Accurate cage selection: essential for reliable, long-term bearing operation
16-01-2012 | BARDEN CORPORATION | PLYMOUTH UK
In applications involving rolling element bearings, proper attention to cage design and materials selection is essential in order to ensure reliable, long-term bearing performance, says Gary Hughes, Product Engineering Manager at The Barden Corporation.
The basic purpose of a cage is to maintain uniform ball spacing, but the cage may also be designed to reduce torque, and to minimise build-up of heat and resist harsh environments.
In separable bearings, the cage is designed to retain the balls in the outer ring so the rings can be handled separately. Cage loading is normally light, but acceleration and centrifugal forces may develop and impose cage loading. Also, it may be important for the cage to accommodate varying ball speeds that occur in certain applications.
Cages are piloted (guided) by the balls or one of the rings. Typically, low to moderate speed cages are ball-piloted. Most high-speed cages have machined surfaces and are piloted by the shoulder of either the inner or outer ring.
High precision deep groove and angular contact ball bearings are available with several types of cages to suite a variety of applications. While cost is always a factor with bearing cages, many others enter into cage design and cage selection, including:
- Low coefficient of friction with ball and race materials
- Compatible expansion rate with ball/ring materials
- Low tendency to gall or wear
- The ability to absorb lubricant
- Dimensional and thermal stability
- Suitable density
- Adequate tensile strength
- Creep resistance
- High temperature
- Corrosive atmospheres
Deep Groove Bearing Cages
Deep groove bearings are typically assembled by offsetting the rings, inserting the balls, centralising the rings, then spreading the balls and inserting the cage. Due to this assembly method, it must be possible to either insert the cage from one side (snap type cage) or insert half the cage from either side and then secure the two halves together (clinched or riveted cage).
For low to moderate speed applications, Barden offers a range of pressed steel cages. The crown cage is the basic cage for the smallest miniature bearings. Larger miniature bearings are supplied with clinched ribbon cages, and the largest bearings tend to be supplied with riveted ribbon cages. One of the benefits of these low speed cages is that they are resistant to operating temperatures above 300 deg C.
Both the crown and ribbon types are used at moderate speeds and are ideally suite to bearings that are grease or oil lubricated and equipped with seals or shields. The W-type is a low-torque pressed metal cage developed by Barden, and is available in many instrument sizes. It is loosely clinched to prevent cage windup (a torque increasing drawback of some cage designs) in sensitive low-torque applications
For higher speeds, Barden offers both the one-piece phenolic snap-in TA-type, and the two-piece riveted phenolic, aluminium-reinforced T cage. The aluminium reinforcement provides additional strength, allowing this high-speed cage to be used in most standard width sealed or shielded bearings, whereas the TA cage offers low vibration in applications where this extra strength is not required.
Angular Contact Bearing Cages
Phenolic is also the cage material used by Barden in many of its miniature and instrument angular contact bearings. The cages are machined halo (B and H) types with high-speed capability. A feature of these cages is their outer ring land guiding, which allows lubricant access to the most desired point - the inner ring/ball contact area.
H-type phenolic cages are one-piece units with a through pocket halo design. They are suitable for high-speed general-purpose applications. The B-type cage is also intended for high-speed, general-purpose applications, but in separable bearings. The design of this cage includes ball pockets, which hold the balls in the outer ring when the inner ring is removed.
For high temperature applications, the cages of Barden's larger spindle and turbine angular contact bearings are machined from bronze or steel - the latter silver-plated. Most of these designs are also outer ring land guided for optimum bearing lubricant access and maximum speed capability.
Non Standard Cages for Specialised Applications
In case where a standard cage does not meet end user requirements, Barden's Product Engineering Department is able to develop specialised cages for unusual applications. These employ materials as diverse as silver-plated steel, bronze alloys, high performance polymers and porous materials such as sintered nylon or polyimide, which can be impregnated with oil to provide reservoirs for extended operational life.
Most aerospace bearing applications are powerful, lightweight units. This means that the bearings too must be minimum weight while operating at relatively high speeds and loads. Due to these requirements, many aerospace bearings utilise either one-piece snap phenolic cages or two-piece aluminium-reinforced riveted phenolic cages.
Bronze cages are also used in aerospace systems, particularly in high speed, high temperature conditions such as the generator bearings used in aerospace auxiliary power units (APU's). However, in some high speed, high temperature applications, silver-plated steel cage designs are the preferred option. In the case of lubricant failure, this silver-plating acts like a solid, dry lubricant, allowing the bearing to continue running for a short period of time in an emergency situation. An additional benefit of this type of cage is the reduced weight and increased strength when compared to a bronze cage.
Similarly, jet engine bypass valves require bearings that need to run in temperatures of 200 to 300 deg C. These bearings utilise stainless steel cages (although not silver-plated) or full complement bearings with no cage.
In smaller-sized aerospace bearings such as those used in gyroscopes, phenolic, fibre-reinforced cages are often the best solution, particularly in slow speed, low temperature environments. These cages are vacuum-impregnated with oil for the life of the bearing. A more recent alternative to this is a sintered polyimide cage, which offers higher (up to 15 per cent) oil retention and much higher operating temperatures (over 200 deg C).
In space applications, where the bearings must operate in a vacuum at relatively slow speeds and low loads, cages are often made from PTFE, molybdenum sulphide and glass-fibre reinforced material which acts as a dry lubricant.
Turbomolecular and Dry Pumps
For oil or grease-lubricated turbomolecular pumps, cage design and material play a critical part in bearing performance and life.
For grease-lubricated turbomolecular bearings, phenolic cages are typically used, as they can operate at high speeds and offer extended lubricant life.
For oil-lubricated turbomolecular bearings, machined plastic (polyamide-imide) cages are now the preferred option. These provide good resistance to high temperatures and help to feed oil into the bearings. Here, the cage is designed as part of the oil circulation system, redirecting the oil to difficult to reach areas of the bearing.
In dry pump applications, although the bearings operate at slower speeds, they have to resist higher contamination levels and poor quality lubricants over the complete life of the bearings. Therefore, steel riveted cages are the preferred choice. However, current developments in high strength plastic (PEEK) cages are pushing this technology into higher speed applications, replacing steel riveted cages.
Comprehensive details regarding bearing cage selection are contained in Barden's Speciality Products catalogue.
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