METALS & METAL PARTS: Protection from Galling
By George Osterhout-Orion Industries, Chicago March 1, 2005
Bonded films extend life of zinc die-cast parts
The near-net shape and complex configurations of die cast zinc parts make them cost effective for thousands of industrial, appliance, tool, and housewares applications. Their limitation, however, lies in dynamic conditions where mating surfaces rub together repeatedly, such as on cams, plungers and pushrods, slides and carriage guides, bearing surfaces, thrust surfaces, gears, valve and switch parts, wear or end plates, latches and pivots.
When zinc parts rub against other parts – particularly other zinc parts – galling (adhesive wear) occurs. The more rubbing that takes place and the greater the load, the worse galling becomes, and the surfaces tend to cling to each other. In the early symptoms, galling causes moving parts to hesitate, plungers to stick, levers to bind. Occasionally, they chatter as they stick, release and stick together again. This is the slip-stick tribological phenomenon. Eventually, part-to-part welding occurs and the mechanism fails.
The susceptibility of zinc parts to galling is extreme and is defined by an empirical “adhesive wear constant.” The greater the constant, the greater a material’s tendency to gall. Zinc sliding on zinc is the worst case, with an adhesive wear factor of 160 x 10-3. This is almost eight times worse than that for stainless steel, a material notorious for galling.
If mating surfaces of zinc parts can be lubricated in a way such that the lubricant is contained, galling is almost completely eliminated. But, designing and applying sealed systems for lubricants is costly, defeating the benefits gained from the process that produces most zinc parts, die casting.
Low-friction coatings such as PTFE have been tried on these parts, but most are quickly worn away in service. Another problem with fluoropolymers is that the high cure temperatures of conventional coatings soften and distort zinc parts.
Fig. 1. Stratified film for die castings involves applying a slightly discontinuous bronze film (middle layer) to the surface of the zinc die cast part, (bottom layer) then infusing a layer of PTFE (top, dark layer) into the bronze.
Dimension Bond, Chicago, has developed a cost effective technique (ZincGlide) to bond a thin film of porous bronze to these parts, to which a low-friction material such as PTFE is applied (See Fig. 1). Because the bronze layer is porous, it traps the fluoropolymer within the pores. With the outer layer of PTFE-rich surface, it wears like a metal-backed bronze/PTFE bushing. The bronze layer provides rigidity and extra wear resistance, while the fluoropolymer provides low friction. This bonded bearing surface eliminates galling, lowers friction and creates lubricated-for-life components in many applications. Plus, it is cost effective.
Literally any surface can be wear protected by the composite film. It can be applied to any surface, regardless of contour or complexity – to a specified thickness ranging from 25 microns to 125 microns. Parts almost never have to be re-sized to accept the film.
ZincGlide(sm) Properties of bonded, low friction, anti-galling film
The first films tried on zinc were derived from those that were developed to replace bushings in shock absorbers, and are designated L26/T80 by Dimension Bond. These wore well, but to achieve adhesion to zinc die castings took much research and lab work. The result of this investment is an advanced family of bonded materials for zinc, designated ZincGlide.
Fig. 2. End plates for fuel pumps (small discs) and for air pumps (large disc) employ bonded films to prevent galling from impellers and vanes. Because galling is progressive, metal-to-metal contact between the end plates and impellers could result in seizing.
A good example of how the ZincGlide film eliminates galling is demonstrated by end plates on small pumps (See Fig. 2). The end plate has a bearing surface formed into it that absorbs small thrust loads from the impeller. Both the end plate and impeller are die cast because complex surfaces can be formed in one shot. However, if the end plates are untreated, the impeller begins to gall the end plate during operation. A number of conventional “fixes” were tried to protect the friction area, including inserting a wear pad or liner, but none proved satisfactory or cost effective. The end plate is now finished with the L26 bonded material, applied to the contact area only. No more failures due to galling have been experienced. The bonded film cannot migrate or move and prevents assembly errors.
Fig. 3. Contact surfaces between a cam and latch were galling so badly that the parts would bind after only a few cycles of operation. A bearing-type film was bonded to the parts to eliminate the galling.
In another application, the ZincGlide film was used to extend the life of a cam-type latch where joining force is progressive (See Fig. 3). Both components are zinc die castings. In its original configuration, the contact surfaces wore quickly even with grease. The L26 film was applied to the surfaces, solving the problem, providing lifetime lubrication.
Fig. 4. Meshing zinc gear teeth become noisy as they scuff against each other under load. However, bonding the bearing-like film to the teeth prevents zinc-to-zinc contact and the galling that goes with it.
In another application, small zinc gears in an office copier were binding due to loads that pressed the teeth tightly together at relatively low speeds. The tooth area experienced scuffing, then progressive galling until the gears would no longer function (See Fig. 4). However, the binding was eliminated when a film of the low-friction, long wearing DB-bonded bearing surface was applied to the teeth of both mating gears.
Fig. 5. Mechanical cranks have high line loads that accelerate galling and resulting lock-up between the gear and mating rack.
Finally, zinc gears in a crank failed because the scuffing quickly became galling, and rapid erosion of the surface took place. Grease was tried as a solution, but it only picked up airborne grit and became abrasive. Now, the bonded ZincGlide film is applied to the tooth surfaces and rack rail, providing lifetime lubrication (See Fig. 5).