at high speeds or at low speeds with high torque. Grid couplings can operate up to 400 hp/100 rpm. Being more rigid, their misalignment capabilities are not as great as elastomeric couplings, so alignment of the motor and pump shafts is more critical.
Disc couplings are extremely uniform in design and can rotate at high speeds. Pumps operating at high RPMs are good applications for these well-balanced, smooth operating devices that, like elastomeric couplings, require no lubrication. Disc couplings, however, are quite complex designs and are extremely sensitive to misalignment and axial movement. Flexing of a disc or metallic component beyond its yield point can cause fatigue, and axial movement can cause failure.
Size Matters
With coupling selection, pump users should beware of the trap that "more is better." Oversizing a coupling can result in a reduction in needed flexibility or misalignment compensation, and a coupling that is too large can put additional stresses on the pumps and motors being coupled. On the other hand, a coupling with too much misalignment capacity may be too soft or compliant, which can cause vibration or an unbalanced condition in rotation.
"For turbine-driven equipment running at 3,000 rpm or higher, you may want a disc coupling," Konkol said. "For high torque, a gear or grid coupling may be the right choice.
"It is all about putting the right coupling in the right application, so that is where the manufacturer can be a valuable asset in the selection process. That engineering knowledge base can be a real value-added service to the user."
