material handling systems

¾

Design data book

65

Basics of Conveyor

Recommend Documents