Gear Box Design Report

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Machine Design & CAD - II POWER TRANSMISSION SYSTEM DESIGN PROJECT REPORT Gear Box Design Khalil Raza Bhatti

QUAID-E-AWAM UNIVERSITY OF ENGINEERING, SCIENCE & TECHNOLOGY, NAWABSHAH SINDH.

Content Introduction Problem Definition Project Objectives Design Methodology Working Drawing Conclusion 07ME90 | MD & CAD -II

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Introduction Gears are the most common means of transmitting power in mechanical engineering. There are tiny gears for devices like wrist watches and there are large gears that some of you might have noticed in the movie Titanic. Gears form vital elements of mechanisms in many machines such as vehicles, metal tooling machine tools, rolling mills, hoisting and transmitting machinery, marine engines, and the like. Toothed gears are used to change the speed, power, and direction between an input and output shaft. This site is all about Gears. Visit the pages linked below to know more about different types of gears: 

Bevel Gear Reducer

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Hypoid Gears

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Bevel Gearbox

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Gear Pump

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Bicycle Gears

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Gearbox

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Fixed Gear Bicycle

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Speed Reducers

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Gear Coupling

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Differential Gear System

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Gear Cutting

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Worm Gear Reducer

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Gear Manufacturing

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Worm Gear Speed Reducer

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Gear Technology

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Spur Gear

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Helical Gears

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Bevel Gears

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Worm Gears

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Gear Design

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Gear Manufacturer

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Gear Lube

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Gear Ratio

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Gear Wrench

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Hydraulic Gear Pump

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Planetary Gear

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Gear Reducers

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Plastic Worm Gears

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Worm Gear Drives

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Worm Gear Speed Reducers

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Spider Gears

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Gear Oil

07ME90 | MD & CAD -II

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Problem Definition: What is the gear box how does it transmit the power? A Gearbox Is Used in Turbines, Windmills, Grinders.. Gears are used for increasing the torque of the source of rotary motion having high angular momentum and low torque. This high torque is necessary for performance of work. This phenomenon of increase in torque is called gear reduction and is brought about by coupling of a smaller gear called the pinion with a larger gear. This results in reduction of torque at the expense of angular momentum. Such a gearbox is called a reducer. One more application of gears is to change the axis or plane of rotary motion with or without gear reduction. When you open a gearbox you will see that the inner construction is very simple. Inside you will find two gears coupled with one another. The gears may be of spur, helical, cycloid, worm or bevel type. In case of gear reduction, the diameter of the output is larger than that of the input gear. If only a change in direction is required, the size of the gears is the same. Spur gears are used for heavy load but are noisy. If the load is comparatively lesser, helical gears are preferred as they are silent in operation due to gradual engagement. If change of plane of rotation is required, hypoid gears are used. The gear may be either of metal or plastic. This entire arrangement is enclosed in metallic or plastic housing. The point of contact of the gear teeth is well lubricated with gear oil. The gear oil must be very clean and free of abrasive materials to avoid wearing of the gears. A gearbox is used in turbines, windmills, grinders, etc. to change the direction of the rotary motion. In automobiles a gear box is used for transfer of to power of the engine to the wheels through a differential.


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Project Objectives Project Objectives are as Follows:

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Factors to Keep in Mind in Gear Design Advantages of Gear Technology Gear Ratio Speed Reducers, Gearboxes, and Gear-Heads Design of Gear box

Factors to Keep in Mind in Gear Design Sometimes in gear design (for e.g. in the case of spur gears, i.e. driver and driven) gears are to be designed for a specific velocity ratio and distance between central shafts. For the purpose of understanding this gear design better, let:

x = Distance between the centres of two shafts N1 = Speed of the driver T1 = Number of teeth on the driver d1 = Pitch circle diameter of the driver N2 T2 and d2 = Corresponding values for the driven or follower, and Pc = Circular pitch We know that the distance between the centres of two shafts, x = (d1 + d2)/2 and speed ratio or velocity ratio, N1/N2 = d2/d1 = T2/T1

From the above equations, we can calculate d1 and d2 (or T1 and T2) and the circular pitch (Pc). The values of T1 and T2 as obtained above, may or may not be whole numbers. But in gear design, since the number of teeth is always a whole number, a slight alteration must be made in the values of x, d1, and d2, so that number of teeth in the gear design is a complete number. 07ME90 | MD & CAD -II

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Advantages of Gear Technology The advantages of gear technology over other transmission means are: 

Gear technology gives positive drives and constancy of speed ratio without any slippage

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In Gear technology, the drive is very compact due to short centre distances in such drives.

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Gear technology has high efficiency, service, and simple operation.

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Gear technology drives are capable of driving loads subjected to shock at speeds up to 20 m/s

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Maintenance of gear technology drives is inexpensive and if properly lubricated and operated, gear drives have the longest service life compared to other drives.

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Gear technology can be used where precise timing is desired.

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Gear technology can drive much heavier loads than other drives.

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Gear drives can be used for a wide range of transmitted power.

Gear Ratio The gear ratio of your transmission, timing belt/chain and even your analog clock are what is responsible for rotational movement and the speed each piece or part achieves. Setting the correct gear ratio is vital, especially in the automotive industry. The wrong gear ratio will rob you of power, performance and even keep your vehicle from running at all, in the case of a timing belt of chain. If you've ever seen two gears working together, one turning the other as the teeth of the two gears mesh, you've seen a perfect example of gear ratio. This complex sounding conundrum is nothing more complex and complicated than the teeth of two gears 07ME90 | MD & CAD -II

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meshing as they turn. It can also mean two sprockets connected with a chain or two pulleys with a drive belt. The best example of the sprockets and chain combination is probably the standard timing chain. This vital piece of engineering is the driving force behind most modern vehicles, though timing belts are still used by some manufacturers. A correct gear ratio is the driving force behind anything that contains rotational motion. Engines, transmissions, clocks and even windup toys use gears with the correct gear ratio to produce the motion needed to turn whatever needs to be turned. Whether it's belts and drive shafts or tiny plastic axles and wheels, the correct gear ratio is incredibly important. One of the best examples of getting the correct gear ratio would be replacing a timing belt. If the proper teeth are not selected when putting the new timing belt back on, the vehicle will be out of time. In short, it will run either very poorly, or not at all. That's because the pulleys and gears must meet at exactly the right point for the rotation to match. If the rotational speed of the gears or pulleys doesn't match then you have the incorrect gear ratio. Gear ratio is also used to increase the speed of gears and pulleys. If you have a large gear turning a smaller gear, the gear ratio will increase the rotational speed of the small gear or pulley, dramatically. A gear ratio is written as any mathematical ratio: 2:1, 3:2, etc. In an example, if the large pulley rotated once per every two revolutions of the small pulley, you would have a gear ratio of 2:1. Gear ratio is an observable factor, as well; look under your hood sometime, or inside a clock or anything else containing gears; you'll see that larger pulleys and gears usually turn more slowly than their smaller counterparts. This knowledge is used to create high speeds within engines and transmissions. Gear ratio and teeth on the gears are inextricably related. If it weren't for the teeth on the gears, slight differences in circumference and other manufacturing inconsistencies would lead to an incorrect gear ratio inevitably. Since the majority of gears use teeth, those inconsistencies don't matter; the teeth make up the difference and provide for a lack of slippage. Pulleys, on the other hand, are frequently the same size and have a rubberized, or non-slip, outer covering. This combines with the autotensioner to keep the belt firmly seated around the pulley, rather than dangling down below the vehicle. Gear ratio within a transmission is incredibly important. Gear ratio is what's responsible for your vehicle's acceleration and top speed. Both wide and close gear ratios have benefits that are inherent to that type, though most modern transmissions do a good job of running the middle ground between these two extremes.


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Speed Reducers, Gearboxes, and Gear-Heads Speed reducers are a combination of sets of gears as found in gearboxes and gear-heads, having shafts and bearings, assembled in an enclosed metal housing. The purpose of application of speed reducers is to convert input to a known output speed, there-by reducing the RPM at the output with increased torque. In its present form of new invention, the multiple speed gear reducer has an input shaft and a power output shaft, with a series of train of load sharing gears interconnecting the shafts at the output and input. The gear trains also interconnect two or more reaction gears of different sizes. These reaction gears are of different sizes, which are connected to the input shaft coaxially, producing relative rotational motion between the gears and the input shaft. These are then operatively connected to the gear trains. Each of the reaction gears has a clutch, which selectively restricts the gear trains to attain a predetermined gear reduction ratio between the input and output shafts. There are separate coaxially arranged sleeves put around the input shaft, which is used to mount these reaction gears. The clutches on the sleeves control the reaction gears in order to maintain the specific gear ratio. Speed reducers are used in machine tools to set the different speed that is required for different kinds of machining. The common types of reducers that are used are the pulley pairs of different diameter, gearboxes, and stepped electric motors. Gear boxes which are termed as speed reducers are generally found in different machine tools, automobiles, transmission products, cranes and hoists, etc. The most common type of speed reducers are:       

Worm gear reducers. Bevel gearboxes. Helical gear reducers. Parallel shaft gear reducers. Planetary gear reducers. In-line helical reducers. Right angle reducers.

Gearboxes or speed reducers are constructed out of high performance hardened and toughened shaved gears, which have been designed to operate smoothly with loss of little power. The gears and the shafts are assembled in a metallic housing using shafts 07ME90 | MD & CAD -II

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and bearings. The input to the speed reducer comes from an electric motor shaft. The gears are of different sizes, having variety of numbers of teeth and coming in different diameter. The gear combination that is required to be meshed into each other is selected by the set of levers and clutches, and this combination enables the speed reducer to run in varied fixed steps. The output of this speed reducer came from the output shaft. The speed reducer needs to be thoroughly lubricated for the system to run smoothly. In designing a speed reducer, the number of teeth of that of the larger gear is divided with that of the smaller gear. This ratio will provide you with the one you want in a helical and bevel gear set. For example, with the large gear having 28 teeth, if the smaller one has 14, the speed reducer ratio between the input and output would give you a ratio of 2:1. However, in worm gears the ratio is designed in a different way, where the number of teeth in the gear, is selected with respect to the number of that of the worm threads. If the worm has 2 threads with the gear having 60, then the speed reducer ratio would be 30:1.

Design Methodology Gear Boxes Design Introduction Prime movers such as Electric motors, internal combustion engines, steam engines and turbines produce rotary motion at certain speeds and with certain torques at optimum efficiency. The motion produced is rarely equal to the motion required to do the necessary work and gear trains are required to translate the motion economically at maximum efficiency. It may be possible to use a low cost method of translating the motion e.g. a timing belt vee belt or chain transmission system. However these methods are limiting in their scope and are subject to regular maintenance and replacement. The engineered gearbox generally provides the optimum solution. Many companies provide motorised gear units with the electric motor mounted directly onto a gearbox providing the drive conditions (torque and speed ) exactly as required by the user. Once installed to the manufacturers instructions the only maintenance required is regular lubrication. Gearboxes can be engineered to allow gear ratio changes to enable output shaft speed while keeping the input speed and torque at the same value. The primary advantage for using a gearbox for changing speed is to enable the full power to be transmitted at the different speeds. Electric motors and other prime movers are rated for a maximum torque at the optimum speed. If the speed is reduced using electronic controls the resulting developed torque is not proportionally increased. Gearboxes also allow the input shaft and the output shaft to be in different directions.


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Simple Drive Train Rules 1) For any pair of meshing gears the angular velocity ratio is given by 2 /1 = ω z 1 /z2 = ω d 1 / d 2 1 = Input speed (rads/s) 2 = Output speed (rads/s) z 1 Number of teeth on input gear z 2 Number of teeth on output gear d 1 Pitch Circle Dia of the input gear d 2 Pitch Circle Dia of the output gear The sign is - (Reversing) if both gears are external and + (Same direction)if one gear is a ring (internal gear) 2) For a train of gear wheels the overall angular velocity ratio is given by 2 /1 = ω Product of teeth No's Driving gears / Product of teeth No's Driven gears = ω Product of pitch diameters (Driving gears) / Product of pitch diameters (Driven gears) The sign is - (Reversing) if there are an odd number of pairs of external gears

Gearbox Design Features The design of the gearbox includes the following features.. 

Input and output shaft relative positions and orientation


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            

Support of external forces on shafts Design and rigidity of casing Type, dimensions and strength of gears Method of changing speed /direction if required Design and strength of gear shafts Gearbox bearings Gearbox Seals Lubrication Noise and vibration Couplings to shaft Fixing /support of gearbox Heat dissipation Maintenance provisions

Gearbox Examples A gearbox is loosely defined as an enclosure for housing gears. Examples of gearboxes are numerous and some are listed below:         

Watch mechanism Bicycle axle gear (Sturmy Archer-3 speed) Sprocket to wheel axle (Sturmy Archer-3 speed) Power tool gear units - allowing speed reduction, change and reversing Automobile synchromesh gearbox -5 speed + reverse - Engine to drive shafts Machine tool integral - Electric motor drive to spindle and travelling motions Wind turbine gearbox - Turbine to generator Steam turbine - speed reduction turbine to generator Marine - Gearbox - turbine /diesal prime movers to Prop shaft Cranes -Gearbox usind for lifting and travelling motions

These are all specialised applications and the notes on this page relate to gear units manufactured as separate units for mounting in transmission systems. The normal method of fixing an enclosed gearbox in industry is to mount it on a rigid horizontal baseplate designed to absorb vibration. The rotary motion is transferred to the input shaft and from the output shafts via flexible couplings. There are a number of variations as listed below

Mounting    

Foot mounted on vertical surfaces Foot mounted below horizontal surface Flange mounted onto the prime mover Shaft mounted with a torque arm to prevent rotation of gearbox


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The sketches below show examples of gearboxes mounted in different ways. These are only illustrative sketches and should not be considered as gearbox designs.

Working Drawing:

When using a gearbox in a non-standard mounting position the lubrication system should be checked for suitability

Gearbox Casing The large gearbox casings are generally castings from cast iron or steel. Cast iron is a rigid material with excellent vibration damping properties. Fabricated

steel

gearbox

are

used

for

small

batch

quantities.

Gearboxes used for the transmissions in vehicles are often made from cast aluminium this is primarily to save weight. The tiny gearbox units are made from a variety of materials including cast zinc alloys. The important criteria in the gearbox casing design are listed below..


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         

Inclusion of safe lifting points to allow installation Support of the shaft bearings and hence the gear loadings; The transfer of the developed gear forces to supporting structure or further drive element; Containment of lubricant and exclusion of foreign matter; Providing a safety and noise barrier; Dissipation of the heat generated by gear friction Aiding testing, installation, and maintenance by containing all element in one unit; Providing convenient access to internals for inspection & maintainance Aesthetic benefits Enable accessible location of nameplate with all of the gear unit details

Gearbox shafts A variety of shaft designs are available including the following     

Plain shaft with keyway Plain shaft suitable for Friction drive coupling system Splined shaft Hollow shaft with internal keyway Flange

The selection of the drive shaft system is generally based on space considerations, on the design of the prime mover of the driven component and on the loading pattern. A proprietary gearbox is design to best fit in with the existing drive arrangements.

Shaft Orientation The shafts transfer of motion to and from a gearbox can be supplied in a variety of designs some of which are listed below.    

Inline shafts .....Epi-cyclic. spur, helical, harmonic Parallel shafts .... spur, helical Shafts at angles but non intersecting....Helical, Worm, Hypoid, Spiroid gears Shafts at angles and intersecting... Bevel gears

Worm gears and bevel gears are most commonly supplied with shafts at Right angles.

Conclusion: The basic theme of our project is to understand the design of gear box. 07ME90 | MD & CAD -II

Gear Box Design Report

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