Cages are key to reliable, long term bearing operation, says Gary Hughes.
The basic purpose of a cage is to maintain uniform ball spacing, but the cage may also be designed in order 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.
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; and 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°C.
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.
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, particularly in high speed, high temperature conditions such as the generator bearings used in aerospace 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.
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Gary Hughes is Product Engineering Manager, The Barden Corporation, Estover, Plymouth, UK. www.bardenbearings.co.uk