FEATURE CLUTCHES, BRAKES & COUPLINGS
WHAT DAMAGES COUPLING SYSTEMS? Keeping a servo driven system running at peak efficiency is no simple feat.
Misunderstanding performance criteria such as misalignment, torque or rpm can be all it takes to cause a critical and costly failure. So what can damage your coupling driven system and how can you avoid these problems in the future? Robert Watkins, vice president of sales and applications, Ruland Manufacturing, Marlborough, Massachusetts, comments
1. CHOOSING THE WRONG COUPLING Unsurprisingly, one of the most effective and common ways to wreck your system is by selecting the wrong coupling. To avoid coupling failure, there are many factors a design engineer must keep in mind – balancing criteria such as torque, rpm, shaft size, tolerances, operating environment and misalignment, is paramount to selecting the right coupling.
2. NOT IDENTIFYING MISALIGNMENT Most servo applications have one or more forms of misalignment, which can cause stress to system components such as bearings, as well as the coupling itself. Misalignment is often caused by a tolerance mismatch from the driving side of a system to the driven side, which can be caused by factors including parts from different manufacturers, inaccuracies in assembly, system/motor movement during operation, system component wear, poor mounts and thermal shaft expansion. Each coupling style can accommodate different amounts of misalignment. It is important to understand the nature of existing misalignment to determine if a high misalignment coupling is needed at the expense of factors such as torque, or if corrective system adjustments are necessary before selecting a coupling.
5. INSTALLING THE COUPLING INCORRECTLY There is no faster way to undo the work of selecting the perfect coupling and optimising system parameters than installing it incorrectly. For example, uneven torqueing of the screws, incorrect shaft penetration, installing off-centre and compressing or stretching the coupling can lead to failure or premature wear of sensitive system components. The safest option is to follow the manufacturer installation instructions, especially when accompanied by videos.
6. BUYING GENERIC COUPLINGS Not all couplings are created equal, or for the same purposes. Some are manufactured with common specifications, tolerances and designs, such that they are
Ruland servo couplings
nearly indistinguishable from many others on the market. While this may be suitable for systems with limited
3. EXCEEDING RPM RECOMMENDATION System requirements determine speed and in precision driven servo systems it is possible to have speeds of 2,000, 5,000, 10,000 or even 25,000rpm. Unfortunately, not every coupling can handle higher speeds, and exceeding the manufacturer’s rpm rating can cause coupling failure or damage to system components. Even if the coupling is rated for high rpm, greater speeds increase the
effects of misalignment. For example, a disc coupling might accommodate very slight angular misalignment at its maximum rated speed of 10,000rpm without adverse effects on the coupling or system components but will cause damage at a speed of 15,000rpm with the same misalignment. Designers must know the maximum operating speed the coupling will experience, and also understand how manufacturers determine ratings – with performance factors in isolation or everything at max.
4. NOT CONSIDERING COUPLING WEAR Couplings are designed to be the wear element in most systems to protect more expensive components such as bearings and motors. Each coupling wears differently and will fail in a different way. Beam and bellows couplings will completely fail, stopping power transmission when they reach the end of their service life. Disc, jaw, and oldham couplings will lose zero-backlash, but still transmit motion. Designers must consider if the coupling requires maintenance or
Broken bellows coupling
replacement when reaching the end of its service life. Beam, bellows, and disc couplings are maintenance free and require complete replacement after a failure, whereas oldham and jaw coupling performance can be restored by replacing the insert after failure.
34 APRIL 2019 | DESIGN SOLUTIONS
performance requirements, precision systems often require or benefit from couplings with additional capabilities. For example, balanced designs are not the industry standard for most couplings. In an application like printing the reduced vibration afforded by a balanced design is a necessity – less precise couplings would cause banding and expensive waste or downtime. The five servo couplings in the picture above all have balanced holes for a balanced design. 1: oldham coupling 2: jaw coupling 3: bellows coupling 4: disc coupling 5: beam coupling (pictured from centre, clockwise).
7. SELECTING THE COUPLING LATE IN THE DESIGN PROCESS Late selection can limit which coupling is used in the system and the performance it delivers. For example, a system requiring high torque and speed may need to use a disc coupling but ultimately have an envelope that is too small to fit a double disc type, forcing the designer to select a single disc type. While it may meet the speed and torque requirements, single disc
couplings cannot accommodate parallel misalignment, meaning that the system will require greater precision during installation to eliminate the chance of parallel misalignment, likely adding cost and complication. To make the design process easier, manufacturers may have CAD, detailed product information, and technical support available on their website.
8. FAILING TO TEST While everything may look correct in the design, it is hard to determine suitability until the coupling is run under common use conditions. Extensive testing prior to use in live systems can help maximise coupling and system performance. Manufacturers can assist in the design process with technical support and by providing product samples to ensure proper coupling selection.
Ruland Manufacturing
www.ruland.com Rigid coupling on a shaft
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