Bearings
Purpose and classification of bearings, plain and rolling-element, radial and thrust; cleaning, inspection and lubrication; defects including brinelling, spalling, corrosion and overheating; correct fitting.
Bearings, summary notes
- Bearings support a rotating or sliding load with minimum friction. The two broad families are plain (journal/bush) bearings, which slide, and rolling-element bearings, ball and roller, which roll.
- Rolling bearings are classified by the load they take: ball and roller bearings carry mainly radial load, thrust bearings carry axial load, and angular-contact and tapered-roller bearings carry both.
- The common defects are BRINELLING, indentations in the race from a static shock load or from being driven in through the rollers, SPALLING (fatigue flaking of the race surface), corrosion, and overheating shown by discoloration.
- False brinelling is fretting wear at the roller/race contacts caused by vibration while the bearing is stationary, e.g. during transport.
- Rolling bearings are cleaned in the correct solvent, dried without spinning them with an air jet (which can overspeed and burst them), inspected, and re-lubricated with the specified grease or oil, never a substitute.
- A bearing is fitted by applying the pressing force to the ring that takes the interference fit, press on the inner ring for a shaft fit, the outer ring for a housing fit, never through the rolling elements.
- Grease seals and shields retain lubricant and exclude dirt and moisture; a failed seal quickly destroys the bearing.
- ⚠ Exam trap: never drive a bearing home through its rolling elements, that brinells the races. Press on the ring that is being fitted.
- Load types
- radial (across the shaft) · axial/thrust (along the shaft)
- Fitting force
- apply to the ring taking the interference, never through the balls/rollers
Evenly spaced indentations are found in a bearing race matching the ball spacing. Name the defect and a likely cause.
Brinelling, indentation of the race. It is commonly caused by a shock load, or by driving the bearing onto its shaft through the balls instead of pressing on the inner ring.
Why must a rolling bearing not be spun with a compressed-air jet to dry it?
With no load and air-driven, the bearing can reach speeds far beyond its rating, and a rolling element can be thrown out or the bearing can disintegrate, a safety hazard as well as destroying the bearing.
Bearings concept map
Bearings
Bearings quiz
Bearings, quiz
1. Brinelling of a bearing appears as:
Indentations in the raceSurface rust onlyDiscoloured grease only2. A bearing must never be driven onto its shaft by pressing on the:
Rolling elementsInner ringShaft shoulder3. A ball bearing carries mainly:
Radial loadNo loadOnly torque4. Spalling of a bearing race is:
Fatigue flaking of the surfaceA lubrication gradeA type of seal5. A rolling bearing should NOT be dried by:
Spinning it with an air jetWiping with a lint-free clothWarm-air drying while held still6. For a shaft (inner-ring) interference fit, the pressing force is applied to the:
Inner ringOuter ringCage7. False brinelling is caused by:
Vibration while the bearing is stationaryExcessive rotational speedOver-lubrication8. A failed bearing seal is serious because it:
Lets lubricant out and dirt inIncreases the load ratingReduces friction9. A thrust bearing is designed to carry:
Axial loadRadial load onlyNo load10. Rolling bearings are re-lubricated with:
The specified grease or oilAny available greaseA lighter oil to reduce drag