Maintenance Technology May 2015 - (Page 30)
BEARING SECRETS
PART 2
The Inner Life
of Bearings:
Lubricant
Selection And
Application
Frequencies
T
his overview of bearing geometries and operating conditions will help simplify the sometimes difficult process of choosing
the right lubricant. Part 1 of this series (Lubrication Technology,
February 2015 issue) explored the three types of common industrial bearings. This concluding installment offers quick tips from
the real world on how to select the correct lubricants for these
bearings and their recommended frequency of application. The
approaches vary.
1. OIL-LUBRICATED BALL AND ROLLER BEARINGS
Lubricating ball and roller bearing isn't difficult. Assuming the
lubricant is clean and contains no water, its most critical aspect is
viscosity. The three heavy horizontal lines in Fig. 1 show a general guide to the minimum recommended viscosities for ball, roller
and roller thrust bearings. The figure shows that for a tapered
roller bearing running at 140 F (40 C), an ISO 46 is the minimum
recommended. (Note: This chart is not applicable for slow-speed
applications-those below approximately 50 rpm-and is drawn
for oil with a viscosity index of 95.)
Ball, radial roller and roller thrust bearings have different
geometries, which necessitate different lubricant viscosities:
■ As a ball bearing rolls, it only has point contact and, thus,
tolerates variations around its path.
■ A plain roller or tapered roller bearing has a line of contact,
which requires a greater film thickness to prevent damage.
■ In an operating thrust bearing, the roller describes an arc
with a tremendous difference in surface speed between the innermost and outermost contact points, which means it requires
an even thicker film to prevent skidding damage.
The typical lubricating oil for rolling-element bearings
will be clean, free of water, and contain the correct additives
for the bearing's ambient working conditions. For example,
in water-contaminated applications, it is good for the oil to
incorporate additives that reduce the chance of corrosion, but
extreme-pressure (EP) additives are only helpful in very-lowspeed applications.
2. GREASE-LUBRICATED BALL AND
ROLLER BEARINGS
A typical grease is roughly 80% oil, 5% additives and 15% thickener. The role of the thickener is to act as a reservoir for the oil,
allowing it to slowly separate as the temperature rises and flow
into the bearing; the viscosities shown in Fig. 1 are still critical.
But because heat tends to deteriorate a thickener over time and
BY NEVILLE SACHS, P.E.
30
LUBRICATION TECHNOLOGY MAY 2015
LubricationTechnologyOnline.com
Special Section
http://www.LubricationTechnologyOnline.com
Maintenance Technology May 2015
Table of Contents for the Digital Edition of Maintenance Technology May 2015
Maintenance Technology May 2015 - Cover1
Maintenance Technology May 2015 - Cover2
Maintenance Technology May 2015 - 1
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