Fretting Corrosion in Gearbox Hollow Shafts
19th February 2009
Hollow shaft gearboxes have been used for many years to facilitate the initial fitting to a drive shaft, similar to the shaft of a conveyor head pulley or perhaps the input shaft of a pump. When new, these hollow shaft speed reducers can be fitted very easily. For a short time, they can be removed reasonably easily. However, after a few years of operation, the hollow shaft reducer will become 'stuck' onto the shaft, and resist all attempts to remove it from the shaft. The most common reason for this 'stubbornness' is due to a special wear process labeled 'Fretting Corrosion'. This isn't to be confused with an electrical version of fretting corrosion, although there are similarities.
What is Fretting?
When two pieces of metal, pressed together by an external static load, are then subjected to a transverse cyclical load such that one contacting face is relatively displaced cyclically parallel to the other face, in the presence of high contact stress, wear on the mating surfaces occurs. If the magnitude of this displacement is less than about 0.1 mm, the wear is termed 'fretting'.
How does Fretting occur?
Fretting occurs by contacting asperities on the mating surfaces continually welding together then breaking apart again. This procedure leads to surface pitting and the transfer of metal particles from one surface to another. Additionally, the small fragments of metal which are broken off oxidise, forming oxide particles which, for most engineering metals, are harder than the mating parts. These particles become trapped between the two surfaces and cause abrasive damage and scoring, labeled Fretting Corrosion. In the case of a hollow shaft gearbox mounted via a solid shaft, the eventuality is that the reducer cannot be removed easily. It has, effectively, 'welded' to the shaft and will strongly resist free axial displacement.
How to prevent Fretting Corrosion?
Prevention in the hope of elimination of this corrosion has been the aim of many, many people over the last hundred or so years. They have found that some forms of lubricant (e.g. SKF LGAF3E) reduce fretting corrosion, unless it allows the cyclical amplitude to increase in which case it can exacerbate the corrosion. Unfortunately, in a hollow shaft mounted speed reducer located by a shrink disc, the introduction of a lubricant between the drive shaft and the gearbox hollow output shaft will have disastrous results. In extreme cases, fretting corrosion has been known to reduce the surface endurance limit (before fatigue sets in) to around 20% of the original value.
The best ways found to minimise Fretting Corrosion are:
- Use of lubricants and surface coatings where possible.
- Increased surface hardness of shaft and hollow bore.
- Use of barriers to limit ingress of corrosive environment to mating surfaces.
- Reduce the loads on the mating surfaces.
Of these, barriers to the outside environment such as an 'O' ring sealant on the head pulley side and a shrink disc on the outside are the only practical attempts at a solution found so far for gearbox hollow shaft mounting. To the best of our knowledge, no one has found Utopia and eliminated this Fretting Corrosion completely.