Page 95 - Electric Drives and Electromechanical Systems

88 Electric Drives and Electromechanical Systems

The required peak input speed and acceleration can be determined,

V L 1 1
u i ¼ ¼ 12:7 rev min ¼ 1:3 rad s
pD
_ u i ¼ 13:3 rad s 2
and hence the torque requirement to accelerate the belt drive can be determined,
T in ¼ 1:54 10 3 Nm

nnn

3.4 Bearings

In the case of a rotating shaft, the most widely used method of support is by using one or
a number bearing. A considerable number of different types of bearing are commonly
available. The system selected is a function of the loads and speeds experienced by the
system; for very high-speed application air or magnetic bearings are used instead of the
conventional metal-on-metal, rolling contacts. When considering the dynamics of a
system, the friction and inertia of individual bearings, though small, must taken be into
account.

3.4.1 Conventional bearings
The bearing arrangement for a rotating component, e.g., a shaft, generally requires two
bearings to support and locate the component radially and axially relative to the
stationary part of the machine. Depending on the application, load, running accuracy
and cost the following approaches can be considered:
Locating and non-locating bearing arrangements.
Adjusted bearing arrangements.
Floating bearing arrangements.
The locating bearing design, Fig. 3.11A, one end of the shaft provides radial support
and at the same time locates the shaft axially in both directions. It must, therefore, be
fixed in position both on the shaft and in the housing. Suitable bearings are radial
bearings which can accommodate combined loads, e.g., deep groove ball bearings. The
second bearing then provides axial location in both directions but must be mounted with
radial freedom (i.e., have a clearance fit) in its housing. The deep groove ball bearing and
a cylindrical roller bearing, shown in, illustrate this concept.
In the adjusted bearing arrangements, Fig. 3.11B, the shaft is axially located in one
direction by the one bearing and in the opposite direction by the other bearing. This type
of arrangement is referred to as cross located and is typically used on short shafts.
Suitable bearings include all types of radial bearings that can accommodate axial loads
in at least one direction, for example the taper roller bearings shown.