Tom Bishop, P.E.
EASA Senior Technical Support Specialist

When a shaft is in need of repair, often the first step is to determine the corrective method required. Econom­ics and best practices are typically sig­nificant factors in the decision-making process in selecting the method of repair. The types of shaft repairs that will be dealt with here are:  opposite drive end bearing journal, drive end bearing journal and a bent shaft. The objective is not to detail the repair processes, but to identify the most common methods appropriate to the types of repair and considerations associated with each method. Table 1 summarizes the methods for various load conditions.

Opposite drive end bearing journal repair options

Among the options for repairing a defective opposite drive end bearing journal are welding, metalizing, plat­ing, metal-filled repair epoxies, and steel bushing. Welding (see Figure 1) of the journal can produce a durable repair with the shaft surface having a near original appearance. The method allows a rapid buildup of material to fill in the damaged area. This combined with a relatively modest equipment expense makes welding an economical repair method. In addition to the skill of the operator, a key part of the weld­ing process is to be certain to relieve any residual shaft stresses. 

Thermal spraying or “metalizing” (see Figure 2) is relatively quick and effective for smaller bearing journals. However, some end users do not accept it as a repair method. Make certain the customer accepts this repair method before proceeding with it. The metalized surface appearance is rather grainy. In some cases the interference fit of the bearing compresses the metal­ized material, thus resulting in a looser than intended fit.  

If the bearing journal is for a sleeve bearing, the metalized surface finish may be too rough. It has been reported that the metalizing tends to degrade due to the lubricating oil penetrating the material. 

The shaft can also be restored by undercutting it and plating it back to size. In many cases, the shaft finish and dimensions after plating will not require any further machining. Tank plating, requiring immersion, is not usually performed in a service center and it can cause hydrogen embrittlement. Brush (selective) plating (see Figure 3) creates a molecular bond to the shaft material and does not create residual stresses. The greater the electrical cur­rent at which it is applied, the faster the rate of deposition, giving it a potential economic advantage. However, this may be more than offset by the cost of environmental controls and disposal of the used plating solution.

Metal-filled repair epoxies (see Fig­ure 4) produce a shaft surface that has a distinct appearance that differs from the original. Nonetheless, they bond well to the underlying substrate and can result in a durable repair for a direct coupled rolling element bearing journal. Coupling fits and radial loads tend to break down the epoxy as do temperatures above about 200° F (93° C). Metal-filled repair epoxies are also not suitable for use as a sleeve bearing the jour­nal repair method. The material can be degraded by the lubricating oil and rapidly degrades if there is metal-to-metal contact. A potential benefit is that since the epoxy material is typically non-conductive it can be used to form part of the path for insulating the bearing against shaft currents.    

Another method that can be used is to undercut the shaft to a point past the bearing journal and install a steel bushing that can be retained by an interfer­ence fit. If the labyrinth area inboard of the journal is worn, the sleeve can be made long enough to replace. Therefore repair that section of shaft as well as the bearing journal. The resulting journal will have the appearance of the original and the only evi­dence of the sleeve will be the location where it abuts the original shaft.

Drive end bearing journal repair options
The considerations for the various repair options for the opposite drive end bearing journal also apply to the drive end. Welding is usually not suitable for shafts that will be subjected to cy­clical or heavy radial loads. Also, the use of a steel bushing is not practical due to the length of shaft beyond the bearing journal which would require undercutting the shaft extension all the way from the end of the shaft to a point behind the bearing journal. Further, in the case of sleeve bearing journals, there are typically shoulders on both sides.  Metalizing should not be used if the bearing journal section of the shaft is subject to flexing, or if the application is a radial load, as the metalized material has a tendency to chip or separate. This method is also not usually suitable for use with interrupted surfaces such as a keyed shaft extension.

Another option for drive end bear­ing journal repair is to stub the shaft (see Figure 5). Although this method can be effective for direct-coupled applications, it should be avoided if the drive end shaft has a radial load, such as with a belt drive. In general, unless the service center has had successful experience with shaft stubbing, it is a method that should be avoided. Further, some end users prohibit stubbing of shafts.

Bent shaft repair options
Repair of a bent shaft usually is as­sociated with the drive end of the shaft, often due to excessive radial load, a drive end bearing collapse or both. In less frequent situations the entire shaft is bent or bowed and sometimes twisted, in which case it is usually best to make a new shaft. 

The repair of a bent shaft may be done by using a cold-pressing method. Or more often, it is accomplished by using the combination of the heat of a torch (see Figure 6), rapid cooling, and an experienced technician to bring the shaft straightness to within an acceptable tolerance. As an alterna­tive when using heat to remove a shaft bend, some technicians do not rapid cool the shaft but allow it to cool for several hours in ambient air. The shaft straightened with these methods should be rechecked in 12-24 hours. Often the shaft will have a memory and return to the bent shape.

There are other methods such as peening that can be used to straighten shafts. Whatever method is used, a high degree of operator skill and ex­perience with the method is required. The possibility of the shaft bending after repair due to residual stress should be considered before using these methods.

Categories: Repair procedures & tips, Shafts

Tags: Shafts Shaft repair

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