Basics of Bearing Life:
L10= (C/P)10/3 x a x106/60
L10 estimated bearing life in hours; estimated as the 90% reliability hours
C = radial rating of the bearing
P = radial load on the bearing
a = life adjustment factor; ex: oil viscosity, oil quality, etc.
n = rotational speed in rev/min
Doubling load reduces life to one tenth
Doubling speed reduces life by one half
Drive Alignment and Belt Tension:
From this we can see that bearing loading is very important. The engineer has done the calculations and
sizing are done for defined conditions, but it is important that the customer assures that with belt tension
and drive alignment, the design loads are not exceeded. Bearing life is reduced substantially when these
are not maintained within design limits (recommendations are in the service manual).
How often should the oil be changed?
It depends on:
• Oil type and viscosity
• Operating environment
• Oil temperature
Some studies have found that fatigue life of ball bearings is reduced 48% when mineral oil lubricant is
contaminated with just .002% water. When the water was increased to 6% the bearing life was reduced
by 83%. With steam, condensable loads and water injection this should watched closely. Blower vents
play an important part in mechanical seal durability and should be kept free of liquids by draining often.
The only sure method is to use oil sampling and change as indicated. Oil samples should be taken
immediately after shutting down the blower. This assures accurate sampling of all solids in the oil. Also,
a fresh sample of oil should be submitted on the initial test that will be used as a base line for all future
tests. Three elements normally are reported: oil health, contaminants and wear metals.
Tuthill Vacuum & Blower Systems
4840 West Kearney Street
Springfield, Missouri USA 65803-8702
o 417.865.8715 800.825.6937 f 417.865.2950
Detecting a Failure:
Bearing failures can occur at very inconvenient times and can propagate a lot of collateral damage in the
failure process (gears, housings, rotors, etc.). SPM’s (shock pulse monitors) are commonly used to detect
bearing failures. With blowers, this detection is not as simple as other equipment as the blower generates
a strong discharge pulse which can cause false triggers in the alarm. These can be overcome with
software and adjusting alarm set points when the system is new and in good condition. Changing the
blower discharge pressure will impact the vibration level and will probably require resetting the alarms.
Selecting an Oil:
Oil viscosity and grade should always be selected per the manufactures recommendation. Using lighter
oil will reduce gear and bearing life while running heavier oils will increase blower oil temperature and
reduce gear life.
Synthetic vs Mineral:
Synthetic even though more expensive than mineral oil is recommended in most applications. It
advantages include broader viscosity control with temperature changes, better elevated temperature
lubrication and longer service life. Also, more resistant the water contamination.
Consider this when discharge temperatures are above 250F. Reducing oil operating temperature will
extend oil life and therefore bearing life. Oil temperature should be maintained between 180F and 200F.
Cooling coils are available if the discharge temp exceeds 250F.
Always run the blower at the correct oil level. Too much oil is just as bad as too little. Too much oil
increases the load on the gears and will likely result in a gear failure. Too little will result in a lubrication
failure (failed bearings and gears). Only check the oil when the blower is shut off. The levels are not
accurate when it is running.