Page 195 - Tribology in Machine Design

180 Tribology in machine design

difference between static friction and kinematic friction. Here, a coefficient
of friction for rotating bearings, is defined as the tangential moving force at
the mean radius of the active area divided by the applied normal load.
Hydrostatic bearings are used for reciprocating platens, for rotating
telescopes, thrust bearings on shafts and in test rigs to apply axial loads to a
member which must be free of any restriction to turning. Journal bearings
support a rotating shaft by a different method, but the larger bearings will
have a built-in hydrostatic lift for the shaft before it is rotated, to avoid
initial metal-metal contact.
To give stability to the pad, three or four recesses should be located near
Figure 5.6
the edges or in the corners, as shown in Fig. 5.6. However, if one pump is
freely connected to the recesses, passage of all the fluid through one recess
may occur, tipping the pad and giving no flow or lift at an opposite recess.
Orifices must be used in each line from the pump to restrict the flow to a
value well below the displacement of the pump, or a separate pump can be
used to feed each recess.
Air or inert gases are used to lift and hydrostatically or hydrodynami-
cally support relatively light loads through flat, conical, spherical and
cylindrical surfaces. Unlike oils, air is nearly always present, it does not
contaminate a product being processed by the machine, its viscosity
increases with temperature, and its use is not limited by oxidation at
elevated temperatures. Its viscosity is much lower, giving markedly less
resistance to motion at very high speeds. Air and gases are compressible,
but the equations derived for incompressible fluids may be used with minor
3
error if pressure differences are of the order of 35 to 70 x 10 Pa.
Numerical example
Design an externally-pressurized bearing for the end of a shaft to carry
4536 N thrust at 1740r.p.m., with a minimum film thickness of 0.05mm
using SAE20 oil at 60 °C, pumped against a pressure of 3.5 MPa. The
overall dimensions should be kept low because of space restrictions.
Assume that the mechanical efficiency of the pump is 90 per cent.

Solution

The choice of a recess diameter, d, is a compromise between pad size and
pump size. Since a small outside diameter is specified, a relatively large ratio
of recess to outside diameter may be tried, giving a 3.5 MPa uniform
pressure over a large interior area. Let d/D = 0.6. From eqn (5.12)

From a viscosity-temperature diagram, the viscosity of SAE20 oil at 60 °C
is 0.023Pas, and from eqn (5.14), the pump must deliver at least