617 Taper roller bearings

Description of bearing construction (Fig. 6.7) The taper roller bearing is made up of four parts; the inner raceway and the outer raceway, known respectively as the cone and cup, the taper rollers shaped as frustrums of cones and the cage or roller retainer (Fig. 6.8). The taper rollers and both inner and outer races carry load whereas the cage carries no load but performs the task of spacing out the rollers around the cone and retaining them as an assembly.

Taper roller bearing true rolling principle (Fig. 6.8(a and b)) If the axis of a cylindrical (parallel) roller is inclined to the inner raceway axis, then the relative rolling velocity at the periphery of both the outer and inner ends of the roller will tend to be different due to the variation of track diameter (and therefore circumference) between the two sides of the bearing. If the mid position of the roller produced true rolling without slippage, the portion of the roller on the large diameter side of the tracks would try to slow down whilst the other half of the roller on the smaller diameter side of the tracks would tend to speed up. Consequently both ends would slip continuously as the central raceway

Fig. 6.7 Taper rolling bearing terminology

member rotated relative to the stationary outer race members (Fig. 6.8(a)).

The design geometry of the taper roller bearing is therefore based on the cone principle (Fig. 6.8(b)) where all projection lines, lines extending from the cone and cup working surfaces (tracks), converge at one common point on the axis of the bearing.

With the converging inner and outer raceway (track) approach, the track circumferences at the large and small ends of each roller will be greater and smaller respectively. The different surface velocities on both large and small roller ends will be accommodated by the corresponding change in track circumferences. Hence no slippage takes place, only pure rolling over the full length of each roller as they revolve between their inner and outer tracks.

Angle of contact (Fig. 6.7) Taper roller bearings are designed to support not only radial bearing loads but axial (thrust) bearing loads.

The angle of bearing contact O, which determines the maximum thrust (axial) load, the bearing can accommodate is the angle made between the perpendiculars to both the roller axis and the inner cone axis (Fig. 6.7). The angle of contact O is also half the pitch cone angle. These angles can range from as little as 7% to as much as 30°. The standard or normal taper roller bearing has a contact angle of 12-16° which will accommodate moderate thrust (axial) loads. For large and very large thrust loads, medium and steep contact angle bearings are available, having contact angles in the region of 20 and 28° respectively.

Area of contact (Fig. 6.7) Contact between roller and both inner cone and outer cup is of the line form without load, but as the rollers become loaded the elastic material distorts, producing a thick line contact area (Fig. 6.7) which can support very large combinations of both radial and axial loads.

Cage (Fig. 6.7) The purpose of the cage container is to equally space the rollers about the periphery of the cone and to hold them in position when the bearing is operating. The prevention of rolling elements touching each other is important since adjacent rollers move in opposite directions at their points of closest approach. If they were allowed to touch they would rub at twice the normal roller speed.

The cage resembles a tapered perforated sleeve (Fig. 6.7) made from a sheet metal stamping which

Fig. 6.8 Principle of taper rolling bearing has a series of roller pockets punched out by a single impact of a multiple die punch.

Although the back cone rib contributes most to the alignment of the rollers, the bearing cup and cone sides furthest from the point of bearing loading may be slack and therefore may not be able to keep the rollers on the unloaded side in their true plane. Therefore, the cage (container) pockets are precisely chamfered to conform to the curvature of the rollers so that any additional corrective alignment which may become necessary is provided by the individual roller pockets.

Positive roller alignment (Fig. 6.9) Both cylindrical parallel and taper roller elements, when rolling between inner and outer tracks, have the tendency to skew (tilt) so that extended lines drawn through their axes do not intersect the bearing axis at the same cone and cup projection apex. This problem has been overcome by grinding the large end of each roller flat and perpendicular to its axis so that all the rollers square themselves exactly with a shoulder or rib machined on the inner cone (Fig. 6.9). When there is any relative movement between the cup and cone, the large flat ends of the rollers make contact with the adjacent shoulder (rib) of the cone, compelling the rollers to positively align themselves between the tapered faces of the cup and cone without the guidance of the cage. The magnitude of the roller-to-rib end thrust, known as the seating force, will depend upon the taper roller contact angle.

Contact area

Fig. 6.9 Roller self-alignment

Contact area

Fig. 6.9 Roller self-alignment

Fig. 6.10 Force diagram illustrating positive roller alignment seating force

Self-alignment roller to rib seating force (Fig. 6.10) To make each roller do its full share of useful work, positive roller alignment is achieved by the large end of each roller being ground perpendicular to its axis so that when assembled it squares itself exactly with the cone back face rib (Fig. 6.10).

When the taper roller bearing is running under operating conditions it will generally be subjected to a combination of both radial and axial loads. The resultant applied load and resultant reaction load will be in apposition to each other, acting perpendicular to both the cup and cone track faces. Since the rollers are tapered, the direction of the perpendicular resultant loads will be slightly inclined to each other, they thereby produce a third force parallel to the rolling element axis. This third force is known as the roller-to-rib seating force and it is this force which provides the rollers with their continuous alignment to the bearing axis. The magnitude of this roller-to-rib seating force is a function of the included taper roller angle which can be obtained from a triangular force diagram (Fig. 6.10). The diagram assumes that both the resultant applied and reaction loads are equal and that their direction lies perpendicular to both the cup and cone track surface. A small roller included angle will produce a small rib seating force and vice versa.

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