Flexible vs rigid couplings: Part 3

This is the third installment in a weekly 3-part series on flexible vs rigid couplings running through December. Here, Ruland&#;s Bobby Watkins shares his insight into the best options for different coupling applications. This excerpt was taken from Watkins&#; presentation during the Design World Webinar, Flexible vs Rigid Couplings.

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A little bit about Ruland Manufacturing. We have manufacture all types of couplings: Rigid couplings, servo insert couplings, bellows, metal bellows, metal disc, oldham types and beams, so we offer the full range of types of motion control couplings.

The types of misalignment earlier were covered pretty well, and again, talking about rigids in particular at this point, they&#;re probably the most misapplied, maybe the most misunderstood, of all the couplings. They&#;re not going to accommodate any of these types of misalignment, but they do have a purpose and they&#;re very, very effective when used in the right circumstance.

Again, why is misalignment a concern? If you&#;ve got misalignment and your coupling is not addressing that misalignment, you&#;re going to have problems, either with broken couplings or bearing failures, and again, if you&#;ve got misalignment present in your system, no doubt, you need a flexible coupling to solve those issues for you. Backlash, windup, and bearing load.  Some of the types of motion control couplings have windup. Some of the types of couplings like the servo insert couplings and the beam couplings, have some windup, some vibration absorption capabilities, shock absorption capabilities, and these are important features to have in certain applications where you don&#;t necessarily want or need high torsional stiffness, but you need some vibration dampening. The bellows type, disc type, those are high torsional stiffness types, and of course, the rigids are probably the ultimate torsionally stiff coupling. Let&#;s see.

This was talked about earlier, if you use the wrong coupling in an application, you&#;re going to have bearing load issues and no more so than with the rigid. If you put a rigid in an application where there is misalignment, bad things are going to happen and probably very quickly. You&#;re going to need bearings, destroy components, and bearing load consideration is very, very important as part of the design. Managing misalignment, again, this is just showing a few different types. The following image is exaggerated, by the way. It would be nice if bellows couplings could really forgive that much in angular, but they can&#;t.

It makes a good marketing picture, I guess. Again, selection of the correct coupling is critical. Address the application misalignment. This is a huge issue that we see in the field. Many, many types of couplings, bellows couplings in particular, fail if you install them either compressed or extended. That&#;s a very important thing to address.  We see quite a bit of problems with customers on installation issues.

Rigid couplings. The benefits. Best torque transmission capabilities, call that highest torque transmission capabilities, size for size. Almost zero windup, the most precise coupling available. It&#;s basically like welding your shafts together. Suitable for shaft support in push-pull applications, relatively low cost. The drawbacks, as we&#;ve talked about. It has no misalignment capabilities, no allowance for thermal expansion. Even if you have your shafts perfectly aligned, but you&#;ve got a high-speed stepper or servo motor cranking up at 10,000 RPM, you&#;re going to get some thermal expansion of that motor shaft. The rigid coupling is not going to accommodate it, it&#;s going to push that load right back on the bearing, and you&#;re going to cook your servo or stepper motor bearing. You&#;re going to cook the motor. Something to keep in mind is that even if you&#;ve got perfectly aligned shafts, you&#;ve got to watch out for that thermal expansion of motor shaft. We tell customers to be careful once you get over about 5,000 RPM. You start getting over 5,000 RPM, you&#;re probably going to have thermal expansion as an issue at that point.

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One of the realities you have to deal with is mounting rigid couplings. If you don&#;t have a design in place to make your alignment easy, they can be very difficult to align. You&#;re shimming, you&#;re using lasers&#; I&#;ll give you an example on an actuator. A linear actuator, a ball screw actuator, if you use clearance holes on your motor bracket flange, what you do is you bolt the motor up to the flange, slide the coupling on, and tighten down the coupling first. The coupling will pull the motor into alignment for you. You use the rigid coupling at assembly to pull your motor and your ball screw shaft into alignment with each other and then bolt down the motor last. If you build in the correct clearances it works extremely well. We sell a tremendous amount of rigid couplings in actuator applications where there&#;s a motor bracket.

If you don&#;t have a motor bracket, you can set up your bracketry in such a way where you&#;ve got some clearance and forgiveness built in. You put the rigid coupling on, let it pull things around it into alignment, then go tighten your components down. This way of doing it works very, very well. However, if you&#;ve got high RPM applications, this isn&#;t going to help you because you&#;re still not accounting for thermal expansion. One of the other things about rigid couplings that we run into, being a manufacturer, is we&#;ll have customers call us and they&#;ll say, &#;You know, I&#;ve tried five different types of couplings. I&#;m breaking them all. I just want to put a rigid coupling in there, and the problem&#;s going to go away.&#; This is a common call that we do get, and it&#;s typically because the customer has a bad alignment issue and he&#;s breaking flexible couplings because he&#;s asking more than they can give. We try to protect customers from themselves sometimes because there&#;s a rush sometimes to put a rigid in there as a fix-all for problems with flexible couplings. If you build in a way to do your assembly, you can use the coupling to do the alignment for you. It becomes very simple. Again, very accurate, high torque, and it can be a great coupling if used correctly in the right application.

The Oldham couplings are wonderful. They&#;re a little compromise, if you will. They&#;re not as torsionally stiff as a bellow or a disc, but they are stiffer than a jaw coupling. They&#;re a good compromise. They work in a tremendous amount of applications where you don&#;t quite need the high torsional stiffness of a bellows. The beauty of the Oldham is it&#;s very, very forgiving, especially in parallel. They&#;re becoming very, very popular, in stepper and servo drive systems. They&#;re stiff enough, they&#;re cost effective, they&#;re easy to install, they&#;re very, very forgiving. The beam type coupling, is typically for encoder applications and very light duty applications. They have a lot of windup to them, which if you ask too much of a beam coupling, all that windup&#;s going to give you some positioning error, but again, they work very well for transducers and encoders and applications where there&#;s not a lot of torque.

There&#;s been a lot of talk about the bellows as well&#; high torsional stiffness, very, very popular. You can get the high speed, high torsional stiffness out of this that you cannot get out of a rigid. You can get well over 5,000 rpm. Then, of course, the curve-jaw coupling, or the servo insert on a jaw coupling. It&#;s going to give you that vibration absorption, that dampening, where in a lot of applications, if you&#;ve got a very aggressive move profile, you&#;ve got shock loads. If you use the rigid or a bellows, which don&#;t have any ability to absorb shock load, it can either destroy couplings or beat up bearings and so forth. A servo jaw coupling gives you that vibration and shock load absorption. The disc coupling has similar qualities to the bellows coupling. High torsional stiffness, high RPM ability, and it can forgive a little more in angular than the bellows couplings do, but not quite as popular. The bellows couplings seem to be much more popular, especially here in North America.

Product selection. Again, multiple solutions exist when selecting a coupling. Hopefully I covered everything.

Ruland 
www.ruland.com

Rigid couplings are the simplest and most economical type of couplings, consisting of two flanges, hubs, or sleeves that are bolted or clamped together around the shaft ends. They provide a fixed and rigid connection between the shafts, without allowing any relative movement or misalignment, making them suitable for applications with precisely aligned shafts with steady and uniform loads. Rigid couplings have many advantages, such as being easy to install and maintain, compact and lightweight, reliable and durable, and cost-effective. However, they cannot accommodate misalignment between the shafts, absorb shock or torsional loads, or compensate for thermal expansion or contraction of the shafts; all of which can cause serious damage to the shafts and coupled equipment.

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