Greasing Bearings - How Much is Enough?

By: Thomas Yoon

Previously, we have talked about using suitable greases for different applications. Basically, we want to use low temperature greases for low temperature applications and high temperature greases for high temperature applications. The reason is quite simple - we want the grease to form a thin film of lubricating oil between the rubbing surfaces.

If we use high temperature grease for normal temperature applications, the chances are the grease will still be in semi-solid state and will not flow to cover the contact surfaces of the moving components during operating conditions.

Assuming you have chosen the correct grease, how do you determine how much you need to put into the bearing?

Excessive grease lubrication can easily cause overheating. The grease gets churned around within the moving parts of the bearing and has nowhere to go. The temperature rises. The grease becomes the wrong temperature selection even though the application is correct.

A general rule to follow is that the bearing should be filled completely but the free space in the housing only partially. This gives room for the grease to be ejected from the bearing on start-up.

However, there is some grease, the so-called "totally-filled" greases like lithium soap greases that can allow filling up to 90% of the free space in the housing, without risk of a temperature rise. This is because they are special. Their stability at high temperatures is excellent and can be utilized over a wider temperature range than sodium soap greases.

By filling up all the free space, impurities are effectively prevented from entering and damaging the bearings and the lubricating intervals can be extended.

For most other greases, the general rule applies.

Bearings can be divided into two categories - non-separable and separable bearings. No matter which bearing type it is, the general practice is to fill up the space between inner race, the outer race, and the rolling components (ball or roller) on both sides of the bearing.

Because of its consistency, the grease should be able to remain in place without dropping off. In this way, we can ensure that the rubbing contact surface actually has grease on it.

For relubrication, how much is enough? The following formula gives a good indication:

G = 0.005 DB

where,

G = grease quantity in grams
D = bearing outside diameter in mm
B = total bearing width in mm

By practicing proper lubrication, the bearings should be able to last for a long time. However, bearings can still fail if it has not been installed properly or for other reasons.

Until next time...

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