Bearings

Bearings with Sliding Contact.A bearing is a machine part which supports a moving part and confines its motion. That part of a shaft which rotates in a bearing is called a journal. Bearings in which one rubbing surface slides over another are called plain bearings and may be divided into two classes; those with a continuous rotary motion and those with an intermittent motion. To the first class belong journal bearings, which carry a load acting at right angles to the shaft axis, and thrust bearings, which take a load acting in the direction of the shaft axis. To the second class belong bearings of parts having a rocking motion as wrist pins, or a linear reciprocating motion, as crossheads. Bearings with a continuous rotary motion form the great majority of all bearings. They are also only ones in which an oil film pressure sufficient to support the journal can be created by the journal itself. Bearings with an intermittent motion must depend for proper operation either on an outside source for obtaining the necessary oil pressure or on an abundant oil supply and a low specific bearing pressure.

The failure of a bearingwith a sliding contact, or the need of replacement, may be due to excessive wear of the bearing, surfaces, overheating, or cracking of the bearing metal.

Excessive wear.Wear is caused by metal-to-metal contact. Wear cannot be entirely eliminated, but it can be appreciably reduced by providing sufficient bearing area and adequate lubrication.

Overheating.Overheating is primarily caused by metal-to-metal contact because of an excessive load or improper lubrication. Lubrication decreases the danger of overheating. However, overheating may occur even with proper lubrication if the heat dissipation of the bearing is not adequate.

Cracking.The bearing metal may crack if it is subjected to heavy shock loads, such are taking place in the running gear of internal-combustion engines when the compressive stresses in the bearing metal exceed its endurance limit. The remedies are to lower the specific bearing pressure or to use a bearing metal with a proportionately higher endurance limit. However, even in this case the presence of an oil film is useful as a shock absorber.

From the discussion of causes of bearing failure it is evident that reducing friction by proper lubrication means making provision to interpose a liquid film between the rubbing surfaces, and thus to substitute fluid friction for frictional resistance between the metal surfaces of the journal and its bearing.

Theory of Journal Lubrication.Most bearings supporting rotating machine parts are lubricated with oil. Some are lubricated with heavy grease; others are lubricated with water; and a few can run apparently dry if the surface is impregnated with oil or graphite.

According to the hydrodynamic theory of lubrication of rotating journals with oil, the following three phases of lubrication maybe considered: (a) starting of the journal from rest; (b) operating with imperfectly lubricated surfaces; (c) running with perfectly lubricated surfaces.

If two lubricated surfaces are pressed together by a load, the pressure tends to expel the lubricant from between the surfaces. When machinery stands at rest, a large amount of the lubricant will be squeezed out and the metal surfaces will come more or less into contact. When the shaft starts to rotate, the friction of the journal against the bearing is high and a certain amount of abrasion will always occur.

In thin-film lubrication, or imperfect lubrication, there exists an unstable condition, and the metal surfaces may therefore touch each other from time to time. Where there is no metal-to-metal contact, lubrication is termed thick-film lubrication, or perfect lubrication.

Methods of Lubricating Bearings.Bearings may be lubricated intermittently, continuously with a limited supply of lubricant, or continuously with abundant amount of lubricant.

The lubricant for intermittent may be oil or grease.Oil may be applied by dropping it from an oil can into an oil hole in the cap of the bearing.

Grease may be applied by forcing it from a compression grease cup screwed into the bearing cap or by forcing it into a hollow space in the bearing by means of a pressure gun.

Any of these methods, generally speaking, provides only thin-, film lubrication. The coefficient of friction is variable and uncertain.

Continuous lubrication with a limited supply of lubricant may be obtained by use of a grease cup with spring action; by use of an oil reservoir with a wick which carries the oil by capillarity; or by use of a drop oil cup.

Only continuous lubrication with an abundant supply of lubricant insures thick-film lubrication. It may be obtained by means-of ring oiling, splash lubrication; or flooded lubrication, under pressure or without it.

Bearings with Rolling Contact.In a bearing with rolling contact the shaft is supported on rollers or balls. A bearing of this type has the theoretical advantage of reduced friction, but to be practical it must fulfill the following conditions:

a) Unavoidable sliding should be reduced to a minimum.

b) The rolling elements must be properly guided in their motion.

c) All rolling elements should be of exactly the same size.

d) The rolling elements and their guides, or raceways, must be extremely hard an very smoothly polished.

e) The pressure should be approximately normal to the surface of contact.

f) The rolling elements must not be overloaded.

Advantages.Well-manufactured bearings with rolling contact in properly designed applications have the following advantages over bearings with sliding contact:

a) They maintain accurate shaft alignment over long periods of time.

b) They can carry heavy momentary overloads without failure.

c) Power loss caused by friction is small except at high speeds.

d) They are particularly suitable for very low speeds.

e) Starting friction is very low.

f) Lubrication is simple and requires but little attention.

g) Replacement in case of failure is easy.

Disadvantages.Against the advantages just mentioned, rolling-contact bearings have the following disadvantages:

a) The design of the shaft and housing is more complicated.

b) The first cost is higher.

c) The housing diameter is larger, except with some need bearings.

d) The resistance to shock loads is lower.

e) There is more noise especially high speeds.

f) They are

Classification.Bearings with rolling contact may be divided into two main classes; ball bearings and roller bearings. Each of these classes may be subdivided into the following types: (a) radial bearings; (b) thrust bearings and (c) radial-thrust bearings, which can take both radial and axial forces.