Page 388 - Shigley's Mechanical Engineering Design

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Shafts and Shaft Components 363
Axial Layout of Components
The axial positioning of components is often dictated by the layout of the housing and
other meshing components. In general, it is best to support load-carrying components
between bearings, such as in Fig. 7–2a, rather than cantilevered outboard of the bear-
ings, such as in Fig. 7–2c. Pulleys and sprockets often need to be mounted outboard for
ease of installation of the belt or chain. The length of the cantilever should be kept short
to minimize the deflection.
Only two bearings should be used in most cases. For extremely long shafts carrying
several load-bearing components, it may be necessary to provide more than two bearing
supports. In this case, particular care must be given to the alignment of the bearings.
Shafts should be kept short to minimize bending moments and deflections. Some
axial space between components is desirable to allow for lubricant flow and to provide
access space for disassembly of components with a puller. Load bearing components
should be placed near the bearings, again to minimize the bending moment at the loca-
tions that will likely have stress concentrations, and to minimize the deflection at the
load-carrying components.
The components must be accurately located on the shaft to line up with other
mating components, and provision must be made to securely hold the components in
position. The primary means of locating the components is to position them against a
shoulder of the shaft. A shoulder also provides a solid support to minimize deflection
and vibration of the component. Sometimes when the magnitudes of the forces are
reasonably low, shoulders can be constructed with retaining rings in grooves, sleeves
between components, or clamp-on collars. In cases where axial loads are very small, it
may be feasible to do without the shoulders entirely, and rely on press fits, pins, or col-
lars with setscrews to maintain an axial location. See Fig. 7–2b and 7–2d for examples
of some of these means of axial location.

Supporting Axial Loads
In cases where axial loads are not trivial, it is necessary to provide a means to transfer
the axial loads into the shaft, then through a bearing to the ground. This will be partic-
ularly necessary with helical or bevel gears, or tapered roller bearings, as each of these
produces axial force components. Often, the same means of providing axial location,
e.g., shoulders, retaining rings, and pins, will be used to also transmit the axial load into
the shaft.
It is generally best to have only one bearing carry the axial load, to allow greater
tolerances on shaft length dimensions, and to prevent binding if the shaft expands due
to temperature changes. This is particularly important for long shafts. Figures 7–3
and 7–4 show examples of shafts with only one bearing carrying the axial load
against a shoulder, while the other bearing is simply press-fit onto the shaft with no
shoulder.

Providing for Torque Transmission
Most shafts serve to transmit torque from an input gear or pulley, through the shaft, to an
output gear or pulley. Of course, the shaft itself must be sized to support the torsional stress
and torsional deflection. It is also necessary to provide a means of transmitting the torque
between the shaft and the gears. Common torque-transfer elements are:
• Keys
• Splines
• Setscrews