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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