Reproduced  with permission of Miniature Bearings Australia

The most common types of cage (also called retainers) used for miniature ball bearings are the TWO PIECE STEEL RIBBON and the ONE PIECE STEEL CROWN type cages. The crown type is better suited to low torque , low speed applications. These two types of cage are interchangeable in the majority of applications. These cages are usually made out of a variety of grades of stainless steel depending upon the manufacturer. Grades usually used are AISI302, AISI410 or AISI430.

Larger bearings (over 10mm bore size) usually utilise a TWO PIECE STEEL RIVETED cage allowing for higher loads, and more vibrational and acceleration stresses to be placed on the bearings.

High speed applications require bearings with a crown type retainer manufactured from various types of synthetic materials including

• NYLON
• DELRIN^® ACETAL RESINS
• PHENOLIC LAMINATES.

Please note however that the term "high speed" is relative to the bearing size. ie. "High speed" for a 10mm bearing may not be "high speed" for a 4mm bearing. 

DELRIN^® is a registered trademark of DuPont.

Strict cleanliness is an essential condition for the good operation of all miniature bearings. RMB achieves this perfect cleanliness by

• full temperature control and air filtration of all production departments
• ultrasonic cleaning of all components after different stages of manufacture
• cleaning by specially developed methods, of all component parts before assembly
• the assembly of parts in special CLEAN ROOMS, in which the temperature, humidity and dust content of the atmosphere are controlled to precise limits
• strict observation of the rules of cleanliness enforced on all personnel working in these CLEAN ROOMS
• the repeated cleaning of finished bearings by processes specially designed and perfected for small bearings
• the use of specially filtered lubricants
• the packing of finished bearings in absolutely clean sachets or tubes hermetically sealed

The above examples give an indication of the efforts made by RMB to supply their customers with bearings of the highest standard of cleanliness. For premium bearing performance it is essential that this standard is maintained by the end user. This may be achieved by observing the following points:

• It is the degree of precision and of cleanliness called for by the application which determines to what point the following recommendations should be carried out.
  
• Assembly components, shafts, spindles, housings, springs etc should be manufactured to the specified tolerances, and the surface finish should be satisfactory for the application in question. All the components should be free from faults, corrosion, burrs etc. Cleaning of all component parts before mounting the bearing should be done outside the workroom where the bearings are to be mounted and all necessary care should be taken to avoid recontamination during transport from cleaning to place of assembly (assembly shop).
  
• the bearings should be assembled in a place specially arranged for the purpose and separated from other departments. It should if possible conform to CLEAN ROOM standards, with dust free atmosphere, temperature and humidity control. Machining should not be done in the same room.
  
• the personnel employed in the assembly of high precision bearings should be subject to special rules of cleanliness. It is usual to equip them with overalls and head-dress of special non-fibrous material. It should be strictly forbidden to smoke, eat, make-up etc within the CLEAN ROOMS.
  
• bearings should be removed from their packing only immediately before assembly. If the package contains several bearings, it should be opened in such a way that only one bearing may be taken out at a time.
  
• bearings should be manipulated with tweezers or other special tools. One should never touch high precision bearings with fingers unprotected by rubber or plastic finger stalls.

The primary purpose of lubrication is to reduce friction and heat. A secondary purpose is to inhibit corrosion. Using the correct lubricant is therefore critical to overall bearing performance.

Most bearings are charged with a general purpose grease which caters for all but the most extreme operating conditions. Special lubricants are available for critical applications and when extreme conditions exist. Specialised lubricants are used in applications such as Dental Handpiece Turbines, Military Applications, Aircraft Instruments, Sound Recording and QC equipment, Computer Applications and more.

Standard materials used for the races of our miniature ball bearings are AISI440C STAINLESS STEEL and SAE52100 CHROME ALLOY BEARING STEEL.

AISI440C Stainless Steel bearings are heat treated to a hardness usually from Rockwell C 58 to Rockwell C 61+. All RMB Swiss Product is manufactured to a MINIMUM of Rockwell C 61. The maximum recommended operating temperature is 177^o Celsius (350^o Farenheight).

SAE52100 Chrome Steel Bearings are usually slightly harder than AISI440C bearings. RMB heat treats their SAE52100 Chrome Steel Bearings to Rockwell C 61-64. The maximum recommended operating temperature of chrome steel bearings is 120^o Celsius (248^o Farenheight).

Operating temperatures higher than those shown above will need special treatment. Please consult MBA.

See our sections on Seals and Shields and Cages for information about the materials used in these parts.

The noise level of bearings depends essentially on the following factors

• the geometric precision and quality of the tracks and balls
• the cleanliness of the bearings and the lubricant
• the precision with which the play is taken up when mounting the bearings
• the value and direction of loads
• the rotation speed

In the absence of recognised standards, RMB measure the noise level of their bearings by comparison with a standard and fix, in agreement with their customer, an acceptable limit.

Example part numbers

Please note that these part numbers are only examples and are not readily available.

3. Flange Specification
Blank - No Flange	F - Flanged Bearing

4. Inner Ring Specification
Blank - Inner Ring Flush with Outer Ring	W - Extended (Wide) Inner Ring

7. Closure Type

10. Snap Ring Specification
Blank - No Snap Ring	NR - Snap Ring on O.D.

Shields - Made from STEEL or STAINLESS STEEL. Shields do not contact the inner ring of the bearing and therefore do not increase the running torque. Shields can be either crimped into the inside of the outer race (type ZZ) or held in the outer race by circlips (type ZZS). The ZZS type are removable to allow inspection and maintenance of the bearing. RMB FILMOSEAL is also a type of shield which utilises a film of lubricant between the inner race and the specially designed shield to more effectively exclude contaminants.

Seals - Usually made from NITRILE. Seals are either CONTACT (type 2RS) or NON-CONTACT (type 2RU). Contact type seals which are secured in within the outer race and contact the inner race greatly increase start up and running torque, thereby reducing running speed. They also provide a very effective method of excluding contaminants and retaining the bearing lubricant. Standard Nitrile seals have a maximum recommended temperature of 100^o Celsius (212^o Farenheight). Some bearings are also available with PTFE seals offering very effective sealing whilst minimising torque. Teflon seals can operate at temperatures up to 300^o Celsius (572^o Farenheight). Note however that whilst the seals can withstand these temperatures the bearing itself will need special treatment if it is to be used at any temperature greater than those shown in our materials section.

Snap Ring - A wire ring situated in a groove on the outer diameter of the bearing. Used for location of the bearing once fitted. The snap ring is usually only available on bearings over 10mm bore size, however there are a number of smaller bearings available with snap rings. 

Flange - An external "rib" protruding from the outer bearing surface. The flange is an integral part of the outer ring of the bearing; it is non removable. The flange is provided to assist in the correct fitting of the bearing. Most sizes of bearing available through Miniature Bearings Australia are available in both flanged and unflanged versions.

Showing various levels of Radial Play. The starting amount is also adjusted by other factors such as housing tolerances, loads and temperature.

Because of the complexity of measurement and taking into consideration manufacturing tolerances, radial play is always indicated between two limits. The normal radial play of RMB radial ball bearings is from 6 to 15 µm (.0002 to .0006"). The normal radial play of Japanese radial miniature ball bearings is 5 to 15 µm (.0002 to .0006"). On request, bearings may be supplied with reduced or increased radial play as per the following chart.

Play is not a criterion of the quality of a bearing but if badly chosen or unfavourably influenced during mounting, it can adversely affect the operation and even reduce the working life of a bearing.

Also see the Bearings FAQ Section

IReproduced  with permission of Miniature Bearings Australia