Pump and Systems, June 2009
After spending money on a powerful new pump and the motor to run it, the next big decision is how to connect the two in a way that maximizes their efficiencies and protects them from normal wear and tear.
Coupling the driving and driven shafts would be simple if they were perfectly aligned, machines did not vibrate and the shafts never actually moved. In the real world of power transmission, however, a flexible coupling can accommodate for the flaws and dynamics inherent in most systems.
"Even if you can get the shafts aligned perfectly, you are doing it when the equipment is not actually running," said Paul Konkol, marketing and business development manager, industrial couplings, for Altra Industrial Motion.
"Once things start moving, temperatures change, operating conditions change, foundations settle. So you need a coupling, but not just any coupling. You need the right coupling, in the right size, for the right pump application."
The basic function of all couplings is to transmit power, accommodate misalignment and compensate for axial movement (end movement of shafts). Sometimes, a coupling is asked to absorb shock or vibration. Selecting the right coupling depends on four basic conditions of shaft misalignment or movement.
- Parallel misalignment occurs when the two shafts do not share the same rotation axis. Their end faces may be parallel, but their center axes are laterally displaced with respect to each other.
- Angular misalignment applies when shafts are neither coaxial nor parallel. The angle at which the shafts are misaligned may be symmetrical or asymmetrical.
- End float occurs when either or both shafts display axial movement, moving in and out. A sleeve-bearing motor, for example, "floats" as the rotor hunts for the magnetic center of the winding. Temperature variation can also cause thermal expansion and variation in position of the shafts.
- Torsional flexibility is the torsional movement in planes perpendicular to the shaft axis. Shock or vibration typically causes this. A torsionally flexible coupling absorbs and dampens these movements.
Figure 1: The four types of misalignment that couplings can accomodate.
The basic construction of the most flexible couplings consists of two flanges or hubs, which attach to the shafts being coupled, and a connecting element that may be metallic (such as in disc couplings), or a sleeve made from elastomeric material such as EPDM rubber, neoprene, Hytrel or urethane, or a mechanical connection (as in a u-joint or gear coupling).
One example of a four-way flexible coupling with good torsional flexibility is an elastomeric coupling that operates in shear (as opposed to compression). The elastomeric flexible member can deform or stretch in shear, displacing under a load. The amount of torsional displacement or elastomer "wind-up" is a measure of the shock that can be absorbed. It also compresses easily and stretches to handle end float.