A tolerance ring is a corrugated metal strip that acts as a designed-in interface between two round metal components –  obvious examples are between a bearing and its housing or a bearing and a shaft, but although this is the most common example there are hundreds of other applications where a part has to be secured to a shaft or into a housing.

The main reason that tolerance rings are used is to make assembly fast and easy.

Firstly the tolerances that the two round parts have to be machined to are much looser than otherwise required and secondly the natural spring in the corrugations allows the two parts to be pushed together much more easily.

For example when installing a standard deep groove ball bearing ISO part no 6204 into a housing you would take the OD of the bearing, in this case 47mm and make the housing (say) 20 to 30 microns bigger (i.e 47.020 perhaps tolerancing that to ±0.010mm (and then perhaps you might struggle to assemble) ±10 microns tolerance is a challenge even using modern CNC equipment.

Let’s look at using the tolerance ring to hold the same bearing in the housing.

A chart which can be found at http://www.euro-bearings.com/tol11.htm) shows us that for the 6204 bearing we need to use a AN47-514 and that the housing bore needs to be made within the range 48.89 to 48.98, a range of 0.090 so, aiming for the mid-point, the tolerance becomes +0.045 /- 0.045  which is a lot wider and easier to hold than  +0.010/-0.010  with obvious time/cost savings to be made.

The above chart lists the relevant inner (BN type) or outer  (AN type) by ISO standard bearing sizes and is a useful reference for electric motor rewinders and anyone who is constantly assembling standard bearings.

A tolerance ring can also be used to transmit torque. To enable torque transfer  the tolerance between the two parts needs to be tighter. The charts available on the Euro-Bearings web site gives this data: http://www.euro-bearings.com/tol1.htm

If a part transmits torque then it must at a certain loading fail to transmit  torque or 'slip' and again this slipping feature can be a useful feature of using tolerance rings in preventing damage to parts due to excessive torque – low cost, but very effective. Torque values are shown in the charts mentioned.

At  some future date after use, if parts need to be separated without the use of force, heat, or any real effort, then that too might be a very good reason to design tolerance rings into a product.  Being able to disassemble parts easily  is a useful feature in any product capable of future repair.

Assembling parts with a consistent amount of force is another feature that tolerance rings offer. 

Their simple spring effect helps to protect machinery from vibration, allows for angular misalignment  and provides a secure mounting system across  a range of temperatures too.

Tolerance rings can be used  in a wide variety of applications, for example:

•     Bearings onto shafts and into housings

•  Pulley to shaft connections

•  Fan to shaft connections

•  Mounting knobs and handles

•  Mounting stators in electric motors

•  Mounting magnets

•  Refurbishment of housings

•  Torque control

•  Pump vanes to shafts