Application Note 19

Demand for ever higher torque to be transmitted through shafts of minimum weight have resulted in a trend towards hollow shafts and the traditional splined and keyed couplings are being replaced by parallel smooth face couplings, similar to those encountered in collet chucks that derive their coupling strength from smooth high pressure contact. Couplings that rely upon interlocking splines or groves suffer from the considerable disadvantage that groves act as stress concentrators. Therefore with keyed shafts, the shaft diameter must be made larger than would otherwise be needed to carry the rated load to provide a safe fatigue limit. Smooth surface shaft couplings overcome this problem but they suffer from limited grip due to the natural coefficient of friction between smooth metal pairs. Contact pressure being the principle variable, they tend to be disproportionately bulky and heavy to achieve high pressure contacts.

Most smooth high torque shaft-to-shaft couplings operate like two back-to-back collet chucks with a cone/taper mechanism that clamps a collet or split ring onto the shaft. It is common to incorporate a flexible member between the back-to-back clamps to accommodate minor shaft misalignment. The strength of the coupling depends upon the dry coefficient of friction between the collets and shaft which can vary between 0.1 and 0.4 for steel. The variability in coupling strength is a problem and is influenced by factors like cleanliness and roughness which determines the actual area in contact. Research has shown that with the best machining and surface finish, the actual area of contact is likely to be less than 1% of the designed overlap. Smooth drive shaft couplings (as with all collet chucking devices) therefore depend on over-sizing to compensate for the limit of dry friction. Satisfactory coupling performance is dependant upon good installation and the onus for this falls entirely upon the user. Trib helps maintain high levels of frictional coupling even when the user fails to fully tighten the coupling. The conditions under which much maintenance work is carried out are far from ideal and Trib helps reduce the risk of malfunction due to sub-optimum assembly. It also makes possible quite different "quick-to-fit" coupling designs that are easier to assemble and potentially weigh less.  

The following graph compares the performance of a typical taper collet coupling before and after treatment with Trib. Trib can be applied as a dry treatment by rubbing lightly with a Trib-Tool.

The untreated performance is shown by the lower dark blue trace above. A trace application to both surfaces is shown as the centre magenta and the saturated application to one surface is shown as the upper red trace. 

The Trib treatment has two effects. First it raises the coefficient of friction form the range 0.1 to 0.4 up to between 0.5 and 2.0+ depending upon how well it is applied,  the conditions of the surfaces and the clamping pressure. And second, because the increase in friction due to the Trib affect follows an inverse power-law as shown by the imposed blue curve on the graph below, it makes the assembled coupling less sensitive to clamp pressure variation. This non-linear behaviour is a characteristic of Trib joining.

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.