Page 201 - Improving Machinery Reliability

172 Improving Machinery Reliability

P = imposed equivalent load, lb
PD = pitch diameter, in.
rpm = operating speed
To = outlet oil temperature, OF
Ti = inlet oil temperature, “F (To - Ti, generally about 50°F)
H, = specific heat of oil in Btu/lb/”F (usually -46-.48)
= .195 + .000478 (460 + Ti)

Conversion: (lbs of oilhin) x (0.135) = gallmin
From this equation it would seem that larger quantities of oil are needed than
could reasonably be expected from either oil ring or flinger methods. This discrepan-
cy takes on several orders of magnitude if we realize that with oil-mist lubrication
three rows of bearings with a 3-in. bore diameter use 3 grams of lube oil per hour
and can operate for years.
Observing the bearing manufacturer’s coefficient of friction numbers we note that
the value for angular contact bearings exceeds the one for radial ball bearings by a fac-
tor of 1.5. It stands to reason that angular contact thrust will either require more cool-
ing oil than pure radial bearings, or will run warmer than pure radial bearings. Also,
getting the right amount of oil to an angular contact thrust bearing will both be more
critical and more difficult than getting the right amount to a typical radial bearing.

Bearing Selection Can Make a Difference

API-610, 8th Edition, requires ball-type thrust bearings, if used, to be dual single
row, 40” (0.7 radian), light preload, angular contact type (7000 series), installed
back-to-back. A lot of controversy revolves around this specification clause, with
some bearing manufacturers expressing concern that the term “light preload” does
not adequately quantify the desirable preload. Also, users occasionally report more
failures with preloaded bearings than with conventional bearings. However, neither
observation has presented a dilemma to this writer. Here is why.
The overall intent of providing preload is to prevent axial shuttling of the rotor
and skidding of the rolling elements in a bearing. Skidding can be extremely detri-
mental to rolling element bearings and we have often observed the unloaded half of
a duplex-mounted bearing generate more heat and fail before the loaded half showed
any distress. However, using a preloaded bearing may mandate lowering the custom-
ary interference fit between shaft and bearing inner ring. Unless this is done, addi-
tional heat may not be carried away by certain oil ring and/or flinger arrangements
often found in centrifugal pumps. Also, if marginal lubrication was provided to
begin with, preloading may be “the straw that breaks the camel’s back.” It is in those
instances that flush ground bearings may offer adequate operating preload to assure
quiet operation, minimize ball skidding, and reduce the risk of thermal runaway.
The introduction of PumpacO thrust bearings by the MRC Company has allowed
many users to extend pump mean-time between failure (MTBF). MRC’s thrust bear-