Roller Guidance in Spherical Roller Bearings
True rolling means there is no slipping, skidding or skewing. Simple plane geometry shows that it will occur when a cone rolls on a cone.
Rollers operating in a tapered roller bearing operate in this fashion. The tapered roller (which is a section of a cone) sees true rolling motion because as it tracks on the outer ring, it circumscribes a cone (A). Similarly, the rollers tracking on the inner ring circumscribe a cone (B). The apexes of these cones meet along the axis of the bearing.
Because there is true rolling, there is no skewing or slipping and the bearing could theoretically operate without a cage.
Spherical Roller Bearing Internal Design
Now, let's consider spherical roller bearing internal design. The two basic types incorporate either a symmetrical roller (C type) or an asymmetrical roller (B type). Both have relative advantages and can generally be substituted one for the other. NTN has standardized on the B design for bearings over 50 millimeters bore.
FEATURES:
| C Type: |
B Type: |
- symmetrical rollers
- no inner ring ribs
- window type cage
- floating guide ring
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- asymmetrical rollers
- 3 integral inner ring ribs
- finger type cage
- center guide flange
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The B type was the first spherical roller bearing design. Its design concept was that the asymmetrical shape of the roller results in non-parallel radial forces on the roller. This condition causes a small thrust force on the roller which acts to seat the roller against the center guide flange. The rollers, thus guided will track true, with no wobble or skewing.
The C type was introduced in the early '70's and sold as an improvement to the B type because the dynamic capacity had been increased. This was accomplished by eliminating the center guide flange and side ribs which allowed for longer rollers.
With the center guide flange gone, there can be no axial (seating) force on the roller. Therefore the roller profile had to change.
In the C style bearing the roller is symmetrical. I.e. the largest diameter is at the axial center of the roller. The forces on the roller must act in the same plane and must be equal and opposite.
It follows that the lines of contact, which are perpendicular to the forces on the roller, must be parallel.
Therefore, theoretical cones circumscribed by the roller contact with the inner and outer rings do not share the same apex.
As a result, the roller does not know which cone to track on. This is lack of guidance and the effect is skewing, wobble and generation of heat.
In order for the apexes of these two cones to meet along the bearing axis, the inner ring raceway must be "tilted" as shown
This "adjustment" changes the roller profile. The roller is now asymmetrical.
The geometry of the asymmetrical roller is such that if the lines of contact with the raceways are extended, they converge at the bearing axis. The result is guidance similar to that found in a tapered roller bearing.
Now that the raceway contacts converge, the radial forces on the roller do not exactly oppose each other. This results in a light seating force on the roller as previously mentioned.
A center guide flange is needed to accommodate the seating force.
There are some who would say this contact between the roller end and the center guide flange generates heat. It is a sliding contact.
The roller end has a large spherical radius and the face of the center guide flange is flat which produces a wedge-like contact. As the roller end face passes by the flange, lubricant is drawn into the wedge. This behavior is a result of the same hydrodynamic effect that makes plain bearings so effective.
The efficiency of this process is dependent on the relative speed between the roller and the flange. Our testing has shown that, within limits, the bearing temperature decreases as the rotational speed increases.
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