Due to the nature of tube manufacturing, a design engineer has to compromise.

If the OD of the metal clad rubber bushing (MCRB) is on maximum, and the ID of the sleeve is on minimum, then one will have problems inserting the MCRB into the sleeve(too tight - may damage tooling - possible plastic deformation).

On the other hand if the MCRB is on minimum end of the tolerance and the sleeve is on the maximum, one might not achieve the required press fit and therefore have potential warranty issues (MCRB may loosen over time - big warranty / safety issue).

Simply put: Press fitting a standard MCRB into a sleeve is not a robust process due to tolerance issues.

The standard tolerance for a 63.00 mm tube is 0.20 mm as per ASTM A513. his applies to the inside and outside diameters. This in theory means that there is a possible variation of 0.40 mm between the sleeve (outside) and metal cladding of the MCRB (inside).

“Sizing” of the metal cladding while using half the tolerance of the sleeve, significantly reduces this variation. However these practical improvements cannot eliminate the variation of at least 0.20 mm (+/- 0.10) that will remain.

Our testing based on the above illustration, resulted in a push-in force of 4.15kN - 12.14kN. The variation in push-out force is 3.8kN to 9.6kN. The graphs opposite show the difference between the maximum measured force and the minimum measured force.

The variation, both push-in and push-out, illustrates the instability of the traditional press fitting process. The variation increases the probability of failure during the warranty period.

Knurling makes the whole press fit process much more robust since it will allow the MCRB to “adjust” its diameter to create a perfect fit that grips the steel sleeve. The knurls will adjust to allow for easy insertion, while at the same time grip the sleeve to make disassembly very difficult (eliminating possible warranty claims).

The above graphs clearly show the superiority of the knurled assembly over the non-knurled variety. Variation in insertion force is much smaller. More importantly, the variations in push-out forces are much smaller in the knurled assemblies. This results in consistently high push-out results, with minimal variation.

To summarize, a knurled bushing gives you a press fit assembly that is of superior consistency compared to a non-knurled assembly.

A traditional knurling process (on a lathe) will produce bushings (MCRB) that are much more favourable to a robust assembly than unknurled bushings. However, due to the nature of the process, the variation of the knurled diameter will depend on:

A) the starting diameter of the cladding

B) flexibility of the rubber within the MCRB

C) rigidity of the knurling lathe

D) skill of the operator

Email: info@robicgroup.com

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