Bearings

There are so many different types of bearings, this page is going to be all about the rotating type of bearing and we will leave the sliding bearing for another day.

Air Bearing – uses an air cushion to separate its rotating surfaces.

Attitude Angle – the angle between the steady state preload through the bearing centreline, and a line drawn between the bearing centre and the shaft centreline.

Axial Load Bearing – a bearing in which the load acts in the direction of the axis of rotation.

Babbitt – a soft antifriction metal used to line bearings. Made of tin, antimony, lead, and copper.

Ball Bearing – designed to reduce rotational friction and support radial and axial loads.

CAD image showing complete bearing and then a section through the bearing. Note that there is normally some form of cage that maintains a spacing between the balls and different options for then sealing the balls to prevent dirt getting into the space.

Bearing Frequencies – faults in any of the four bearing components will generate specific frequencies dependent upon the bearing geometry and rotating speed.

  • BPFO – Ball Pass Frequency, Outer Race
  • BPFI – Ball Pass Frequency, Inner Race
  • BSF – Ball Spin Frequency
  • FTF – Fundamental Train Frequency – The rotation frequency or rate of the cage supporting the rolling elements in an anti-friction bearing.

Bearing Life – this is defined as the length of time, or the number of revolutions, until a fatigue spall of a specific size develops.

  • L10 (B10), is the life in hours or revolutions in which 90% of the bearings selected will obtain or exceed.
  • Median life (average life), L50 (B50), is the life in hours or revolutions in which 50% of the bearings selected will obtain or exceed.

Bearing Misalignment – a misalignment that results when the bearings supporting a shaft are not aligned with each other. The bearings may not be mounted in parallel planes, cocked relative to the shaft, or distorted due to foundation settling or thermal growth.

Bearing Nomenclature – each bearing manufacturer has specific codes applied as prefixes and suffixes to their bearings. These codes inform the user of the construction, materials, clearances, and other factors used in the construction of the bearing. Consult the individual manufacturer’s handbook for specific code meaning.

Cage – a device used to seperate the rolling elements of a bearing.

Cage Pocket – a section of a bearing cage that retains the ball.

Connecting Rod Bearing – The bearing located in the large end of the connecting rod by which it is attached to the crankshaft.

Different connecting rod end types and bearings

Double Row Bearing – has two rows of rolling elements. Normally used where radial loads are high and a compact bearing solution is required.

Friction Torque – the torque (TF) caused by the frictional force that occurs when two objects in contact move.

TF = 0.5 µ P d

where: TF = Friction torque [Nm], P = dynamic load [N], µ = coefficient of friction, d = shaft diameter [m]

For a steel shaft and a bronze bearing, µ = 0.1 to 0.25

Hydrodynamic Bearing – a bearing which supports the shaft on a thin film of oil, the fluid-film layer is generated by journal rotation. Often referred to as stable lubrication due to the fact that as the lubrication temperature increases, the viscosity drops. This results in a lower coefficient of friction, that causes the lubrication temperature to drop.

Hydrostatic Bearing – a bearing which supports the shaft on a thin film of oil, the fluid-film layer is generated by externally applied pressure.

Needle Roller Bearing – a special bearing which has a small OD when compared to its ID. The rolling elements take the form of needle rollers that allow high radial loads to be taken. Not capable of taking axial loads.

CAD of a needle rooler bearing, this particular bearing has two seperate rows of needle rollers.

Open Bearing – a ball bearing that does not have a shield, seal or guard on either of the two sides of the bearing casing.

Plain Bearing – a relatively simple and inexpensive bearing typically made of two parts. A rotary plain bearing can be just a shaft running through a hole.

Critical aspects of design:

  • radial load
  • radial accuracy
  • specification of the diametrical clearance between the journal and the bearing
  • lubrication
  • temperature range of operation
  • required lifetime of bearing
  • cooling requirements

Roller Bearing – a bearing in which the relatively moving parts are separated by rollers.

Sleeve Bearing – a journal bearing, usually a full journal bearing. This type of bearing has no rolling elements and the shaft rides on a film of oil.

Spalling – a flaking of the surface of a bearing.