Thread: Need help with smoothing movement - Grinder Creep

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Hey Guys!

I built this prop last year for halloween. It was mostly a success but you can see as the gear goes around it slows a lot on the upstroke and then speeds up on the down stroke. Is there anything I can do it make it more even?

It just a wiper motor turning the wheel and the crank is attached to the wheel. I am running it off of a ATX power supply on one of the 5v outputs.

Any suggestions are welcome!

- DC

First thing..........That is a way cool prop!!!!!! Very nice job on it, I would like to see a how to or at least the drawings for it !!

Now for the meat of your questio ....#1 Do you need it to be DC or can you run it on AC....either way what I would do is use a low rpm high torque motor with a sprocket or belt drive. The crank is going only in one direction ...correct? Do you have it set up as a cam driven mechanisim? If so I thinf that is where the problem is , on the downside of the rotation the motor is trying to lift more weight than it can .If you use a "regular" motor as suggested before there will be no real extra weight to lift. An alternative could be to eliminate the arm on the wiper motor and use a coupling to drive the gear directly, since the arm is what gives the oscillation.

How exactly is the wiper motor driving that sprocket? Would really like to see the 'guts' of that prop in action.

I can see as you have gone to the effort of posting this, that you feel there is an issue here. I am by no means an expert in human mechanics, though here is my 2 cents on the matter.

It's already spot on. A smooth operation would only remove from the authenticity. I have in the past used several hand cranked meat grinders and have never used one which free spinned. Granted perhaps this massive machine has a huge flywheel and achieves a free spin.

In a perfect world one could turn the crank at a constant speed. Due the gearing/mechanical differences in the gears and slack points one must exert more pressure on the up stroke. The down stroke is also smoother/easier to achieve as the arms/upper torso's weight is used to push down.


To put it shortly, you model exactly displays the inevitable forces that one must exert to move the device, physics meh! Don't change it.

I have a couple of mates ova atm and they agree, the stall adds not detracts.

If anything I believe it should be more staggered with more stall points. Think of the process..... meat (smooth), tendons (slow), bone (stall). What if we hit a solid skull, I would like to see a brief stall, reverse of polarity for a second then back to forward motion.

how are you controlling the speed of the wiper motor? With a PWM circuit or lower voltage, lower voltage will reduce torque.
I had a similar problem with my FCG, i used a counter weight to offset the slow down, that way the motor is always lifting the same amount of weight

I personally would leave it just like it is because it looks more real than if it turned more freely. It looks like the person doing the turning is tired from all the grinding....more natural.

To get what you asked for just remove the arm on the motor and connect the wheel to the bolt coming from the center of the motor. If that isn't long enough then use a female to female coupler that screws on the bolt and then you can extend it by screwing a longer bolt into the coupler. If you are already doing that then you will need to go up to the 12v

Just curious if you've tried running the motor in the other direction to see if that makes any difference. Or have you tried running it without the skeleton holding the crank. Just want to make sure the skeleton arms aren't creating the extra drag on the up rotation.

I think by just securing the base, would give it more realism. Its an awesome prop though! Kudos to you

In answer to some of the previous questions here are some details.

I am using a monster guts wiper motor. The arm has been taken off and I am using one of the threaded extension rods so that the motor spindle can fit through the plywood and through the particle board gear. The gear is secured with thread locks (those round threaded discs with 3-4 claws pointing down to grab into the wood).

Since people seem to like the movement, I think I will just give it a small tweak with a lead counterweight placed behind the gear opposite of the crank handle.

Thanks for all your input guys!

Is all of the weight of the gear and crank being supported by the shaft of the wiper motor ? If you, you are probably getting a good bit of side loading on it. In the long run it could have some negative effects on the motor and gearbox. If it becomes an issue, you could always support the weight of the gear with some kind mandrel and let that take the brunt of the weight. If it is not too heavy, a lazy susan style bearing might even work.