Rule of Twos – Factors that influence the job of lubrication

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I am often asked: “How much grease does this bearing need?” or “What oil flow rate does this bearing need?”

The unfortunate thing about these two questions is that there is no simple answer if the only information given is the bearing part number and its speed (RPM).

There is more than one factor involved in selecting the amount of lubrication for a bearing, but to simplify things, consider breaking it all down into twos.

There are two common types of (rolling element bearing) lubrication: grease and oil.

There are two main jobs for each lubricant type:

• Grease

1. To lubricate.
2. To seal (prevent ingress and remove contamination).

• Oil

1. To lubricate.
2. To maintain an operating temperature at the rolling contact interface.

There are two main lubrication mechanisms:

• Film separation

1. an oil film
2. a dry film

• Boundary lubrication (some surface contact, that is, metal to metal).

There are two main factors that control oil film:

1. Relative speed of the two surfaces.
2. The operating viscosity of the oil at the interface.

Two drops for two weeks

Bear in mind that the only job of the lubricant is to separate the surfaces. So how much is needed for this task?

A researcher, E.R. Booser, ran an experiment in which he ran a ball bearing on only two initial drops of oil at 36,000 RPM for two weeks at 100°C before encountering any failures, writes Piet M. Lugt in his book, Grease Lubrication in Rolling Bearings (Wiley Tribology Series, December 2012). From this we can deduce that not very much grease or oil is needed for lubrication. The question about the amount of grease or oil to select is rather about the other factors of temperature control and cleanliness.

By keeping the “rules of twos” in mind, typical questions can be answered:

Question: Which is a better lubricant, grease or oil?

This all depends on the application:

Is the application heavily contaminated?
> Grease probably better

Does the machine have high shaft speeds?
> Oil is probably better

Are you looking for a simple low-cost design?
> Grease is probably better

Is there heat associated with the process?
> Oil is probably better

Question: Which provides a better lubricating oil film: oil or grease?

Given that the speed is high enough for (elasto) hydrodynamic lubrication (fast enough to create an oil wedge between the rolling surfaces), oil is better as there is typically more than enough oil available to create the film. With grease, there is very little oil available in the lubricating gap. Typically, the lubricating gap is in starvation mode and there is typically some degree of boundary lubrication.

Question: How much oil or grease do I need?

Very little for lubrication, but for “other jobs” the following must be considered:

• Operating temperature of application
  • Is the (base) oil selected viscous enough at the operating temperature to provide a film?
    • If not, then there needs to be cooling (oil flow) to take away the heat and control temperature so the oil can form a separating film.
  • Degree of contamination
    • This will control how frequently you will add grease to take away the contamination.
    • This will help select the appropriate filtration with oil circulation.
    • This will help determine how frequently you will need to change an oil bath.
  • Degree of machine vibration
    • This will control how frequently you will replenish the grease (as vibration causes grease to leave the bearing cavity).
  • The operating temperature
    • This will cause aging of the oil (base oil of the grease/deterioration of the additives).
  • The orientation of the shaft
    • Grease will not stay put in a bearing with a vertical shaft.
  • Outer ring rotation
    • Outer ring rotation centrifuges the grease away from the bearing and can cause oil/soap separation.

There are in fact formalized calculations that can be used to determine how much and how frequently a bearing needs lubrication. These calculations are available on the internet and on smartphone applications that can be downloaded from the respective app stores. But before you start, think twice before you jump into the calculations. Remember the factors that influence the second job of the lubricant.

Douglas Martin is a heavy-duty machinery engineer based in Vancouver. He specializes in the design of rotating equipment, failure analysis and lubrication. Reach him by email at mro.whats.up.doug@gmail.com.

This article is featured in the February 2018 issue of Machinery and Equipment MRO.