PART II Design of a Mechanical Drive

2-1 Objectives and content of Part II A mechanical drive is called sometimes a power transmission that serves the following functions: 1) It receives power from some kind of rotating source such as an electric motor, an internal combustion engine, a gas turbine engine, a hydraulic or pneumatic motor, a steam or water turbine, or even hand rotation provided by the operator. 2) The drive typically causes some change in the speed of rotation of the shafts that make up the drive so that the output shaft operates more slowly or faster than the input shaft. Speed reducers are more prevalent than speed increasers. 3) The active elements of the drive transmit the power from the input to the output shafts. 4) When there is a speed reduction,there is a corresponding increase in the torque transmitted.conversely,a speed increase causes a reduction in torque at the output compared with the input of the drive.

2-1-2 The process of designing a mechanical drive 1)The nature of the driven machine: it might be a machine tool;an electric drill;the axle of a farm tractor; the propeller shaft of a turbojet for an airplane; 2)The level of power to be transmitted: From the examples just listed 3)The rotational speed of the drive motor or other prime mover: Typically the prime mover operates at a rather high speed of rotation. The shafts of standard electric motors rotate at about 1,200,1,800,or 3,600 ( revolutions per minute (r.p.m.). 4)The desired output speed of the transmission: This is highly dependent on the application r.p.m.

What should be done by designer? Choose the type of power transmission elements to be used: gears, belt drives, chain drives, or other type. Specify how the rotating elements are arranged and how the power transmission elements are mounted on shafts. Design the shafts to be safe under the expected torques and bending loads and properly locate the power transmission elements and the bearings. Specify suitable bearings to support the shafts and determine how they will be mounted on the shafts and how they will be held in a housing. Specify keys to connect the shaft to the power transmission elements;couplings to connect the shaft from the driver to the input shaft of the transmission or to connect the output shaft to the driven machine;seals to effectively exclude contaminants from entering the transmission;and other accessories.

Place all of the elements in a suitable housing that provides for the mounting of all elements and for their protection from the environment and their lubrication.