Bearings, Seals and Fits
This section covers Bearings, Seals and Fits. The exam tests bearing type selection, lubrication, seal function, and interpretation of clearance, transition and interference fits.
Core concepts
- Sliding bearings have a shaft running in a lubricated bush; rolling bearings use balls, cylindrical rollers or needles between races. Ball bearings suit radial and light axial loads; roller bearings carry heavier radial loads; thrust bearings carry axial load.
- Lubrication reduces friction and wear and removes heat. Clearance must allow an oil film but not cause excessive vibration.
- Seals retain lubricant and exclude dirt: oil seals, O-rings, gaskets, gland packing and labyrinth seals. A gasket seals static joints; an O-ring seals by controlled squeeze.
- Fits describe the relationship between hole and shaft. Clearance fit leaves a gap; interference fit makes the shaft larger than the hole and requires force or heating; transition fit may be slight clearance or slight interference.
- In hole-basis notation, H is the basic hole. Lower-case letters denote shaft deviation.
Worked example
A drawing specifies H7/g6. H7 is the hole basis, and g6 is a shaft with a small negative deviation, so the shaft is slightly smaller than the hole. The result is a clearance fit suitable for a sliding or locating part. By contrast, H7/p6 is an interference fit for a press-fit hub, and H7/k6 is a transition fit. The reasoning is in the letter position relative to the basic hole.
Traps that cost marks
Do not call every rolling bearing a ball bearing. Do not use an interference fit where parts must slide. Do not over-pack a bearing or ignore seal lip direction. A seal that runs dry may burn.