Most of us probably never notice the spinning cylinder mounted between the pump and motor, except how easy it is to dis/assemble when a pump or motor is changed out. Otherwise, the disc coupling never factors into our daily routine.
While maintenance-free couplings can be ignored for decades, knowledge of how a disc coupling works and how different designs fit best with different applications will help in choosing the best coupling for the job. After all, a disc coupling's job is to protect expensive connected equipment, so it is essential to have the right disc coupling.
Subtle style and design differences result in varying torque load capacities, misalignment compensation, ability to withstand shock from rough or reversing applications, service life and other functional parameters.
Why a Disc Coupling?
For dozens of general industrial applications, and many specialized applications, a disc coupling efficiently transmits torque, accommodates misalignment and compensates for end movement between driving and driven equipment.
Disc couplings offer precise positioning with zero backlash, even in rough or reversing applications. They can transmit high torque loads with power efficiencies of 99+ percent, long operating life (typically from 20 to 30 years), no maintenance, precision balance and in some units, the center spacer can cover long spans.
Obviously, the application dictates the requirements of the disc coupling, so always consult a coupling engineer to ensure an optimized coupling selection.
Typical disc coupling specifically designed for process pump and general purpose applications. This all-metal disc coupling requires no lubrication and incorporates a plug-in feature to allow installation and removal without disturbing the pump alignment. Photo supplied by TB Wood's .
Two Styles
Disc couplings come in two basic styles: the traditional style and the drop-in style. Traditional disc couplings feature five-piece construction with two hubs, one spacer and two flex discs along with hardware (Figure 1). The three-piece drop-in style couplings have fewer parts-two hubs and a factory-assembled spacer unit (Figure 2).
Figure 1. Traditional style, five-piece coupling
Figure 2. Drop-In Style, three-piece coupling
While the traditional style has a lower initial cost, the drop-in style has a lower overall cost of ownership due to faster installation time. Because it is largely factory assembled, the drop-in style has fewer loose hardware items to drop and lose during installation. The traditional is lighter in weight, but the drop-in meets a higher balance class.
Traditional style disc couplings may not be readily balanceable and may lack good balance retention/repeatability after disassembly and reassembly. The drop-in is a well-balanced unit with piloted fits that ensure good balance repeatability.
Design Basics
The basic functions of a flexible coupling are to transmit power, accommodate misalignment and compensate for end movement.
Transmit Power
Generally power loss from driver to driven component is small, especially with disc couplings that are nearly 100 percent effi
cient. Torque at a speed [T (LB.IN.) = HP x 63,025/RPM] varies widely, of course, depending on size and design features, but disc couplings are typically among the most power dense of the flexible couplings used in general purpose applications.
Accommodate Misalignment
Disc couplings excel at compensating for both angular and parallel offset misalignments (Figure 3), and each manufacturer will provide specifications for individual units. However, disc couplings are not like common elastomeric coupling types where a single flexing element accommodates all types of misalignment.
Figure 3. Misalignment types
With disc couplings, a single disc only accommodates angular misalignment. Double-flexing couplings are needed to compensate for parallel misalignment. Double-flex styles are consequently required in all but a few special cases (Figure 4).
Figure 4. Single and Double Flexing Couplings. In this illustration, parallel misalignment imposes angular misalignment "A" equally to each flex element in the "double hinging" effect shown. Angular offset "B" of either shaft applies only on one flexing element and is additive to the parallel component to get the total misalignment "C."
Compensate for End Movement
The ability to compensate for end movement (Figure 5) is noted as the "free end float rating" in coupling catalogs. All couplings are suitable for use with electric motors, but some units meet NEMA end-play standards for sleeve bearing motors without the use of limited end float (LEF) devices.
Figure 5. End Float Compensation
While the purpose of a disc coupling is to protect connected equipment, careful equipment installation will help. Because driving or driven equipment are often sensitive to reaction forces of the coupling to misalignment, it is important to minimize misalignment at installation and ensure equipment has adequate end float. Alignment benefits the equipment, after all, not the coupling. In some cases, when expected thermal growth of the equipment is known, the coupling is misaligned at installation, so it runs "hot" with little to no misalignment.
Nuts, Bolts and Discs: How It Works
With disc couplings, the bolting arrangement between the two outer hubs and the inner spacer is an important design feature. Three (or two, or four) holes are bolted to the driver hub, and an equal number are alternately bolted to the driven hub. Between hubs and spacers are flex discs, otherwise known as disc packs, flex elements, flex blades or flex packs (Figure 6). Whatever you call them, it is important to know that the individual blades are not "shims" that can be removed to adjust for equipment spacing variations.
Figure 6. Driver/driven hub bolt hole pattern
Torque is transmitted in tension through half of the links of the flex discs. A link is the metal area of the flex disc between the bolt holes (Figure 7). The remaining links operate in compression and are generally not considered in the ratings. Which links are in tension versus compression is determined by the rotational direction. The compressed links tend to
