Page 102 - The Tribology Handbook

Tilting pad thrust bearings A17

Oil flow STARTING UNDER LOAD
Oil is circulated through the bearing to provide lubrica- In certain applications, notably vertical axis machines the
tion and to remove the heat resulting from the power loss. bearing must start up under load. The coefficient of
It is usual to supply oil at about 50°C and to allow for friction at break-away is about 0.15 and starting torque
a temperature rise through the bearing of about 17°C. can be calculated on the Mean Diameter.
There is some. latitude in the choice of oil flow and The specific load at start should not exceed 70% of the
temperature rise, but large deviations from these figures maximum allowable where acceleration is rapid and 50%
will affect the performance of the bearing. where starting is slow.
Where load or torque at start are higher than accept-
The required toil flow may be calculated from the power able, or for large machines where starting may be quite
loss as follows:- slow a jacking oil system can be fitted. This eliminates
friction and wear.
Power Loss (kW)
Oil flow (litredmin) = 35.8 X BEARINGS FOR VERY HIGH SPEEDS AND
Temperature Rise ("C) LOADS
Power Loss (hp)
Oil Flow (US gals/min)= 12.7 x Traditionally the thrust pads are faced with whitemetal
Temperature Rise (OF) and this is still the most commonly used material. But,
with increasingly higher specific loads and speeds the pad
surface temperature will exceed the permissible limit for
whitemetal - usually a design temperature of 130°C.
Two alternative approaches are available:-
EQUALISED PAD BEARINGS 1 The pad temperature may be reduced by
Where the bearing may be subject to misalignment, either (a) Directed lubrication - see Figs. 17.10 and 17.11.
due to initial assembly, or to deflection of the supporting
structure under load an alternative construction can be (b) Adopting offset pivots; accepting their dis-
adopted, although with the disadvantages of increased size advantages.
and expense.
The equalised pad bearing is shown in Fig. 17.12. The (c) Changing the material of the pad body to high
pads are supported on a system of interlinked levers so conductivity. Copper - Chromium alloy.
that each pad carries an equal share of the load.
Misalignment of the order of up to 0.1" (0.002 slope) can 2 Alternatively the pads can be faced with materials able
be accepted. Above this the equalising effect will diminish. to withstand higher temperatures but at increased cost.
In practice the ability to equalise is restricted by the
friction between the levers, which tend to lock when under 40% Tin-Aluminium wiIl operate 25°C higher
load. Thus the bearing is better able to accept initial than whitemetal. Has comparable boundary
misalignment than deflection changes under load. lubrication tolerance and embeddability with
better corrosion resistance.

Copper-Lead will operate 40°C higher than
whitemetal. Poorer tolerance to boundary lubri-
cation and embeddability. Requires the collar
face to be hardened.
.A.
1 I
Polymer based upon PEEK can be used at
temperatures up to 200°C and above. Compar-
able embeddability and better tolerance to
boundary lubrication. Suitable for lubrication by
water and mainly low viscosity process fluids.
Ceramic Pads and Collar Face, made from
silicon - carbide, these can be used up to 380°C
and specific loads up to about 8 MPa (1200
p.s.i.). They are chemically inert and suitable for
lubrication by low viscosity fluids such as water,
most process fluids and liquified gases.

LOAD MEASUREMENT

SECTION 'AA CAGE RING The bearings can be adapted to measure thrust loads
SPLIT-LINE
using either electronic or hydraulic load cells. The latter
can provide very effective load equalisation under mis-
alignment and may be used to change the axial stiffness at
Fig. 17.12. An equalised pad thrust bearing will to avoid resonant vibration in the system.
A17.6