Troubleshooting Bearing Lubrication

Lubrication is essential for proper bearing life, but even so, about 70% of bearing failures occur for reasons other than their lubrication quality or quantity. Still, users of industrial equipment will very often blame the lubricant used when a bearing failure occurs.

We often hear the term ‘lubrication failure’, implying that there was no oil or grease in the bearing. In most cases the answer is not that simple, because the question that should be asked is: “Why did the lubricant fail to prevent damage to the bearing?”

The answer to this question is not so obvious, because it involves investigating much more than the lubricant.

Lubrication-related failures occur primarily as a result of three possible situations. The lubricant used was either unsuitable, inadequate, or excessive.

Unsuitable lubricant: This is a lubricant that, when used in a particular bearing application, does not contain the suitable additives, is of an incorrect viscosity, or may not be designed for use in such an application or temperature range (i.e. grease should not be used where oil is recommended, and vice versa).

Inadequate lubricant: Viscosity of the oil, either as oil itself or as the oil content in grease, is the most important property of any lubricant. The viscosity/temperature relationship is critical to the quantity of lubricant that any bearing might require at a given time. If the viscosity is too high (thick) relative to the temperature, insufficient oil will flow to (or through) the bearing.

If the viscosity is too low (thin), the oil will not be sufficient to maintain a separating film between the rolling elements and raceways of the bearings. In either case, the asperities (microscopic machined high points) of the bearing component surfaces may contact each other, initially causing a frosted or smearing condition, followed by adhesion at the contact points (see fig. 3). Failure of the bearing will be inevitable.

As a general guideline, non-vibrating, lightly loaded bearings operating at temperatures of 70C or less and running at high speeds can operate very effectively using an anti-wear or R & O (rust and oxidation) oil with a viscosity range of ISO 32-46 cSt (centistokes). Bearings running at higher temperatures may require higher viscosity oil of 68 cSt, particularly at heavy loads. These applications may also require oils with EP (extreme pressure) additives.

For applications where the ambient temperatures are at 0C or less, viscosity index-improved oils of ISO 15 cSt or 22 cSt should be used.

Minimum oil viscosities for low- and medium-speed bearings at normal operating temperatures should not be less than 13-20 cSt and not less than 30 cSt for rolling element thrust bearings, with the generally accepted optimum viscosity in the range of 13-50 cSt.

This optimum viscosity range depends upon bearing RPM, size, type and load. For high-speed bearings such as spindle bearings, minimum viscosity is 6-10 cSt. There are several methods of calculating the ISO viscosity selection for bearing lubrication. These methods include the formulae in Table 1.

In grease applications, the results will be the same if the viscosity/temperature relationship is such that oil will not flow from the grease thickener in sufficient quantities to protect bearing surfaces under all operating conditions. For this reason grease consistency grades are critical in these applications, in addition to the viscosity of the oil contained in the grease.

The National Lubricating Grease Institute (NLGI) has established nine consistency grades, based on the worked penetration of greases under test conditions.

These grades run from triple zero (000), which has a consistency similar to a high-viscosity oil for typical use in a centralized lubrication system, to a number six, which is a block grease.

Common greases used in machinery applications with ambient temperatures in a range of 60-75F (16-24C) might require a consistency of two or three with the appropriate oil viscosities, to ensure sufficient separation of the oil from the grease thickener under these operating conditions. These grease selections would also depend upon bearing speeds (see Tables 2 and 3).

Excessive lubricant: This is frequently the cause of higher than normal bearing operating temperatures. Excessive grease or oil quantities cause internal friction within the lubricant, which in turn promotes excessive temperatures, causing oxidation and premature lubricant and bearing failure.

Oil levels that are too high and excessive quantities of grease in bearings cause a churning action within the rotating components and the result will always be an increase in temperature.

Oil of too high viscosity, or grease with a too high consistency, will also increase operating temperatures. Care must be taken therefore, when investigating high temperatures, that the troubleshooter consider not only the possibility of excessive lubricant, but that the correct lubricant for the application is in use.

As a rule of thumb, if the troubleshooter cannot hold a hand comfortably on the bearing housing, whether on an electric motor or gear reducer, the temperature is too hot.

Another quite common error made by some inexperienced technicians is to fill new sealed bearings with grease using a syringe under the bearing seal. This is a serious mistake! Churning will occur, internal temperatures will rise, oxidation will take place within the lubricant and premature bearing failure will result.

Sealed bearings are shipped from the manufacturer with approximately 20% of the bearing cavities grease-filled. No more lubricant is required. Many bearings fail as a direct result of excessive lubrication.

The standard tube of grease used in the common grease gun contains 400 grams of grease and typical cylindrical roller bearings with a 6-in. OD and 4-in. ID, operating at 1,800 RPM, only requires about 35 grams of grease applied every two-and-one-half months or 1,825 hours, when the bearing is operated in ambient room temperatures of 65F (18C). However, when determining re-lubrication intervals, bearing operating temperatures must be considered.

If we use the example noted previously and the actual operating temperature of the cylindrical roller bearing is 130F (54C), the regreasing interval should be reduced to about 900 hours (or every five weeks) using 35 grams of grease.

The general rule of thumb is this: The service life of grease-lubricated bearings is reduced by half for every 27F (15C) increase in temperature above 160F (70C).

If, for example, the calculated relubrication interval for a given bearing is 1,000 hours at 70C, this interval must be cut in half to 500 hours if the actual operating temperature is 85C (185F).

To calculate the required amount of grease in ounces to relubricate a bearing in service, use the following calculation:

Bearing OD (in.) X Bearing Width (in.) X the constant 0.114 = grease quantity in ounces (i.e.: 3 in. OD X 0.75 in. Width X 0.114 = 0.25 oz. of grease.

Where bearings are specified in metric dimensions, the following calculation may be used:

Bearing OD (mm) X Bearing Width (mm) X the constant 0.00018 = grease quantity in ounces

The initial grease pack for a bearing before installation should be three times either of the above results.

Troubleshooting tips

The first action that should be taken by the troubleshooter when investigating the cause or causes of a bearing problem is to familiarize oneself with the following conditions, regardless of the bearing’s location or the type of machine in which it is installed.

What is the recommended operating temperature of the bearing? Compare this with the actual operating temperature using an accurate testing device, such as an SKF ThermoPen or a hand-held Flir Thermacam infrared camera.

Measure the noise level using a device such as the UE Systems ultrasound tester. If noise l
evels are increasing above those normally experienced, it could indicate insufficient lubricant, vibration, premature spalling, or reduced internal clearance, due to higher than normal temperatures or poor bearing installation. (Keep in mind that noise is frequently accompanied by high temperatures).

If insufficient lubricant is suspected, determine if the bearing has the correct amount of oil in the housing and add more if necessary. If the bearing is grease-lubricated, pump a shot or two of grease into the bearing. If the noise level does not change after a few minutes, insufficient lubricant is not the cause.

Often, noise is associated with mechanical looseness or some other condition that may cause vibration at or near the bearing. A stroboscope will very quickly indicate whether or not a vibration is present.

Vibration or noise may also be the result of an overloaded bearing or a bearing rotating at excessive speeds; using a digital tachometer, speeds can quickly and accurately be determined, then compared with specifications.

Often noise is associated with defective seals that may be rubbing on the bearing’s shaft. This condition may also increase the operating temperature (near the lip of the seal) as well as cause lubricant leakage or seepage past the seal. A groove at the seal lip may be observed.

If a leaking seal is obvious or suspected, contamination may have entered the bearing, causing premature damage. An oil or grease sample should be obtained and analyzed for higher than normal wear metals and contaminants, particularly dirt or water.

In a grease-lubricated bearing, a leaking seal combined with high temperatures might indicate that the grease has reached or exceeded its ‘dropping point.’ Confirm that the grease in use is the recommended product and that it has not been mixed with an incompatible type of grease.

Incompatibility also might lead to a seal leak, as the two incompatible greases react with each other and oil separates from the thickening agents in either (or both) greases.

On machines using ring oiling of the bearings, ensure that the rings are in fact rotating. If they are worn or not rotating, oil will not be picked up and distributed by the ring. High temperatures, noise and eventual premature bearing failure will be the result.

On centralized lubrication or oil mist systems, ensure that the system is calibrated properly and is distributing the correct amount of grease or oil to the affected bearings.

Finally, the troubleshooter should be thoroughly familiar with the machine itself, its overall operating conditions and the processes or applications for which the machine is used. Above all, remember that about 70% of bearing failures are not lubricant or lubrication related, although they may appear to be.

Click here to view the tables from the story

Contributor Lloyd (Tex) Leugner is the principal of Maintenance Technology International Inc. of Cochrane, Alta., a company that specializes in the resolution of maintenance and lubrication problems and provides training for industry. He can be reached at 403-932-7620 or texleug@shaw.ca.

References: The Practical Handbook of Machinery Lubrication, 3rd Edition, L. Leugner; SKF Bearing Maintenance Handbook, The SKF Manufacturing Group; Care and Maintenance of Bearings, The NTN Bearing Corporation; Failure Atlas for Hertz Contact Machine Elements, 2nd Edition, T.E. Tallian.