Match the bearing to the load

Although the function of all bearings is to minimise friction between moving parts in a mechanical system, some applications require much higher levels of accuracy. While it may not be as critical for relatively low accuracy applications such as conveyors, handling equipment and cheap drills to operate quietly and accurately, it is much more important that bearings used in industrial robots, electric motors, quality power tools and sensitive instruments deliver smooth operation to ensure a long life.

Radial & axial loads

Where accuracy and smoothness are required, it is important for engineers to consider the forces acting on the bearing. Because a bearing typically supports the free motion of a shaft about an axis of rotation, two forces normally act on the bearing: a radial load and a thrust load.

A radial load acts perpendicular, at 90 degrees, to the axis of rotation, while a thrust load — also known as an axial load — acts in parallel to the axis of rotation. Any misalignment of the shaft can also result in a moment load, a tilting force that can increase wear.

Applying a thrust load to a bearing can be beneficial. For example, applying a permanent thrust load (preload) to the inner or outer ring, using washers or springs, can eliminate play in the bearing and provide more accurate rotation. Conversely, applying an excessive load can be catastrophic. If engineers fail to adequately match a bearing to the radial and thrust loads in an application, it can drastically reduce the life of the bearing.

When calculating bearing life, it is important to consider load ratings, these are a measure of how quickly the rotating elements of a bearing will experience fatigue and the total number of revolutions a bearing can withstand before it fails. These ratings can be categorised into static load ratings and dynamic load ratings.

A typical radial ball bearing, which is designed primarily for radial loads, has a maximum static and dynamic load capacity. The static load capacity is the maximum radial load that a bearing can withstand before the load causes a total, permanent deformation of the bearing balls or the raceway equal to one ten-thousandth of the ball's diameter.

Although a bearing may be able to tolerate a high static load, it will do so at the loss of accuracy and smoothness, making it impractical for use in high accuracy environments, such as electronics manufacturing and in robots used in food and beverage production. The typical static-load rating for a stainless steel bearing is approximately 75–80 per cent of the load rating for chrome-steel bearings, due to the hardness of chrome steel.

The dynamic load rating, on the other hand, is the ability of 90 per cent of a group of identical chrome-steel bearings, with only the inner ring rotating, to endure a radial load of a constant magnitude and size for one million revolutions before the first signs of fatigue develop.

The greater the load, the higher the level of stresses the balls and raceways will be subjected to. This will lead to more rapid wear and a shorter bearing life. Fatigue failure results in the ball path being eroded, leading to spalling, where a fracture on the surface of the raceway causes material to be removed, ultimately leading to failure.

Excessive loading can also lead to other signs of fatigue such as overheating, degradation of the lubricant and abrasion caused by flakes of particulate matter.

In situations where a bearing primarily carries a radial load with only a small thrust load, engineers can calculate the bearing life using the formula: 16666/rpm x (dynamic load rating / radial load)3.

To demonstrate how a heavy load will shorten bearing life, imagine a miniature bearing with a dynamic load capacity of 100kgs and a speed capacity of 40,000rpm used with a high radial load of 80kgs and a relatively low bearing speed of 5000rpm. Applying this bearing-life formula, we can see that the bearing life would be unacceptably short:

16666/5000rpm x (100kgs dynamic load rating / 80kgs radial load)³
= 3.333 x (1.25)³
= 6.51 hours

While a bearing used in the guidance system on a missile might only need to last for up to sixty seconds, a bearing used in a shower pump may need to last for ten years or more. In these situations, it is the responsibility of the manufacturer of the product to determine an acceptable bearing life span.

Choosing the right bearing

There are a variety of measures that manufacturers can take to ensure long bearing life. The first step is to limit the radial load to between 6–12 per cent of a bearing's dynamic load rating. Although a bearing is able to tolerate a much higher load, its life will be shortened.

Other basic factors include reducing the speed of the application, avoiding heavy loads at high temperatures, minimising the risk of contamination by dirt or dust and spreading the load between a number of bearings to extend life.

The next step is to choose the right material. Although they are corrosion resistant, bearings made from AISI440C or KS440 stainless steel will support approximately 80–90 per cent of the load ratings for chrome-steel bearings during rotation.

Engineers should also consider using full complement bearings for high radial loads at slower speeds. These are bearings with no retainer and extra balls. Under this type of heavy loading, lubrication can also be squeezed out under the heat and pressure of operation. To ensure that a thin film of lubricant continuously coats the surfaces of the balls and raceway, engineers should consider using a lubricant with additives such as molybdenum disulfide that exhibits anti-wear properties in pressurised environments.

In SMB Bearings' experience as a specialist in thin-section, corrosion resistant and miniature bearings, choosing the right type of bearing can also make all the difference. While all radial ball bearings have some thrust load capacity, it's often better to use heavy duty bearings that have deep raceways if greater thrust loads are present as these can withstand axial loads of up to 50 per cent of the static radial load rating.

Although thin-section bearings — where the difference between the inner and outer diameter of the bearing is small — are great for compactness and saving weight, they can only support axial loads of between 10 and 30 per cent of the bearing's static radial load rating due to the shallower raceways. Additional radial loads or moment loads will reduce thrust load capacity even further. Excessive thrust loads on a thin-section bearing can cause the balls to ride dangerously close to the top of the raceway.

For applications with high thrust loads, engineers should ensure that the bearing is installed properly. Poor installation may also lead to misalignment that can drastically reduce the axial load capacity.

In situations with heavy thrust loads, it is advisable to choose a combination of radial and thrust bearings or choose an alternative such as an angular contact bearing. This is typically the same size as a radial ball bearing but is designed to handle much higher thrust loads.

By choosing the right type of bearing and considering key factors in the battle to control radial and thrust loads, engineers can ensure they continue to innovate while delivering the highest levels of accuracy, smoothness and bearing life.