Bearing Basics

Bearings require lubrication in order to minimize wear and friction. Proper lubrication increases bearing life.

There are two broad classes of bearings: Sliding bearings or rolling element bearings.

Sliding bearings are frequently referred to as plain bearings, journal bearings or sleeve bearings. Ways are also considered sliding bearings.

Sliding bearings are mounted either vertically or horizontally, and the bearing material is usually softer than the shaft or surface being supported. This insures that debris are absorbed in the bearing material rather than the harder, more critical shaft. In addition, the bearings are designed to wear out before the shaft. Sliding bearings are made from materials such as bronze, Babbitt, lead, nylon, or plastic. They are designed to operate under hydrodynamic (full fluid film) or boundary (thin film) lubrication regimes.

Rolling element bearings are frequently referred to as anti-friction bearings. They are a class of bearing in which the moving surface is separated from the stationary surface by elements such as balls, rollers or needles that roll in a controlled manner. If properly lubricated, the load capacity and life of a rolling element bearing is limited primarily by the fatigue strength of the bearing steel.

Improper lubrication of rolling element bearings is a serious issue. Those who oil rolling bearings often overfill these bearings. Too much oil in a bath lubricated bearing can cause churning and oil foaming. Generally, the correct amount of oil should only be half way up the bottom roller when the bearing is at rest.

Bearings are also frequently lubricated by grease, and over-greasing the bearing housing is another common mistake. Excess grease can cause excess pressure, which in turn can lead to seal rupture. Too much grease can also lead to heat buildup. Excess heat often leads to both grease and bearing failure. The proper amount of grease is 1/3 to ½ the volume of free space in the pillow block base for the bearing. Details on how to determine the appropriate amount of grease for a bearing are discussed below.

AMSOIL Barges In    A large East Coast barge manufacturer moves 350 ton barges in and out of the water for servicing on two rail-based 16-wheel trolleys. The purchasing people invested in a $1-per-tube heavy duty petroleum-based lubricant to grease the wheels. These wheels are frequently subjected to high pressure and water. While using the initial grease, their winch could not pull the trolleys and the barge out of the water. The manufacturer instead used tugboats to push the barges out while cranes and other equipment on land were used to supplement the winch. Eventually, several of the winch wheels seized and as a result a crane was damaged.    Luckily for the manufacturer, a solution was found. Someone working for the company sourced a grease made by Lubemaster called Nautica. Nautica is an excellent quality grease and worked well for the job. After Nautica was installed, both the tugboats and the cranes were no longer needed. However, the downside for the manufacturer was that the grease that worked cost over $6 per tube.    Eventually, the barge manufacturer was introduced to AMSOIL. The Manufacturer tried AMSOIL GHD grease, which is a heavy duty, moly fortified NLGI #2 grease with a low 1.8% water washout. The manufacturer has been very pleased with its performance. And the purchasing department is also pleased, since the cost of AMSOIL GHD is about $4 less per tube when compared to the Nautica Grease.

Why Do Bearings Fail
When bearings operate in a clean environment, the primary cause of damage is the eventual fatigue of the surfaces where rolling contact occurs. However, when particulate contamination enters the bearing system, it is likely to cause damage such as bruising, which can shorten bearing life dramatically.

Furthermore, when dirt from the environment or metallic wear debris from some component in the application is allowed to contaminate the lubricant, wear can become the predominant cause of bearing damage. If bearing wear becomes significant due to particulate contamination of the lubricant, changes will occur to critical bearing dimensions which could adversely affect machine operation.

The important parameters influencing bearing wear are contaminant particle size, concentration, hardness and lubricant film thickness. Increases in any of these parameters will increase bearing wear. Increasing lubricant viscosity will reduce bearing wear for a given contamination level.