Retainers keep the balls evenly spaced around the raceway to prevent ball to ball contact and allow higher speeds.
They also help to retain grease around the balls and raceways. For greater accuracy and to prevent additional friction, it is important that the retainer is not allowed too much radial movement. To achieve this, the retainer is guided by either the balls or one of the rings.
Metal Crown / Ribbon
These standard retainers are manufactured from carbon steel for chrome bearings and AISI304 or AISI420 grade stainless steel for stainless bearings. These were often made from brass which also offered a high temperature capability but this is much less common due to higher cost of brass and advances in steel technology.
For higher temperatures, stainless steel is usually recommended. The crown cage and ribbon cage perform the same function but the crown cage is used primarily on smaller miniature bearings and thin-
- Good for low to medium speeds
- Can withstand higher temperatures according to the type of steel (see "Bearing Material" section)
- Crown type -
inner ring guided - Ribbon type -
mainly ball guided
Polyamide Crown (TW)
This moulded PA66 retainer has better sliding characteristics than the steel cage and produces fewer fluctuations in running torque. It is suitable for higher speeds than the steel type. It is also available with fibreglass reinforcement for greater rigidity and even higher speeds. The polyamide retainer has good low noise properties. This retainer is not suitable for low temperature applications as it loses elasticity below about 30°C. In vacuum applications, it may become brittle.
- High speed and low noise
- Temperature range approx -
30 to +120°C - Ball guided
Polyethylene Crown (PE)
This low speed retainer is made from high density polyethylene (HDPE) and used in 316 stainless steel bearings. It has very good corrosion resistance so can be used in the presence of seawater and many chemicals.
- Very corrosion resistant
- Temperature range -40 to +80°C
- Inner ring guided
PEEK Crown (PK)
PEEK cages are often used in ceramic bearings, 316 stainless steel bearings and PEEK bearings. They are highly corrosion resistant, have a wide temperature range and are suitable for vacuum use.
- Very corrosion resistant
- Low outgassing so suitable for vacuum use
- Temperature range -
70 to +250°C - Inner ring guided
PTFE Crown (PT)
This retainer is used in ceramic bearings, 316 stainless steel bearings and PTFE bearings. It is highly corrosion resistant and has a very wide temperature range.
- Very corrosion resistant
- Temperature range -
190 to +220°C - Inner ring guided
Full Complement (F/B)
A full complement (or full ball) bearing contains extra balls and has no retainer. It is used for its greater radial load capacity although axial load capacity is very small. These bearings can only be used at low speeds and bearing torque is increased due to ball to ball friction. Improved steel and hardening techniques have increased the load capacities of bearings with cages and the full complement bearing is much less common now.
- Higher radial load capacity
- Much lower speeds than caged type
- Low axial load
- Increased bearing torque
Combating common retainer problems
From lubrication failure to misalignment bearing failure occurs for a range of reasons. Retainers, however, can succumb to two common problems:
Hooping
The phenomena when the retainer wobbles like a hula-hoop causing torque spikes in the rotating assembly. The retainer should track in a true circumferential plane concentric with the pitch diameter of the balls.
Hang-up (wind-up)
When an axial load is applied to static bearings that have the axis of their shaft in a horizontal mode, the balls fall downward to a position where they are unequally spaced prior to the load being applied. When the axial load is applied it squeezes the balls between the inner and outer raceways. Now that the balls are held securely in unequally spaced positions they cause the retainer to bind. This binding is called “retainer hang-up.” Once bearing rotation commences the retainer is stressed and some of the balls may skid causing damage that will initiate premature bearing failure.