Safety first at higher bearing speeds with SKF Explorer single row angular contact ball bearings
2019 April 30, 09:00 GMT
Running machinery at higher speed helps to raise performance. But there is a danger of component and system failure unless parts, such as bearings, are redesigned to handle these increased loads.
If the rotating machinery industry had a mantra, it would be ‘faster, faster’ - this is particularly relevant for equipment such as pumps and compressors. These industry workhorses are increasingly expected to deliver higher outputs. The way to increase power density is to boost rotation speed, but with this comes potential instability, increased wear and possible failure.
For example, pumps are increasingly expected to operate continually while receiving minimal maintenance. Many factors affect performance, including operating conditions, the materials it is made from and its mechanical design. In this final regard, the bearings used will have a huge effect on long term reliability, energy efficiency and life expectancy.
Pump bearings are subject to high axial loads, temperatures and mechanical vibration. In operation they may also lack lubrication, putting them under more strain, which can cause power loss, excessive heat generation, increased noise or wear and early failure. Such conditions have similar effects in other machinery, such as compressors, meaning that an answer was needed to help these assets work at higher power densities.
In response, SKF has designed a new SKF Explorer class bearing that copes with higher rotational speeds. The bearing has a new raceway geometry and a brass cage, which reduces noise and vibration while adding robustness. The changes have helped to boost the limiting speed by around 30% compared to earlier designs. However, by reducing the contact angle, from the standard 40° down to 25°, SKF has boosted limiting speed by a further 20%, for an overall 40% improvement.
The single row angular contact ball bearing is ideal for applications including screw compressors, pumps and gearboxes, reducing the total cost of ownership through improved reliability and energy efficiency. Sealed versions, to prevent contaminants entering the bearing during installation and operation, are also available for applications that are more difficult to maintain.
The main design changes involved the cage, raceway and bearing contact angle.
Inside the cage
The new cage design is fundamental to improving robustness: brass is stronger than steel, which helps to reduce cage contact forces and increase cage strength, improving the tolerance to shock loads and vibration, to allow higher speed capability even under severe conditions.
Changes to the cage went beyond switching to a new material. It has also been redesigned to give a toroidal shape to the cage pockets, reduce the cage pocket axis angle and optimise cage pocket clearance. SKF performed finite element analysis (FEA) on the design to compare the attributes of different design variants, such as assessing cage pocket geometries. Following FEA, a number of physical tests were carried out, including many high-speed tests to verify cage performance.
An improved ball-to-cage contact helped stabilise temperature behaviour at high speeds. A new oval cage pocket design reduced clearance in the axial direction and helped cut noise and vibration levels by 15%.
Similarly, changes to the raceway geometry also helped boost performance.
The raceway profile of ball bearings is usually a circular arc. Under severe operating conditions, with axial shock loads and shaft misalignment, the contact ellipse can reach the edge of the shoulder and cause high stress peaks. These higher loads can lead directly to early bearing damage and failure. To reduce this risk, the raceway geometry for bearings with 25° contact angle was improved, adding a second circular arc with larger osculation.
This reduces the risk of ellipse truncation. Single row angular contact ball bearings with this new raceway geometry can accommodate axial forces up to three times higher, without ellipse truncation, compared to bearings with constant raceway radius.
The new geometry slightly increases contact pressure, because contact area has been reduced. The pressure increase depends on the axial and radial loads acting on the bearing, but under typical conditions the increase is always below 1%.
Perhaps the most fundamental change has been to develop a bearing with a contact angle of 25°. Single-row angular contact ball bearings typically have a 40° contact angle as standard, though high-speed applications with moderate axial loads prefer a smaller contact angle.
If a single-row angular contact ball bearing is loaded only in the axial direction, contact forces between balls and rings increase with smaller contact angles. As the bearing rotates, centrifugal forces cause contact angle changes. The contact points between the inner and outer rings, and the balls, will move outwards. This causes a contact angle variation, which leads to ‘sliding’ between balls and rings. However, the contact angle variation is much smaller for bearings with smaller contact angles, which reduces sliding and lowers cage pocket forces. This is why bearings with a 25° contact angle can work at higher speeds without cage fracture.
Designers need ways to satisfy the ongoing quest to run machinery at ever-increasing speeds. These new bearings ensure that rotating machinery can run at cooler temperatures and with greater energy efficiency, to survive the most punishing of conditions and ensure a longer life, free of failure.