Couplings are a critical part of system performance in high-tech applications, yet they are often one of the last components to be specified. Selecting the proper coupling ensures the equipment will meet performance requirements and have a long, trouble-free life. Poor coupling selection can lead to high maintenance costs, frequent downtime, and imprecise positioning.
High-tech systems are found in almost all industries including semiconductor, medical, agriculture, printing, and aerospace. Precise positioning systems generally consist of a driving component, such as a stepper servomotor, and a driven component, such as a ball screw, with the coupling used for power transmission (Figure 1).
The diversity in industries using high-tech systems creates a wider variance in how couplings are applied, even if the applications are similar. For example, precision equipment is used to finely open and close panels controlling the amount of radiation being administered to a patient with brain cancer. This same application is used on the aperture mechanism of high-precision telescopes researching exoplanets from space. These applications, while serving the same function, have different system requirements. Focused radiation requires a coupling that meets hygienic requirements in a relatively controlled environment. Telescopes need a coupling that can survive the launch into space, operate in a vacuum, and perform for long periods of time without failing.
There are numerous considerations to make when designing a coupling (Figure 2). These may include:
- Physical space requirements. Shaft sizes, spacing between shafts, and overall envelope size should be the first considerations for specifying a coupling.
- Misalignment. Designers often misapply couplings where shaft misalignment is greater than the coupling can accommodate by failing to adequately calculate tolerance stacks and manufacturing inconsistencies (Figure 3). This can lead to poor performance and frequent system maintenance.
- Environmental conditions. Most high-tech applications exist in controlled climates without exposure to extreme environments. However, they can appear in environments including extreme temperatures (hot and cold), vacuum, and chemical exposure that can affect coupling performance.
- Operating conditions. Designers must account for factors such as speed (RPM), rotational cycling, torque, duty cycle, and acceleration/deceleration rates.
- Performance criteria. System performance expectations including level of positioning accuracy and repeatability, settling time, and overall responsiveness must be considered when selecting a coupling.
Selecting a coupling that meets application requirements involves understanding the strengths and weaknesses of each style. This article reviews six different types of motion control couplings: rigid, beam, bellows, miniature disc, zero-backlash jaw, and oldham. They are all zero-backlash because this is critical to the performance of high-tech applications.
Beam couplings utilize continuous cuts in the body to transmit torque and accommodate misalignment. They are a good fit for high-tech systems that operate at a moderate speed (maximum of 6,000 RPM), have significant misalignment, and require some dampening.
Two common variations of the beam coupling are single-beam types with one long, continuous cut, and multiple-beam types that have one or two sets of shorter cuts overlapping each other. Multiple-beam type couplings offer higher torque, torsional stiffness, and parallel misalignment capabilities. Single-beam couplings have better angular and axial flexibility.
Beam couplings are generally available in aluminum for low inertia and stainless steel for increased torsional rigidity. Designers must be careful when using stainless steel as they have significantly higher inertia and cost. It is generally advisable to consider alternative coupling styles.
Rigid couplings are manufactured out of several materials and are available in a number of styles. For high-tech applications, clamp style is preferred to set screw. Clamp type couplings do not mar the shaft, have higher torque capabilities, and require no maintenance. Aluminum is preferable due to its low inertia. The rigid coupling has the highest rated torsional rigidity, making them ideal for high-tech applications that require precise movements over short increments, such as 3D printers.
Rigid couplings have the highest bearing loads of all couplings and require perfect shaft alignment. Thermal expansion under high speeds is also something to monitor, as rigid couplings have no means to accommodate the resulting stresses that have the same effect as misalignment on the bearings.
Disc couplings are either composed of two hubs joined by a flexible metallic center disc (single), or two hubs and a center piece joined by two metal discs (double). Double-disc couplings can accommodate parallel and angular misalignment. Single-disc couplings fit in a smaller envelope and only accommodate angular misalignment.
Both variations are torsionally rigid with low bearing loads and inertia. The flexible discs allow for high misalignment, especially in the double-disc design. Furthermore, disc couplings can handle upwards of 10,000 RPM. The combination of these factors makes disc couplings a common choice in semiconductor and solar equipment.
The disc coupling is best suited for applications where accuracy and strength are emphasized due to high torsional rigidity, and is not a good choice when dampening is needed. They are delicate and can damage easily if installed incorrectly.
The bellows coupling is constructed of two aluminum or stainless steel hubs connected — either by welding or an adhesive — to a metallic bellows. The thin wall of the stainless steel, bronze, or nickel bellows adds to responsiveness and accuracy. Bearing loads are low and constant throughout all points of rotation, and bellows couplings can accommodate all forms of misalignment. The combination of high torsional stiffness and low inertia (with aluminum hubs) allows for a high level of system responsiveness.
All of these strengths do not compromise the coupling’s torsional rigidity, which is even greater than the disc coupling. Running speed capability is on par with the disc couplings at about 10,000 RPM. Bellows couplings are also sensitive to installation and can be damaged easily if not properly installed (Figure 4). The bellows coupling is a great option for printing, which requires accuracy and no dampening to prevent banding.
Zero-Backlash Jaw Couplings
The zero-backlash curved jaw coupling is composed of three parts that press fit together — two aluminum hubs and an elastic insert referred to as the “spider.” The design of the jaws of the hubs helps to reduce deformations that the spider may undergo during normal operation. Spiders are available in different durometers to balance stiffness with dampening capabilities. The high dampening characteristics of curved jaw couplings make them an ideal fit for stop-and-go inspection systems.
Jaw couplings can have high bearing loads and are not suited for systems requiring high misalignment. Another consideration is that jaw couplings are failsafe, meaning the jaws of the hubs will lock together and continue to transmit torque if the spider fails. This may or may not be desirable, depending on the system.
Similar to the jaw coupling, the oldham coupling is also made up of three components — two aluminum hubs and an insert — that press fit together.
The oldham coupling is a good starting point for system design and has a few clear advantages: high parallel misalignment capabilities with low bearing loads, interchangeability of the center disc, and the ability to act as a mechanical fuse. They are commonly used in valve actuation applications where precise regulation of the valve is required.
Oldham couplings can only handle small amounts of angular misalignment and axial motion. Nylon inserts may be chosen to dampen vibrations; however, the press fit is not tight enough to ensure zero-backlash operation. Speed capability for all oldham couplings is relatively limited, around 4,500 RPM. Oldham couplings have long life capabilities, but eventually the disk will fail completely or wear to the point that it loses zero-backlash properties and needs replacement. When the disk is replaced, full performance of the coupling is restored.
High-tech systems only operate as well as the components that make them up. Each coupling has its own strengths and weaknesses that make it useful in a variety of applications. However, no one coupling can be applied to all situations. Based on this rationale, the zero-backlash coupling should be considered in the early stages of system development. If the coupling’s performance characteristics meet the operating goals of the system, the user can be assured of maximum system performance and longevity.
This article was written by William Hewitson, President of Ruland Manufacturing Co., Marlborough, MA. For more information, Click Here .