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Lubrication for Life, Lubricant Selection is Often Overlooked in the Design Process - Part 1 

For rolling element ball bearings, determining the right lubricant is essential to maximize the life of the
bearings and machinery and in turn, save money, time, and manpower. Unfortunately, many engineers
fail to consider the multiple factors involved in the selection process. In this technical article, AST shares
their holistic, strategic tips to help you identify the right lubricant for your product.

Introduction
Lubrication plays a vital role in the performance and life of rolling element bearings, yet its significance
is often underestimated. The most important task of the lubricant is to separate parts moving relative to
one another (balls or rollers and raceways) in order to minimize friction and prevent wear. A lubricant
that is designed for specific operating conditions will provide a load bearing wear protective film. The
ideal condition is when this film separates the friction surfaces. In addition to providing this load bearing
film, the lubricant should also allow for the dissipation of frictional heat to prevent overheating of the
bearing and deterioration of the lubricant. The correct lubricant will also provide protection from
corrosion, moisture, and the ingress of contaminants.

Lubricants used in rolling element bearings should have the following characteristics:

● Maintain a stable viscosity over a broad range of temperatures
● Good film strength that can support loads
● Stable structure that provides for long service life
● Non-corrosive and compatible with adjacent components
● Provide a barrier against contaminant and moisture that does not leak out of the bearing

Types of Lubricants
● Oils: Both petroleum based and synthetic oils are available. Examples of synthetic oils are
silicone, diesters, PAO’s, and fluorinated compounds. Bearings lubricated with oil exhibit less
start-up and running torque and have higher speed capability. However, because oils are subject
to evaporative losses, their service life in a bearing is less than that of grease. Miniature and
instrument bearings are often only lubricated once for the life of the bearing, making the choice
of lubricant critical. Larger bearings are subject to re-lubrication as part of the machinery
maintenance cycle.These bearings are often lubricated via oil recirculation systems that are
designed into the machinery or equipment. Key characteristics to consider when selecting an oil
include temperature range, viscosity, and evaporative rate.

● Greases: Greases consist of a base oil with a thickener added. These thickeners consist primarily
of metal soaps (lithium, sodium, aluminum, and calcium), organic (ureas), or inorganic
compounds. While these thickeners greatly influence the characteristics of the grease, the
lubricating properties of the grease are attributable to its base oil. Additionally, grease can
contain additives that improve its performance. Additive types include antioxidant,
anticorrosion, anti-wear, fillers, fortifiers, and extreme pressure fortifiers. Temperature range,
base oil viscosity, and stiffness or penetration level are key characteristics to consider when
selecting a grease. Most greases used in rolling element bearings are NLGI grade 2.

● Solid Films: These are non-fluid coatings applied to the friction surfaces to prevent wear. They
are used in harsh situations such as extreme temperatures, vacuum, or radiation where oil or
grease cannot survive and are typically selected as a last resort, or option. These coatings
include graphite, MoS 2, silver, gold, or PTFE. Hard coatings include TiC or chrome. Solid films are
engineered on a specific application-by-application basis.

The lubricant selected and amount used also impact the maximum operating speed and torque, both
starting and running. In miniature bearings, the lubricant can impact the noise level. Filtered greases and
oils are recommended for use with miniature or instrument bearings.

Learn about the selection factors AST engineers recommend for selecting the ideal lubricant in
Lubrication for Life – Part 2. Coming October 2.