Edited by Leslie Langnau, Managing Editor
High tech applications in industries such as medical, semiconductor, solar, packaging, and printing require precision components to maintain a high degree of accuracy and repeatability. The shaft coupling is an important component of such systems, used, for example, to connect servomotors to linear systems. A variety of servo couplings are available, including rigid couplings, beam couplings, bellows couplings, disc couplings, oldham couplings and zero-backlash jaw couplings.
In the past, design engineers would not consider using rigid couplings in high tech applications due to their large mass and common misconceptions about their performance. However, small-bore rigid couplings, especially those made from low inertia aluminium, are used for their torsional stiffness, high torque capacity, and zero-backlash performance.
Honed straight bore rigid couplings improve fit and alignment for applications that require strict control of tolerances. Precision honing assures precise roundness, straightness, and collinearity of the bores and a larger percentage of shaft contact for greater torque transmission ability.
Designed with multiple sets of two or three spiral cuts, four and six beam couplings suit light duty applications. Manufactured from aluminium, the multiple cut design has low inertia and accommodates angular and parallel misalignment as well as axial motion. The single piece design with multiple cut patterns yields backlash-free operation and allows for a small amount of shock absorption. Stainless steel versions suit applications requiring higher torque transmission and corrosion resistance.
Six beam couplings have three sets of short spiral cuts for high torque capacity and torsional stiffness. This is useful for reversing servo motor applications where it is vital to maintain the reliability of the system. Four beam couplings are more compact with two sets of longer spiral cuts for extra flexibility with low bearing loads. A smaller size helps reduce inertia allowing the coupling to be highly responsive while retaining torsional stiffness. The four-beam coupling is good for precision applications that use delicate components such as encoders and tachometers where large amounts of windup could lead to inaccurate readings.
Some bellows couplings combine aluminium hubs with a stainless steel bellows. This lightweight design results in a torsionally stiff coupling that offers a high degree of accuracy and repeatability. The thin walls of the bellows allow the coupling to flex easily while remaining rigid under torsional loads. For this reason the bellows coupling accommodates small amounts of parallel and angular misalignment, and axial motion while maintaining a low bearing load.
An important feature of the bellows coupling is its balanced design, which reduces vibration in applications up to 10,000 RPM. These couplings are also zero-backlash with no moving parts assuring a long, maintenance free life. The characteristics of the bellows coupling make them well suited for high-end servo, stepper, encoder, and positioning applications.
Disc couplings are comprised of two aluminium hubs and multiple flat stainless steel disc springs. With similar performance capabilities as the bellows, the thin disc springs allow for greater misalignment with a small decrease in overall performance. Available in single and double disc styles, disc couplings are torsionally stiff and suit precise zero-backlash systems with speeds up to 10,000 rpm.
Although ideal for applications where a compact installation is required, single disc couplings cannot accommodate parallel misalignment. Therefore, double disc couplings have a center spacer separating two sets of disc springs that can bend independently from each other. This design increases misalignment capabilities and allows parallel misalignment. Some center spacers are available in a choice of anodized aluminium for better performance or insulating acetal for electrical isolation.
Oldham couplings consist of two aluminium hubs with drive tenons that slide on a floating center disk to transmit torque with zero-backlash. This design suits applications that require a large amount of parallel misalignment with a low bearing load. The three piece design results in a highly customizable shaft coupling that can easily combine inch to metric, different bore dimensions, and keyed shafts. The center disk is available in acetal plastic for high torsional stiffness and zero-backlash, or nylon for shock absorption and noise reduction.
For many light duty industrial and high tech applications, oldham couplings are electrically isolating due to the non-conductive material of the center disk. The center disk will also act as a torque limiter during overload to protect machinery components. When the disk fails, it breaks cleanly and does not allow any transmission of power.
Zero-backlash jaw couplings
Similar to the oldham coupling, the jaw coupling is also a three piece design comprised of two aluminium hubs and an elastic element called the “spider.” Made of an advanced polyurethane material, the spider dampens impulse loads, minimizing shock to the servomotor and other sensitive equipment. Available in three durometers, the spider lets you tailor the performance of the jaw coupling, such as torsional stiffness, misalignment, and dampening requirements. Metric and inch hubs (set screw, clamp style, keyed, or keyless) are interchangeable and can be combined into a single coupling.
Jaw couplings have a curved profile hub that press fits onto the spider assuring zero-backlash. The curved jaw profile concentrates the forces to the center of the spider’s limbs, improving the effectiveness of the elastomer material. Raised contact points on the spider limbs help maintain proper spacing between the two hubs, assuring electrical isolation and full angular misalignment capabilities. Jaw couplings are considered failsafe because the jaws of the two hubs interlock for direct power transmission if the spider was to fail, allowing the application to be safely shut down for maintenance.
Ruland Manufacturing Company