Applications:
- Automotive industry
- Wind Energy
- Shipping
- Aviation
- Office equipment
- Bicycles/roller skates
- many more…
Definition:
The term bearing is derived from the verb “to bear”, as bearings are designed to bear radial and thrust loads or a combination of the two.
Contacting surfaces through which load is transmitted, whilst allowing relative motion between two elements of a machine.
Types of bearings:
Generally, of two types:
- Sliding (known as ‘plain surface’, ‘plane’, ‘hydrodynamic’ or ‘journal’) bearings.
- Rolling element (or ‘rolling contact’ or ‘antifriction’) bearings.
Both usually are assisted by lubrication
Journal bearings - Typical loads:
- Typical application is to allow rotation of a shaft, withstanding radial loads perpendicular to the axis of the shaft
- When a tilting pad design is applied to the bearing, axial or thrust loads applied in parallel to the shaft axis can also be supported
Types of lubrication:
Boundary Lubrication
Full Film Lubrication
‘Mixed Film’ lubrication
Hydrostatic lubrication
Hydrodynamic lubrication
Self-lubricating
Hydrostatic lubrication:
Oil film dependent on feed pressure and clearance.
The separating pressure is supplied by an external oil pump.
Hydrodynamic lubrication:
Hydrodynamic bearing design is a complex procedure, dependent on (amongst
other factors) speed, load, viscosity of lubricant, clearance and surface finish
Motion of the journal generates pressure in the lubricant separating the two surfaces
Previously done by iteration, now typically done by dedicated software
Self-lubricating bearings:
Metallic components with high porosity that have the ability to provide their own lubricant
Lubricant is impregnated within the sliding layer of the bearing
The lubricant is released through pores in the sliding layer, lubricating the bearing surface
Configuration and material layers of the shell:
- Solid
- Split
Lubricant viscosity dependence on
temperature:
- Most commonly known lubricants are oils and greases
- Bearing performance is dependent on the type of lubrication occurring and the viscosity of the lubricant
- Lubricant viscosity is highly dependent on temperature
Design of boundary lubricated bearings:
- General considerations in the design of a boundary lubricated bearing include:
coefficient of friction, load capacity, relative velocity between the stationary and
moving components, operating temperature, wear limitations and production
capability - PV factor is an indicator of a boundary lubricated bearing to withstand frictional
energy dissipated in the bearing
PV= Load capacity x sliding velocity
- At the limiting PV value, the temperature will be unstable and failure will occur
rapidly. A practical value for PV is half the limiting PV value
Bearing Selection:
- Determine speed of rotation, and load to be supported
- Set bearing proportions (common practice is to set the initial ratio of diameter to length equal to between 0.5 and 1.5. If the diameter is known as an initial trial, set L=D)
- Calculate load capacity (to be able to get to PV)
- Determine tangential speed of journal, V= ω r (m/s)
- Calculate PV factor, and multiply with safety factor (a factor of 2 would be typical)
- Select bearing from published data
Rolling element bearings:
- The rolling bearing consists of 2 races (inner and outer rings) in relative motion and between them a row of balls (or rollers) in contact with both. Balls enable relative motion between the races.
- Balls or rollers roll inside the raceways which can be more or less deep
- A cage keeps the balls equi-spaced around the bearing
- The seal can be made of metal for dust protection, or it can be made of plastic preventing escape of lubricant
Common materials of rolling element bearings:
- Chrome/Stainless steel
- Ceramic (hybrid and full ceramic)
- Plastic (acetal, PTFE, and nylon), carbon graphite, etc.
Note: plastic is usually used when load/temperature isn’t too high.
Rolling element bearings: Classification:
- Rolling element bearings can be classified according to the loading conditions, i.e., radial, thrust (axial) or combined
- They are also classified according to
the configuration of their rolling elements, e.g. balls, rollers.
Radial Load:
- Radial bearings refer to bearings that support primarily radial loads
- Typical examples are deep groove ball
bearings, cylindrical roller bearings, and
needle bearings
Thrust Load:
- A variety of radial bearings have also axial
configurations in order to withstand an axial
load
Clearance:
- Radial clearance
- Axial clearance
Ball bearings:
- Among the most used bearings, mostly for radial loads, fair axial load capacity
- The raceway grooves on both inner and outer rings have circular arcs of slightly larger radius than that of balls
- Cage is made of pressed steel that gives it high load capacity and flexibility
- Ball bearings have point contact (at least in theory); each ball contacts the race in a very small patch → high contact stresses
- Double row bearings can withstand higher radial loads.
- Self-aligning ball bearings have an inner ring with two grooves placed side by side, while outer ring has one hemispherical groove, allowing inclination of two rings during functioning and better self-alignment.
Angular contact ball bearings:
- Inner and outer ring raceways are displaced relative to each other in the direction of the bearing axis
- They can withstand both radial and axial loads
- Contact angles of 15°, 25° , 30° , and 40° are available
- If doubled up, they can handle thrust loading in two directions
Cylindrical roller bearing:
- Straight rollers
- Linear contact with the raceways, rather than point contact, giving them high radial load bearing capacity and higher shock resistance
- Pressed steel or machine brass cages are usually used
- They are used when high accuracy is required, they can manage only limited
thrust loads
Needle roller bearings:
- Contain many slender rollers with a length 3 to 10 times their diameter
- Needle Roller Races
- Poor axial load capacity
- Good radial load capacity
- Thanks to their compactness, they are used when space is limited
Spherical roller bearings:
- Barrel shaped rollers
- Capable of carrying both heavy radial loads but also some axial loads in either direction
- Suitable for low & medium speed applications
- Spherical roller bearings can accommodate misalignment
Tapered roller bearings:
- Tapered inner and outer roller races
- They can withstand both axial and radial loads
- Poor misalignment capacity
