Rubber-in-compression couplings help variable frequency drive cut energy costs

Mitsubishi Materials of Japan have solved a problem with a new Scherbius control system after installing rubber-in-compression couplings from Renold Hi-Tec Couplings of Halifax. Mitsubishi Materials installed new Scherbius control systems on two induced-draught fans at one of the company's cement works on the Japanese island of Honshu. The new system was intended to cut energy costs and to increase efficiencies by allowing the fans to be operated at optimum speed. However, after installation Mitsubishi Materials' engineers encountered high levels of vibratory torque at speeds close to the desired 1,000rpm operating speed. Until a solution to the problem could be found, and to avoid any possibility of damage to the motor, engineers were forced to introduced a barred speed range between 890 and 940rpm.


Renold Hi-Tec Couplings were consulted as the company had a good track record of successfully solving similar problems at other sites. The problem lay in the characteristics of the new Scherbius control system, which is essentially a type of variable frequency drive (VFD). The system uses electronic switching techniques to digitally create different frequencies from the supply frequency to control the motor speed. Each switching event will cause a small torque pulse in the motor which, in effect, is a twist of the drive shaft. If the frequency of the switching coincides with the natural frequency of the drive system it will result in resonance, which can cause catastrophic fatigue failure.

To solve the problem, Renold Hi-Tec's engineers first conducted a transient analysis of the complete drive train to simulate the system's natural frequencies and levels of vibratory torque that were measured on-site. The results were as expected and showed that as soon as the desired operating speed was approached, high reversing torques would impose keyway stresses three times higher than with the original control system.

The calculations were then repeated with one of Renold Hi-Tec's PM60 rubber-in-compression couplings instead of the original gear-type coupling. By careful selection of the rubber hardness and damping characteristics of the rubber elements, Renold Hitec's engineers managed to shift the resonance points away from running speed and reduce vibratory amplitudes to an acceptable level.

After installing the new couplings, on-site measurements were again taken and the results showed that keyway stresses at resonance had reduced by a factor of four. This was actually higher than was predicted by the calculation. The barred speed range was subsequently lifted and the fans allowed to operate at the optimum speeds to recover the maximum power savings from the new control system.