TRIB-JOINTS™ are high strength (grip) press fit joints. They are stronger than adhesive joints and are easier to make than conventional welds.

Application Note:1

Trib-Joints are strengthened mechanical friction joints, such as press-fit joints, whose strength is raised by the introduction of a friction enhancing chemical agent that promotes exceptional amounts of cold pressure asperity welding between rubbing metal surfaces.

Therefore Trib-Joints are mechanical joints with enhanced asperity welds that result in extensive material transfer due to many cold welds forming and shearing as the parts are pressed together. Trib-Joints behave like very high friction joints and are quite different in nature to friction welds or adhesive bonds. This chemical induced action between the metals can quadruple the frictional coupling in a press-fit join. 

Trib-Joints are created by applying a cold welding agent to the surfaces to be joined,  preferably and most conveniently done with 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 high adsorption. After use the used pad is peeled off and discarded revealing a new clean pad for making the next joint. If a swab or syringe is used the surfaces must be carefully prepared to ensure consistency.

The  photographs below illustrate the effect of treatment with a Trib-Tool.

It is important that the strength of the joints is kept below the strength of the parts to avoid distortion during initial assembly. Over life Trib-Joints tend to gain strength due to random diffusion. If joints are heated after assembly diffusion at asperity welds occurs to strengthen the joint.

Asperity welds only form at actual metal to metal contact points. The actual number of contacts and their patterns between two well machined metal surfaces are surprisingly sparse and difficult to record and therefore a simulation is used here to illustrate the situation, done with an oil pastel rubbed on paper that was previously rolled between highly polished steel rolls to imprint their roughness. As pressure on the crayon is raised so the number and size of contacts rise simulating the plastically deforming asperities. In practice Trib chemical action enhances metal deformation by momentarily lowering the local yield strength during plastic deformation and thereby increases the number and size of asperity contacts (and welds), thus raising friction. The traces below illustrate how, from left to right, the asperity contacts between two rubbing steel bodies increase as pressure rises. The orange trace illustrates a typical of the contact pattern between two rubbing Trib treated steel surface showing how the asperity count and contact area rises more quickly than on the mauve trace which is untreated but of similar roughness. The average contact pressure at the far left would be low (<10N/mm˛) and the right had side would be high (>200N/mm˛). These traces illustrate that a Trib Joint may have many small asperity welds interspersed by un-welded areas, there being significantly more and stronger cold-pressure welds on the orange trace due to Trib action and despite the un-welded areas these joints are easily made stronger than the parent metal parts.

Trib-Joints therefore are chemically enhanced mechanical friction joints, whose strength is increased by chemical-mechanical interaction. Since they are mechanical joints their function is analysed below in terms of classical mechanics with associated friction effects, and later the chemical aspects are reviewed.

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. 

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

Design Parameters.

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

 F = µAp                                               (1)

 T = µAp d/2                                          (2)

where µ is the joint coefficient of friction discussed above, A is the nominal area of the joint (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-Joints can be modelled satisfactorily, accuracies of +/- 20% being typical. 

Joint 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 joints 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 joints 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.

During rubbing between treated surfaces, deforming asperities are momentarily softened by chemical action and the contacting metals flow and intermix. When deformation stops they recover full strength virtually instantly. This results in many small welds distributed across the rubbed overlap area. There is no chemical cure involved or significant heat generated, hence there is no cooling time after pressing a joint together.

The actual chemical action is complex and not fully explained, but there is strong evidence from wear studies that chemicals can soften metals under very specific conditions. In essence when certain chemicals become trapped between rubbing asperities they may be damaged and shed single atoms of hydrogen that is absorbed into the deforming metal, this appears to lower the yield strength of the near surface molecular layers, thus promoting more deformation and flow. When plastic deformation stops the single atoms of hydrogen rapidly diffuse out and the metal recovers full hardness and strength. The hydrogen is also a powerful oxygen scavenger which probably contributes to asperity welding.

The 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 is much more mobile within a metal matrix and is able to rapidly diffuse in and out upon cessation of plastic deformation, hence Trib-Joints do not exhibit any tendency to fatigue failure. 

Trib-Joints behave like shrink fit joints in fatigue and there is plenty of evidence in the literature that shrink fit joints enjoy good fatigue strength.

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. Both couplings resist shear.

Therefore a Trib-Joint comprises many point welds interspersed with interlocking disrupted surface distributed over a relatively large rubbed 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. Sleeve Trib-Joints can be designed to 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 rubbing contact. A high asperity contact count provides more welds sites and a more consistent joint. Thus the smoother and straighter the parts -the better the joint. In practice if a joint is less well machined, then increasing the interference allows good joints to be made, providing at least one part can deform slightly to make a snug fit. Practical Trib-Joints 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 basis for the above explanation is found in the science of Tribology -the study of interacting surfaces in relative motion, the name being derived from the Greek word "to rub". The classical explanation of friction between contacting surfaces is that the resistance to slip is the result of many micro-welds and shears that occur at asperity contacts. On a micro scale metal surfaces made by conventional machining are relatively rough. The actual contact between two of the best achievable machined surfaces is said to be less that 1% of their overlap areas when in light contact one with the another. The contacts occur at high spots or "asperities". Because of the high local loading these contacts deform, cracking their oxide layers and exposing new clean metal. If clean metals come into contact they spontaneously bond, forming cold pressure welds . As surfaces slide one over the other, asperity welds form and shear - this gives rise to resistance described as friction friction. Under dry conditions if natural oxide is damaged it recovers almost instantly in air. If a lubricant is used it tends to keep the surfaces apart to prevent actual contact and therefore reduce friction. If asperity contact does occur with lubrication and a weld may form, then upon shearing reactive additives in the lubricant deposit a film that repair the oxide and minimise further welding and galling.

Trib-Joints are made by introducing trace amounts of a anti-lubricating chemical between the rubbing surfaces that actually increases friction by encouraging asperity welding to bind the surfaces and resist further motion. It is well known that some cold welding occurs randomly on dry surfaces when oxide recovery is impeded. For example aluminium will readily attach to a dry drill. However it was found that dry pick-up adhesion is much weaker than the cohesion that results from the anti-lubricant treatment.

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

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.