Question

a. A shaft is used to transmit power. What does this mean?

b. Explain the general process of choosing a material for a shaft.

c. Discuss the considerations that must be made when selecting the layout of a shaft.

Answer 1

a.) in case of power transmission we use shafts. By means of a rotating shaft we can transfer power from one machine to another. We provide some torque in the transmission shaft, due to that the shaft rotates on its axis, and the load is also rotated by the shaft.

So the power has been transmitted = torque load angular speed of shaft

b.) usually moving parts in high speeds are made of ductile materials. For hiegher strength we use alloy steel to build shafts. Shafts are generally formed by hot rolling, and finished by simple turning operation.

Design of shafts are done by considering allowable shear stress and angle of twist. shear stress is generaterd due to pure torsiona and due to less rigidity of shaft material , twist happens which is not expected. So according to our torque requirment we calculate both the shear stress and angle of twist, and choice material which can withstand these.

For heigher level engineering we also determine the optimal speed in which the shaft will run. In critical speed of shaft, due to eccentricity of center of mass of shaft and centrifugal force on that, shaft may break under bending stress. And that is calculated for fatigue load, which determines the life of shaft.

c.) in general we use cylindrical shaft. But tapered shafts are also used in diferent cases. Sometimes the same shaft has diferent diameter for diferent length. All these are done considering the torsion applied in diferent section of the shaft.

another consideration should be there, that is stress concentration factor. A true design of a shaft can never be completed without fatigue stress consideration. But in the basic level we assume that no fatigue load is there.

Even the load itself is dynamic. If it is sayed that 100Nm torque is being applied on the shaft, we consider it to be 150Nm for fatigue safety. This one is a vital consideration