Design Of Gearbox For Machine Tool Applications

Design procedure of Gear Box for automobile and machine tools

This article gives the complete design of a Gear Box. This includes a step by step guide to the design of each component of a Gear Box, such as gear design, shaft design, casing design, etc.

Importance of Gear Box

What is a Gearbox? A Gear Box is actually a speed variable device which changes the speed of power transmission device according to the requirements of torque. We normally use various power transmission devices like lathe machine, drilling machine, milling machine or any other machine tools which require power at different torque or speed as per the will of the operator or based on the operation performed. High torque operations require low speed and vice verse. E.g. when we drive the vehicle at the steeper road, we lower the speed to have increased torque. Power and torque are directly proportional but speed and torque are inversely proportional according to the formula:

Power transmitted = 2 x PI(=3.14159) x Speed x Torque/60.0 Watt

Hence, to perform different operations that require different torque or power and thus, we can fulfill the requirements by changing the speed. This speed variation can be done by using different gear having different teeth cut on it on various shafts at the proper distance to have perfect mesh in between the two gears. In the case of lathes, milling machines the speeds are fixed by the required cutting feed and/or cutting speed for a range of materials. Low strength materials can be cut at higher speed and high strength material required higher torques so that speed needed is very low compared to that of low strength materials. Also in the case of automobiles maximum and minimum speeds of the vehicle determines the size of the gearbox.

Those students who are in final year mechanical engineering, can take Gear Box design as their final year project. You can refer to various other options for final year project topics here.

Design procedure for Gear Box for automobile and machine tools

Design procedure for Gear Box

One has to follow a procedure to complete a task and the design of an important part of an automobile also required a design system to follow. Follow this procedure to have an idea about how to design a complete Gear Box with each component's individual design.

Determine the maximum and minimum speeds of the output shaft. This will determine the complete speed ratio. Calculate the number of steps or speed reduction stages for this range. This depends on the application as well as space optimization. Higher reduction stages require more space because of more number of gears and shafts requirements.

Select the type of speed reducer or gearbox based on the power transmission requirements, gear ratio, positions of the axis, space available for speed reducer. Also, make sure that for low gear ratio requires single-speed reduction. Select worm gears for silent operation and bevel gears for intersecting axis.

Determine the progression ratio which is ratio maximum speed and minimum speed of output shaft of Gear Box. The nearest progression ratio should be a standard one and is taken either from R 20 or R 40 series.

Draw the structural diagram and kinematic arrangement indicating various arrangement possibilities during speed reduction or increment.

Select materials for gears so that gear should sustain the operating conditions and operating load. Normally cast iron is chosen for housing and cast steel or other alloy can be selected as per the load requirements.

Note down the maximum power output in Horse Power (H.P) or transmission power and revolution per minute of short i.e. rpm of each shaft.

Determine the center distance between the driven and driver shaft based on the surface compressive stress.

Determine the module of gear by beam strength as well as fix the number of teeth required.

Calculate the diameter of the shafts by torque requirements and bending moment considerations.

Calculate the key size, shape or type of transmission keys for each gear.

Select appropriate fit and tolerance for mating parts like shaft and gear.
Select bearings types based on the loading and operating conditions. Also, make sure to include consideration of maximum speed and expected life of gears and gearboxes. Normally ball bearings and roller bearings are used for the small size of gearbox because they have lower thrust requirement due to low-speed reduction.

Make the shaft stepped or provide collar to prevent axial displacement of bearing and gear.

Provide suitable clearance between gears and walls of the housing of a gearbox and based on these considerations design the casing/housing of the gearbox.

Complete the design of the casing. One should Provide fins if necessary to have increased heat transfer by convection and conduction. Put inspection hole/manhole as well as a drain hole to drain lubricating oil. Also, provide an oil level indicator to have the proper amount of oil during operation, if not put, this will lead to failure of gear and shaft due to overheating or due to frictional failure. Provide transportation hooks for transporting the gearbox from one place to another or during installation and maintenance. Also, design gear changing lever accordingly.

Draw neat and clean working drawings in suitable software like AutoCAD, Pro-Engineer etc., indicating required details during manufacturing or assembly.

One can also perform finite element analysis of the complete Gear Box after it completely designed.