HYDRAULIC PROPERTIES AND SYSTEMS Hydraulic Fluid The efficient operation of any hydraulic system depends as much on the liquid, which transmits the power as the mechanical components. components.
Basic requirements of a hydraulic fluid. Low compressibility. Adequate fluidity to permit efficient transmission of power. Primary function of a hydraulic fluid. Transmit power. Lubricate the moving parts. Additional desirable qualities of a hydraulic fluid. Provision of seal between moving parts. Minimise wear. Protect the system from corrosion. Low freezing point. High boiling point. Will not damage seals and flexible tubes. Mineral Oil Petroleum base hydraulic oil is the most commonly used oil for industrial and mobile applications as it has the following desirable qualities: Good lubrication. Prevents rust. Good sealing ability. Note: Mineral oil is not suitable for industrial applications where fire resistance is required. Fire Resistant Fluids A fire resistance fluid is one which is difficult to ignite. It is used in hydraulic applications where there is a danger of a ruptured line or leaking fitting spraying the hydraulic liquid onto a source of ignition.
Basic types: Emulsions, both water in oil and oil in water. Glycol water base. Synthetics. Phosphate esters. Halogenated compounds Silicone.
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Emulsions Emulsions, particularly water in oil and Glycol in water, obtain their fire resistance from the snuffing and cooling action of the steam which evolves when the fluid contacts an ignition source. Phosphate Esters and Silicon types have inherent fire resistant characteristics because of their chemical composition.
Hydraulic Fluid Comparison Chart
Petroleum
Fire Resistance Bare Flames Hot Surfaces
WaterEmulsion
WaterGlycol
Phosphate Ester
Poor
Fair
Very Good
Good
Poor
Fair
Good
Very Good
Cost compared to Petroleum
1
1
Stability
Excellent
Good
Excellent
Excellent
Very Good Excellent
Excellent
2 to 2.5 cost of petroleum oil
3.5
Lubricity in Pump Balance Vane. Gear.
Excellent
Good
Excellent
Excellent
Temperature Limits
-18 0 to54 C
10 to 0 49 C
0 to 49 C
0 to 54 C
Corrosion Protection
Very Good
Good
Good
Compatibility
Excellent
Very Good (except paint)
Very Good (except paint)
Very Good Good (except paint, rubber, plastic)
0
0
0
0
Excellent 0
0
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HYDRAULIC COMPONENTS Hydraulic Pumps
Hydraulic pumps convert mechanical energy into hydraulic energy. They provide the driving force to move liquid under pressure thus transmitting power (Power = Pressure x Flow Rate). A major advantage of the hydraulic pumps is that they can deliver power in a package of small size and weight, unmatched by any other power-transmitting device. The hydraulic pump performs two functions during its operation:
Creates a negative pressure (vacuum) at its inlet allowing atmospheric pressure to force liquid from the reservoir into the inlet port of the pump.
It delivers (forces) the liquid out of the discharge port into the hydraulic system.
Note: Resistance to the delivery of the liquid from the pump causes pressure in the system. Categories ; 1.
Positive Displacement Pumps (Hydro-static pumps)
Pumps are classified into one of two groups. These pumps have a positive mechanical seal between the inlet and discharge ports. There are two basic categories of Positive Pumps:
Fixed Displacement.
Variable Displacement
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2.
Non-Positive Displacement Pumps (Hydro-Dynamic)
These pumps do not not have a mechanical mechanical seal between the inlet and discharge discharge ports; therefore the discharge flow rate is greatly influenced by the pressure at the discharge port.
Types of Hydro-static; 1.
External gear pumps (Hydro-static pump)
Gear pumps have a positive and fixed displacement, and are often used for the following reasons:
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They cost less to manufacture than most other pumps.
Simple in construction, in most pumps the gears are the only moving parts.
They can operate against pressures up to 21MPa and sometimes higher.
The pump consists of drive and a driven gear enclosed in a closely fitting housing. As the teeth of the two rotating gears come out of mesh, a low-pressure void is formed at the inlet of the pump. Oil from the reservoir is forced into this lowpressure void by either atmospheric pressure and/or gravity. The oil at the pump inlet port is trapped between the gear teeth and pump housing. It is carried around to the discharge port of the pump and forced out through the discharge port into the hydraulic system. Note: Frictional and load resistance on the discharge flow from the pump will cause pressure . 2.
Axial In-Line Piston (Plunger) Pump (Hydro-static pump)
These pumps have a cylinder block, which is mounted on the drive shaft and rotates with the shaft. The pistons stroke in the bores of the cylinder block, which are parallel to
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the axis of the block. The heads of the pistons are in contact with a tilted plate called a swashplate. The swashplate does not rotate but it can be tilted back and forth. It is mounted on a pivot and is controlled either manually or by an automatic servo device. If the angle of the swashplate were fixed, the pump would operate as a fixed displacement pump, delivering the same amount of oil with each revolution. If the swashplate is tilted the angle between it and the pistons causes the pump to have a displacement, proportional to the angle of the swashplate. The angle of the swashplate controls the distance the pistons stroke their bores. The greater the angle the further the pistons stroke, and more oil is discharged from the pump with each revolution. 3.
Un-Balanced Vane Pumps (Hydro-static pump)
The unbalanced vane pump uses the same basic principle of a turning rotor with vanes working inside a fixed rotor ring. However, the operating cycle only happens once each revolution. So this pump has only one inlet and one outlet port. The slotted rotor is offset in a circular ring, resulting in the discharge pressure “induced force” acting against the pump shaft and its bearing, resulting in increased wear and potential pump failure.
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ANCILLARY EQUIPMENT Hydraulic Reservoirs Capacity A hydraulic reservoir should contain fluid so that its working level is always maintained high enough to prevent a “whirlpool” effect at the pump inlet. It should also have enough capacity to hold the system’s fluid when the equipment is in the shut down state. As a general rule the reservoir should contain at least three (3) times the pump capacity per minute. Construction 1. Welded steel with large clean out / inspection cover. 2. Internal surface should be protected from rust. 3. Breather or vented cap is used on most reservoirs with the inclusion of an air filter screen. 4. Drain plug. 5. Baffle plate to separate return line fluid from the suction line. The baffle plate prevents: Turbulence. Foreign material, air etc returning into the inlet. 6. Sight glass to provide a visual check on fluid level. 7. Drain line should be constructed so that it is always below the fluid surface.
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Accumulators Purpose: An accumulator is primarily a device for storing pressurized hydraulic fluid. Function:
1. 2. 3. 4. 5.
As a shock absorber. To provide oil make - up in a closed circuit. To compensate for leakage in a system. To provide a source of power supply in emergency. To maintain steady delivery pressure over a period of time without keeping the pump operating.
Types:
1. Weight loaded 2. Spring loaded 3. Air or Gas Accumulators
Accumulator Weight Loaded or Gravity Type
The weight-loaded type consists of a movable piston and a weight. As hydraulic oil is pumped into the cylinder, the piston pushes the weight higher, increasing the potential or stored energy of the weight. The potential energy is released with the downward motion of the weight. An accumulator of this type is custom built for a particular installation. A single large accumulator may provide service for a number of different machines.
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Accumulator Spring Loaded Type
The spring-loaded type consists of piston loaded with a spring. Adjustment of the spring is sometimes provided. As the oil is pumped into the accumulator the piston compresses the spring. This energy is stored in the spring and is released when required. The pressure on the oil is not constant for all positions of the piston because the spring force depends on the compressed length of the spring. Usually this type of accumulator delivers only a small amount of oil at low pressure. Accumulator - Air or Gas Type
Hydraulic oil is nearly incompressible. This means that a large increase in oil pressure results in a small decrease in the volume of air or gas. Oil cannot therefore be used to store useable energy. But gas can be compressed to store energy. The gas acts in a similar manner to the spring
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Categories;
Non-separator types consist of a fully closed cylinder, air nitrogen or inert gas if forced into the cylinder to pre-charge the accumulator. As a greater quantity of oil is pumped into the accumulator the gas above the oil is compressed still further storing the energy in the compressed gas. This type of accumulator should be mounted in a vertical position because the gas must remain at the top of the accumulator. Aeration can occur with this type of accumulator. To prevent the gas being exhausted into the oil, only about two thirds of the accumulator volume can be used for the air or gas volume.
Heat Exchangers
Heat in hydraulic circuits is generated through the dissipation of pressure energy that produces work. Hydraulic components, pumps, valves, etc contribute to heat generation by internal friction. Heat build-up results in decreased efficiency and shortened system life. Heat build-up can cause: 1. Deterioration of hydraulic fluid. 2. Shortened seal life. 3. Accelerated wear of moving parts. 4. Safety hazards. 5. Power loss. 6. Increased cost due to viscosity change. 7. Loss of lubricity.
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Pressure Gauges
Pressure gauges are used in fluid power equipment to provide: 1. An indication of the operating pressure, especially where the pressure must be chosen by the operator. 2. An indication or alarm of abnormal pressure within the system.
Pressure and pressure change within a system must be correct for proper operation of hydraulically powered or controlled equipment. The pressure gauge indicates this pressure and helps to prevent malfunctions. Gauges can also be calibrated in values proportional to pressure, such as total force exerted by a hydraulic cylinder.
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Types: 1. Bourdon tube type 2. Spring loaded piston
Bourdon Tube Gauge
Bourdon tube type gauges are used to measure from vacuum to above 140Mpa (20,000 psi)
Advantages of Bourdon Tube Gauge:
1. 2. 3. 4. 5.
Accuracy Ruggedness Reliability Simplicity Low cost
Operation:
One end of a tube usually formed into a segment of a circle is fastened to a socket, which connects to a pressure source. The tube is flat on opposite sides. When pressure is applied inside the tube, the walls deflect and tend to assume a round cross section. This sets up stresses that increase the coiling radius and the free end moves a small amount. This movement is translated into rotary motion of an indicating pointer by linkage and or gear arrangements.
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Spring Loaded Piston Gauge
Spring loaded piston gauges are less likely to be damaged than the Bourdon type because they do not have levers, gears, cams or bearings. They are not as accurate as Bourdon gauges but are most suitable for fluctuating pressures.
Operation: The fluid acts on a piston, which moves in a cylinder against the resistance of a spring. A carrying bar or indicator moves with the piston along a calibrated scale.
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a calibrated scale.
Spring Loaded Pressure Gauge
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Gauge Calibration Calibration is the process of ensuring that the quantity indicated by a measuring instrument is an accurate indication of the actual quantity being measured. Gauge calibration is a sensitive and exact procedure, which requires skill instrument shop.
Fluid Conductors Pipe Size Outside diameter of pipe conforms to the standard thread sizes and remains constant regardless of wall thickness. Pipe sizes are designated by a dimension (this size was originally the inside diameter of the pipe). Pipe Threads NPT National Pipe Taper NPTF National Pipe Taper (Dry Seal) BSPT British Standard Pipe (Dry Seal) BSP British Standard Pipe
The NPT and BSP threads seal by flank contact. The NPTF and BSPT (Dry Seal). Threads seal by destructive interference fit along the thread crest. A thread sealant (pipe dope or Teflon tape) must be used in assembling NPT and BSP pipe threaded joints and is recommended for NPTF and BSP threads.
Flow Meter
The flow meter shown is a device used to measure the rate of fluid flow of a fluid.
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It consists of a vertically mounted tapered glass tube through which the fluid flows. The fluid enters at the bottom (small end) flowing to the top (large end), causing the indicator to rise upwards in the tube to indicate the flow rate. Since the tube is tapered the space between the wall and the indicator increases as the indicator rises, allowing more flow through. The indicator will rise to a height corresponding to the flow rate; the flow rate is read from the graduations on the meter.
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Tubing
Fluid conductors
All types of tubing are made of relatively malleable materials. Thus, tubing can be bent easily to reduce the number of fittings necessary for fabrication. Sizes Tubing is manufactured in standard size and is classified by the outside diameter. Wall thickness is usually expressed in mm, as a decimal of inch or as a gauge number.
Tubing Type Steel tubing is the only tubing material permitted by J.I.C. (Joint Industry Conference) standard without restriction. There are two types - seamless and electric welded.
Seamless tube is produced by: the cold drawing of pierced or hot extruded billets. Welded tube is made by shaping a cold rolled strip of steel into a tube and then welding and drawing it to size.
Copper Tube The use of copper tubing is restricted because it acts as an oil-oxidation catalyst and tends to work harden when flared.
In addition, copper tubing has poor resistance to vibration. Vibration will also cause the copper to work harden, making it brittle and likely to fracture. The use of copper tubing is limited to stationary applications at low pressure and to air circuits.
Aluminium Tubing Aluminium tubing of seamless quality has good bending and flaring properties and is suitable for low pressure applications.
Plastic Tubing Plastic lines are made from three basic materials - PVC, Polyethylene and Nylon.
Plastic lines are limited in their pressure rating.
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Pipe Application Since bend radii affect pressure loss in lines, a minimum of 3½ diameters is recommended for bends. Tubes are joined by flared or flare-less fittings.
Advantages of tubing include its adaptability to bending and flaring, vibration resistance and heat conductivity.
Pipe Fittings Pipe Fitting Table
Figure M164.2.01
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Tube Fittings
370 Flare 0
The 37 flare provides excellent results for connections when tubing is flexible.
450Flare 0
The 45 flare may be used with flexible tubing and will withstand pressure up to 5,000PSI
The 45 -inverted -inverted flare flare provides provides protection for the seat and thread
This design provides excellent results for highpressure hydraulic applications
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Tube Fittings
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Tube Fittings
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Hose
The use of hydraulic hose permits relative motion between components.
Advantages 1. Use where there is severe vibration. 2. Compensation for manufacturing tolerances in piping. 3. Absorption of hydraulic shocks. 4. Where connections and disconnections are frequently made.
Reinforcement Construction Materials used include natural or synthetic yarns or fibres, metal wires or combination of these materials. The reinforcement may be braided, spiral wound or both. Each group of wires is termed a Plait and each wire an End. The number of ends in a Plait varies. A bonding material is applied between each component of the hoses.
Size Flexible hose for fluid power application range in standard sizes measured in internal diameter.
Hose Fittings A hose fitting couples the hose to pipe or tubing or accessories. A fitting consists of two major parts: the portion that provides a means for attaching or connecting to an accessory or to other fluid lines.
Types of Fittings 1. Non-Reusable 2. Reusable
The non-reusable fitting is crimped or swaged and is squeezed onto the hose and in the even of failure the fitting cannot be reused. Reusable fittings can be removed from a failed hose and installed on a new length of hose.
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Hose
The information on this chart was sourced from Parker Hose and Fit ting Catalog # 4400 March 1990 p.15
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Actuators
Hydraulic Actuators (Linear) Hydraulic actuators perform the exact opposite function from pumps; they take energy out of a hydraulic fluid and convert it into extension of a shaft (movement) with the ability to overcome resistive force (load).
The force developed is a product of the piston area and the maximum pressure. Force= Area x Pressure 2 e.g. 0.2m x 1 MPa 6
0.2 x 1 x 10 200000 newtons or 200kN
Operating Principal Fluid is applied to one side of the Piston and the opposite side of the piston is exhausted.
When calculating the force developed on the reaction stroke (Rod End) the pressure does not act on the total piston area, the area of the rod must be subtracted from the piston area.
Travel Speed Cylinder travel is controlled by the quantity of fluid pumped into the cylinder.
Cushions Hydraulic cylinders may be supplied with cushions on the rod end, blind end or both ends. The cushion consists of a closed chamber close to the end of the stroke; the fluid is trapped and metered out slowly in order to slow the cylinders movement.
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Cylinder Types
Standard Double Acting Provides a power stroke in both directions. This is the standard type used for the majority of applications.
Single Acting Where thrust is needed in only one direction, a double acting cylinder may be used with the active end vented to atmosphere through a breather in the case of an air cylinder, or vented to the reservoir below the oil level, in the case of an oil hydraulic cylinder.
Double Rod Are used where equal displacement is needed on both sides of the cylinder. Sometimes the extra end issued to mount cams for machine tool applications.
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Cylinder Types
Ram Type Single Acting This type has only one fluid chamber, and is usually mounted in a vertical position. Used on lifting cranes.
Telescopic Type Are used where collapsed length must be shorter than could be obtained with a standard cylinder. They are available with up to 5 sleeves. Commonly used for tray elevation on tip trucks.
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Rotary Actuators SEMI-ROTARY ACTUATOR
Fixed Vane Housing
Moving Vane Rotor
Rotary actuators produce oscillating power by rotating an output shaft through a fixed arc. They produce high instantaneous torque in either direction and require only small space and simple mounting. The actuators consist of a chamber or chambers for containing the working fluid and a moveable surface against which the fluid acts. The moveable surface is connected to a shaft to produce the output motion.
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Types of Rotary Actuators
1. Linear Cylinder - with crank arm. 85 to 110 degrees. 2. Rack and Pinion - rotation available. available. 90, 180, 360 degrees. 3. Scotch Yoke - 90 degrees rotation. 4. Vane - Up to 280 degree rotation. 5. Helix - Rotation 100 to 370 degrees. 6. Sprocket - this type of unit is available with shaft rotation up to five (5) complete turns. 1800 degrees.
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Fluid Motors A fluid motor is a device, which converts fluid power into mechanical force and motion. Fluid motors are similar in basic construction to a hydraulic pump. Basic Types 1. Fixed displacement 2. Variable displacement
In a fixed displacement motor, a fixed quantity of fluid is used for each revolution; the speed will remain constant as speed is controlled by the quantity of fluid introduced into the motor. In a variable displacement motor the quantity of fluid can be varied by different methods to control the speed of rotation. Fluid motors can be applied directly to work applications, they provide excellent control for acceleration, operating speed, deceleration, and smooth reversals and positioning. The use of fluid motors in operating units is called: Hydrostatic Transmission. Design types of hydraulic motors
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Piston Motors These follow a variety of configurations, the majority are multi cylinder units, which may be in-line, vee, H, Flat 4 or Radial. Other types include conventional reciprocating engine design such as crank-less motor using a rotating piston cylinder assembly with slotted piston ends traversing an elliptic cam ring. The most popular configuration is the "vee” for four cylinder motors and Radial for three cylinder and upwards. The number of cylinders can be odd or even and 3, 4, 5 and 6 cylinder radials are all common.
Seals and Sealing Static Seals A seal that is clamped between two mating parts is called a static seal. The clamped parts do not move, but the seal may move due to increases and decreases in pressure.
Examples:
1. 2. 3.
Pipe thread connections Mounting Gaskets Seal rings on cylinder ends
Dynamic Seals These are installed where movement takes place between two mating parts.
Examples:
1. 2. 3.
Spool Seals Gland Packing Piston seals
Seal Types
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O-Ring This is the most commonly used fluid seal. It is moulded in synthetic rubber and has a round cross section. O-Rings can be used in static and dynamic applications.
T- Ring Seals These seals are used in dynamic applications, i.e. reciprocating cylinders and pistons. They are moulded in synthetic rubber. They act in a similar manner to Orings but because of shape does not have the tendency to roll in their grooves. H. Section seals are similar. Lip Seals These are dynamic application seals and are used on rotating parts, ie. hydraulic motors. They have positive sealing properties, which are assisted by increase in pressure. These seals are made in rubber or leather supported by a pressed steel housing. Single and double seal construction are available. Cup Seals A positive seal used on cylinder pistons. The pressure forces the lips against the cylinder wall. This type of seal is backed up with backing plate and can be used for very high pressure. Piston Rings These are of square section and are made from cast iron or Teflon. Used on cylinder pistons, in sets of two or more and when plated offer less resistance to motion than leather or synthetic seals. Compression Packing Compression packings are usually of the U or V form and are formed or moulded to shape. They are made of leather or synthetic rubber and are generally used in multiple packs. They are compressed into a gland by a ring that is tightened against a female gland support ring. Excessive tightening of the gland ring or nut will accelerate wear.
Some designs of packing are adjusted by a series of springs instead of the gland nut. This permits for correct tension on the packing and increases the seal life also decreasing the problem of over tightening of the packing.
Seal Materials Synthetic rubber elastomer are used extensively for the manufacture of O-rings and other types of seals. They are compatible with most oils.
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Nitrile (buna N) - is the standard for most service. It performs satisfactorily in petroleum lubricants and hydraulic oils, petrol, water, alcohol and many other fluids. Temperature range: - 40 to 125 degree C. Neoprene - provides excellent performance in refrigeration fluids and has good resistance to oxygen, weathering and many combinations of chemicals plus moderate resistance to petroleum oils. Temperature range : -55 to 140 degree C. Ethylene Propylene Water, steam, brake fluids, acids, alkalis and phosphate eaters, all may be sealed with O-rings of this compound. Oxygen and weathering resistance is also excellent. Temperature range: -55 to 150 degree C. Silicone This type of material material is limited to static service. It is compatible with a variety of fluids. Temperature range: - 65 to 250 degree C. Fluorocarbon (viton fluorel) Compatibility with nearly all fluid types and high temperature stability are attributes of this compound. Temperature range: - 31 to 225 degree C.
Where seals are used, a good surface finish on the contacting metal surfaces is required. Groove finish should be 32 micro inches or better and where the ring rubs over a cylinder or rod, the finish needs to be 16 micro inches or better. Back up rings are usually made of leather, rubber (hard) or teflon.
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Directional Control Valves
Directional Control Valves ( DCV) provide a means of controlling when and where fluid is required. The valves start, stop, accelerate, decelerate and control direction of motion of actuators. They are used to: 1. Hold an actuator in a fixed position and then direct fluid to move it in either direction. 2. Permit free flow of pump output to tank at low pressure when high pressure is not needed. 3. Vent relief valves by electrical or mechanical control. 4. Isolate branches of a circuit. 5. Select pressures by opening appropriate lines to relief pressure control or to various pumps.
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Classification:
A.
Directional valves may be classified according to the type of internal control element: sliding spool, rotary spool, rotary plate, poppet and ball.
B.
Directional valves may also be classified as two-way, two-wa y, three-way three-wa y or four-way, to indicate the type of circuit for which they are applicable.
The term one-way valve can be applied to a check valve, which has two ports but permits flow in one direction.
Poppet and Ball Control Valves These valves are two way or on-off valves; they are used as simple shut-off valves.
Operating Pressures Range: 550 P.S.I. (4 MPa) to 5000 P.S.I. (35 MPa) A four-way valve controls a double acting cylinder or hydraulic motor in both directions by admitting flow to either side of the actuator.
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Spool Valve
Purpose : To direct or block the flow of oil to a required circuit. Types: 1. Two positions (forward, reverse) 2. Three position (raise, hold, lower) 3. Four position (raise, hold, lower float)
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Configuration: The major types of sliding spools are:
These configurations refer to the spool in the centre position. Although a particular spool may be desirable for a specific operation, its effect in the circuit must be considered. For example, an open centre valve may be desired to open the pressure and cylinder ports to tank, but if other valves are to be actuated while the valve is centred, the open centred type cannot be used because the fluid will be lost to tank. Thus a closed centre or partially closed centre spool is needed.
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Construction: 1. Hardened, ground and lapped steel spool accurately fitted to a cast iron or aluminium alloy body. 2. Throttling slots machined in the lands to allow partial flow. 3. Machined grooves around the valve land to assist in lubrication, sealing and centring to the spool in the bore.
Actuation: 1. Manual Manual operation of a valve is used when the time element is not precise, and an operator must initiate the required action. Because of the relatively slow action of manual valves, flow forces become more critical and maximum capacity of the valve may be lower than for pilot or electrically operated valves. 2. Mechanical In a mechanical operated valve, a roller or other suitable connector replaces the operating lever or knob (e.g. table control on surface grinding machine). 3. Air Actuated Air actuated valves can exert higher forces than other means and can control higher hydraulic flows with lower pressure air and without excessively large actuators. 4. Electric As electric solenoid is mounted on one or both ends of the valve. When an electric current is passed through the coil windings, the spool is actuated. Because of the ease of running electric wires, this type of control can be actuated from a distance and is ideal when control is required in a hazardous situation.
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DCV Actuation
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Pressure Control Valves
Function: Protect the system from damage of excessive pressures. Set the upper limit of force exerted by a linear or rotary actuator. Precise control of system pressure permits hydraulic clamping of delicate products, control of straightening parts after heat treatment.
Wide ranges of pressure control valves are available and will be either normally open or normally closed valves. The valves are named by the function and control they perform in a hydraulic circuit. The normally closed type blocks flow between two ports until an established pressure is reached.
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The normally opened type is used for reducing, priority valves and pressure switches. These valves permit flow through the valve until an established pressure is reached; then the flow is restricted or blocked completely.
Relief Valves System Protection
A "Safety "Safety Relief " valve should be built into all hydraulic circuits; control of maximum pressure level is the first consideration. Dependability outweighs considerations such as noise, chatter, etc. Safety relief valves are not expected to operate unless there is a malfunction in the system. They may be non-adjustable or have the adjustment protected from tampering.
Operation: A relief valve crates an orifice between the pressurized supply line and secondary lower pressure area. Normally the relief valve is closed until the pressure level reaches the pre-set value. As the system pressure rises, flow through the valve increases until the entire pump output volume passes through the valve. When system pressure drops the valve should close smoothly and quietly.
Valve Loading Relief valves may be loaded by a spring or weight, which makes the spool or poppet to a close position. Fluid under pressure moves the valve member against the spring or weight to provide the fluid flow through the valve.
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Pilot Operated Relief Valve
Pilot operated relief valves use pressurized fluid to assist the main spool spring or weight to hold the passage through the valve closed. Fluid is directed through a restricted passage from the input supply to a control chamber. The pressurized fluid acts against a piston and adds to the spring force. A small capacity relief valve that is usually a poppet or piston type limits the resultant force available. This piloted relief permits the assist fluid to pass back to the low-pressure area at a pre-determined pressure. A pilot operated relief valve can also be combined with an unloading function, by diversion of the pilot pressure fluid back to tank. The relief valve control is reduced to the value established by the main spool spring. Some directional control valves may be combined with the relief mechanism to provide either of two pressure levels. A piloted operated relief valve may be provided with a separate drain if restrictions are expected in the major return tank line.
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Balanced Piston Relief Valve
The balanced piston or spool (shown on diagram) is named because in normal operations it is in hydraulic balance.
Pressure at the inlet port acting under the piston is also sensed on its top by means of an orifice drilled through the large land. T he piston is held on its tank seat by a light spring if the inlet pressure less than the valve setting When the pressure reaches the setting of the adjustment spring, the poppet is forced off its seat limiting pressure in the upper chamber.
The restricted flow through the orifice into the chamber results in an increase of pressure in the lower chamber. This unbalances the hydraulic force and tends to raise the piston off its seat. When the difference in pressure between the upper and lower chambers is sufficient to overcome the force of the light spring (approx 150Kpa) the larger larger piston unseats permitt permitting ing flow flow directly to tank. Increase flow through the valve causes the piston to lift further off its seat but since this compresses only the light spring very little over-ride is encountered.
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Pressure Reducing Valves
Pressure reducing valves control pressure downstream by limiting its outlet pressure. These valves are used to control clamping pressures and variations in system pressures. Operation: A normally open valve, it provides a free flow passage through valve from inlet to secondary port, until a signal from outlet (downstream) side tends to throttle the passage through the valve. Pilot pressure from downstream acts against an adjustable spring holding the valve open. Application: 1. To control light clamping pressures for fragile objects.
2. To vary operating pressures in branch circuits, from normal downstream pressure.
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Pressure Sequence Valve
Function: A sequence valve is used to cause an operation to take place in a circuit in a definite order or sequence. It is also used to maintain a minimum pressure in the primary line while other operations take place. Operation: The fluid flows through the primary passage to operate the first phase until the pressure setting of the sequence valve is reached. At this stage flow is diverted to the secondary port to operate a second phase. Application: To clamp a work piece with the first stage, then hydraulically control the cutting tool as the second stage.
These valves are drained externally since the secondary port is under pressure when the valve sequences. If the pressure were allowed in the drain passage it would add to the spring force and raise the pressure required to open the valve.
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Pressure Switches
Use: Pressure switches are used to make or break (open or close) electrical circuits at selected pressures to actuate solenoid operated valves or other devices used in the system.
Operation: An electrical switch is operated by a push rod which bears against a plunger whose position is controlled by hydraulic and spring force. The pressure at which the switches operate is selected by turning the adjusting screw to increase or decrease the spring force.
When the preset pressure is reached the plunger will compress the spring and allow the push rod to move down causing the snap action switches to revert to their normal condition.
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Hydraulic Filters A hydraulic filter is used as a device to remove soluble contaminants from a liquid. These contaminant particles are trapped by porous material within the filter. When properly matched to a hydraulic system, a filter not only serves as insurance but can also significantly reduce down time due to abrasive wear caused by unfiltered particles. Filter Rating Filters are rated on their ability to retain contaminants of certain size. Absolute Filtration Rating The diameters of the largest hard spherical particle that will pass through a filter under test conditions. This is an indication of the largest opening in the filter element. Filtration Ratio The ratio of the number of particles greater than a given size ( m) in the influent (still) fluid to the number of particles greater than the same size in the effluent fluid. Mean Filtration Rating A measurement of the average size of the pores of the filter element.
Note: Filtration ratings are in units of meters. meters. -6
1 micron = 1 m = 10 metre Nominal Filter Rating An arbitrary value indicated by filter manufacturers. How abrasive Wear Occurs Particles the same size as or slightly smaller than the clearances interact with surfaces to cause abrasive wear. Large particles as shown on the sketch cannot get into the critical clearance areas, very small particles less than 1 micron usually flow through without abasing either surface.
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Filter Types Strainers Strainers are constructed of fine mesh wire screens, or of screening elements consisting of specially processed wire of varying thickness wrapped around metal frames. Strainers offer small resistance to flow and are used in pump inlet lines when pressure drop must be kept to a minimum.
Strainers can be installed singularly or in parallel depending on the supply demand of the pump. Any exposed inlet fitting must be airtight; strainers require periodic cleaning, they prevent catastrophic failures caused by chips and other large contaminants. This is the principal reason that suction strainers are used on the inlet of many hydraulic pumps.
Filter Material
Filters Filter elements are made of various materials such as paper, cellulose felt, glass, and sintered powders of metal, ceramics and plastics.
These elements usually screen much finer than strainers and can remove particles as small as 2 microns. Filters offer protection against abrasive wear, which in most components is caused by silt (1 to 5 microns in size). This type of filter should be used with strainers capable of chip control.
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RATING
Filter Material Materi al 100 mesh screen 200 mesh screen Sintered woven wire mesh Resin impregnated paper
Ultra-fine Paper Cellulose Felt Glass Sintered metal Ceramics Plastic
m
Chip Control Cleanable Cleanable Cleanable
Nominal
Mean
Absolute
135 70 10
140 74 17
220 105 25
Disposable
10
18
30
Silt Control Disposable
0.45
0.9
3
Disposable
0.5 - 100
2 - 50
Disposable
2-65
13-100
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Filter Location Location depends on the function for which the filter is to be used.
Filters used for chip control should be located as close as possible to the components to be protected. Example of such filters includes suction strainers protecting pumps; screens mounted ahead of orifice and filters and screens mounted ahead of control valves. Filters with the primary function of silt control can be located almost anywhere in the system. Examples include filters mounted in the pressure lines ahead of critical components such as servo valves or hydraulic motors, return lines to tanks and filters located in a separate cleaning loop in conjunction with a pump providing continuous cleaning up of the hydraulic system. For the best silt removal filtration, the filters should be full flow filters in a location that passes all of the system flow.
Classification: Filters can be classified as Full Flow or Proportional. In a full flow filter, all of the fluid enters the unit, passes through the filtering element. Although the full flow type provides a more positive filtering action it offers greater resistance to flow particularly when it become dirty. For this reason a full flow filter often incorporates a valve to automatically by-pass the element (return line filter).
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Air Service Unit
Air Preparation and Conditioning The quality of compressed air used in today's industry varies greatly from the extremes of totally oil free and sterile air as required by the food industry, to the well lubricated dry air used in manufacturing machines.
To produce these wide variations of compressed air conditions, various equipment must be employed, these being 1. Filters to remove unwanted contaminants. 2. Pressure regulators to control pressure. 3. Lubricators to supply correct amounts of lubricants.
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Air Service Unit Filter All compressed air systems will contain contamination of some type, these being:
Moisture Solids Gases and odours Oil
Moisture is the most common contaminate in either droplet or vapour form. Vapour is very difficult to remove as once the droplets size is below one (1) micrometer, mechanical separators of the cyclonic type are unable to remove the vapour and other means must be employed, eg. air dryers. Operation: Dirty moist air enters the filter via the main body and is directed down into the bowl as it passes from main body to the bowl, the air passes through a set of directional vanes these impart a swirling motion to the air, this cyclonic motion tends to cause heavy contamination to be flung by centrifugal force out to the bowl where it gradually slides down the wall and into a quiet zone at the bottom of the filter. This quiet zone exists below the splash shield, heavy contamination in this area is unable to be picked up by the airflow and carried onto the filter element.
The heavy contamination is made up of water and oil droplets larger than one (1) micrometer and solids such as dust and rust particles.
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The air then proceeds through the filter element that may have a pore size from 5 to 30 micrometers; the filter element removes the remaining solids larger than the pore size employed. Solids -Solid pollution exists in compressed air systems in the following forms: Intake dust. Compressor wear products. Rust and scale from supply network.
Contaminates of this type are adequately removed by the selection of appropriate filters of the conventional type. It is important to note that intake filters fitted before the compressor remove between 96% and 99% of air bound solids from intake air to compressor. Liquid Oil and Aerosols - Highly efficient coalescent filters are commonly employed for the removal of liquid oil and aerosols. Operation: Air is passed into the centre of the filter element, which is made up of microfibres enclosed in a shell of metal or plastic, this is then surrounded by open pore plastic foam. The air passed though the boro-silicate glass fibres and the droplets of aerosol impinge onto the fibres and run down the fibre to a crossover point, when many droplets run together they coalesce into a large drop, which is forced through the filter and appears on the outer surface of the filter as more and more drops appear their weight causes them to run down the surface and finally collect in the porous form from where it can be easily collected and removed from the system. Coalescing filters are prone to be contaminated by solids and therefore should be preceeded by a pre-filter. Vapours and Gases - Micro-fibre filters cannot remove vapours and odours, Vapours and gases must be removed by catalytic conversion by absorption onto activated surfaces.
The most common activated surface is ‘activated carbon’, though it is not too effective with carbon monoxide, carbon dioxide, acetylene, light hydrocarbons and sulphur dioxide. These require catalytic conversion. Note: The catalysts are adversely affected water vapours.
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Air Service Unit Pressure Regulator Pressure Regulators (Pressure Reducing valves )
Air regulators are an essential part of correct air preparation within a compressed air system, their purpose is to control the pressure being supplied to equipment, be it a hand tool requiring full system pressure or a machine control circuit requiring very low pressure. The advantages of using regulators within a pneumatic system are:
Supply air at constant pressure regardless of flow. Operate the system more economically by minimizing the amount of pressurized air wasted. Promotes safety by operating the tool or actuator at reduced pressure. Extends component life, by decreasing wear rates. Increase operating efficiency. Able to have controlled pressures within a single system.
Balanced Poppet Self Venting Regulators operation The poppet as the name implies, is pressure balanced. The poppet is exposed to the same reduced pressure on both the top and bottom surfaces, therefore the effects produced by reduced pressure fluctuations are cancelled out, and the sensitivity and response are greatly improved. This type of regulator has a very large flow capacity and good pressure regulation.
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Venting excess system pressure As downstream pressure is applied to the diaphragm, it causes the diaphragm to move down sufficiently to cause the top poppet to close thus preventing further flow, and a bottom poppet valve to open allowing the increased pressure to be dumped to atmosphere.
Compressed Air Lubrication Most moving parts in an air system require some form of lubrication for efficient operation, eg. cylinders, valves, motors. Some pneumatic components are lubricated using an airline lubricator; this introduces oil into the system in an atomised state. Note:
Correct lubrication decreases friction, prevents corrosion. Most new pneumatic automation equipment components do not require lubrication. (refer to manufacture specifications)
Operation: Air flowing through an automatic airline lubricator creates a differential between air line pressure and pressure to the lubricator reservoir. This pressure differential causes oil to be lifted from the reservoir into a sight feed dome and dripped through a metering tube into a body section where the oil atomises and mixes with the air flowing through the lubricator.
With an oil-mist lubricator, the air passes a deflector to atomise the oil. Both methods depend on an increase in air velocity to atomise the oil. The lubricate air then carries the oil-mist to the component. Oil Fog - coarse droplet Oil Mist - fine mist.
Lubricators are mounted downstream of the pressure regulator.
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Oil Fog mounted as close as possible to the equipment they are to serve.
Oil Mist can be installed above or below WORK STATION.
Air Service Unit servicing se rvicing
Maintenance: DO NOT allow the collected condensate to fill the filter bowl. Cleaning Plastic Bowls Remove all air pressure from the air service unit before servicing. Remove the bowls taking care not to damage seals. Clean the bowls with a soft cloth and warm soapy water. NEVER use solvents to clean plastic (polycarbonate) bowls. Cleaning Filter Elements Blow out with compressed air in the opposite direction to the normal airflow. Replace with a new element if required. Lubricator Use only a light mineral oil. Take care not to overfill the lubricator. Ensure the correct drip rate is set for the application.
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Review questions for Learning activity
1.
Section 2
– Fluid Power
What fluid is used in a standard Industrial Pneumatic Pneumat ic system? ……………………………………………………………………………
2.
What fluid is used in an industrial hydraulic system? ……………………………………………………………………………
3.
List three basic types of fire resistance fluids. ……………………………………………………………………………. ……………………………………………………………………………. …………………………………………………………………………….
4.
List the two categories of air compressors. compressor s. ………………………….…………………………………………………. ………………………….………………………………………………….
5.
List a compressor to represent each of the two categories. ………………………….…………………………………………………. ……………………………………………..………………………………
6. Give another description for for each of the following hydraulic pump classifications. Positive Displacement Pump or? ……………..…………………… Non- Positive Displacement Pump or?……………..………………
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7. Identify the positive displacement pump category from the given symbols. Symbol
8.
Description
Label the five missing items of the hydraulic reservoir reservo ir shown.
……………………………………………………………………….. ……………………………………………………………………….. ……………………………………………………………………….. ……………………………………………………………………….. ………………………………………………………………………..
9.
State the main purpose of an accumulator. accumulator . ……………………………………………………………………… ………………………………………………………………………
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10. List three accumulator types. 1. ………………………………………………………… 2. ………………………………………………………… 3. …………………………………………………………
11. Complete the following accumulator function statement. To maintain steady delivery …………………………………..
……………………………………………………………... ……………………………………………………………...
12. Air receivers are used to:
…………………………………………
…………………………………………
…………………………………………
…………………………………………
…………………………………………
…………………………………………
13. Air Receiver Size is determined by: …………………..………………………………………… …………………..…………………………………………
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14. Heat build-up in hydraulic systems can cause:
15.
…………………………………………
…………………………………………
…………………………………………
…………………………………………
…………………………………………
…………………………………………
…………………………………………
After-coolers are used to? (Tick your response/s)
.
Cool Hydraulic Systems Cool Pneumatic systems
16.
Name two types of pressure gauges. 1. ………………………………………………………. 2. ……………………………………………………….
17.
What is the angle of JIC flare fittings? …………………………………………………………..
18.
What is the angle of SAE flare fittings? ……………………………………………………………
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19.
What function do hydraulic linear actuators perform?
…………………………………………………………………….. .……………………………………………………..……………... …………………………………………………………………….. ……………………………………………………………………..
20.
Describe a single acting hydraulic Ram. …………………………………………………………….……….. ………………………………………………………..……………. ……………………………………………………………………...
21.
What would be a typical application for a telescopic hydraulic actuator? actuator ? ……………………………………………………………………. ………………………………………………………..……………..
22.
Complete the following statement. statement . A fluid motor is a device which …………………………………. .……………………………………………………………………
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23. Name three design types of fluid motors. 1. ………………………………………………………….. 2. ………………………………………………………….. 3. …………………………………………………………..
24.
What does the fluid-power term DCV represent? …………………………………………………………………
25.
Label the 4 DCV centre configurations shown.
26.
What is a major function of a pressure relief valve in a hydraulic circuit?
…………………………………………..
…………………………………………..
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27. List two applications for a pressure-reducing valve.
………………………………………………………………. ………………………………………………………………
………………………………………………………………. ………………………………………………………………
28. A pressure-reducing pressure-reduc ing valve is: (Tick your response/s)
.
Normally open valve. Normally closed valve.
29. State an application for a pressure sequence valve.
…………………………………………………………………...
……………………………………………………………………….
30. List three locations for filters in hydraulic systems.
……………………………………………………..
……………………………………………………..
……………………………………………………..
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31. Identify the symbol shown.
……………………………………………….
32. Label the three components of an air service unit.
33. What is the most common contaminate of compressed air? …………………………………………………………
34. How is this contaminate removed from the compressed air?
………………………………………………………… ………………………………………………………… …………………………………………………………
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35. Complete the following statement. When cleaning the plastic bowl of an air service unit, you should NEVER. ………………………………………………………………………………
36. Complete the following. (Tick your response/s)
.
Oil fog compressed lubricators produce: Produce coarse oil droplets. Produce a fine oil mist. Note: Teacher will re-direct student to the relevant section to locate the correct responses and if necessary will initiate group decision on any of t he areas covered by th ese review questions.
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