How to Choose the Right Super Precision Bearing for your Application
In high precision applications, even the smallest inconsistency in motion or alignment can have a big impact on performance. Choosing the right super precision bearing requires balancing speed, load, accuracy, stiffness, preload, temperature and bearing arrangement against the application's operating conditions.
But where to start? We’ve put together all the factors to consider when choosing a precision bearing for your application.
Find out what the different types of bearing arrangements are and where they are used
Speed and Limiting Factors
Operating speed is one of the first considerations when selecting a super precision bearing, particularly for applications such as machine tool spindles, grinding machines and high-speed motors.
Every bearing has a speed capability influenced by factors including design, size, internal geometry, cage material, preload and lubrication. As rotational speed increases, so does the heat generated by friction within the bearing. If this heat cannot be controlled, it can reduce lubricant life, alter bearing preload and ultimately shorten service life.
Manufacturers provide speed ratings to help with bearing selection, but these should be treated as part of the overall calculation rather than in isolation. The actual maximum operating speed of a bearing will depend on the conditions in which it is being used.
For very-high speed applications, engineers may also consider the bearing’s speed factor, often expressed as n x dm where n is rotational speed and dm is the bearing’s mean diameter. This can be particularly useful when assessing bearing design and lubrication requirements.
If high speed is the priority, a smaller contact angle, low-friction cage design and carefully controlled lubrication may all help increase speed capability.
Hybrid super precision bearings, which combine steel rings with ceramic rolling elements, can also be considered for particularly demanding high-speed applications. Their lower rolling element mass can help reduce centrifugal forces and friction at high rotational speeds.
Radial vs Axial Loads
The next consideration is the type and direction of load that the bearing needs to support.
Radial loads act perpendicular to the shaft, while axial loads act parallel to it. Many precision applications experience a combination of the two.
Angular contact ball bearings are widely used in super precision applications because their internal geometry allows them to support both radial and axial loads. However, the amount of axial load they can accommodate is closely related to their contact angle.
It is important to establish:
- The expected radial load
- The expected axial load
- The direction of the axial load
- Whether loads are constant or variable
- Whether shock or impact loading is likely
- The required level of system rigidity
Where substantial loads need to be accommodated, the bearing arrangement can also be used to distribute them more effectively. Bearings may be installed in double, triple or quadruple sets depending on the load, stiffness and accuracy requirements of the application.
Required Accuracy and Runout
Runout describes the amount by which a rotating surface deviates from its intended axis of rotation. In applications such as precision machining, even very small levels of radial or axial runout can affect surface finish, dimensional accuracy and repeatability.
There is little benefit in specifying a bearing capable of extremely high speeds if the application cannot tolerate the resulting level of runout, or vice versa.
Before choosing a bearing, consider the maximum acceptable radial runout, axial runout, shaft displacement, and variation in rotational accuracy.
The tighter the accuracy requirement, the more important the bearing’s precision class becomes.
It is also worth remembering that bearing accuracy is only one part of the system. Shaft and housing tolerances, mounting accuracy, fits, contamination and installation procedures can all influence the final runout of the assembled spindle or machine.
Specifying a very high precision bearing cannot compensate for inaccuracies elsewhere in the system.
Bearing Size and Arrangement
Bearing size affects several performance characteristics, including load capacity, stiffness, speed capability and heat generation. In general, increasing bearing size can improve load-carrying capacity, but larger bearings also have higher peripheral speeds and may not be the best choice for extremely high-speed operation.
The bearing arrangement is equally important. For example, angular contact bearings can be mounted in back-to-back, face-to-face or tandem arrangements.
A back-to-back arrangement can provide high moment stiffness and support axial directions and may offer greater tolerance of certain alignment errors. A tandem arrangement increases axial load-carrying capacity in one direction.
More complex arrangements can use three or four bearings to achieve the required combination of load capacity, stiffness and speed.
Read our guide to the different bearing arrangements and where they are used
Contact Angle
The contact angle is defined as the angle formed between the line connecting the points of contact of the rolling element and raceways and a line drawn perpendicular to the bearing axis. Changing the contact angle changes the way a bearing handles radial and axial forces.
A smaller contact angle is generally suited to higher rotational speeds and applications where radial loads dominate. A larger contact angle increases the bearing’s axial load-carrying capability and can provide greater axial stiffness, although this may come at the expense of maximum speed.
Common super precision angular contact bearings are generally available with contact angles around 15° or 25°, although the options vary between bearing manufacturers and series.
Where the application involves significant axial loads or requires high axial stiffness, a larger contact angle may be preferable. For extremely high-speed spindle applications with relatively moderate axial loads, a small contact angle may be the better choice.
Preload Requirements
Preload is the controlled internal load applied to a bearing before the external operating load is introduced.
Correct preload can increase bearing stiffness, improve running accuracy, reduce vibration and help maintain consistent rolling element contact.
Too much preload, however, increases friction and heat generation, and at high speeds, excessive preload can result in rapidly rising operating temperatures and reduced bearing life. However, too little preload can lead to insufficient stiffness, unwanted movement and reduced accuracy.
The optimum preload therefore depends on the priorities of the application.
For example, a high-speed spindle may require a relatively light preload to keep friction and temperature under control, while an application prioritising rigidity and machining accuracy may require a higher level of preload.
Temperature also needs to be considered because thermal expansion of the shaft, housing and bearing components can alter the effective preload once the machine reaches operating temperature.
Many matched super precision bearing sets are supplied with a predetermined preload to simplify assembly and achieve more consistent performance.
Find out more about bearing preload and how to select the right preload for your application
Operating Temperature
Operating temperature shouldn’t be considered separately from speed, preload and lubrication because all four are closely linked. As bearing temperature increases, the shaft, housing and bearing components expand. Because different components may heat up and expand at different rates, the internal conditions within the bearing can change during operation.
This is particularly important with preloaded precision bearings. A preload that is correct when the machine is stationary and cold may increase as the spindle reaches operating temperature.
When selecting a bearing, engineers should therefore consider:
- Normal operating temperature
- Start-up and warm-up conditions
- Maximum expected temperature
- Temperature differences between shaft and housing
- Lubricant temperature limits
- Thermal expansion of surrounding components
- Heat generated at maximum operating speed
Bearing Accuracy and Tolerance Classes
Super precision bearings are manufactured to tighter dimensional and running tolerances than standard bearings.
Tolerance classes control characteristics such as bore and outside diameter accuracy, ring width and acceptable radial and axial runout and thus should reflect the accuracy the machine needs to achieve.
For applications requiring very high rotational accuracy, bearings may be specified to standardised precision classes such as P4 or P2, or to manufacturer-specific super precision grades.
It is also essential that the shaft, housing, mounting surfaces and assembly process can maintain that level of accuracy. A P2 bearing installed onto a poorly manufactured shaft will not produce a P2-accurate system.
Matching the Bearing to the Application
There is no single specification that makes one super precision bearing the best choice for every machine. Selection involves balancing speed, load, stiffness, temperature, accuracy and bearing life.
Machine tool spindles
Machine tool spindles typically demand a combination of high rotational accuracy, rigidity and speed.
Angular contact super precision bearings are frequently used, often as matched sets. The correct contact angle and preload will depend on the cutting forces, required spindle stiffness and maximum operating speed.
For particularly high-speed spindles, hybrid bearings with ceramic rolling elements may also be considered to reduce centrifugal forces and heat generation.
Grinding machines
Grinding applications can place particularly demanding requirements on bearing accuracy.
Small variations in spindle movement can affect surface finish and dimensional accuracy, making low runout and stable operating temperatures important considerations.
Depending on the machine, high-speed capability may also be required, making bearing preload and lubrication especially important.
High-speed motors
In high-speed motors, reducing friction and controlling temperature are major priorities.
Bearing design, cage material, lubrication and rolling element material can all influence the achievable speed.
Hybrid bearings may be beneficial in some high-speed motor applications because ceramic rolling elements are lighter than their steel equivalents and offer useful electrical insulation properties.
Conclusion
Choosing the right super precision bearing means looking beyond a catalogue part number. Shaft and housing geometry, fits, preload, lubrication, temperature, installation accuracy and operating conditions can all affect a bearings final performance.
If you’re unsure which super precision bearing best meets the requirements of your machine, speak with our specialist super precision bearing division. By looking at the complete application rather than a single specification, we can help you identify the most appropriate bearing design, arrangement and specification for the job.