Inertial Dampers
| Using inertial
dampers to reduce resonance Stepper motors are affected by resonance. The resonance can become so severe that the stepper will stall and lose position, and stalling can damage the stepper. Resonance is caused by parts vibrating in concert with each other. It can be lessened by altering a variety of factors including micro-stepping, acceleration rates, speed, looser or tighter mounting hardware, thicker or thinner stepper mounts, longer or shorter mounting bolts, and a myriad of other variables. Resonance sounds like a grinding or rattling noise; this is usually followed by a high pitched whine as the stepper stalls while still receiving step signals. .........................
A damper attached to the stepper's shaft can reduce resonance. It is essentially a wheel that turns with the stepper; however, it is not simply a flywheel. |
| The damper reduces resonance by remaining slightly out of sync with the stepper. The out of sync vibrations help to counter the stepper's harmonics, which otherwise would crescendo and cause the stepper to stall. |
 A simple damper made from a 2-1/2 inch caster works well; it performs better than one made from 1-3/4 inch caster. The caster has a 5/16 bore; an axle is made by drilling a 1/4 inch hole in the head end of a 5/16 inch bolt. This hole accepts the stepper's shaft. A hole is drilled and tapped for a set screw in the side of the head; the set screw is brass colored in the left image. Washers sandwich the caster, and the spring that is on the threaded end of the bolt. A lock-nut is used to adjust the spring's tension against the caster. The caster does not easily freewheel, but it will turn on the axle bolt. The caster slips so it cannot be used as a knob to turn the stepper. A longer bolt with a wing nut on the end works for hand turning the stepper. Results were the same as the skate wheel version shown below. |
| This unit is easy to make and
all of the parts are off the shelf from
Lowes. The difference in performance is impressive. The cheap leadscrews with the Xylotex 425s can approximately double their speeds when the damper is part of the system. The rapids for the Xylotex improved dramatically from an undamped 25 ipm to a damped 60 ipm. ......................
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Background and Experiments A variety of different shop built dampers were tried here since commercially made dampers are expensive and difficult to find.  A system shown on the right is a hub of wood with threaded rods that hold weights. The rods also act as set screws on the stepper's shaft. The outside nuts adjust the weights' distance from the hub, and the other nuts are over sized for the rods so they can slide and rattle on the rods. The two small through bolts keep the wood from splitting. This unit worked well and was adjustable; however, this propeller on the back of the stepper was a hazard, but it was a good proof of concept. ...........................
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The next option tried was
made
of one of the knobs shown on Hold Down
page. Recesses were drilled in the knob's back, and ball bearings were sealed in the holes with a plastic cover. A 1/4 inch aluminum hub was pressed out of a stray XL pulley and pressed into the knob. There seemed to be no difference whether the bearings were larger or smaller, BB shot was also used in varying quantity and the results were all similar. This unit also worked relatively well, but did not seem to help in a wide range of speeds. |
 It appeared that what was
needed
was a resilient and flexible damper that was not so stiff as to have
its own specific harmonics. Commercial dampers will use fluids
or elastic compounds to address this. A thrift store roller-blade wheel, left image, was tried but its stiff hub, even with slop built into the homemade design, would cause stalling on deceleration as the wheel snapped back on the stepper's shaft. Another roller-blade wheel that did not have the hard plastic hub, but was made entirely of softer plastic was tried. Image below right. It worked well. Two notches were cut in the bore to hold a stud that would prevent the wheel from freewheeling. |
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The axle is made of 1/4 inch ID and 5/16 inch OD copper pipe. Left image above. The bearings do not turn completely, they are simply used as flexible bushings to hold the axle.  A hole is drilled through the copper pipe and a cut nail
is pressed
through it. The pipe to nail fit is tight; otherwise the hole in
the copper will quickly stretch and the nail will rattle in the hole
and allow too much play.The nail fits in the notches that were cut into the plastic wheel. The fit should be relatively tight, a little play is okay, but too much allows the wheel to coast too far, and it will cause the stepper to stall on deceleration. The damper is held onto the stepper's shaft with a set screw that is tapped into a 3/8 inch nut. A matching hole is drilled in the copper pipe so the set screw tightens against the stepper's shaft rather than the pipe. ...............
The unit can be used to turn the stepper when the axis is disabled. Turning the damper to force-turn a powered stepper will tear the roller-blade wheel's plastic. This unit worked well but the wheel is not easily found. The caster version shown at the top of the page worked just as well and was made of home center components. |