Here we have a Demag KBA range brake motor that has recently arrived for repair. Can you spot the obvious fault?
 
This particular motor is used to drive a rubber top chain conveyor; the application is to drive the belt forward with stops / starts every few seconds. The picture shows the remains of a keyway on the shaft and what is left of the once rectangular key. The rotor shaft failure on this motor could have been caused by a number of factors. The keyway has rolled very badly which could have been the result of a poorly fitting coupling with worn bore. It could also have been down to a worn key or poorly fitted key. Once there is some wear in the system, the aggressive stop start action of the motor will accelerate the wear problem eventually resulting in the condition of the shaft as you can see here.
 
Upon further investigation it was discovered that the motor had been used for a considerable amount of time without the flexible element fitted, not as shown in the picture, this element was fitted later by the user but too much later. Essentially, with a slack fitting of the coupling and key, this has led to a premature failure of the motor shaft.
 
Demag KBA motors utilise a conical rotor design and instead of having an electromagnetic brake with its own energising coil, they rely on the magnetic field of the motor to pull the rotor into electrical centre when powered up. Upon powering down there is a spring which pushes the rotor backwards causing the brake lining on the fan to connect with the fan casing, thus stopping the motor.
 
This type of brake is very reliable and extremely fast acting which enables these motors to work in very arduous environments with demanding duty cycles. However like all engineering components they still need to be fitted correctly and given a little TLC now and then.
 
If the wear on the shaft is only minor this can be rebuilt and re-machined. However as may be the case with this motor it may require a new shaft or complete replacement rotor to be fitted.
 
#mechanicalengineering #demag #brakemotors #couplings