Two-bearing arrangement:

DB type (also called “0” type); DF type (also called “X” type); DT type.

(Fig.4)

Three-bearing arrangements:

TBT; TFT; TT

(Fig.5)

Four-bearing arrangements:

QBC; QFC; QT; QBT; QFT

(Fig.6)

Five-bearing arrangements:

PBC; PFC; PT; PBT; PFT

(Fig.7)

The diagrams show the visual representations of these pairing arrangements, with the bearings positioned in different orientations to manage the axial and radial loads. The specific arrangement used depends on the application requirements and loading conditions. Proper bearing pairing helps optimize load capacity, stiffness, rotational accuracy, and service life of the bearing system.

(Table 1) and (Fig. 8 )shows “universal bearings”, where the inner and outer rings are ground to the same height. This allows C1=C2, and meets a certain preload grade (usually A, B, C). Such bearings can be arbitrarily paired into DB, DF, DT arrangements, but are only suitable for these three arrangements. If three-bearing, four-bearing, or five-bearing arrangements are needed, the pairing type must be specified when ordering. The manufacturer can then grind the appropriate protruding heights based on the pairing type to meet the design requirements.

In summary, universal paired bearings offer flexibility for two-bearing arrangements, but the pairing type needs to be predefined for arrangements with more than two bearings. The manufacturer will adjust the inner/outer ring heights accordingly to achieve the desired preload in different multi-bearing arrangements.

Taking (Figure 3) as an example, to apply preload F to the two sets of bearings, there must be a clearance C between the inner ring end faces before loading. The left bearing protrudes by C1, and the right bearing protrudes by C2. C=C1+C2.

For selective preload, C1 may not equal C2, as long as C1+C2=C.

For universal preload, C1 must equal C2, and the inner ring width and outer ring width of each bearing set must also be equal.

In summary:

Selective Preload:

C1 may not equal C2

Total clearance C=C1+C2

Universal Preload:

C1 must equal C2

Inner and outer ring widths of each bearing set must be equal.

Total clearance C=C1+C2

All rolling bearings have a rotational speed limit. As the bearing rotation speed increases, the temperature rise caused by internal friction also increases. The limit speed is an empirically determined allowable speed that does not cause burn, overheating, or inability to operate continuously. Therefore, the limit speeds of various bearings depend on factors like bearing design, materials, loads, lubrication method, and cooling conditions around the bearing.

Additionally, the limit speeds listed in bearing tables for oil lubrication and grease lubrication assume standard bearing designs, normal loads (P≤0.09C, Fa/Fr≤0.3). If the bearing has contact seals, the seal peripheral speed determines the limit speed. For non-normal loads, the limit speed decreases. For paired bearings, the limit speed decreases 10-40% depending on number of bearings and precision grade.

Therefore, when selecting bearings, these factors must be fully considered. The listed limit speeds are reference values that may need to be lowered based on actual operating conditions. Proper speed selection helps maximize bearing performance and service life.

Preload is based on arrangement which is a key factor of bearing speed.

Excessive preload can lead to abnormal heat generation, increased frictional torque, reduced fatigue life, etc. Insufficient preload results in low bearing stiffness, which can also lead to premature bearing failure at high speeds. Therefore, operating conditions and the purpose of preloading should be fully studied to determine the preload amount.

Product samples from various companies provide recommended preload values, but they are formulated based on respective test methods and experiences, so the recommendations can vary quite a bit. Some companies classify preload into four levels – minimal, light, medium and heavy. Others use three levels – light, medium and heavy. The recommendations should not be blindly followed. Proper preload should be selected based on specific requirements through practice.

In general, light, or minimal preload is chosen for spindle bearings of CNC grinding machines with automatic tool changers, medium or heavy preload for lathe spindles, and medium or heavy preload for milling machine spindles.

There are two situations for the shaft system during operation. One is that there is a certain radial and axial clearance during operation. The other is that there is no clearance during operation. When there is no clearance during operation, preload must be applied when arranging the bearings. The main purpose is to increase the rigidity of the shaft system to meet the working performance requirements of the shaft system.

Purpose of Preloading Angular Contact Ball Bearings

The main purposes of preloading angular contact ball bearings are:

1.To determine the axial and radial position of the bearing, and at the same time suppress shaft vibration. To increase the rigidity of the bearing.

2.To prevent axial vibration and abnormal noise caused by resonance.

3.To prevent and restrain the rolling, orbital and spin slippage of the rolling elements.

Ways of Preloading Angular Contact Ball Bearing Arrangements

(Fig.1 Fig.2)

There are two types of preloading methods for angular contact ball bearing arrangements – positional preload and constant pressure preload.

Positional preload utilizes the offset between the inner and outer ring end faces (when axial clearance is eliminated and angular contact is formed without external force) to create a certain gap between the inner or outer end faces, as shown by C in (Fig. 1.) When an F force is applied as shown in (Fig. 1) to make C=0, a preload is applied to the bearing. This method is positional preload. The larger C is, the greater the preload force.

Constant pressure preload is shown in (Fig. 2.) The inner rings of the two bearings are fixed to the shaft, while the outer rings are slidably fit in the bearing housing. A constant preload f is always applied to the bearings by a spring. This type of preloading is constant pressure preload.

The two preload methods have their own characteristics:

1.Under the same preload force, positional preload has better stiffness compared to constant pressure preload.

2.With positional preload, preload force can easily change due to temperature differences between inner and outer rings and changes in radial clearance. Constant pressure preload is less affected by preload force changes caused by temperature changes.

In summary, positional preload is generally used to increase bearing stiffness, while constant pressure preload is suitable for high-speed rotation or preventing axial vibration of the shaft system.