Page 220 - Tribology in Machine Design

Sliding-element bearings 205
Figure 5.24
centre of curvature is indicated by a cross. Generally these bearings give
Many of the bearings with unconventional geometry have been de-
veloped principally to combat one or another of the causes of vibration in
high-speed machinery. It should be noted, however, that the range of bearing
properties due to the different geometric effects is so large that one must be
relatively careful to choose the bearing with the proper characteristics for
the particular causes of vibration for a given machine. In other words, there
is no one bearing which will satisfy all requirements.
5.6.1. Journal bearings with fixed non-preloaded pads
The bearings shown in Fig. 5.24 are, to a certain extent, similar to the plain
journal bearing. Partial arc bearings are a part of a circular arc, where a
centrally loaded 150° partial arc bearing is shown in the figure. If the shaft
has radius R, the pad is manufactured with radius R + c. An axial groove
bearing, also shown in the figure, has axial grooves machined in an
otherwise circular bearing. The floating bush bearing has a ring which
rotates with some fraction of the shaft angular velocity. All of these bearings
are called non-preloaded bearings because the pad surfaces are located on a
circle with radius R + c.
Partial arc bearings are only used in relatively low-speed applications.
They reduce power loss by not having the upper pad but allow large vertical
vibrations. Plain journal and axial groove bearings are rarely perfectly
circular in shape. Except in very few cases, such as large nuclear water pump
bearing which are made of carbon, these are crushed in order to make the
bearing slightly non-circular. It has been found that over many years of
practical usage of such bearings, that inserting a shim or some other means
of decreasing the clearance slightly in the vertical direction, makes the
machine run much better.
Cylindrical plain journal bearings are subject to a phenomenon known
as oil whirl, which occurs at half of the operating speed of the bearing. Thus,
it is called half-frequency whirl. Axial groove bearings have a number of
axial grooves cut in the surface which provide for a better oil supply and
also suppress whirl to a relatively small degree. Floating bush bearings
reduce the power loss as compared to an equivalent plain journal bearing
but are also subject to oil whirl. All of these bearings have the major
advantage of being low in cost and easy to make.
5.6.2. Journal bearings with fixed preloaded pads
Figure 5.25 shows four bearings which are rather different from the
conventional cylindrical bearings. The essence of the difference consists in
that the centres of curvature of each of the pads are not at the same point.
Each pad is moved in towards the centre of the bearing, a fraction of the pad
clearance, in order to make the fluid film thickness more converging and
diverging than it is in the plain or axial groove journal bearings. The pad
good suppression of instabilities in the system but can be subject to