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Trib-Technology theory - how to enhance friction in friction couplings

Application Note:1

 

Trib-joins are chemically enhanced friction joins, whose friction coupling is raised by the introduction of a friction enhancing chemical agent that promotes cold pressure asperity welding between pressed together metal surfaces. Although they employ many tiny welds, these joins behave like very high friction joins and are quite different in nature to friction welds or adhesive bonds. The chemical induced action between the metals can quadruple sliding friction and increase static friction by 40%.

The friction enhancing agent is applied to the surface by wiping, brushing or rubbing - conveniently applied with a frictional rubbing tool called a Trib-Tool. The Trib Tool comprises a stick of mildly abrasive non-woven pads stacked one on one in a spill proof hand held case, each pad is impregnated with the friction enhancing chemical. Upon rubbing, the pad cleans the surface and seamlessly releases the chemical onto the cleaned surface to ensure optimum wetting and adsorption. The chemical agent is a safe to touch clear liquid, whose viscosity can be varied to suit the application.

The friction enhancing agent is applied to the surface by wiping, brushing or rubbing - conveniently applied with a frictional rubbing tool called a Trib-Tool. The Trib Tool comprises a stick of mildly abrasive non-woven pads stacked one on one in a spill proof hand held case, each pad is impregnated with the friction enhancing chemical. Upon rubbing, the pad cleans the surface and seamlessly releases the chemical onto the cleaned surface to ensure optimum wetting and adsorption. The chemical agent is a safe to touch clear liquid, whose viscosity can be varied to suit the application.

The  photographs below illustrate the effect of the friction enhancing agent between soft steel press fitted parts.

 

The above 12mm diameter mild steel pin is shown on the left after treatment with a Trib-Agent, a similar pin is shown in the centre after pressing in and pulling out of a mild steel collar with 15 micron interference. A close up of the disrupted welded area on the right shows excessive material flow due to the Trib effect.  

The photographs below show soft mild steel transferred onto a hard ground pin.

On the left is the 10mm bore of a mild steel collar and on the right a hard ground pin with similar 15 micron interference. The pin which was tapered slightly at its end and the initial part of the pin was untreated, thus the Trib effect started some way along the bore and this prevents cocking distortion. Clearly there is much less disruption of the hard pin surface and soft material has transferred (welded) onto the pin during final positioningIt will be appreciated that the majority of the disruption shown occurs upon disassembly.  The disruption between press-fit hard steel parts being barely visible and if the parts are shrunk together thermally or by hydraulic deformation (oil injection) the surface disruption cannot be detected!

Despite the huge amount of asperity welding, the joins still behave like friction joins thus Trib-joins may be designed to yield upon overload, and if they slip they can strengthen due to continued presence of the friction enhancing agent and arrest slip.

The disrupted surfaces above suggest Trib-joins will be vulnerable to fatigue crack initiation, but extensive tests and a history of successful applications in highly demanding applications have proved otherwise. In fact the distributed nature of the joins provides fatigue behaviour similar to shrink fit joins, Trib-joins have been proven very satisfactory in applications with both torsion and axial cyclic loading. 

After introducing the friction enhancing agent the assembly forces can easily rise above the strength of the parts when  making press-fit joins, therefore it is important that the strength of the joins is kept below the strength of the parts to avoid distortion during assembly. Over life Trib-Joins tend to gain strength due to random diffusion. If joins are heated after assembly diffusion at asperity welds occurs to strengthen the join.

The technology was first conceived in 1990 as a means of improving press-fits. As evidence of its economic value it has been used since 2003 in safety critical engine management systems, with millions of joins running world wide in hostile environments.

Mechanical Theory.

Consider two bars arranged so the lower bar is fixed and the upper bar is slid across the lower bar thereby to provide a sliding single point contact as illustrated below. This single point contact mimics a single sliding asperity contact.

A normal load W is applied to the upper bar in the direction of the vertical arrow. 

The lower bar is fixed and the upper bar is dragged in the direction of the horizontal arrow. The area bounded by the red line has been rubbed with a Trib-Tool, that has transferred into its oxide a friction enhancing agent.

A tangential reaction force is transmitted across the sliding contact and designated as force F. 

µo = F/W

The effect of the chemical treatment is shown below where initially the friction is low when sliding between untreated surface, but rises significantly once it encounters the treated area shown in red above..

The coefficient of friction µ varies with mean join pressure, a result expected from plastic theory. Experimental joins in torsion displayed the behaviour shown above right. The simplest empirical relationship fitting the trend is µ = µo + µ1/p. The value of µo can be found from crossed bar tests, but µ1 is obtained from test joins. Pin-on-disk measurements of friction tests support the finding of pressure sensitivity.

Design Parameters.

Trib-joins are most effective in axisymmetric format, with a solid or hollow bar fitting in a circular hub. For cylindrical (parallel shaft) joins, the axial force F  and the torque T a join withstands are given by equations (1) and (2),

 F = µAp                                               (1)

 T = µAp d/2                                          (2)

where µ is the join coefficient of friction discussed above, A is the nominal area of the join (length x circumference), and p is the interface pressure (obtained from Lamé thick cylinder theory in the elastic case). Most of the observed behaviour of axisymmetric Trib-joins can be modelled satisfactorily, accuracies of +/- 20% being typical. 

join interface pressures should exceed 20N/mm2 for reliable predictions. In practice most of the 20 % variability is attributed to variation of surface roughness and cleanliness, with care joins have been made in precision assemblies in significant quantities with less than 5% variation in push on and push off force.

To achieve above mentioned pressures joins should be toleranced to H7/p6 or better for thick wall parts. Parts that are able to deform during assembly essentially become self sizing and toleranceing can be relaxed accordingly. 

Metallurgical and Chemical Considerations.

Asperity welds between touching metals can only form at asperity contact points. The actual number of contacts between two well machined metal surfaces are surprisingly sparse perhaps covering as little as 2% of an apparent overlap and this is difficult to illustrate. With the empirical schematic coloured bars below we attempt to illustrate how the contact between pressed together steel surfaces might be expected to increase. These were prepared by stroking a crayon across fine paper and progressively increasing the loading on the crayon, thus the crayon contact density rises simulating the plastically deforming asperities between contacting metals. The affect of the Trib chemical action is to enhances metal deformation thereby raising friction. The orange trace illustrates a contact pattern between two pressed together steel surfaces after being treated with friction enhancing agent. The contact count and contact area rises more quickly on the orange than on the mauve trace which is representative of a dry interface of similar roughness. Each microscopic cold pressure weld is surrounded by a residual elastic field that holds the asperities pressed together. Upon releasing the residual elastic stress the welds spontaneously crack and the join can be undone.

Based on research relating to metal transfer mechanisms between sliding metal pairs in bearings, it is hypothesised that as two steel surfaces are pressed together and/or slide one against the other in the presence of a friction enhancing agent, the film thickness at and around each asperity contact reduces to a point at which it becomes unstable and releases atomic hydrogen onto virgin clean metal exposed by oxide cracking. The hydrogen is absorbed into the deforming asperity and momentarily softens the asperity causing enhanced metal flow and intermixing, which is classic cold pressure welding. Upon cessation of deformation the single atoms of hydrogen rapidly diffuse out of the asperity allowing full recovery of strength. This results in many small welds distributed across the contacting overlap area. There is no chemical cure involved or significant heat generated, hence there is no cooling time after making a join.

Hydrogen is also a powerful oxygen scavenger which probably contributes to asperity welding.

Mention of hydrogen triggers fears of hydrogen embrittlement, a phenomena that plagues fusion welding processes. Embrittlement occurs when h2 molecules accumulate at grain boundaries forming small pockets of gas that act as stress raisers and crack initiators. The hydrogen we refer to above are single atoms of hydrogen which are much more mobile within a metal matrix and are able to rapidly diffuse in and out upon starting and cessation of plastic deformation respectively, no evidence has been found of hydrogen embrittlement in Trib-joins. Trib-joins behave like shrink fit joins in fatigue and there is plenty of evidence in the literature that shrink fit joins enjoy good fatigue properties.

As the surfaces on press-fit parts rub when pressed one into the other, new welds progressively form and shear under compression, some re-weld others form strong mechanical interlocks between the disrupted surfaces. 

Therefore a Trib-join comprises many point welds interspersed with interlocking disrupted surface distributed over a relatively large contact area and held together by an outer constraint. It is essential the outer part maintains the compressive force holding the joined faces together. If this is relaxed individual welds break as the compressive elastic forces surrounding the welds relax. OverlapTrib-Joins can easily exceed parent part strength in tension, compression and torsion.

It is essential that the surfaces to be joined should be clean and in good contact. A high asperity contact count provides more welds sites and a more consistent join. Thus the smoother and straighter the parts - the better the join. In practice if a join is less well machined, then increasing the interference allows acceptable joins to be made, providing at least one part can deform slightly to make a snug fit. Practical press fit Trib-joins are designed to operate below the seizure threshold to prevent distortion as the parts are forced together. Hitherto seizure was erratic and unpredictable. With our surface treatment and design software -seizure can be precisely predicted and controlled. 

The slip arrest characteristic shown below provides a fail safe function capable of preventing catastrophic failure, however it does result in cumulative degradation of the join and after a limited number of such events then routine service is required.

The green trace in the graph below is typical of dry metal to metal friction grip experienced between a tool like a drill or router gripped with a collet chuck. Trib treatment boosts frictional coupling between most static metal interfaces and uniquely changes stick/slip behaviour once slip starts as shown below.

Given a steady contact pressure between smooth metal pairs, dry friction between faces follows the green curve below where static friction is greater than dynamic friction. Significantly after Trib treatment, dynamic friction (grey curve) rises above the static level and arrests slip and prevents fretting, thus for example when used on a tool shank clasped by a chuck Trib causes an extra grasping action as friction grip automatically rises to eliminate slip!

HIGH-GRIP permanent swaged, press-fit and shrink-fit torque joins

The graph below shows how the torque strength of friction joins made with H7/p6 mid range pre-fit is boosted by treating their surfaces with TRIB treatments. The green line shows a join of similar interference made with untreated surfaces. The blue line is a join treated with a moderate Trib-Gel, providing a join that will yield and absorb energy before shearing the shaft. The red line shows a very sensitive Trib-Gel that creates a join that matches parent metal strength and the shaft fails in shear with no detectable slip only elastic wind up.

To learn more click on applications or read FAQ's for a more detailed explanation of Trib joining

Footnote: See relevant topics in Joining Questions for more information on the practical aspects of Trib-joins.

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for more information email: info@tribtech.com

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The TribTech name derives from  "tribos" - Greek for 'rubbing'. 'TribTech' is a trade name used by Ball Burnishing Machine Tools Ltd. Registered Office 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 based on aspects of  tribology, the science of surfaces. All rights reserved by Ball Burnishing Machine Tools Ltd. Last modified: 29th Sept 2016 copyright © 1999/2016. 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. Be aware this site uses cookies, your continued use implies you accept these.