The Inner Life of Bearings, Part 2: Lubricant Selection And Application Frequencies

BY Neville Sachs, P.E.

This overview of bearing geometries and operating conditions will help simplify the sometimes difficult process of choosing the right lubricant. Part 1 of this series 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.

Fig. 1. A general guide to recommended viscosities for ball, roller and roller thrust bearings

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-low-speed 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 change the rate of oil separation, some operating conditions will require more oil than others.

Figures 2 and 3 reflect empirical charts developed for and used successfully in setting up lubrication programs in several large manufacturing plants and paper mills. (Note: These charts were not the result of laboratory research, but substantial trial and error.)

Fig. 2. A simple guide to re-greasing intervals for bearings

The information in Fig. 2 can help determine basic re-greasing intervals for ball and roller bearings. The five curves on the chart are for five different bearing bores (shaft diameters). The approach is to use the shaft speed and diameter to define the basic interval.

Referring to Fig. 2, we can see that the suggested re-greasing interval for a 3-in. diameter shaft rotating at 1750 rpm would be 8000 hours. That value, though, has to be modified by the factors that correct for the actual conditions surrounding the bearing, as shown in Fig. 3.

Fig. 3. Modifying factors for recommended re-greasing intervals of ball and roller bearings based on operating conditions. Click to enlarge.

If that 3-in. (75 mm) shaft in Fig. 2 were horizontal, had two spherical roller bearings on it and was running at about 160 F (71 C) in a dry but slightly dusty environment with very low vibration, the corrected re-greasing period would be found by multiplying the original 8000 hours by each of the correction factors as follows:

Actual period = 8000 hours x F_t x F_c x F_m x F_v x F_p x F_b
= 8000 hours x 0.5 x 1.0 x 1.0 x 1.0 x 1.0 x 0.1 = 400 hours

Accordingly, if the referenced plant were running two eight-hour shifts per day, five days per week, the example bearings would need to be greased every five weeks, and would likely be rounded off to once per month. (Note: This approach does not apply to very-slow-speed bearings.)

3. Oil-lubricated plain bearings

The procedure needed to select the correct lubricant for higher-speed plain bearing applications (over 100 rpm) is beyond the scope of this article. As mentioned in Part 1, and in Ken Bannister’s article this month on the path to bearing reliability, if the oil supply to the bearing exceeds the leakage, the bearing will develop its film. Complicating the situation is the fact that the leakage rate will depend on the clearance, ambient temperature, oil ISO grade and the load, and the correct lube specification requires a long and detailed calculation.

Remember that bearings with babbitt contact faces are far more tolerant of contamination than those with bronze or aluminum contact faces. (Note: Bronze and aluminum lack the embedability of babbitt, and any contamination with solid particles approaching the film thickness can lead to upset conditions.)

4. Grease-lubricated plain bearings

The key to successful lubrication of grease-lubricated plain bearings is the additive package in the grease. Decades ago, it was common to find lead in bronze bearings because the lead would liquefy and provide a lubricant film under high pressures. Today’s environmental concerns have virtually eliminated lead as a lubricant component. Science replaced it with various chemicals, including molybdenum disulfide, sulfurized isobutylene compounds and potassium-boron compounds. These formulations have led to outstanding results. Still, there can be a huge difference in performance from one formulation to the next.

When it comes to setting up a re-lubrication schedule for grease-lubricated plain bearings, it is imperative to consult with—and heed the advice of—an experienced lubrication professional and/or your lubricant supplier. The same is true for other important decisions in your lubrication program.

Neville Sachs has extensive experience in machinery reliability and lubrication. The author of two books on failure analysis and a contributor of sections to other books, he has also written more than 40 articles. A Professional Engineer, Sachs holds STLE’s CLS certification, among others. Contact him at sachscracks@att.net.