Trib-Tech improves shrink fit joint strength with diffusion bonding - useful for attaching gears to shaft or gear rings to gear wheels

Application Note 5

Shrink fits are often unable to transmit the higher power that modern gears are capable of handling. They rely on dry friction for their coupling strength; this can be more than doubled by applying our patented Cold Welding Agent between the surfaces as the joint is made.

These joints are made stronger still if electrical current is flowed across the joint during and after assembly. The currents concentrate at asperity contacts to heat and soften them so they deform and more contacts form to increase actual contact area. The Cold Welding Agent acts as an oxygen scavenger when heated within the confines of the joint to reduce metal oxide and localised diffusion occurs. These diffusion enhanced joints are patented and called Trib-Bond’s and are an improvement over our patented Trib-Joint’s. This provides a low cost method of making diffusion bonded parts in open atmospheres.

It is important that good contact is established between faces when making Trib-Bonds. It is helpful if the parts are gently rubbed one against the other until the joint locks to activate the Cold Welding Agent and form cold welds. The following diagram shows a ring gear shrunk onto a web which in turn is shrunk onto a shaft in the presence of friction enhancement treatment applied to the cool surface. The induction coils selectively heat the joints to form high strength partial diffusion bonds.

A competitive joining method is electron beam welding but this requires expensive equipment and is limited because of the very high powers needed to penetrate deep into large joints The term "Bonded Shrink or Press Fits" is used to describe adhesive enhanced joints made with an interlocking layer of plastic trapped between the joined surfaces. The plastic is a thermoset usually formed when an anaerobic adhesive cures. When first made these joints show a useful increase in torsional coupling. However unlike Trib-Bonds which tend to gain strength over life due to diffusion, adhesive bonded press fits risk losing strength due to fatigue and material degradation if subjected to thermal cycling as occurs in gear boxes because there may be up to a ten fold difference in thermal expansion rate of the cured adhesive over the joined metals.

for more information email: info8@tribtech.com

The TribTech name derives from "tribos" - Greek for 'rubbing'. 'TribTech' is a trade name used by Ball Burnishing Machine Tools Ltd. of 12 Brookmans Av. Hatfield, Herts. AL9 7QJ. United Kingdom;  Company Reg. No. 1408807, VAT Reg. No. 421 6210 04; a knowledge based company that develops, patents and licenses technology. All rights reserved by Ball Burnishing Machine Tools Ltd. Last modified: May 01, 2008 copyright © 1999/2008. The information and data provided herein should be considered generally representative for the tools and technologies described. In all cases users should carefully evaluate the tools and technologies to determine their suitability for a particular purpose.