By Mike Santora
Couplings are simple yet precisely engineered devices that connect shafts to transmit torque and compensate for misalignments. But many variations of coupling designs exist to complement machine dynamics and meet specific application criteria.
Couplings are either torsionally rigid or flexible. Flexible couplings compensate for misalignments, while rigid designs handle drive components already in alignment. Temperature, misalignment potential, mechanical resonance and several other factors make some couplings better than others for given applications. Here are just a few different coupling types and the applications where they’re most common.
Rigid couplings are torsionally stiff and used when shafts are already in alignment. These couplings have two drawbacks: They fail if parallel shaft misalignment exceeds one thousandth of an inch, and they are susceptible to vibration and cannot run at high speeds.
Clamp or compression couplings provide high torsional strength. Their two-piece design lets engineers easily remove these couplings for maintenance. Rigid couplings go in servo systems that require high torque capacity, moderate speed and low cost.
Some engineers see flexible couplings as the most compliant components in mechanical motion systems. This attribute makes torsional stiffness critical in maintaining positional control. Many designs need a shaft to start and stop multiple times per second, a dynamic requirement that necessitates a torsionally stiff coupling to reduce settling time between cycles. Regardless, flexible couplings frequently win out because of their torque capacity. Flexible couplings damp vibrations in both continuous-motion and intermittent applications.
Flexible bellows couplings are common in motion applications that need precision control and misalignment compensation. There are three components: a metal bellows and two hubs. One hub connects to the driving element; the other hub connects to the driven element. The bellows connects the two hubs. The bellows element of this coupling compensates for all three types of misalignment. Because of this design, metal bellows couplings are suitable for most applications. Bellows couplings are found in CNC machines, robotics and encoders.
Jaw couplings have two metal hubs that engage through a spider insert made of elastomer customizable to meet specific design needs. Because they’re less stiff than other couplings, flexible-jaw couplings work best in constant motion applications—including those in oil and gas, water and waste management, construction and aerospace.
Engineers use Oldham as an alternative to straight jaw couplings on general industrial equipment such as pumps, valves, gearboxes and conveyors. They are versatile and offer long life, even in designs that exhibit significant misalignment. Their three-piece design—two hubs and a torque-transmitting center—makes them easy to install and disassemble.
Oldham couplings are manufactured in a variety of materials to satisfy different application demands—the need for zero backlash or vibration reduction, for example. They address parallel misalignment and limit axial motion.
Disc couplings work with servomotor and demanding applications because they transmit high torque, operate at high or changing speeds and handle misalignments. Disc couplings are often selected for automotive, construction and aerospace applications.
No matter the material or subtype, the one-piece design of beam couplings makes them easy to maintain. The couplings are also zero backlash with spiral cuts to transmit torque over spaces with significant angular, parallel or axial misalignment. Beam couplings are often selected in motion control applications for robotics, for attaching servomotors to leadscrews and attaching shafts to encoders.
Offset couplings require less space when connecting offset shafts in large systems. With these couplings designers can create smaller and more efficient machines.
Engineers can mount the offset couplings to shaft hubs or directly to existing machine flanges. They are available for shaft displacements of 0.156 to 17.29 in. and torque capacities to 50 to 460,000 in.-lb. Design configurations abound.
One option is to comb couplings with zero-maintenance bearings to boost uptimes and eliminate the need for lubrication. Such sealed bearings don’t have lube fittings, so make for a streamlined coupling setup—one that sheds contaminants and lets the couplings work in tight spaces and challenging environments.
Offset couplings work in embossing systems, presses, paper industry equipment and pharmaceutical machinery.