Flexible vs rigid couplings: Part 2

This is the second installment in a weekly 3-part series on flexible vs rigid couplings. Here, Ringfeder’s Ed Cole shares his insight into the best options for different applications. This excerpt was taken from Cole’s presentation during the Design World Webinar, Flexible vs Rigid Couplings.

Ringfeder is a manufacturer of transmission components, but today specifically we’re going to be talking about bellows type couplings. Here’s a quick summary of some of the servo coupling requirements that you’ll find out in the field. Number one and most importantly, we need backlash free operation, and position and accuracy maintenance. Number two, the couplings need to be balanced for high rotational speeds because many of the applications, will be running at 5,000 or 10,000 rpm or more, potentially. The couplings need to be designed so that they have low and moderate torques, what we see, is usually in the neighborhood of up to 100 newton meters, but we make our bellows servo insert couplings, which can go as high as 5,000 newton meters. We’ve covered the whole range. We also need to handle misalignment, specifically angular, axial, and parallel misalignments.

Here’s some of the applications that you may find a servo coupling in. Any automation system that has shaft driven linear motion, which would be machine tools, packaging equipment, test and measuring systems and semi conductor manufacturing equipment–what all these types of equipment have in common is that they’re very precise in location. This is what’s perfect for the bellows servo insert type coupling. They’ll deliver the precision that you need to meet your requirements.

Basically you have two different types of choices for servo applications. You can either go with the rigid metal bellows coupling or the servo insert coupling. In the Ringfeder world, the rigid metal bellows couplings would be something like our EKN or DKN series, and for the servo insert type couplings, that would be our GWE type couplings. Within these two types, there’s different types of connections. Most commonly, we have a clamping style where there’s a side set screw. We also have other types as well. We have a plug-in style and a clam shell style that’s available.
How do you pick a coupling? How do you pick between a bellows coupling and a servo insert type coupling? Many applications, in reality, don’t strongly favor one coupling or the other. You could use either one. Metal bellows or servo insert coupling can satisfy the basic requirements. It’ll meet the torque, it’ll meet the shaft size requirements and life cycles and misalignment. If you don’t have a preference, typically companies will use what they’re used to. They’ll use the same thing. Bellow seems to be a bit more popular for us.

Even though they’re very similar, each of these couplings do have subtle advantages between them. Here’s some of the construction details of a metal bellows coupling.

Basically they’re fabricated from three different individual components and put together. You have your two hubs, and you have your bellows in between, which are your flexible element. Typically, the bellows itself is made out of stainless steel, and typically the hubs are made out of aluminum, but for larger sizes, we’ll make the hubs out of steel.

Here’s an example of one of our DKN 20 bellows couplings that’s used in the machine industry, and in this particular instance, the coupling is used to compensate for some misalignment between the motor and the ball screw. In this case, the motor is rotating in both directions.

Again, most of these applications, the high torsional stiffness relates to the accuracy of the machine, and that’s what provides it.

Here are some construction details. Looking at the servo insert type coupling, typically, again, these are made from two hubs with a spider or damping element in between.

 

Here are some of the differences, though, between the bellows coupling and the servo insert coupling. Basically with the servo insert coupling, you can have adjustable flexibility, not that there’s a dial on the coupling, but when you select your coupling, spiders or other elements have different hardnesses to tailor to your application. What we do and most other manufacturers do, is color code the element which corresponds to a hardness.  If you have the hardest element, that will be closer to the stiffness in a metal bellows. Not quite as stiff, but it’ll be stiffer in that direction. If you go with the softer insert, that will maximize your damping and misalignment capabilities as well. That’s one major difference. If you have vibration and you have a bellows type coupling, it’s going to transfer the vibrations through the coupling to the other side. With a soft insert, that will do some damping.

Also note, couplings are part of a system. They work in conjunction with other power transmission components, such as gear boxes and linear actuators. It’s important to choose couplings whose design works well with these other components.  In one particular instance, shown here:

we see a test band that runs in a climate chamber. This is a fairly difficult application because the temperature can vary anywhere from -40 to 80° C, and also 97 % humidity. In this case, special design required that our DWE 5104 be made out of stainless steel. Also note that some of the other components here, we’re working with, in this case, a magnetic hysteresis clutch on the left in the picture.

Ringfeder
ringfeder.com