Page 297 - Analysis and Design of Machine Elements

2~3 mm Shafts 275

(a) (b) (c) (d)
Figure 10.8 Axially locating and fastening elements on shafts.

Locknuts and lockwashers in Figure 10.8b are suitable for locating elements at the end
of shaft. It can carry large thrust force, but the screw thread may reduce the strength of
shaft. A retaining ring (or snap ring) is frequently used to hold an element in place axially
on a shaft or in a housing bore. As shown in Figure 10.8c, a retaining ring is installed in
the groove cut in the shaft after the bearing is retained in place. Since the groove can be
regarded as two sharp-filleted shoulders positioned close together, stress concentration
is fairly high [1]. Therefore, retaining rings are located where stresses are reasonably low.
A tapered surface between shaft and mounted element is often used on the overhang-
ing end of shaft. A collar and screw are then used to lock the mounted element tightly to
the shaft, as shown in Figure 10.8d. This approach is convenient for assembly and disas-
sembly and can carry large thrust loads. When thrust loads are small, it may be feasible
to rely on press fits to maintain an axial location [2].

(2) Circumferentially locating and fastening elements on a shaft
While transmitting torque and rotatory motion, it is necessary to prevent relative rota-
tion between mounted elements and shafts. Keys, splines, setscrews, pins, interference
fit and tapered fit, can be used to circumferentially locating and fastening elements on
ashaft.
Power transmission elements like gears, pulleys, sprockets and so on are attached
to a shaft by keys or splines to transmit medium to high torques, while setscrews
(Figure 3.3e), interference fit (Figure 10.8c) and pins (Figure 4.7b) are used for light
service. Both keys and splines can be made with a reasonably loose slip fit to permit the
hub to slide axially along the shaft. They are the most effective and economical means
for torque transmission. Setscrews are inexpensive, yet not suitable for applications
where loosening would impose a safety hazard. Interference fit is commonly used in
bearing assembly. However, it has a strict requirement for the manufacturing of the
mating surface and it is inconvenient to assemble and disassemble elements.

10.4.5.3 Machinability and Assemblability of Shafts
The structure of shaft should facilitate manufacture, assembly, measurement and main-
tenance. The dimensions of fillet radius, chamfers, keyways, grooves and suchlike on
different shaft segments with similar diameters has to be consistent. The keyways on
different segments of a shaft should be on the same generatrix.
A grinding-relief groove should be provided for the grinded shaft segment to prevent
the grinding operation from going all the way to the shoulder. Similarly, a relief groove
should be designed for screw fabrication, as shown in Figure 10.9.