Thread: Where can I get a new crank arm?

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Hi everybody!

I have finished the basic mechanism for my rocking granny, and it's awesome. I love it! The only problem is that the rocking motion that she does isn't quite as pronounced as I'd like, particularly because the rocking chair I found seems to be kinda heavy. It still looks decent, but I think if I had a longer crank arm, she'd have better motion and the chair would rock better.

The problem is I have looked at the only stores I know of here that could carry such a thing, and the people that I asked in these stores looked at me as if I'd grown a second head. :P

Does anybody know of any online stores where I can get a crank arm that would fit a windshield motor? I got the motor from Monsterguts, but they don't seem to offer a longer crank arm part, unfortunately.

If I could make it, I would, but since it's gotta have those grooves that fit the motor so it spins right, I don't think I can.

Any help would be appreciated!

Thanks!

I've never used a MG motor but I've used dozens of wiper motors to make props move, and I don't think I ever used the original crank arm that came with them. If you make the hole in the crank arm the same size as the true diameter of the motor shaft (the solid part inside the splines), the crank metal will form around the splines as you drive the arm on with a nut. This is especially true of malleable metals such as aluminum. Put some threadlock on the shaft threads and wrench that nut on tight and it should work. I've moved some pretty heavy props with crank arms made this way and they've never slipped (knock on wood).

Try it! I think you'll be OK.

I've made lots of aluminum crank arms for MG motors. I use 1/8" X 1/2" flat stock. Drill a 1/4" hole where you want the motor shaft to go, and break the edge of the drilled hole on one side of the arm. You can do this with a small file or a larger drill bit. Drill an appropriately-sized hole (for the bolt attachment to the chair) where needed. Place the arm on the motor shaft with the filed side toward the motor and tighten the nut. The splines on the shaft will bite into the arm. Be sure to use the lock washer to keep it in place.

Before you change the arm can you post a picture of where it's mounted to on the prop? That most likely could be the problem and not the arm's length.

One other thing could be causing the rocker not to move like you would like. The speed of the motor. Since your chair is very heavy the slower speed could be keeping the prop from getting the momentum to move the rocker at a faster pace.

Either one could be the cause of the slow moving rocker and not the length of the arm on the crank. A short video and pictures would really help us find the problem.

Mordessa,

I had the same problem with my chair. The easiest fix that I found - I zip tied a weight to the top of the wood (2x6). You can adjust the rocking motion by the amount of weight added.

Hope that helps!

I did the same thing daspizz23 did.
Gave the rocking motion a bit more momentum. You might try that first.
Or move the point where the crank attaches to the board a little bit higher.

worst case scenario, if you have to get a new crank arm, I believe that Monster Guts is offering them now for wiper motors.

Originally Posted by Otaku

I've made lots of aluminum crank arms for MG motors. I use 1/8" X 1/2" flat stock. Drill a 1/4" hole where you want the motor shaft to go, and break the edge of the drilled hole on one side of the arm. You can do this with a small file or a larger drill bit. Drill an appropriately-sized hole (for the bolt attachment to the chair) where needed. Place the arm on the motor shaft with the filed side toward the motor and tighten the nut. The splines on the shaft will bite into the arm. Be sure to use the lock washer to keep it in place.

I've had great success with this technique.

Hi!

I want to thank you guys for all your help with this! I'm embedding two vids here.

The first one is what she looks like in motion, so you can see what I mean about the rocking not being great.

http://www.youtube.com/watch?v=XfPBsxVab4U

This second one is what the mechanism looks like while in motion. It's kind of loud, but we're hoping that we can drown out the sound with sound effects and sound dampening clothing on her.

http://www.youtube.com/watch?v=boBrtCWdumA

Do you still think, after seeing the videos that adding weight might help? I haven't tried that yet.

I do have the motor set to the faster speed, I was considering trying to set it to the slower one to see if that helps at all, but I wanted advice first as the wires are kinda hard to reach at this point. Hehe

Using an aluminum crank arm does sound ideal, from what you guys are describing. I assume that would be pretty easy to bend into the proper shape, too?

But again, I can't find ANYTHING like that here in Sweden. The only stores that I can find that may carry something even remotely close to that sort of product do not have any alternatives. No aluminum flat stock, nothing. That's why I'm wondering if you guys might know of an online store where I can get what I need.

I've checked with Monsterguts, but I don't see any mention of crank arms, except for an extender which doesn't seem to be at all what I'm looking for.

Thanks again for helping me out with this!

The length of the crank arm isn't the problem it's the piston arm (that's the arm connected to the wood/back of your prop) that's keeping the prop from having more bend. Let me explain, and I will do it with pictures because I think I can make more sense that way.

If you connect the arm lower on the back you will get more forward movement



Look at the bending that I get in this prop by connecting the piston arm lower







But before you do anything I would just add more weight because from the video it looks like the force of the prop going forward isn't enough tom keep the rocker from going backwards. So you having the two forces pulling against each other (prop moves forward and the rocker goes back) instead of them going in the same direction at the same time. So the easiest thing to do is add weight like DaveintheGrave & daspizz23 recommended.