How to Select the Right Bearing Part 7

Components Surrounding the Bearing

Concluding the How to Select the Right Bearing series, Koyo Bearings shares with us the considerations you must take for the components around your bearing. The original post on their website here.

To catch up on the previous Bearing Selection blog posts, read each one below:

How to Select the Right Bearing Part 1: The Order of Priority for Selecting Bearings and the Types of Bearings

How to Select the Right Bearing Part 2: Selecting Bearing Arrangement

How to Select the Right Bearing Part 3: Bearing Dimensions and Service Life

How to Select the Right Bearing Part 4: Bearing limiting speed, running accuracy, and fits

How to Select the Right Bearing Part 5: Bearing Preload and Rigidity

How to Select the Right Bearing Part 6: Bearing Lubrication

To ensure the intended performance of the machine, it is important to layout the components surrounding the bearing in accordance with the usage conditions.

It is also, of course, necessary to consider the cost performance of maintenance and inspection with mounting and dismounting.

Table 1: Bearing selection checklist

This post will cover three points:

  • The design (including accuracy and roughness) of the shaft and housing
  • Bearing mounting dimensions
  • Sealing devices

1. The design of the shaft and housing

Table 2 introduces the points to be aware of in the design of the shaft and housing.

Note that it is necessary to perform the design from a comprehensive perspective, also taking cost performance into account.

Table 2: Points to be aware of in the design of the shaft and housing

Shape and dimensions of the shaft

A shaft should be designed as thick and short as possible so that the force applied to the shaft during operation does not result in significant distortion and bending that would decrease the service life of the bearing. If there is limited installation space in the radial direction, it is also effective to select a double-row bearing or a wider bearing from width series 2 or higher.

Shape and dimensions of the housing

A housing with rigidity should be designed so that the force applied to the housing during operation does not result in significant distortion of the housing that would decrease the service life of the bearing. With housings made of aluminum or another light alloy, rigidity can be provided by inserting a steel bushing (see Figure 1).

Fig. 1: Example of light alloy housing

The expansion coefficient of a light alloy housing greatly differs from that of the bearing, so care needs to be taken when selecting the fit.

Fitting surfaces of the shaft and housing

The fitting surfaces of the shaft and housing should be finished to acquire the required accuracy and roughness (see Table 3).

The shoulder end-face should be finished to be perpendicular to the shaft center or housing bore surface.

Under general operating conditions, the fitting surface is finished by turning. However, if the conditions require minimal vibration and noise, or if the bearing is used under severe operating conditions, a ground finish is required.

Table 3: Accuracy and roughness of shafts and housings (recommended)
*Refer to the figures listed in the following when the basic tolerance IT is required

The fillet radius of the shaft and housing

The fillet radius (ra) of the shaft/housing should be smaller than the chamfer dimension of the bearing. Generally, it should be a simple circular arc, but, when the shaft is given a ground finish, a recess (grinding undercut) may be provided (see Figure 2).

Fig. 2: Fillet radius

If the fillet radius must be larger than the bearing chamfer (ra2 > ra), a spacer should be inserted between the inner ring and shaft shoulder, or between the outer ring and housing shoulder (see Figure 3).

Fig. 3: Example of spacer inserted between bearing inner ring and shaft shoulder

The shoulder heights of the shaft and housing

The shoulder height (h) of the shaft and housing should be decided to make the bearing easy to dismount. The shoulder diameter of the shaft should be smaller than the outside diameter of the inner ring, while the shoulder diameter of the housing should be larger than the inside diameter of the outer ring (see Figure 4).

Fig. 4: Shoulder heights of shaft and housing

Locknuts and screw threads for shaft mounting

If applying an axial load or preload, locknuts or screw threads are attached to the shaft. These nuts/screws are set to be as perpendicular to the shaft axis as possible (see Figures 5 and 6).

Fig. 5: Shaft locknut for mounting
Fig. 6: Screw thread for mounting (attach the endplate)

Split Housings

In a split housing, machine a recess on the bore surface where housings meet and finish the meeting surfaces smoothly, and then assemble the housings together (see Figure 7).

Fig. 7: Recess on bore surface area meeting surfaces of the split housing

2. Bearing mounting dimensions

The “mounting dimensions” of a bearing are the dimensions necessary to mount the bearing. The mounting dimensions are the fillet radius and shoulder diameter of the shaft and housing.

Based on the standard values indicated in Table 4, our catalog lists the mounting dimensions for each bearing number (see Table 5). Additionally, the grinding undercut dimensions for ground shafts are given in Table 6.

Table 4: Shaft/housing fillet radius and shoulder height of radial bearings (excerpt)
Table 5: Mounting dimensions by bearing number (example; deep groove ball bearings; excerpt)
Fig. 8: Mounting dimensions of deep groove ball bearing
Table 6: Grinding undercut dimensions for ground shafts (excerpt)

3.Sealing devices

Sealing devices for bearings not only prevent foreign matter (dirt, water, metal powder) from entering but prevent lubricant inside from leaking, preventing damage to the bearing itself. It is necessary to consider which is the most suitable sealing device as well as the proper lubricating method for your bearings’ operating condition

Sealing devices come in two types: non-contact and contact.

Non-contact type sealing devices

Non-contact type sealing devices, which eliminate friction because they do not have a contact point with the shaft, are especially suitable for operation at high rotation speed and high temperature.

There are three varieties of non-contact type sealing devices: oil groove, flinger (also called slinger), and labyrinth.

Oil groove

This kind of sealing device has three or more grooves at the narrow clearance between the shaft and housing cover.

Except when used with grease lubrication at a low rotation speed, it is usually accompanied by other sealing devices.

Fig. 10: Example of flinger (slinger) structure

Labyrinth

Labyrinths provide clearance in the shape of engagements between the shaft and housing. They especially prevent lubricant leakage at a high rotation speed.

Fig. 11: Example of labyrinth structure

Contact type sealing devices

Contact type sealing devices provide a sealing effect using contact between their end and the shaft and are manufactured from substances such as synthetic rubber or synthetic resin. Synthetic rubber oil seals are the most popular.

Figure 12 shows an example of an oil seal structure.

Fig. 12: Example of oil seal structure and functions

Conclusion

To ensure the intended performance of the machine, it is important to layout the components surrounding the bearing in accordance with the usage conditions.

It is also, of course, necessary to consider the cost performance of maintenance and inspection with mounting and dismounting.

  1. The shaft and housing should be designed to prevent significant distortion and bending of the shaft and housing due to the force applied during operation. The fitting surfaces of the shaft and housing should also be finished to acquire the required accuracy and roughness. If the conditions require minimum vibration and noise, or if the bearing is used under severe operating conditions, a ground finish is required.
  2. Bearing mounting dimensions are determined based on the recommended dimensions for the bearing number of the selected bearing.
  3. There are two types of sealing devices: non-contact and contact. The type best suited for the operating condition should be selected.

This concludes the How to Select the Right Bearing series with Koyo Bearings. Be sure to check out their line of bearings here.

Works Cited

Koyo Bearings. (2021, January 14). Bearing Trivia. Retrieved from Koyo Bearings: https://koyo.jtekt.co.jp/en/2021/01/column02-07.html

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