Motion System Design, Stepper Motor Sizing

Torsional Windup and Load Friction

Author: SEM USA | May 11, 2016

In our last posting, we talked about how a very torsionally flexible coupling could affect the ringing at the load.

A rubber tube

We proposed a one-inch long plastic tube that fit snugly over the motor’s shaft and the load’s shaft and that the torsional windup of this coupling was zero. The moves were snappy and settled out with very little ringing.

Then we eventually made our plastic tube coupling 12” long. The longer the tube the more torsional windup we had. The motor would take a step, but the tube would just twist or windup and it would eventually turn the load. The load ringing would take a long time to settle out.

Now this discussion was taking place in a frictionless environment.

Figure 1: Torsional windup of a rubber tube and load inertia
Figure 1: Torsional windup of a rubber tube and load inertia

Add load friction

Add some load friction and our 12” hose coupling would have to windup until the torque it was transmitting exceeded the sticky-friction or stiction that holds the load still. The load would eventually move if the motor stepped far enough, but when the motor stopped the load would get stuck at some in-between location. This in-between location would be located at a torque value that is less than the stiction value and with some of the motor’s torque “stored” in the torsional windup.

So do you think a stiffer coupling is best? How about we replace the plastic tube with an aluminum tube that again fits over both the motor’s shaft and the load’s shaft?

Aha, those were kind of trick questions because the answer is yes for the first question and no for the second, but why? Weren’t we trying to get the load to be tightly connected to the motor’s shaft?

If a solid coupling were used

If the load is coupled to the motor’s shaft via a solid coupling then we’d have no windup at all. However, both the motor and the load’s shaft need to be perfectly aligned with each other. The shafts need to be perfectly lined up and parallel to each other and the mounting faces need to be perfectly parallel too. Remember the motor shaft and the load shaft are in fixed positions and anything other than perfect alignment will cause the shafts to bend as they rotate.

So what happens if the shafts are slightly off? Well, something has to “give.” That something is the shafts. In order for them to stay aligned as the shafts turn they will bend. (Wasn’t “As the Shafts Turns” a soap opera?) What happens is the motor shaft or the load shaft or both shafts bend at the point they exit their bearings. This back-and-forth bending motion accommodates the misalignment of the shafts. This back-and-forth bending motion takes place every time the shaft makes a move, be it a complete revolution or a partial one. The shafts are being stressed and will eventually break because of material fatigue. It’s what happens when you repeatedly bend a piece of metal back-and-forth. I’ve actually seen a broken motor shaft stay attached to the motor body via a split ring on the inside of the front motor housing bearing, but the shaft was no longer attached to the rotor.

The bottom line: flexible coupling

Bottom line is that you need a good quality flexible coupling to go between the load’s shaft and the motor’s shaft. This desirable flexible coupling needs to be able to accommodate the angular misalignment’s between the motor and load shafts and have very little torsional windup.

More next time.

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