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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About Author :
Thomas Yoon's skill in illustration/writing has enabled him to
produce numerous ebooks on engineering subjects that can be
downloaded at Engineering
E-books. He has also produced an ebook on "50 Secrets of Truth
and Life" that is fully illustrated with cartoons in a
humorous way.
