What is Shaft Coupling Windup and Backlash?

Release Date: August 24, 2016

In this installment of the Ruland Instructional Video Series we discuss the differences between shaft coupling windup and backlash and how they impact system performance.

Transcript

Hi everyone and welcome to another installment in the Ruland Manufacturing Instructional video series on shaft collars and couplings. My name is Bobby Watkins and in this segment, we are going to be covering and reviewing coupling wind-up, coupling backlash, and how both of this affect your system performance and things to be considered in the design stage.

Windup is not backlash. Backlash refers to power transmission error, wind-up is position error without torque error. All coupling have windup to some extent. Zero backlash means zero loss in power transmission. A couple examples showing backlash and windup. This particular coupling has backlash. This is error. As you can see, there is clearance between the spider and the hubs and one rotation on this side does not equate to one rotation on the other. I'll show you. I'm turning this hub, turning, turning, now I'm engaged. The first part of the revolution was lost. I'm not one to one, that is a coupling that has backlash and will cause positioning error. This is a spring which I'm gonna use to show an example of windup. so again, a spring will windup but it retains the torque, goes back to its original position.

Let's think of an example. We've got an linear actuator, driven by our stepper or servomotor system. One rotation of the servomotor, translate into one rotation of your ball screw import on your actuator and let's just say that should translate into one inch of travel on your carriage. So I had my system set up but I have a coupling that has backlash, it has error. My servomotor turns one rotation but because of the error, my ball screw input shaft has only turned .95 of the rotation. That would translate into my system believing I've traveled one inch on my carriage, I've actually only traveled .95 inches. There's your error.

First, we'll go over the couplings that have error, have backlash. You can not use these in a system where positioning is important. This error is gonna cause positioning error on a linear cable and a valve application, it's gonna to cause you to believe a valve may be open and in reality it's partially closed. It's gonna cause errors in systems where you need to maintain position. The advantages of this coupling are that because it has clearance, because it has error, it's easy to align, it's forgiving, and for general use, it's fine. Zero backlash, this is a jaw coupling that has zero backlash. It's a press fit on that spider. There is no error. The advantage is you can use this coupling in precision positioning applications, like a linear slide, like a precision valve application and you will not have error. The disadvantage is, in some cases, the jaw coupling in particular, the alignment requirements, because there is no clearance, little more stringent, little more difficult to align in most cases, versus a coupling that has clearance in it.

Yes, most of the coupling do lose their zero backlash ability over time. I'll show you a couple of examples. An oldham coupling, which has a disk in the middle and to accommodate this alignment, the disk is scuffling back and forth, it's moving and it does wear over time. So over time, the disk abs will wear down and you will develop clearance. So a maintenance program to replace the disks on an oldham is necessary. On a jaw coupling, the spider in the middle, does lose its ability to rebound, to compress and rebound over time. So after extended use, it can develop clearance as the spider's ability to compress and rebound is lost.

Thank you for joining us in this segment, in the Ruland Instructional video series. If you have any question about anything you've seen in this segment, or any application question in general, please find us at Ruland.com.

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