Couplings and Shaft Alignment
In power transmission, a coupling is a device used to connect two or more machine shafts together for the purpose of transmitting power. Coupling design factors in many variables, such as horsepower, load, specific gravity, head pressure, torque, shaft sizes, safety factors etc.
There are two families of couplings – rigid and flexible.
A rigid coupling is most often used when the following exist:
- The driver generates huge amounts of torque.
- There can be zero change in the relative axial positions of the coupled shafts.
The most common applications for rigid couplings are on large turbine generators, and certain process machines where timing of the operation must be kept to exact standards. Rigid couplings are normally less expensive than flexible couplings, and often take up less space. However, they must be carefully aligned to practically zero misalignment. In addition, any dynamic changes in the machines, such as thermal growth and piping movement must be known and carefully controlled, or damage to the machines may occur.
Flexible couplings offer one big difference. They can flex slightly, allowing for small amounts of misalignment. The degree of misalignment tolerable with a flexible coupling depends greatly on the coupling’s design.
In the flexible coupling family, there are many different types. The most common are:
- Elastomeric – Elastomeric couplings use an elastomer, or flexible element, to provide sliding motion between coupling components, thereby minimizing friction, and allowing for slight misalignment.
- Disc pack – These couplings use thin metal discs, which are bolted together, causing a laminate effect. The flexibility of the discs along with rubber bushings in the bolting, compensate for slight misalignment.
- Gear – Gear couplings having mating male and female teeth, which are usually lubricated. Normally, these teeth are crowned, to allow for slight misalignment.
Notice that in each statement, “slight misalignment” is used. Even flexible couplings can only tolerate a few thousandths of an inch (mils) of misalignment between the shaft centerlines. If more misalignment exists, the forces from misalignment are transferred to the bearings, seals, gears, and shafts.
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