Jasper Fisher 
Rexel Motor Repair 
Alton, Illinois 
Technical Education Committee Member 

Like most maintenance and repair tasks, a successful outcome is gen­erally predicated on good planning and preparation. The first steps in the process are often the most critical. 
When preparing to remove a shaft from an armature (Figure 1) or rotor core (Figure 2), first measure and record the location dimensions of all shaft-mounted components. This in­
cludes materials such as bearing spacer collars, flingers, and removable cool­ing fans and the shaft-to-core location dimensions. 

A common reference measurement is from the outer edge of the lamina­tion stack to a bearing journal shoulder and/or the shaft extension end. These location measurements should be made to 1/32” +/-1/64” (0.8 mm +/-0.4 mm) (or better) and be permanently recorded in the job record. 

Heating the assemblies 
Heat is one of a technician’s best friends! Because steel expands linearly at 0.0000059 inches per inch of diameter per degree F (or 0.000012 millimeters per millimeter of diameter per degree C), we can diminish the restraint associated with interfering fits by heating armature- or rotor-shaft assemblies to some 300° – 350° F (150° – 175° C) prior to application of force. 

Hint: Commutators should not be heated above 350° F (175° C) to avoid damaging mica vee-rings, segment mica, or molding – bedding materials. 

There is some very useful information on this topic in the “Shaft” section of EASA’s Mechanical Repair Fundamentals of Electric Motors under “Removal of Shafts From Rotors.” It recommends that for solid shafts with outside di­ameters of 2” (50 mm) or less, con­sider placing the room temperature assembly in a 15 – 25 ton (13,600 – 22,700 kg) press. First, make sure the shaft and ram head are concen­tric, level, and square. Then apply 5 tons (4,550 kg) of force to see if it is sufficient to remove the shaft from the core. 

Hint: An important principle to be aware of is the “push nut / retainer” effect (Figure 3) when a shaft has been pressed into a lami­nated core stack. In order to prevent galling damage to the components, it is critically important that a shaft which has been pressed into a stack of laminations be removed in the same direction by which it entered. 

If the attempt at room temperature is successful, you have saved time and effort.  If the shaft doesn’t budge with some 5 tons (4,550 kg) of force, it’s time to bring heat to the process. 

General Principles 

• Safety is paramount!!! 
• Sturdy supports (e.g., heavy wall pipe with heavy plate disc) and spacer / protective blocks must have strength (tensile and yield) and hardness material properties similar to the shaft being considered for removal. Never use cast iron or similar brittle materials that may break or shatter during the application of force. 
• Shafts must be level, concentric and square with the ram head (face). 
• Protect rotor fan - end ring components (squirrel cage elements). 

Heating options 
One way (again as proposed in Mechanical Repair Fundamentals of Electric Motors) is to apply heat to the outer surface of the rotor / armature with a heating blanket (such as are available from welding supply stores) if dealing with an armature or wound rotor. Another option is to use a trusty propane fired “weed burner” or your rosebud heating torch if dealing with a squirrel cage rotor, while maintaining the 5 ton (4,550 kg) force. 

The plan is for the heat to soak the laminated core body to sufficiently diminish the interference fit such that the shaft can be safely removed with the beginning force level and by a safe increase in press force. 

Caution: Do not exceed safe temperature limits for the component materials! Remember, it is unsafe to ap­proach or exceed the yield strength of the shaft and associated fixtures, press rings, blocks, etc. If the shaft has not budged, the next step is to bore or drill the shaft from the laminated core. 

Hint: Don’t forget to create an accurate drawing of the shaft before profiling it by cutting, drilling or boring. 

For shafts larger than 2 inches (50 mm) it is best to heat the assembly to 300° – 350° F (150° – 175° C) after the press and fixtures, press rings, blocks, etc. have been setup (staged) while at room temperature. Place the rotor assembly in your drying/curing oven or use the heating options stated in the preceding paragraph. 

Insert the preheated assembly into the press setup and apply sufficient force to move the shaft up to the maximum safe limit of the weakest (yield strength) component such as cast (ferrous and non-ferrous) frame heads, cast fan hubs or spacers. Most NEMA size shafts should move when 100 – 150 tons (90,000 – 136,000 kg) of force is applied. Note: An ancil­lary heat benefit is that cured varnish resins or protective enamels may be sufficiently softened so as to facilitate removal. 

Hint: When pressing a shaft with a commutator still mounted and con­nected to the winding leads, consider tasking an assisting technician to care­fully observe (from a safe distance) the coil leads for any sign of compres­sive movement or buckling. 

Categories: Repair procedures & tips, Shafts

Tags: Shafts

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