Selecting mechanical couplings for machinevision and optical-inspection systems requires a trade-off between damping and rigidity.
Machine-vision and optical-inspection systems demand precise motion control, and this requires precision components. One often overlooked but important component is the zerobacklash coupling that connects a motor to a linear stage. For instance, a coupling might connect a servomotor to a ball screw that drives a vision system used for inspection.
Such couplings must have zero backlash to ensure accuracy and repeatability. In other words, there can be no play between components. If the motor shaft turns 90°, a zero-backlash coupling guarantees the ball screw shaft turns the same 90°.
Vision and inspection systems usually operate in one of two general modes. One is the stop-and-image motion system that moves quickly to one spot, takes a picture, and then moves on to capture another image. Rapid starts and stops inherently cause vibration in this type of system. Couplings that dampen these vibrations reduce settling times and increase through-put.
The second type of system operates in a constant scanning mode. Unlike the stopand- go approach, which only requires accuracy upon stopping, this type of motion must be accurate while moving. This necessitates a coupling with high torsional rigidity. Vibration may be less of an issue in devices using the scanning mode.
In addition to vibration damping and torsional rigidity, other important design criteria for accurate and repeatable high-speed motion are the coupling’s inertia and its misalignment, speed, and torque capabilities. Speed and torque are fairly straightforward, but misalignment and inertia are a bit more complex. Designers must consider three types of misalignment — parallel, angular, and axial — and applications often exhibit more than one at the same time. Couplings that cannot accommodate misalignment will likely fail. Depending on the coupling, misalignment can also generate loads that can cause premature bearing failure.
Inertia is the property that governs a coupling’s tendency to stay at constant speed unless acted upon by a force. In other words, inertia relates to the force required to start the coupling from rest or stop it when moving. To a large extent, it’s affected by the coupling’s mass. Experts generally recommend lowinertia couplings for stop-and-go motion, to conserve energy, and lessen wear on components. Inertia is less of an issue for systems in constant motion.
Complicating matters for system designers, there are more than a few couplings from which to choose, and there is a degree of give and take when selecting zerobacklash couplings, as each has strengths and weaknesses. Here’s a look at six different types of zero-backlash couplings, highlighting the benefits of each in regards to the accuracy of machine-vision and inspection systems.
Continuous slots cut in the body let beam couplings transmit torque and accommodate misalignment. They are a good fit for machine-vision systems that operate at moderate speeds ( 6,000 rpm), have they could be problematic in stop-and-go systems. Bearing loads are the highest of all couplings and one must take extra care to ensure that shafts are perfectly aligned. Users must also monitor thermal expansion at high speeds, as rigid couplings cannot accommodate the resulting stresses that would have the same harmful effect as misalignment on the bearings.
Provided the system shafts are properly aligned and there are no thermal-expansion issues, rigid couplings are a good option for scanning applications. They offer excellent torque capacity, have the highest torsional stiffness, and require no maintenance.
The zero-backlash, curved-jaw coupling is a variation on the straight-jaw coupling. (Straight-jaw couplings have inherent backlash and are unsuitable for machine-vision systems.)
Curved-jaw couplings have three parts, two aluminum hubs and an elastic insert — referred to as a “spider” — which press-fit together. The hub jaws limit spider deformations, thus providing zero-backlash during normal operation.
Curved-jaw couplings can operate at high rpm, but their advantages lie in the benefits the spider offers. The elastic material absorbs vibration, making these couplings the best choice for stop-and-go systems that require damping. Further, spiders are available in several hardness grades, letting engineers tailor performance to the application. Harder inserts have more torsional stiffness at the expense of some damping capability.
Jaw couplings are not torsionally rigid enough for scanning motion, even with the hardest elastic spider. And jaw couplings produce high bearing loads when there is significant misalignment between shafts. Another consideration is that jaw couplings are fail-safe, meaning the jaws of the hubs will lock together and continue to transmit torque even if the spider fails. This may or may not be desirable, depending on the application.
Curved-jaw couplings have superior damping characteristics, and the ability to mix and match spiders and hubs is an advantage as well. Jaw-coupling inertia is relatively high compared to other motion- control couplings (on par with an aluminum rigid coupling), but the spider’s damping characteristics negate this issue. The coupling’s balanced design lets it run at high rpm without vibrations. However, if there is high misalignment and rigidity during motion, a different coupling type may be a better choice.
Similar to jaw couplings, Oldham couplings have two aluminum hubs and a press-fit insert. They have a few clear advantages. First, they accommodate high parallel misalignment, because the center disc can slide over the tenons of the hubs. This lack of resistance (other couplings have a springlike resistance to misalignment) results in low bearing loads.
A second advantage is the ability to interchange the center disc. Generally, two materials are used. Acetal provides torsional stiffness while nylon provides damping, much like in jaw couplings. Third, an Oldham coupling has the unique ability to act as a mechanical fuse. Hub tenons will not interlock if the center disc fails and the coupling will cease to transmit torque. Also, the Oldham coupling’s aluminum hubs help keep inertia low.
Oldham couplings work well with parallel misalignment but only tolerate small amounts of angular misalignment and axial motion. Too much angular misalignment and the coupling loses its constant-velocity characteristic, and excessive axial movement will literally pull it apart. Also, while nylon may damp vibrations, the sliding hubs and soft characteristics of nylon mean the coupling is not tight enough to be zero backlash. Speed capabilities for all Oldham couplings are also relatively limited, around 4,500 rpm. Additionally, the sliding tenons eventually wear the insert to the point it requires replacement.
For applications running <4,500 rpm with high parallel misalignment and little angular and axial motion requirements, the Oldham is suitable for either stop-and-go or rigidityminded scanning motion.
Disc couplings come in one of two designs: two hubs joined by one set of flexible metal discs; or two hubs and a center piece joined by two sets of flexible metal discs. Double-disc couplings accommodate parallel and angular misalignment because the two sets of discs can bend in different directions. Single-disc couplings only accommodate angular misalignment.
Both variations are torsionally rigid. The flexible discs allow for misalignment. Bearing loads and inertia are low as well. Further, disc couplings can handle speeds upwards of 10,000 rpm, with low inertia due to the lightweight materials.
Disc couplings are best for applications that emphasize accuracy and torsional rigidity; but they are not a good choice when damping is needed. Disc couplings are, therefore, more suitable for scanning as opposed to stop-and-go vision systems. While rigid couplings are best for accuracy and torsional rigidity, disc couplings permit shaft misalignment while retaining high torsional stiffness. One drawback is disc couplings are delicate and damage easily if installed incorrectly. Applied correctly, disc couplings have outstanding qualities for torsionally rigid applications that have some misalignment.
A bellows coupling consists of two aluminum hubs welded or bonded to a metallic bellows. Nickel and stainless steel are the two most common bellows materials. Bellows normally have a thin wall, which adds to responsiveness and accuracy. The flexible bellows also accommodates all misalignment types and bearing loads are low and constant throughout all points of rotation. With aluminum hubs, the bellows coupling has low inertia which saves the system from unnecessary force and provides even greater response.
All these strengths do not compromise the coupling’s torsional rigidity. Running speed is on par with the disc couplings, up to about 10,000 rpm. The bellows coupling is a great option for machine-vision applications requiring accuracy, though vibration damping is nonexistent. They operate at high speed, have excellent torsional rigidity, while still retaining good misalignment capabilities.