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What is a tolerance ring?

What is a tolerance ring?

A total misconception about tolerance rings is that they are a way of correcting machining errors between two round parts. As Gerry Smith of Euro-Bearings explains, this is not the case.

A tolerance ring is a corrugated metal strip that acts as a designed in interface between two round metal components, with obvious examples being between a bearing and its housing or a between bearing and a shaft. The main reason that tolerance rings are used is to make assembly fast and easy.

How so? 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 installing a standard deep groove ball bearing ISO part number 6204 into a housing you, would take the outer diameter of the bearing, in this case 47mm, and make the housing (say) 20 to 30 microns bigger (ie 47.020mm, perhaps tolerancing that to ±0.010mm). Even then you might struggle with the assembly Let’s consider, too, that holding the ±10 microns tolerance is a challenge even using modern CNC equipment. So, with all of that in mind, let’s look at using the tolerance ring to hold the same bearing in the housing. The online data for the 6204 on the Euro-Bearings website tells us that we need to use an AN47-514 and the housing bore needs to be made within the range 48.89mm to 48.98mm – a range of 0.090mm. So, aiming for the mid point, the tolerance becomes ±0.045mm which, as you can see, is a lot wider and easier to hold than  ±0.010mm, with obvious time and cost savings to be made.

As an aside, the online data lists the relevant inner (BN type) or outer (AN type) by ISO standard bearing sizes and is a most useful reference for rewinders and anyone 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, and again the charts available on the Euro-Bearings web site give this data.

With all of this in mind, tolerance rings can be used in a wide variety of applications, intruding pulley to shaft connections, fan to shaft connections, mounting knobs and handles, mounting stators in electric motors, mounting magnets, refurbishment of housings, and torque control.

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