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Application Note 23

Releasable heavy duty enhanced friction couplings

These couplings employ improvements to technology first patented by us in 1990 for raising the strength of press-fit friction joins by introducing a chemical friction enhancer between sliding metal pairs. This development now provides releasable heavy duty friction enhanced couplings that require relatively low press-fit forces to assemble and disassemble, which was hitherto impractical.

These couplings provide precision location and high stability and are intended for fabricating structures and machine frames.  The principles extend to providing quick release shaft couplings for use in drives, marine propulsion etc. or power generation with gas, steam, water or wind turbines and pipe lines or joining pipes in hazardous environments because no external heat is required in their making.

The couplings employ our commercially proven anti-lubricating chemicals applied between a pre-positioned free-fitting coupling sleeve positioned to overlap the parts being coupled. To make the coupling, the coupling sleeve is cold deformed and held in high pressure high friction contact with the parts being coupled.

The technology optionally exploits anisotropic properties of selectively reinforced fiber composites, especially aluminium metal matrix composite (MMC) by selective reinforcement so that the coupling sleeves are strengthened in their axial direction only, and a light weigh radially stiffened MMC deformer presses the friction enhanced inner face of the coupling sleeve into high friction contact with the parts being coupled. Thus during and after assembly the MMC sleeve and deformer are mutually reinforcing because their reinforcements are at right angles, which provides exceptionally strong light weight releasable press-fit couplings. Patents are pending.

Configurations of hollow dowel sleeve couplings

Typical configurations for coupling structural elements, such as large steel components are shown in schematic diagrams Figs 1 to 4 below in which the couplings act effectively as releasable dowels, which couplings can be made very strong and light .

Fig 1 shows a cross section view of an expandable coupling sleeve straddling bores in touching bodies, to act as a coupling dowel. The coupling is formed as the sliding central body, a deformer that is sized to expand the coupling as it is forced into the coupling and hold it in firm contact with the coupled bodies. The interface between the coupling sleeve is treated with a friction enhancing chemical medium and the interface between the coupling sleeve and the sliding body is lubricated to minimise sliding resistance.

Fig 2 differs from Fig 1 in that the sliding body is divided into two halves so that one half of the coupling sleeve can be positioned and secured in a first body before the second body is located and secured.

The coupling is un-coupled by withdrawing the sliding bodies. By providing appropriate oil ways the oil-injection method can be used to assist with the assembly or disassembly of large couplings, where the injected medium can be either a lubricating or anti-lubricating medium as described in App.Note 24.

Before assembly or reassembly the surfaces need to be abraded clean to a polished bright finish, making them as smooth as practical, with rounded asperities to maximise the contact (bearing) area ratio and removing any previously transferred cold welded material.  Since the cold-pressure welds can only occur at actual inter-metallic contacts, the smoother the surfaces the higher the contact area and stronger the coupling. Surface roughness therefore is a key preassembly quality  parameter.

Figs 3 and 4 differ over Figs 1 and 2 by having a hollow sliding sleeve. The direction of fibre reinforcement is indicated by the direction of hatch used to illustrate the anisotropic parts above.

Configurations of hollow sleeve tubular couplings

Similar basic configurations are illustrated in schematic diagrams Figs 5 to 10 for coupling hollow or solid shafts or flow pipes.

In Fig 5 the coupling sleeve is placed in overlapping relationship with the hollow bodies being coupled and the sliding sleeve that acts as the deformer is slid over the coupling sleeve to squeeze it down into high pressure contact with coupled bodies. A friction enhancing agent is trapped between the coupling sleeve and coupled bodies and a friction reducing agent is introduced between the coupling sleeve and the sliding sleeve to minimise sliding resistance during assembly. This provides a smooth flow bore.

Fig 6 shows a similar coupling to that in Fig 5 except that the coupling is made on the inside, leaving the outside smooth.

Fig 7 and 8 show a further variation for use on thin wall coupled bodies that can be readily deformed. In Fig 7 the coupling sleeve is placed on the outside and sliding sleeve is placed on the inside. In Fig 8 the arrangement is reversed with the coupling sleeve on the inside and the sliding sleeve on the outside.

Fig 9 and 10 are similar to Figs 7 and 8 except that the ends of the coupling tubes are preformed to provide a smooth outside profile in Fig 9 or a smooth inside profile in Fig 10, which is suitable for use as a flow bore.

Figs 7 to 10 show couplings in which the coupled parts (tubes in this case) are trapped and supported by adjacent sleeves, which sleeves are made stronger than the coupled parts, and can therefore be made with similar or dissimilar materials. Thus the configurations in Figs 7 to 10 may employ either all metal, or all non metal or a combination of metals and non-metals. In the case of non-metals the friction is increased by employing either matrix softeners or adhesives. Thus in principle the technology extends to coupling glass reinforced fibre bodies or other composite combinations.

A practical example of a steel tube coupling

Fig 11 shows a coupling between two tubular bodies, which corresponds with to the schematic of Fig 5. To seal such a coupling, the ends are machined square and abut. An elastomeric or deformable metal seal ring can optionally be placed between the abutting faces. An axially reinforced (anisotropic) coupling sleeve is shown divided into quadrants with one quadrant shown removed. Optional location groves are shown towards the pipe ends into which the raised sections on the inside of the coupling sleeve segments locate. The friction enhancing (anti-lubricant) is applied either direct to the pipe adjacent to the coupling sleeve, or applied to the coupling sleeve  or applied to both. A preferred method is to apply friction enhancer to both mating surfaces with a mildly abrasive applicator tool that cleans the metal as it deposits the anti-lubricant. After positioning the coupling sleeve, a second sleeve, the sliding sleeve, which is radially reinforced and has its inner surface pre-lubricated,  this lubricated sleeve is then forced over the coupling sleeve to press and hold the coupling sleeve in high friction contact with the bodies being coupled. The second sleeve slides until it reaches the raised dead stop on the outside end of the coupling sleeve. An optional cir-clip is provided to prevent the sliding sleeve being inadvertently displaced.  The sliding sleeve is most conveniently forced into position with a custom hydraulic clamp arranged to engage between the dead stop end of the coupling sleeve and the distant end of the sliding sleeve.

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.