Rolling bearing greases for low-noise applications
Selection criteria for long service life
Published in antriebstechnik 42 (2003) Nr. 11
by Helmut Miller

Low-noise, high-precision running is often of crucial importance for bearings, e.g. in electric motors, fans and precision-engineered mechanical components. Nowadays, low-noise specialty lubricating greases enable many industrial and engineering applications to be effectively mastered. For selecting the right lubricant, however, the individual components involved pose additional requirements: aspects like elastomer compatibility, thermal stability, speed or the bearing’s running time have to be factored in. Depending on the operating conditions concerned, designers can utilize different grease concepts, and underpin their choice with dependable data on the behavior of selected lubricating greases, as validated by test rig studies. In order to maximize rolling bearing lifetimes, precise knowledge of the lubricant’s performance features is indispensable.

Introduction
As more knowledge was gained over the years, a wide range of low-noise lubricants has been created, so meanwhile numerous applications have been successfully mastered. Nowadays, a distinction is made within this grouping between low-noise types and high-purity low-noise lubricants. The latter are required for applications which demand a lubricating grease of high purity, due to the bearing size, high speed, for example, or other parameters involved.

In the case of lifetime-lubricated, non-regreasable rolling bearings, particularly, there are additional stipulations to meet besides low-noise running: as few lubricants as possible, in minimized quantities, are required to assure reliable function of the bearings in the face of increasingly stringent performance specifications, e.g. in the automotive and mechanical engineering sectors. So the rolling bearing as a machinery component also has to be continually improved by means of the lubricant. One important aspect in this context is the need to monitor and influence all variable parameters.

The maximum lifetime of a system or a machine will usually depend on the weakest link involved; in many cases this will be the lubricant and/or contaminants in the lubricant. It is known that, depending on the nature and composition of the contaminants in a rolling bearing, the lifetime of this machine component will sometimes be dramatically reduced. The lubricant and its properties with reference to the requirements and operating conditions involved can thus make a significant contribution to extending the useful lifetime.

Grease concepts for different requirements
Low-noise, and high-purity low-noise rolling bearing greases are subjected to stringent and very comprehensive monitoring mechanisms, by reason of their composition, and more particularly by reason of the sophisticated production engineering involved - e.g. manufacture under specially monitored environmental conditions – and the very rigorous requirements specified for quality. This meticulous rigor is the foundation for ensuring that the rolling bearing’s nominal lifetime is dependably reached.

The grease’s performance capabilities and properties can be significantly varied by both the basic composition and the production engineering involved. Depending on the requirements or applications, different grease concepts are entailed (see figure 1).

Multifaceted stresses
High-purity, low-noise rolling bearing greases are normally used for lubricating relatively small rolling bearings, e.g. deep groove ball bearings 608. Applications involving larger bearings, e.g. angular-contact ball bearings 7020, are encountered in the high-speed range or in the precision-engineering sector. Vital factors here include good thermal stability of the lubricant, plus good start-up characteristics, good anti-wear properties, good compatibility with seals and elastomers, good corrosion-protection characteristics as well as structural stability.

The higher a bearing’s speed, the more intensive the stresses on the lubricant concerned. The increasing internal friction leads to higher bearing temperatures, which in turn may entail increased thermal stress on the grease. Even the tiniest of particles or impermissible wear during running-in will severely impair the rolling bearing as a tribological system, and besides increased bearing noise may also lead to a total failure of the rolling bearing within a very short time. Solid contaminants, depending on their size, nature and quantities, may damage the rolling bearing’s surface in the contact area. Indentations of this kind will often cause local increases in stress levels during subsequent bearing motion, resulting in premature fatigue damage (figure 5).

The sustained presence of sufficiently hard foreign bodies in the rolling bearing will entail abrasive wear of greater or lesser intensity, which in turn leads to running noises, higher internal clearance and guidance inaccuracy. Calculations by the rolling bearing manufacturers have underlined this. Field results, plus mechanico-dynamic tests with a new generation of high-purity, low-noise rolling bearing greases show that even sophisticated requirements in terms of frictional behavior, anti-wear properties, thermal stability and noise behavior can be met (figures 6 and 7).

Crucial selection criteria
Low-noise rolling bearing greases, and high-purity low-noise rolling bearing greases, are a preferred choice for use in

Precise knowledge of the operating conditions and the running-time requirements means that initial estimates of the right lubricant can be arrived at, and provides a basis for future requirements.

Since the rolling bearings will usually have been adequately dimensioned for the specified applications in terms of the bearing lifetime L₁₀, the determinant factor for lifetime in the case of non-regreasable rolling bearings is the grease service life F₁₀. For regreasable rolling bearings, approximately 50 to 70 % of the grease service life is recommended in many cases. However, in order to exploit the lubricant’s capabilities to the full and to enable the used lubricant to be regenerated, or to reach the next major lubrication deadline, it is advisable to determine the regreasing interval taking into account the actual operating conditions involved.

When selecting a low-noise or a high-purity low-noise rolling bearing grease, the maximum and/or minimum service temperature should not be taken as the sole criterion for the final choice. Lubricating greases, like other technical products, are assigned to preferred application categories, and exhibit disparate performance features. Besides service temperature range and noise behavior, aspects like anti-wear properties, friction torque, running-time requirements, elastomer/plastic compatibility and corrosion protection frequently play a crucial role for selecting the right lubricant. Substantiating the individual performance parameters by means of model tests is part of the classification procedures for a rolling bearing lubricant. Close cooperation with the lubricant producer enables the designer to utilize test results, which is why application engineers should always be consulted. 

Reliable data from model tests
Whether a low-noise or a high-purity low-noise rolling bearing grease meets the noise requirements in a rolling bearing can best be assessed under original or field conditions. In many cases, however, noise testing instruments from rolling bearing manufacturers have proved their worth. Approved test rigs like the GRW noise testing instrument, the FAG-MGG11 and the SKF BeQuiet+ machine are available for advance testing. (See the manufacturers’ literature for information on how these test machines function.) These tests provide additional data on the noise behavior of the lubricating grease inside the bearing; the designer thus obtains vital indicators for selecting the right lubricant.

Summary
Low-noise and high-purity low-noise rolling bearing greases make an important contribution to optimum functioning of the rolling bearings concerned. Since in many cases the components involved are relatively small deep groove ball bearings containing very small quantities of lubricant, the lubricants used should possess not only good noise behavior and, if necessary, a high degree of purity, but also good anti-wear properties. Because the requirements for lifetime, speed, temperature, stress, media influences, etc. may vary considerably between different applications, feedback from the field has repeatedly shown that good applications engineering consultancy and lubricant expertise can maximize a rolling bearing’s lifetime. In the case of non-regreasable rolling bearings, in particular, even the tiniest of changes or modifications to the tribological system, e.g. higher or lower temperature, different seal concepts or materials, different installation methods, etc. may entail significant fluctuations in running time. Given running time expectations for grease-lubricated rolling bearing applications in the range of > 100,000 hours or 15 years, close cooperation between the partners and manufacturers involved is all the more vital. Only when the as-is status is accurately known will the best preconditions be in place for ensuring that the right lubricants are chosen and new lubricant generations developed, for meeting the more stringent requirements involved.