Shaft Design Project

Figure 1: Machine drive tensioner assembly, including shaft, bearings, bushing connection, and sheave. Background and Key Design Features

In a team of 3 students, I designed and machined an aluminum prototype shaft for tensioning a machine drive. The drive belt is tensioend by a constant force of 100 lbf and the shaft rotates between 100-300 rpm, except when undergoing maintenance or a scheduled shutdown. The shaft is supported by two bearings: a pillow block bearing on one end and a ball bearing on the other. At a distance of 3.375" from one of the ends, the shaft must interface with a 4" 32-teeth timing belt sheave. To achieve this connection, the team selected a 15/16" quick-disconnect clamp-on bushing with 3 1/4-20 screws. 12L14 steel was selected for the material of the final shaft due primarily to its high machinability and strength. The complete shaft assembly is shown in Figure 1.

Figure 2: Exploded view of machine driver tensioner assembly. The final cost of the machine drive tensioner assembly is $202.54, with the shaft itself contributing just over $15 in material cost. See Table 1 for a costed bill of materials. Since the shaft is subjected to a high number of cycles at a relatively low load, the primary failure mode considered is fatigue. The shaft was computed to have an infinite fatigue lifetime with a minimum fatigue safety factor (FSF) of 15.702 at the point of maxmimum bending moment. It is also important to note that there is no keyway on the shaft, as the pressured generated at the interface of the fit between the shaft and the clamp-on bushing was determined to be more than sufficient. Given the low torque requirements of the shaft and the frictional force between the bushing and the shaft, the factor of safety for maintaining no-slip at the interface was calculated to be 90.8. An exploded view of the machine drive tensioner assembly is shown in Figure 2.