Design of Bearings Questions and Answers

Answer: A

A spherical roller bearing is best suited to handle misalignment between the shaft and the housing, because its outer race has a spherical surface that allows the rollers to adjust. Thrust ball bearings carry axial loads, angular contact ball bearings carry combined loads and are sensitive to misalignment, and needle roller bearings need good alignment, so they are not suited to misalignment.

Self-aligning ball and spherical roller bearings tolerate misalignment of about 1.5 to 3 degrees, while other types tolerate only a fraction of a degree.

Answer: A

The expected life is below 0.5 million revolutions.

For ball bearings the load-life relation uses the exponent 3:

\[ \begin{aligned} L_{10} &= \left(\frac{C}{P}\right)^{3} = \left(\frac{35}{45}\right)^{3} \\ &= 0.778^{3} = 0.47 \times 10^{6}\ \text{rev} \end{aligned} \]

The load exceeds the basic dynamic rating, so the life must be less than the rated 10⁶ revolutions, and the cube makes it fall below half. Using the roller-bearing exponent 10/3 would give 0.43, still below 0.5. The range 0.5 to 0.8 corresponds to forgetting the cube (C/P = 0.78), and the larger ranges are impossible when P exceeds C.

Answer: C

The true statement is n = 3, assuming that the outer racing is fixed and inner racing is revolving.

The load-life relation for rolling bearings is \(L_{10} = (C/P)^{n}\) with \(n = 3\) for ball bearings (and 10/3 for roller bearings). The basic dynamic load rating C is defined for the standard condition in which the inner ring rotates and the outer ring is stationary, the usual arrangement on a rotating shaft.

An exponent of 1/3 inverts the relation and would make life increase with load, which is absurd. A rotating outer race with a fixed inner race is a different condition, for which catalogues apply a rotation factor, so the rating with n = 3 is stated for the inner-race-rotating case.

Answer: C

The bearing that can NOT be used is the straight roller bearing.

A helical gear produces an axial (thrust) force as well as radial load, because its teeth are inclined to the axis. A straight (cylindrical) roller bearing carries radial load only and has essentially no thrust capacity.

Angular contact bearings and tapered roller bearings are designed for combined radial and axial loads, and a double-row ball bearing also takes moderate thrust in both directions.

Answer: D

Rolling contact bearings are known as antifriction bearings.

Ball and roller bearings carry the load on rolling elements instead of a sliding surface. Rolling friction is much lower than sliding friction, especially at starting, so they are called antifriction bearings.

Thick and thin lubricated bearings are sliding (plain or journal) bearings, classed by the lubricant film between shaft and bush. Plastic bearings are plain bushes made of nylon or PTFE, again sliding rather than rolling.

Answer: B

The bearing that carries load parallel to its axis is the thrust bearing.

A load along the shaft axis is an axial or thrust load, and thrust bearings, such as ball and roller thrust bearings and the thrust collars on lathe spindles, are built to take it.

Needle bearings are roller bearings with thin rollers for radial loads in small spaces. Plain bearings (bushes) and split bearings, which are plain bearings made in two halves so they can be fitted around a shaft, also carry radial load, that is, load at right angles to the axis.

Answer: D

Angular contact bearings are designed to take axial thrust and radial load together.

The raceways are offset so that the line of contact through each ball makes an angle with the bearing axis. A load along that line has both a radial and an axial component, so the bearing carries a combined load, with axial thrust in one direction; they are often fitted in pairs to take thrust both ways, as in machine tool spindles.

A bearing for radial load alone is a plain deep groove or roller bearing, and one for axial thrust alone is a thrust bearing. 'Axial and vertical thrust' is not a meaningful pair, since vertical describes an orientation, not a type of load.