Turning around shaft repairs
May 1st 2007

Harsh environments and operating parameters can result in wear, erosion and corrosion of shafts.

Colin Chapman of Henkel, makers of Loctite brand products, provides some useful information about shaft maintenance using engineering adhesives

Most shaft assemblies are integrated within systems that represent a significant capital equipment cost. Therefore, it is important both to extend their working lives and to ensure reliable performance under a variety of conditions. The use of proactive maintenance – with appropriate products – can minimise costly downtime, increase reliability and, thus, cut costs.

So, just what can be done in this area of proactive maintenance? It goes without saying that mechanical parts do wear. But excessive wear – with all its resultant damage – can be limited. For example, in a new assembly, the fit between a key and the keyway is generally tight. But, over time, wear can result in the fit becoming loose – leading to damage.

However, the application of a medium strength threadlocking adhesive to the key or keyway at initial assembly will help fill the gap in any worn areas and still allow the strength of the joint to be maintained. By using a medium strength product, the assembly can be dismantled in the future should other forms of servicing be required.

Threadlockers are anaerobic in nature, so they will not cure until air has been removed from the joint – action that occurs when the metal parts are reassembled.

The results of using a threadlocker include the elimination of micro-movements that lead to fretting and reduced wear in the keyway. In addition, because the adhesive flows into all the surface irregularities and effectively seals the assembly, corrosion is prevented.

Such adhesives are available in either liquid or semi-solid stick formulations. The former are applied directly from the bottle, while sticks provide the same bonding strength, but from a container that fits easily into the pocket or tool boxes.

Bearing the strain

Where initial machining of a component has not been precise, movement on shaftmounted components such as bearings can occur. Perhaps the load is producing axial forces that are greater than the original calculations – and wear occurs.

Or a spun bearing is caused by insufficient interference or inappropriate loads.

In any of these three scenarios, adhesives can be used to repair the worn shaft and thereby reduce downtime and even prevent expensive parts from being scrapped.

For instance, where the repair involves an original assembly that has either a press or transition fit, then a medium strength retaining adhesive can provide the answer. If the shaft is badly worn, then sleeving can be used as the repair method. This solution involves machining the shaft to create a transition fit and then bonding it to the correct size sleeve. The outside diameter of the sleeve should be the specified shaft dimension.

The surface of the parts should be cleaned with a suitable degreasing agent.

Retaining adhesive can then be applied to the leading edges of both the shaft and the sleeve and the parts assembled.

Further beads of adhesive may be required for long components.

Excess adhesive is then wiped away and the joint left to cure.

Like threadlockers, retainers are available in both liquid and stick formats.

Building up

If rebuilding and bonding of a badly worn shaft is required, then a metal filled epoxy is used for the repair. Using a lathe, the shaft should be undercut in the worn area.

Where the shaft diameter is between 13 and 25mm, the desired undercut is 1.5mm. For shafts up to 75mm, the undercut should be 3mm.

The ends of the worn area are then dovetailed in order to lock the assembly.

The undercutting should then be finished by machining a rough-cut surface – for example, Rz:=100µm. The larger the shaft diameter, the more coarse the cut.

The parts are then cleaned and degreased, and a thin layer of a suitable epoxy adhesive forced into the rough surface finish. The shaft can be turned at a low speed and further adhesive applied with a putty knife to a depth slightly above the desired level. After the adhesive has cured for 12 hours, the repair can be machined to the required dimensions.

The joint between the shaft and the mounted component can be retained with a suitable anaerobic adhesive.

Don't fret about splines

One particularly challenging exercise involves preventing unnecessary fretting of the spline inside a socket. This wear is caused where there is friction and movement in the spline coupling.

The solution is to employ a Moly Paste proactively. Typically, this product will be a thick paste that contains 65% molybdenum disulphide. It is designed to reduce friction and to stay in place through heat, load and vibration to ensure troublefree disassembly from -29 to 400º C.

Full range of uses

The applications listed are but a few of the uses of adhesives for the repair and maintenance of shafts. Taper joints, for example, can be treated in a similar way to their cylindrical counterparts. In addition, they allow for the recycling of parts to avoid scrap and replacement costs, and assist with disassembly operations.

Just as important, engineering adhesives help to ensure reliability and the smooth running of machinery by restoring performance of shafts to 'as new' condition.

In all cases, a discussion with the technical helpdesk of an adhesive supplier can help ascertain the best product for each job.

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