January 15, 2008
Shafts are critical components of many machines, and a damaged shaft can hamper or halt production. Some failed shafts can be repaired and others can't. This article can help you determine which can be repaired and what to consider when attempting repairs.
Photo courtesy of B+B Dynamo + Armature Ltd., Winnipeg, Manitoba, Canada
Shafts are important, highly stressed mechanical elements used to transmit rotary motion from a driver unit to a driven part. Like any other component, a shaft can fail by one of a series of failure modes.
If the shaft is broken in two or more pieces because of fatigue cracking across most of the section, not much can be done to restore it to a useful part, except possibly in exceptional cases by special means (see example in a later paragraph).
But before you throw a broken shaft in the scrap bin, investigate the failure. An investigation can suggest some minor changes that, if introduced in the new part, will improve performance and lengthen useful life.
Worn or damaged shafts are part of everyday life. A failed shaft can shut down any machine when it is most needed for an urgent job. Whoever is in charge of production probably would go out of his or her way to purchase or steal a replacement part at whatever cost.
If a spare part is not readily available, it might be more expedient to pressure the repair shop and request shaft reconditioning in 24 hours at most.
Management should provide all available information about the shaft, including the original drawing of the part, to the person in charge of the maintenance and repair facility.
What is the material? What are the treatments and the hardness? Was the part case-hardened? This information is essential if welding is being considered as a possible repair process.
If this information is not available, it would be wise to seek the help of a metallurgical laboratory before planning the repair procedure. An informative, qualitative analysis of the material can be easily obtained nondestructively by X-ray fluorescence, a commonly used test also available with portable equipment. Having some knowledge about the material's hardness helps to anticipate the kind of problems likely to arise when attempting to weld the material.
Shafts commonly are made of hard steel. Welding procedures can affect repairs in many ways. High-alloy steel is prone to cracking when welded, unless special procedures, such as preheating, are put in place.
The welding heat reduces the hardness and strength of the original material, but may promote the formation of a very hard layer in the heat-affected zone, the region bordering the weld.
Furthermore, the uneven heating may produce difficult-to-manage distortions, so it may be preferable to introduce a mechanical joint that restores the function of the shaft, at least temporarily, until a new part is procured.
In principle, weld repair should be attempted only on annealed material that will be hardened again by heat treatment after welding in the usual way.
Restoring dimensions in a worn shaft is an easier task. Although overlaying with fused powder or copper alloys can be done with an oxyacetylene flame, without melting the shaft material, the process still softens the steel.
Plasma thermal spray is the preferred method, because the shaft material is heated minimally. This is a special process, though, and should be outsourced to an experienced job shop if the skill sets are not available in-house.
An example of an exceptional case hinted at previously is that of the main shaft of an aircraft gas turbine. Manufacturers found that these expensive parts had to be scrapped when the splines at the shaft end became worn or deformed.
Special friction welding machines were developed and built to repair these shafts. The worn splined end is cut off and a new section is friction-welded in place. Before being approved, the suitability of this procedure was demonstrated by examinations and tests, as usual with aircraft applications.