Couplings: The Forgotten Componment?
By David Lockett
You've assembled a complex drive system and all that's left to do is attach the motor. The only problem is that the shafts aren't exactly aligned due to the stack up of tolerances in the mountings and...
By David Lockett
You’ve assembled a complex drive system and all that’s left to do is attach the motor. The only problem is that the shafts aren’t exactly aligned due to the stack up of tolerances in the mountings and bearings. A rigid connection between the motor and drive system will put too much side force on the bearings and create more load than the motor was designed for.
Your solution is a coupling. That’s the last thing many people consider in constructing a drive train. Nevertheless, it is this humble component that not only has to link the two assemblies but also compensate for design errors and operating conditions. Sometimes even the design engineer can get stuck, having designed him or herself into a corner. A common question then is, “How do I make it fit?”
Fortunately, coupling manufacturers are very familiar with this scenario and many are able to help the stressed engineer arrive at the best solution. Every application has its own specific requirements, and accordingly, there is a wide range of different motion control coupling mechanisms from which the engineer can choose. Although most will satisfy the need for zero backlash and the control of misalignment, each has individual characteristics which determine its ideal application.
How do you know whether you need a bellows, membrane, sliding disc or helical beam, or some other design? To recommend the best type for the job, the obvious question your supplier will ask is “What do you need the coupling to do?”
For example, you may need to connect a servo motor to a ball-screw on a high-precision three-axis measuring system. Or you may simply need to control the opening and closing of a valve.
To size the coupling, you first need to consider the amount of torque, the shaft size, speed and the characteristics of misalignment. Is the misalignment angular, radial, axial or a combination of all three? How stiff must the coupling be in torsion to ensure accuracy of the shaft position?
Then, take into consideration the duty cycle. Does the coupling need to work continuously or intermittently, and what will be the nature of its operating environment? Also, what method of fixing do you need? Set screw or clamp — and does it require a keyed shaft?
Coupling manufacturers can help with your coupling decisions. Here are a few coupling types and suggestions for their application:
If the overriding need is for high-speed operation between 5,000 and 10,000 rpm, with high acceleration, high torsional stiffness and minimum wind-up, a bellows coupling is the most likely recommendation. Typical applications are closed-loop servo systems, positioning slides, metering valves and those involving predominantly inertial loads.
There is, of course, a trade-off between misalignment and torsional accuracy. The more misalignment you need to handle, the more you will usually have to compromise torsional stiffness.
Bellows couplings are available with different lengths of bellows and numbers of convolutions that provide different performance characteristics. They are also available in stainless steel and nickel. Stainless steel is generally stronger and suited to higher loads.
The nickel type is the best choice where transfer of intelligence is the main criterion, such as in high-precision position control, velocity control and applications with very high resolution encoders. Nickel bellows couplings provide high torsional stiffness and other distinctive properties, such as good flexibility, good accommodation of radial misalignment and low bearing loads.
Flexible membrane couplings
Another statically balanced design which has similar attributes to the bellows is the flexible membrane coupling. The key factor which differentiates the flexible membrane coupling is its long life. If operated within set parameters, the flexible membrane coupling has near infinite life. Its misalignment capacity is limited but this can be overcome using two flexible stages separated by a spacer.
It is most suited to applications involving high-end servo drives, pulse generators, positioning slides, lead screws and dynamometers.
Helical beam couplings
One of the greatest benefits of the multi-beam coupling is that it is a single-piece coupling that can be manufactured in a wide choice of materials to suit the application.
For example, it can be totally stainless steel for use in clean room applications or in applications where corrosion is an important consideration. These couplings can be electrically conductive or insulating and are often chosen for their slim-line appearance.
The helical beam coupling is available with six beams, which allows for higher radial misalignment. It is also available with three beams, which has a greater torque capacity than the six-beam, and it’s naturally smaller and therefore suitable for applications where space is at a premium. These are the best choices for light-duty power transmission applications.
Sliding disc couplings
Sliding disc Oldham couplings are best employed for lower-speed applications of 3,000 rpm or less. They have exceptional radial misalignment capability and their three-piece construction allows them to be assembled effortlessly in blind or difficult installations.
Typically, sliding disc couplings are used in servo systems, micro stepper drives, ball screw drives and any application that requires high torque from a small package.
A unique feature of this coupling mechanism is that side loads induced on bearings due to misalignment are minimal and constant for any amount of radial offset (within the coupling’s physical limits.)
Another attribute is the resilient torque disc which can be used to dampen transfer frequencies caused by stepper motors. This ‘mechanical fuse’ also protects other elements in the drive chain, and since it is replaceable, it makes the sliding disc coupling a cost-effective option in the event of overload.
If lack of space is a major issue, this coupling type is ideal as it combines the attributes of a sliding disc coupling and a universal joint in a very small space. The Unilat coupling is a design originated by Huco Dynatork.
Exceptional angular and radial misalignment and very low side forces are the performance characteristics of this short-length coupling package. Generally, Unilat couplings are used in conjunction with encoders, resolvers, small pumps, stepper motors and in light push/pull applications.
Having reviewed all the relevant application factors, your supplier or manufacturer should generally be able to provide any of the coupling types in stock. Special requirements can often be met by adapting a standard design but a fully customized design is of course the ultimate option.
David Lockett is joint managing partner with Huco Dynatork, an Altra Industrial Motion brand. The company offers a complete engineers guide to coupling mechanisms, dimensions, performance characteristics and their selection on its website, www.huco.com.
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