Friday, 30 May 2014

Bearing closures - types, uses & selection...

Have you ever wondered why some bearings have rubber seals in, others have metal shields & some have nothing at all?

Well, we'll try & explain for you now:

Bearing closures are available in two basic types - shields and seals. Both closure types are generally ordered as integral components of deep groove bearings.
Closures serve the same purposes with varying effectiveness - they exclude contamination, retain lubricants and protect the bearing from internal damage whilst being handled.  
Closures are attached to the outer ring of the bearing. Seals are defined as closures which contact the inner ring, while shields clear the inner ring. Both are designed so that the stringent precision tolerances are not affected by their use.
Determining the proper closure for an application involves a trade-off, usually balancing sealing efficiency against speed capability and bearing torque. Shields do not raise bearing torque or limit speeds, but they do have low sealing efficiency. Seals are more efficient, but they may restrict operating speed and increase torque and temperature.
Another consideration in closure selection is air flow through the bearing. This is detrimental because it carries contamination into the bearing and dries out the lubricant. It is preferable to use seals if an air flow is present.

A number of types of closures are available:
A metal shielded bearing
Metal Shields - Most sizes are available with metal shields. Shields are designed to prevent larger particles from entering the bearing and also to keep grease inside the bearing. They may be pressed into the bearing’s outer ring (non-removable) or retained by a circlip (removable). As the shields make no contact with the inner ring, they do not increase starting or running torque. Shields on stainless steel bearings are generally made from AISI 304 grade stainless steel.

• Prevent contamination by larger particles
• Reduce lubricant leakage
• Do not increase torque

Rubber contact seal bearings
Contact seals -  The standard bearing seal consists of nitrile/BUNA-N rubber bonded to a metal washer. High temperature teflon seals (up to 250°C) or Viton seals (up to 230°C) are available on some sizes. The inner lip of the seal rubs against the bearing inner ring to provide an effective seal against smaller particles such as dust and moisture while preventing lubricant leakage. Contact seals produce much higher frictional torque levels than shields and reduce the maximum speed of a bearing. Below -30°C nitrile rubber and viton will stiffen and provide a less effective seal so teflon seals or metal shields should be considered for very low temperatures.

• Good protection against contamination
• Greatly reduce lubricant leakage
• Reduce maximum speed by approx. 40%
• Greatly increase bearing torque

• Temp. range –40°C/+110°C (nitrile rubber) or up to 230°C (Viton) and 250°C (Teflon)

A viton seal 
Bearing with Teflon seals

Non-contact seals - These seals are also made of nitrile rubber bonded to a metal washer but do not rub against the bearing inner ring and therefore do not have the same effect on bearing torque and maximum speed as contact seals so can be used for low torque, high speed applications. They offer superior protection over metal shields but do not provide as effective a seal as the contact type.

• Good protection against contamination
• Reduced lubricant leakage
• No torque increase
• Do not affect maximum speed
• Temp. range –40°C/+110°C

An "open" bearing - no seals or shields

Alternatively, if a bearing needs to be extremely free-running, is naturally corrosion-resistant (because it is produced from a corrosion resistant material - stainless steel, plastic or ceramic for example), doesn't require any lubrication & is operating in a clean environment it can be supplied open - no closures used at all!

If you are in any doubt as to what would best suit your particular application, 
please contact us & we'll help you make the right decision.

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