Technical guide to bearing selection for rotating shafts and torsion shafts

Shafts are a key element of almost any machine. In order for these rotating and torsion-loaded components to function reliably, the bearings that carry these shafts must be perfectly matched to the respective application and installation situation. The selection of a suitable shaft bearing (rotary bearing) therefore has a significant impact on the service life and energy efficiency of a machine as well as its noise generation and maintenance requirements.

But which type of bearing is the right one for a particular shaft?

This guide shows how to select the optimal bearing solution for rotating and torsion shafts based on technical criteria and practical framework conditions.

Function and importance of shaft bearings

Shaft bearings allow for a bearing seat (a movable connection) between two elements that move relative to each other, such as a shaft and a housing. They support the shaft, hold it in position, and allow the shaft to rotate about its own longitudinal axis. The various designs differ not only in the extent to which they restrict the individual degrees of freedom (play, axial, radial, angular), but also in their operating behavior, precision, and load-bearing capacity.

Depending on the installation situation, radial forces, axial forces, or a combination of both must be absorbed by the shaft bearings. However, the positioning accuracy, friction losses and heat generation, as well as the service life and vibration behavior of the overall system, are also influenced by the shaft bearings used.

The problem:

in design terms, engineers are often faced with a situation where the shaft is already specified. The function, diameter, and material of the shaft are already defined, and the bearing must adapt to the shaft. In such cases, the inner diameter of the bearing is determined directly by the shaft dimension, while other parameters such as size, design, tolerance, sealing and lubrication, and material can be specifically selected.

What influencing factors must be considered?

Shaft bearing selection begins with the analysis of the application and the determination of the requirements to be met. These requirements determine whether radial, axial or angular contact bearings are required.

In particular, the following must be taken into account:

Speed, load and fluctuating load

High speeds require low-friction shaft bearings. Due to their low contact area between the bearing ring and rolling element, rolling bearings are suitable here.
High loads, such as those found in presses or gearboxes, favor the use of roller bearings or needle bearings, since forces acting on their linear load-bearing surface can be better distributed.

If alternating loads or shock loads are to be expected in the application, the shaft bearings must also be designed for this purpose. Tapered roller bearings or angular contact ball bearings with high internal stability are often advantageous here. These can be operated with clearance or with preload thanks to their adjustability.
Fatigue resistance (material) is also critical for high-frequency load changes (vibrations).

For detailed information on the properties and differences of plain and rolling bearings, please refer to our posts Plain bearings in practice and Differences between ball bearings and plain bearings.

Operating temperature

The effect of the operating temperature on the lubrication and running characteristics of the shaft bearings is not to be ignored. At temperatures above 70 °C, the lubrication behavior and material properties of many lubricants change. If higher operating temperatures are to be expected, high-temperature greases can be a solution.

The material of the bearing (plastic, steel, ceramic) and its temperature resistance must also be considered.

Installation conditions, space requirements and integration into the design

The possibilities for integrating the shaft bearings within the machine should also be considered. In compact systems, the axial height and width of the shaft bearings play an important role; good access to the installed shaft bearings should not be underestimated, as it has a strong influence on subsequent maintenance options and maintenance costs. The different mounting types of housing bearings: pillow block bearings, flanged bearings or integrated bearing solutions offer different possibilities here. While finished housing units such as pillow block bearings or flanged bearings allow for easy integration, individual designs allow the bearing to be integrated directly into the component.

Fit selection

in our example, the shaft and thus the nominal diameter have already been selected. The nominal diameter of the inner ring of the bearing is thus fixed. There are tolerance tables for bearing inner rings and bearing outer rings.

For the integration, 4 tolerance field positions must be considered:

1. Tolerance field position of the shaft
2. Tolerance field position of the bearing inner ring
3. Tolerance field position of the bearing outer ring
4. Tolerance field position of the bore in the housing

Common market tolerance field positions for shafts are g6, h6, h7.

A typical tolerance class for the housing bore is H7, resulting in a clearance fit in combination with the outer ring tolerance. The advantage of this fit is easy installation. If it is necessary to fix the bearing outer ring, an anaerobic adhesive is recommended.

The tolerance field positions for bearing outer and bearing inner rings of radial bearings are defined according to JIS or ISO. The accuracy classes are defined here (e.g. P0).

Tolerance classes for external dimensions of the bearing are typically specified in P-classes (precision classes). The tolerance class for internal clearance is typically given as C-classes (clearance classes),

The selected coupling of the fit between the shaft and bearing as well as the bearing and housing results in the overall bearing clearance in connection with all actual tolerances.

For more information, see our articles: Principles of dimensional tolerances and fit selection and Fit types and tolerances – An overview.

Axial bearing retention

When combining the shaft, bearing, and housing, axial displacement should be prevented. Thus, at least one side should be designed as a fixed bearing in the case of a double bearing of a shaft.

For this purpose, there are various options for the two interfaces outer ring housing and inner ring shaft.

The following retaining elements or securing methods are available, among others:

• form-fitting retainers such as: snap rings, retaining rings, lock washer, or locknuts.
• force-locking retainers such as: claw clamping discs, set collars, or clamping rings
• friction-locking retainers such as: anaerobic adhesives or press fits

For more information, see our article Elements for securing axial bearings.

In addition to the design aspects and the available space, the sealing and protection of the shaft bearings should also be considered. Dust, moisture, and chips can not only have a negative impact on the lubricating behavior and running characteristics of the shaft bearings. In the worst case, they can lead to bearing damage.

Common seal types for rolling bearings are:

• Shields (Z / ZZ): single or double-sided, for dry, clean environments
• Sealing washers (RS / 2RS): protect against moisture, dust, and light splashing water
• Labyrinth seals and cover plates: for harsh industrial environments with heavy contamination

More information can be found in our article: Sealing ball bearings – Seals, sealing washers and cover washers.

Arrangement and combination options

Taking into account the shaft form and shaft dimension, as well as the available space, the shaft bearings must also meet the requirements for accuracy and load. The selection, integration and arrangement of the bearings must therefore be carefully considered.

Here are some installation examples:

Arrangement options for angular contact ball bearings

Angular contact bearings offer an economical way to accommodate axial forces in addition to radial forces. The correct arrangement of the bearings is crucial.

Complete shaft bearing housings

Shaft bearing housings offer the possibility of integrating finished rolling bearing positioning and alignment relatively easily into an existing system. The shaft bearing housings are usually pre-lubricated and sealed by the manufacturer. In addition, they reduce design effort and spare part procurement by providing them as finished assemblies.

In the example, a combination of 2 angular contact ball bearings and a deep groove ball bearing.

The plain bearing as an alternative to the rolling bearing

Unlike rolling bearings, plain bearings do not work with rolling elements, but with the principle of sliding on a lubricating film. Plain bearings are low maintenance and cost effective, but are usually more susceptible to wear than rolling bearings. For very high radial loads, heavy impacts or slow movement, they can be a very good alternative to rolling bearings. For more information, see our post: Differences between ball bearings and plain bearings.

Summary - Bearing Selection Procedure

1. Analysis of shaft function (rotary motion, torque, load)
2. Determination of the type of load (radial, axial or combined)
3. Calculation of loads and speeds
4. Design definition (ball, roller, combination or plain bearings)
5. Determination of fit, lubrication, and sealing
6. Checking the installation conditions
7. Simulation / recalculation of service life (DIN ISO 281)
8. Integration into the CAD system

Practical examples

Example:
High-speed spindle with a speed of approximately 25,000 rpm and low load.
Possible application:
Angular contact ball bearing (pair arrangement) with grease filling

Example:
Drive shaft in transmission (combined load and moderate speed)
Possible application:
Tapered roller bearing (preloadable)

Example:
Conveyor roller shaft (low speed with high load in dusty environment)
Possible application:
Spherical roller bearing with labyrinth seal

Common sources of error:

• Improper fit: leads to play or crushing damage.
• Inadequate lubrication: Main cause of bearing damage.
• Misalignment: dramatically increases bearing load.
• Overload: reduces service life exponentially.
• Insufficient sealing: leads to corrosion and premature failure.