For those who might be interested, I've compiled a 'tutorial' from another source that hopefully explains duplex bearing pre-load. The duplex bearing that is the subject of this thread is a 'back-to-back' paired bearing and it is the inner race that is pre-loaded. As Crab mentioned, pre-installation, the spider case is heated and the bearing frozen to enable an interference fit on the outer race. The inner race is then pre-loaded with the nut.
From another source- "One of the final steps in the bearing manufacturing process is the assembly of the individual bearing components: the outer ring, inner ring, balls and retainer (or ball separator). When the bearings are assembled, it is necessary to have a controlled amount of internal clearance, or looseness between the rings and balls. This is referred to as
radial play in most bearing catalogs.
In certain applications, this internal clearance must be removed for a pair of bearings to operate properly. The application of an
axial load across a pair of bearings – for the purpose of removing free internal clearances – is called
preload.
Benefits of preloading ball bearings include:
- Rotational accuracy and precise shaft positioning
- Elimination or reduction of ball skidding
- Control and reduction of axial and radial deflection under applied load
- Noise reduction
- Load sharing between bearings
Preload is critical in most high-precision and high-speed applications, particularly where rotational and positional accuracy is required.
If, under operating conditions, a bearing has radial play this means that one bearing race can be moved radially and axially relative to the other. With rotation, this looseness translates into wobble or non-repetitive runout. This motion is unacceptable in applications such as machine tool spindles, electric motors, optical encoders, flow meters, and high-speed hand tools.
The application of axial preload forces the balls into contact with raceways, establishing a contact angle which causes the ball set to rotate in a uniform circumferential plane.
Duplex bearings are matched pairs of bearings with “built-in” preload. The inner or outer ring faces have been ground to a precise dimension known as the preload offset. This offset corresponds to the rings axial movement when a specific axial preload is applied. When the bearings are clamped together at assembly the offset faces abut, establishing a permanent, rigid preload in the bearing set.
Duplex bearings have increased radial and axial rigidity. There are three common preload configurations. DB (back-to-back) and DF (face-to-face) can handle bi-directional thrust loads. DT (tandem) can handle very heavy unidirectional thrust loads. At higher speeds, these bearings can run hotter due to the rigid preload. These bearings are commonly used in machine tool and other spindle applications, due to their low deflection rate, minimal runout and ease of assembly.
How much preload should be applied?
In general, provided the design requirements are met, the least amount of preload is desired
What are the benefits of preload?
Rotational accuracy and precise shaft positioning, elimination or reduction of ball skidding, control and reduction of axial and radial deflection under applied load, noise reduction, load sharing between bearings
Does preload have any effect on a bearings operational life?
Bearing life decreases as preload is increased
What other side-effects of preload should I know about?
With increased or excessive preload, stresses are higher and excess heat is generated "