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Proper lubrication for rolling bearings is essential for reliable operation. Given that some leading bearing companies have stated that incorrect lubrication can account for more than 30 percent of bearing failures, lubrication is a key influence that can make or break bearing service and life.
The lubricant provides a separating film between the bearing rolling elements, raceways and cages to prevent metal-to-metal contact. By controlling surface contact, the lubricant is able to minimize the effect of surface contact, namely undesired friction that otherwise would generate excessive heat, metal fatigue and wear. The lubricant must also prevent corrosion and contamination damage.
Grease has become a common lubricant choice for rolling element bearings. The practical benefits are apparent: grease is easy to apply, can be retained within a bearing’s housing and offers protective sealing capabilities.
Composition is Key
Greases are classified by their stiffness or consistency according to the U.S. National Lubricating Grease Institute (NLGI) and are graded from NLGI Class 000 (very soft) to 6 (very stiff). These classifications are based on the degree of penetration achieved when a standard cone is allowed to sink into the grease at a temperature of 77°F (25°C) for a period of five seconds.
The depth of penetration is measured on a scale of 1 to 10 mm. The softer greases allow the cone to penetrate further into the grease, yielding a higher penetration number. Grease consistencies for industrial applications range between NLGI Class 1 and 3 for relubrication of industrial bearings.
Grease composition is roughly 85 percent base oil (mineral or synthetic) and 15 percent soap or thickener and will vary in different greases.
The base oil is the oil inside the grease that separates and protects surfaces under operating conditions. Thickeners stiffen the mixture to enable it to remain stationary around the moving components. A grease is characterized by its type of thickener such as lithium or lithium complex. Performance characteristics are derived mostly from the oil and additive mixture, but in some cases the thickener also provides unique performance enhancement.
By varying oil viscosities, thickeners and additives, grease manufacturers can build a grease to suit predefined applications and operating conditions.
In turn, mixing grease types at a machine can be long-term fatal and should be avoided. Mixing grease types can be the same as contaminating the lubrication, and the result is either softer grease that allows lubricant to flow away from the application at a lower temperature or harder grease that decreases its ability to lubricate.
Based on SKF’s years of experience performing root cause analysis of failed bearings, it can be said that half of all bearing failures in industrial applications can be attributed to poor or inadequate lubrication conditions caused by the improper selection of the basic grease type for the operating conditions, improper relubrication intervals, mixing incompatible greases, liquid or solid contamination, or overgreasing. Therefore, for optimized bearing performance, it is imperative to select the correct grease to deliver the necessary base oil viscosity in the proper amount, at a given operating temperature and at the required acceptable interval.
Although it may be tempting to favor standardization of a single lubricant in order to increase purchasing power by buying in quantity, production machines have specialized rotating assemblies and, in most cases, lubrication requirements are fairly specific.
When selecting bearing lubrication grease, other conditions should be considered in addition to temperature, speed and load. For example, where bearings are subject to heavy vibrations if grease with low mechanical stability were to be applied, the grease matrix may be destroyed by the vibrations and cause premature bearing failure.
As an overall checklist to help guide users, the following factors are among the most significant in selecting the proper grease for bearing lubrication:
Various grease testing parameters and procedures have been developed to provide users with quality and consistency standards. Some represent industry standards, such as those targeting corrosion protection, wear protection, bleed rates, consistency and mechanical stability. The author’s testing approach goes even further.
Among the more notable parameters subject to SKF’s testing procedures include:
Lubricating greases should protect metal surfaces from corrosive attack in service. The corrosion protection properties of rolling bearing greases can be evaluated using a mixture of lubricating grease and distilled water in a bearing (ASTM D1743).
The bearing alternates during a defined test cycle between standstill and rotation at 80 rpm. At the end of the test cycle, the degree of corrosion is evaluated according to a scale between zero (no corrosion) and five (severe corrosion).
A more severe test method is to replace the distilled water with salt water following the standard test procedure.
Testing can advance further by continuously allowing water to flow or wash through a bearing arrangement during the test cycle, placing greater demands on the corrosion protection properties of the grease.
Specially engineered test machines can determine the life and high-temperature performance limit of lubricating grease (ASTM D3336).
Ten deep-groove ball bearings are fitted into five housings and filled with a given quantity of grease. The test is conducted at a predetermined speed and temperature. Both axial and radial loads are applied and the bearings run until failure.
The time to bearing failure is recorded in hours and a Weibull life calculation is made at the end of the test period to establish the grease life. This information can then be used to determine the relubrication intervals for an application.
In applying the test of roll stability, the consistency of a rolling bearing grease should alter slightly or not at all during the working life of the rolling bearing. The change in the grease structure (amount of softening or hardening) can be evaluated by filling a cylinder with a prespecified quantity of grease. A roller is placed inside the cylinder and the complete unit is rotated for two hours at room temperature (ASTM D1403). At the end of the test period, the cylinder is allowed to cool to room temperature and the penetration of the grease is measured.
(Weld Load Resistance) Test
Three steel balls are held in a cup and a fourth ball is rotated against the three balls at a given speed (ASTM D2596). A starting load is applied and increased at predetermined intervals until the rotating ball seizes and welds to the three stationary balls. The test indicates the point at which the extreme pressure limit of the grease is exceeded.
(SKF ROF and R2F Tests for ball and roller bearings, respectively)
This test is conducted by measuring the grease life of bearings size mounted in actual housings. The test method is carried out at normal and elevated temperatures and at higher speeds to shorten testing times.
Groups of bearings must operate satisfactorily without increased torque or wear for a specified time. Using the Arrhenius aging principle and applying it to the test temperature enables an estimation of the grease life at other temperatures. Ball bearings are used for the ROF test and spherical roller bearings for the R2F test.
A further test can be made using the WAM machine that measures the lubricant film thickness of a ball running on an optical disc.
(Grease Noise Level - SKF BeQuiet Test)
Independent test methods have been developed to measure the amount of solid contaminant, either soft clumps of thickener or hard particles, manufactured into the grease based on the amount of noise the grease generates using a high-frequency sound measurement profile.
A set of high-quality, high-sensitivity bearings are lubricated with the candidate grease and placed in housings in the test stand. As the elements roll over the clumps and particles the generate high frequency sound emissions, which are measured in three different frequency bands (high, medium and low) and categorized into five grades or levels.1 The number of vibration peaks above certain reference levels determines the grease noise classification. The outcome is SKF’s Grease Noise Classification (GN), ranging from GN4 for extremely quiet running to GN0 for noisy.
A rating of GN4 is best. Greases with this level of cleanliness, based on the absence of noise-causing clumps and particles, are manufactured under climatically controlled conditions, and should be considered to be premium-quality products.
Grease Life and Relubrication
As noted, the life expectancy of a grease for rolling bearings will rise or fall based on many factors. But when and how should relubrication be triggered? Small, lightly loaded ball bearings may not require any relubrication during a machine’s lifetime, making them prime candidates for sealed or shielded “sealed-for-life” bearings.
Bearings not sealed for life or factory-filled should be filled with grease with sufficient free space in the housing (up to 50 percent) to allow room for the excess grease to be ejected from the bearing during startup. Filling the bearing with grease should be one of the last operations completed when mounting a replacement bearing to ensure cleanliness and minimum contamination. (A complete fill is acceptable with low-speed operating conditions where overheating is not a concern.)
Determining an application’s relubrication intervals will hinge on the influencing conditions, such as temperature, speed, load, bearing arrangement and type. Environmental considerations can also prompt an increase in relubrication frequency. In heavily contaminated settings (quarries and foundries, for example), very short intervals between visits, or perhaps even continuous relubrication will be the rule. Single or multipoint automatic relubrication is used to fulfill these tasks.
In applications where manual lubrication is selected, users should confirm that the lubrication fitting is clean, the right type of lubricant is used and the correct quantity of lubricant supply is set. Grease cleanliness is as important as the proper amount. Extra caution should be given to assure contaminant-free relubrication. If contaminated grease is placed into a system, it can potentially cause more damage than a lack of lubrication altogether.
Over time, the lubricant in a bearing arrangement will naturally lose its lubricating properties. This underscores the necessity for careful attention to original lubricant selection and indicates advantages in partnering with knowledgeable and experienced suppliers from the start.
Kuhl, R. and Haag, C. “Grease Noise in Roller Bearings.” Noise generation in bearings and its effects, influence of the lubricating grease, grease noise tester, test procedures and requirements.
Bichler, Max and Roth, Christian. “Test Rig Uses Noise to Measure Grease Cleanliness.” Practicing Oil Analysis magazine. May-June 2004.