Antifriction or rolling-element
bearings use balls or rollers to substitute rolling friction for
sliding friction. This type of bearing has closer tolerances than do
plain bearings and is used where precision, high speeds, and heavy
loads are encountered.
In antifriction bearings, a lubricant
facilitates easy rolling; reduces the friction generated between the
rolling elements and the cages or retainers; prevents rust and
corrosion; and, in the case of grease, serves as a seal to prevent
the entry of foreign material.
High-quality rust- and
oxidation-inhibited (R & O) oils are generally recommended for
bearings. Extreme-pressure and antiwear additives may also be
desirable under conditions of heavy or high-shock loads. Oils with no
additives can easily oxidize and turn gummy. Only once-through drip
applicators can justify the use of straight nonadditivized oils. In
fact, R & O oils are probably just as inexpensive today.
When temperature control and cooling
are a consideration, oil-circulating systems are the choice. A
bearing lubricated with either oil or grease does not carry away heat
on its own. Table below gives general guidelines for the selection of
proper viscosity oils for antifriction bearings.
Most antifriction bearings are grease
lubricated because of the economics of simple seal and housing
designs. Greased bearings offer adequate protection from dirt and
water, and require infrequent attention.
The selection of the proper type and
grade of grease depends on the operating conditions and the method of
application. Generally, soft greases (e.g., NLGI 1) are preferred for
use at low temperatures and in central systems.
Harder greases (e.g., NLGI 2 or 3)
perform better at high speeds. Ball bearings do best with NLGI 2 or 3
grease, while spherical, cylindrical, needle, or tapered roller
bearings with broad face line contact design require NLGI 2 or less.
Care should be taken not to overgrease
antifriction bearings. Generally, the bearing housing should be
one-third to one-half full. Overfilling can lead to several problems:
ruptured seals, excessive temperature buildup, and eventual failure
due to starvation of the bearing for lubricant.
With repeated greasing and continued
high temperature, the oil in the grease may be driven off, leaving
the soap in the bearing. Soap makes up about 8 to 10 percent of
grease. It is easy to see that if the oil portion of the grease
repeatedly is eliminated, the soap can rapidly fill the voids in the
bearing.
Eventually the bearing will accept no
more grease because it becomes filled with soap, a non lubricant. The
bearing then soon fails, and the unlucky grease sales representative
is told that the grease is poor quality.
Many PC software programs are available
that can aid in the selection of lubricants in the initial design and
in failure analysis. These can be obtained from many lubricant
manufacturers.
2 comments:
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Based on the bearings types and speed we want to choose the grease type. For roller and high speed bearings, grade 2 bearing type is suitable one.
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