(All image credit: R+W America)
The right coupling can improve performance and help avoid operational problems throughout a machine’s life.
When pairing a coupling with a servo gearbox, many things should be considered, including whether the coupling is intended for highly dynamic, high precision motion, or simply for smooth running and efficient acceleration. The potential for torque overload should also be considered.
Additionally, while mechanical overload protection is often categorically disregarded in modern servo drive applications, it is actually a valid and useful technology when considering recent advancements.
Bellows
Bellows couplings offer a high level of accuracy and are ideal for camming, indexing, reversing and positioning. The flexible element is a formed stainless steel bellows, which exhibits two beneficial characteristics: high torsional stiffness, and continuous symmetry. Two hubs are connected to the bellows via adhesive, clamping rings, or welding.
This type of coupling works well in dynamic applications because motion is transferred with zero backlash and a high level of stiffness, resulting in very short settling times and minimal torsional wind-up. This stiffness also helps to minimize rotational positioning errors, along with the continuous symmetry of the bellows, which allows for true constant velocity output rotation, even when misaligned.
Elastomer
Elastomer couplings are a practical choice when the primary objective is smooth and efficient operation. Precision versions are concentrically machined with a preloaded elastomer insert which takes up small amounts of misalignment, while also damping vibration from the motor.
With multiple insert options, it is easy to find a good compromise between damping characteristics and torsional stiffness. Temperature resistant and electrically conductive ATEX rated inserts are also available for demanding or hazardous environments.
The hub design allows for interchangeability, offering a wide variety of configurations, and also improving commercial availability as the individual hubs can be stocked with a single bore diameter and matched-up as needed.
Since these couplings are not as torsionally stiff as bellows couplings, they are normally recommended for constant motion applications, rather than for indexing or positioning. The advantages they offer are a high torque density and relatively low cost, while also damping vibration and shock.
Torque Limits
When it comes to torque overload protection in servo gearbox applications, it is always appropriate to program torque limits in the servo drive parameters, so that the motor does not produce excessive torque.
This is a good way to reduce the risk of creating a torque overload, but it does not protect a drive line from the mechanical overload which occurs when a rotating mass comes to an abrupt stop.
Even relatively light components, such as the rotating shafts and gears of the motor and gearbox can cause significant damage when locked up at high rotational speeds.
Incorporating a zero backlash ball detent clutch into the bellows or elastomer coupling is a great way to help ensure that the energy is released in the case of a machine crash in such a way as to minimize damage to the gearbox and downstream components.
The past decade has brought a number of advancements to mechanical torque limiter technology, making safety couplings smaller, lighter, and faster responding than traditional varieties.
Ultimately, taking the time to evaluate the optimal coupling for an application will help to ensure the best results. With constant advancements in coupling technology, gearbox performance and protection will increase, and making the right selection will help get the most out of your equipment.
This article originally appeared in the November/December 2015 print edition of PD&D.
Filed Under: Gears • gearheads • speed reducers, Motion control • motor controls
