356 Budynas-Nisbett:Shigley's Ill.Design of Mechanical 7.Shafts and Shaft ©The McGraw-Hill Mechanical Engineering Elements Components Companies,2008 Design,Eighth Edition Shafts and Shaft Components 353 Splines are essentially stubby gear teeth formed on the outside of the shaft and on the inside of the hub of the load-transmitting component.Splines are generally much more expensive to manufacture than keys,and are usually not necessary for simple torque transmission.They are typically used to transfer high torques.One feature of a spline is that it can be made with a reasonably loose slip fit to allow for large axial motion between the shaft and component while still transmitting torque.This is use- ful for connecting two shafts where relative motion between them is common,such as in connecting a power takeoff(PTO)shaft of a tractor to an implement.SAE and ANSI publish standards for splines.Stress concentration factors are greatest where the spline ends and blends into the shaft,but are generally quite moderate. For cases of low torque transmission,various means of transmitting torque are available.These include pins,setscrews in hubs,tapered fits,and press fits. Press and shrink fits for securing hubs to shafts are used both for torque trans- fer and for preserving axial location.The resulting stress-concentration factor is usu- ally quite small.See Sec.7-8 for guidelines regarding appropriate sizing and toler- ancing to transmit torque with press and shrink fits.A similar method is to use a split hub with screws to clamp the hub to the shaft.This method allows for disassembly and lateral adjustments.Another similar method uses a two-part hub consisting of a split inner member that fits into a tapered hole.The assembly is then tightened to the shaft with screws.which forces the inner part into the wheel and clamps the whole assembly against the shaft. Tapered fits between the shaft and the shaft-mounted device,such as a wheel,are often used on the overhanging end of a shaft.Screw threads at the shaft end then permit the use of a nut to lock the wheel tightly to the shaft.This approach is useful because it can be disassembled,but it does not provide good axial location of the wheel on the shaft. At the early stages of the shaft layout,the important thing is to select an appro- priate means of transmitting torque,and to determine how it affects the overall shaft layout.It is necessary to know where the shaft discontinuities,such as keyways,holes, and splines,will be in order to determine critical locations for analysis. Assembly and Disassembly Consideration should be given to the method of assembling the components onto the shaft,and the shaft assembly into the frame.This generally requires the largest diam- eter in the center of the shaft,with progressively smaller diameters towards the ends to allow components to be slid on from the ends.If a shoulder is needed on both sides of a component,one of them must be created by such means as a retaining ring or by a sleeve between two components.The gearbox itself will need means to physi- cally position the shaft into its bearings,and the bearings into the frame.This is typ- ically accomplished by providing access through the housing to the bearing at one end of the shaft.See Figs.7-5 through 7-8 for examples. Figure 7-5 Arrangement showing bearing inner rings press-fitted to shoft while outer rings float in the 中加00 <00000加9 housing.The axial clearance should be sufficient only to allow for machinery vibrations. Note the labyrinth seal on the right.Budynas−Nisbett: Shigley’s Mechanical Engineering Design, Eighth Edition III. Design of Mechanical Elements 7. Shafts and Shaft Components 356 © The McGraw−Hill Companies, 2008 Shafts and Shaft Components 353 Splines are essentially stubby gear teeth formed on the outside of the shaft and on the inside of the hub of the load-transmitting component. Splines are generally much more expensive to manufacture than keys, and are usually not necessary for simple torque transmission. They are typically used to transfer high torques. One feature of a spline is that it can be made with a reasonably loose slip fit to allow for large axial motion between the shaft and component while still transmitting torque. This is use￾ful for connecting two shafts where relative motion between them is common, such as in connecting a power takeoff (PTO) shaft of a tractor to an implement. SAE and ANSI publish standards for splines. Stress concentration factors are greatest where the spline ends and blends into the shaft, but are generally quite moderate. For cases of low torque transmission, various means of transmitting torque are available. These include pins, setscrews in hubs, tapered fits, and press fits. Press and shrink fits for securing hubs to shafts are used both for torque trans￾fer and for preserving axial location. The resulting stress-concentration factor is usu￾ally quite small. See Sec. 7–8 for guidelines regarding appropriate sizing and toler￾ancing to transmit torque with press and shrink fits. A similar method is to use a split hub with screws to clamp the hub to the shaft. This method allows for disassembly and lateral adjustments. Another similar method uses a two-part hub consisting of a split inner member that fits into a tapered hole. The assembly is then tightened to the shaft with screws, which forces the inner part into the wheel and clamps the whole assembly against the shaft. Tapered fits between the shaft and the shaft-mounted device, such as a wheel, are often used on the overhanging end of a shaft. Screw threads at the shaft end then permit the use of a nut to lock the wheel tightly to the shaft. This approach is useful because it can be disassembled, but it does not provide good axial location of the wheel on the shaft. At the early stages of the shaft layout, the important thing is to select an appro￾priate means of transmitting torque, and to determine how it affects the overall shaft layout. It is necessary to know where the shaft discontinuities, such as keyways, holes, and splines, will be in order to determine critical locations for analysis. Assembly and Disassembly Consideration should be given to the method of assembling the components onto the shaft, and the shaft assembly into the frame. This generally requires the largest diam￾eter in the center of the shaft, with progressively smaller diameters towards the ends to allow components to be slid on from the ends. If a shoulder is needed on both sides of a component, one of them must be created by such means as a retaining ring or by a sleeve between two components. The gearbox itself will need means to physi￾cally position the shaft into its bearings, and the bearings into the frame. This is typ￾ically accomplished by providing access through the housing to the bearing at one end of the shaft. See Figs. 7–5 through 7–8 for examples. Figure 7–5 Arrangement showing bearing inner rings press-fitted to shaft while outer rings float in the housing. The axial clearance should be sufficient only to allow for machinery vibrations. Note the labyrinth seal on the right