Properly aligned bearings can be critical to the life and health of pumps
Misalignment is a frequent cause of rolling bearing failure. It can cause cage fracture, which will result in seizure of the bearing, pump failure and costly downtime. It can also cause edge loading, which will result in early bearing failure. Typical bearing-life calculation tools assume that the bearing’s inner and outer rings are well aligned. A general acceptable alignment is better than 0.003 radian (10 arcminutes) for ball bearings and 0.0012 radian (4 arcminutes) for cylindrical roller bearings. Rolling bearings are manufactured with great accuracy. Great care must be taken with machining practices and assembly accuracies of the mating shaft and housing to maintain this accuracy. In practice, however, the machining accuracy of parts surrounding the bearing must be considered. Sources of misalignment include:
- Nonconcentric housing bores
- Non-perpendicular shoulders on mating components
- Bent shafting
- Errors during installation
- Baseplate irregularities
- Non-flat mounting surface
- Insufficient rigidity of the mounting surface
Diagnose Misalignment
Misalignment in a failed bearing can typically be diagnosed by examining the rolling element path inside the bearing. As bearings rotate, the rolling elements generate a wear path on the inner and outer raceways. A well-aligned bearing will exhibit a running path down the centre of the inner and outer rings, while a misaligned bearing will exhibit uneven running paths (see Figure 1)
Avoid Misalignment
Misalignment can be avoided by being attentive during the bearing installation process. The first step is the proper design and machining of the mating housing and shaft components. Housings should be rigid to provide firm bearing support. In cases in which two bearings are mounted in one housing, the fitting surfaces of the housing bore should be designed so both bearing seats may be finished together with one operation, such as in-line boring. The recommended accuracy and surface finish of shafts and housings are listed in Table 1 for normal operating conditions (IT values are International Tolerances Grades as per the International Organisation for Standardisation 286). The shoulders of the shaft or housing that contact the face of a bearing must be orthogonal to the shaft centre line.
Table 1 – Shaft – HousingThe fillets of the shaft and housing should not contact the bearing chamfer, while the supporting shoulder diameter still must be large enough to fully support the face of the bearing. During installation, all mating surfaces should be cleaned, and all shaft and shoulder abutting surface edges should be free of burrs.
Bearing mounting methods will vary depending on the bearing type and the type of fit. Because bearings are usually used with rotating shafts, the inner rings require a tight fit. Bearings with cylindrical bores are usually mounted by pressing through the inner ring on the shafts (press fit) or heating them to expand their diameter (shrink fit).
Bearings with tapered bores can be mounted directly on tapered shafts or on cylindrical shafts by using tapered sleeves. Bearings are usually mounted in housings with a loose ?t. However, if the outer ring has an interference fit, a press may be used. End users should always apply a light film of oil on the fitting surfaces first to prevent scoring. When pressing a bearing into a housing, apply press to the outer ring of the bearing. When pressing onto a shaft, press on the inner ring.
Mitigate the Effects on Bearing Life Several bearing solutions are available to help mitigate the effects of misalignment. For example, nylon cages are more flexible than steel cages and can accommodate misalignment better than steel cages. Increasing the internal clearance of the bearing will increase its misalignment capacity. Self-aligning ball bearings can also be used. These bearings have a spherical raceway with a centre of curvature that coincides with that of a bearing. This allows the axis of the inner ring, balls and cage to deflect to some extent around the bearing centre. However, this design can create a smaller contact angle between the ball and the raceway, which results in a lower load capacity compared to a similar sized, deep-groove ball bearing.
The permissible static misalignment in this bearing type is approximately 0.07 to 0.12 radian (4 to 7 degrees) under normal loads. Depending on the surrounding structure, this angle may not always be possible. Because standard L10 calculations assume that the bearing is well-aligned, additional calculations must be made to determine the effect of misalignment on the bearing’s fatigue life. The maximum allowable misalignment of a bearing varies depending on the size and type of bearing, internal clearance during operation, and the load.
Assume the fatigue life without misalignment as Lθ 0, and the fatigue life with misalignment as Lθ. The effect of the misalignment on the fatigue life can be found by calculating Lθ/Lθ=0. Figures 2 and 3 show the effect of misalignment on the life ratio for a deep groove ball and roller bearing, respectively. In these figures, the horizontal axis shows the misalignment of inner/outer rings (rad), while the vertical axis shows the fatigue life ratio Lθ/Lθ=0. As an example of ordinary running conditions, the radial load Fr (N) {kgf} for both figures was assumed to be approximately 10 percent of the dynamic load rating Cr (N) {kgf}, and the shaft fit was machined to the recommended value.
| Figure 2 – Life ratio for 6200 deep-groove bail bearing | Figure 3 – Life ratio for NU315 cylindrical roller bearing |
The decrease of the internal clearance because of the expansion of the inner ring was also considered. Figure 2 was generated using the normal radial clearance for the deep groove ball bearing. The three separate plots represent maximum, minimum and mean effective clearance. The reduction of the fatigue life is limited to 5 to 10 percent up to 0.004 radian of misalignment, therefore, not significantly reducing the bearing life. However, when the misalignment exceeds this limit, life is reduced considerably. In this scenario, an increase of 11µm in internal clearance results in ~0.0015 radian increase in misalignment capacity.
Figure 3 plots three separate clearance classes for a cylindrical roller bearing: normal, C3 and C4 clearance. In comparison to Figure 2, the life ratio is reduced by more than 10 percent with only 0.001 radian of misalignment. Little variation between the different clearance classes exists, despite a total difference of 50µm. Clearly, the roller bearing is more sensitive to the effects of misalignment than the ball bearing, and this should be considered when selecting a bearing type in a new pump design. These figures were generated for typical operating conditions but are not applicable to all pump applications. Reducing or eliminating misalignment is critical to long bearing and pump life. Catalogue-recommended assembly tolerances and installation processes must be followed to prevent bearing misalignment. If misalignment cannot be completely avoided, additional calculations are required to determine the effect it will have on the bearing life. Contact a bearing manufacturer for assistance with these calculations and additional application analyses.
Helena Measham,
Customer Service & Marketing Communications Manager
NSK UK Ltd
Tel: 0500 2327464 Fax:
Web: www.nskeurope.com
Email: info-uk@nsk.com
Address: NSK UK Ltd, Northern Road, Newark, Nottinghamshire, NG24 2JF, UK
NSK Europe Ltd
- 01636 605 123
- info-uk@nsk.com
- Belmont Place, Belmont Road, Maidenhead, Berkshire, SL6 6TB GB
About us
Established over 100 years ago, NSK (Nippon Seiko Kabushiki Kaisha) is a Japanese-listed company that has evolved from a regional ball bearing supplier to a roller bearing specialist and automotive supplier with a global market presence. Today, NSK employs more than 31,400 employees in 30 countries. As per March 2019, NSK achieved sales of 991,4 billion Yen. This result has been driven by ever-increasing investment in research & development, enabling the company to continuously improve the quality of its products and services. This investment supports NSK’s objective of “No. 1 in Total Quality”. In addition to a complete rolling bearing portfolio, NSK develops and manufactures precision components and mechatronic products, as well as systems and components for the automotive industry, including wheel bearing units and electric power-steering systems. In 1963, NSK’s first European subsidiary, Düsseldorf, Germany, was opened and in 1976, the first European production facility in Peterlee, England. Today, NSK Europe supports pan-European sales with production locations in England, Poland and Germany, logistics centres in the Netherlands, Germany and England and technology centres in Germany, England and Poland. In 1990, NSK purchased the UPI Group including the renowned European bearing manufacturer RHP, with its factory in Newark (UK). Additionally, NSK has developed a comprehensive network of authorised sales distributors. NSK Europe is divided into application-based business divisions: Industry rolling bearing technologies & linear and precision technology (EIBU) as well as bearing modules and steering systems for the automotive industry (EABU & ESBU). In this organisation, NSK Europe's 4,260 employees achieved a turnover of over 1,045 million Euros as per March 2019.
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Europe, Africa, Asia, Australia, South America, North America
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Automation, Chemicals, Consultants, Components Electronics, Energy and Power, Food and Beverage, Glass Ceramics Cement, Metals and Minerals, OEM, Paper and Pulp, Pharmaceutical Cosmetics Toiletries, Plastics and Rubber, Recycling, Textiles, Tobacco, Water and Wastewater
