Burning the FP's

Here’s my story for the past two days. Pretty much I have spent all my waking hours (outside of class) trying to get our arm design to work. We have a 5 foot upright (made of 2" square aluminum tubing) with our swinging arm made from two pieces of 80/20 extruded aluminum (one piece slides to increase length with the help of a piston), and then our whole gripper/grabber assembly on the end of that. The whole swinging arm weighs around 4 pounds. To power this we are using a single Fisher Price motor with the stock gearbox. We modified it a little so that our output is now a 1/2" keyed shaft.

At first, we tried using some 1/4" timing belt with pulleys we ordered from McMaster Carr, but there wasn’t enough engagement and the belts would slip. So we quickly rigged up some chains and sprockets (with the same reduction) to see if we’d have enough torque. The arm moved with ease, but the setup was a bit messy (sprockets cockeyed, chains too long/short etc.) and the Victor would trip if we put more than about 20% power to it.


So, we cleaned up the setup a bit, and it looks pretty nice now, and still moves the arm with ease (extended and retracted). I will try and get some pictures, unfortunately the only reference I can provide is this little pic I painted for your viewing pleasure. Slapping that tetra on there though puts about another negative 40 or so foot-pounds for us to work against and trips the Victors and releases a little magic smoke.

So we concluded we are going to have either make our own transmission, use a different motor, or both. Right now, we are thinking about perhaps putting the globe motor on the FP transmission, or maybe using two FP’s in tandem. We would like two arms though so our goal is to save the other FP if it works; we’d definitely rather have one versatile arm over two of what we have now though.

Anyway, anyone have any idea why we are stalling it and tripping the Victors? The way we calculated we should have more than enough torque but oh well :stuck_out_tongue: It’s about a $50 (so far lol) setback that we don’t really want to make bigger.

Seems like it should work with a 4:1 reduction. But a little efficiency loss will go a long way in this arrangement. I’ll bet your 1/2" shaft mod is dragging. Hopefully your bridging the shaft with bearings and not catelevering it out. Those FP’s do not like stall load at all. If you saw smoke once from them, they’re probably toast and need replacing. Adding the globe should solve your problem. Probably power your first 2:1 reduction point. But I’d first look for something rubbing/binding etc. and bridge your shafts.

An FP with the stock gearbox has an optimal efficiency of about 100 rpm. With your two 2:1 reductions your getting about 25 rpm at your arm, or about 150 degrees a second.

Does that correspond with the actually performance you were getting? If so, at that speed, the FP would be stalling with very little actual load. It might be fine with the weight of your arm, but stick a tetra a few feet out, and say hello to the magic smoke.

I would suggest that you try just switching up the sprockets on your arm first, before any major rebuilds. Try two 4:1 reductions, instead of your current two 2:1 reductions. That will give you a little more manageable 6.25 rpm at your arm, or about 37.5 degrees per second and 4 times as much torque.

I agree that 4:1 should be enough (I did calculations for our FP powered arm before) though it would make sense to go down to about 8:1 You need to check your system for any major losses as swampdude said.

I am guessing that the FP motor and tranny is for turning the top arm, is that correct? If so, I am also guessing that you have not accounted for the friction at the turning point. With an 8 lb tetra at the end, the turning friction must be enormous. You may need to put a heavy duty bearing at that point to reduce the friction. I would be careful as you troubleshoot as some teams have reported stressing the FP gearbox to the breaking point. You might also consider some counter weight of some type to balance the load at the top of the arm. Springs or surgical tubing works well in these apps. Pictures would help a great deal.

Yeah I will definitely get some pictures tomorrow to post. We have bearings on all those joints; I know the picture I provided is not very helpful. It doesn’t seem like its binding anywhere, the system makes for a lot of losses though. The top arm is the one we are attempting to swing, yes. I apologize for the poor grammar and overall laziness :stuck_out_tongue:

EDIT: Oh I forgot to mention we aren’t cantilevering it out there, we have bearings and pillow blocks, I will really try to get some pics up here asap. I also forgot to mention the gears did grind a bit at first because the box was not held tightly enough together (if that makes sense, sorry I’m tired and can’t think too clearly right now :)) so we used a couple of wood screws to hold it together. We know we probably fried the motor, but were hoping our buds over at 988 or 989 will help us out if we decide to go with the FP’s :smiley:

Our team was thinking using FP motor too, well two FP motor, with 6:1/7:1, maybe even 8:1.

Don’t you think 4:1 is a bit fast?

120/4=30rpm…(i think 0.3-0.2 rps is good…)…or am i too slow?

It may or may not be too late in the season to try it, but I think this would be a good application for the Nothing But Dewalts FP transmission. You get a 47:1 reduction in the tranny, which gives you a peak power RPM of around 255. Plus you should get a lot of torque out of it. Most importantly of all, you can leave in the anti-backdrive pins, so your motor doesn’t have to melt trying to hold your arm in place.

Team 57 is going with this setup for a telescoping tower for all these reasons, though we’re having troubles getting the parts. The clamshells are on backorder, so we’ve ended up buying a drill to cannabalize the shell from. then when our backordered shell comes in, we’ll have a shiny new drill for regionals.

I wish I had read that earlier. I am beginning to worry we will burn ours trying to hold the arm up. Maybe I should put some fans on it… Probly wont help but whatever.

FPs we’re a big pain is the ASP last year for my team. This year we’re using globes alone at a 10:1 reduction (I think - that’s NOT my department). :wink:

gear that devil down… even if it worked with the origional 4:1 reduction, that arm is flying through the air, and when the arm+robot+tetra all runs into a brick wall, thats a lot of intertia waaaaay up in the air… we learned the hard way last year what inertia does to an arm, and what happens when intertia takes over…not a pretty sight… the tip of our arm, fully extended, will move no faster than 16in/sec…
good luck

Figure out how to use a spring/surgical tubing/weight to counter the force of gravity on your arm. The arm should ideally be neutrally balanced so the motor has to pull it down into the starting position. That will be your best shot at not burning up the FP motor.

Yep, you gotta gear that F-P motor down quite a bit to make it useful. We are gearing one of ours down by 894:1, which is 7x the reduction of the F-P gearbox alone.

Andy B.

Yeah, right now we are gonna use a two stage sprocket set up to gear it down. First will be about 2:1 second will be 4.5 to one. In addition to the FP gearbox. Thats… somewhere in the vicinity of 1100:1. Then multiply nine times 40 foot pounds (the supposed torque of the FP motor) and our arm should theoretically have about as much torque as some cars.

We tried two stock FP Gearboxes with an additional 6:1 reduction (180*6= 1080 total reduction) and it powered our 7 lb, ~4.5 ft arm but drew 30 Amps each when 13 lbs were loaded at the end of the arm. See attached pics #297 & 303.

Not a good scenario for those FP’s - design it so they don’t have to draw more than 15-20 Amps at full load, otherwise they just might not last. We made a new transmission that has 260:1 reduction (see attached pic #311) and then the additional 6:1 for a total of over 1500:1 reduction. Hopefully this will keep the motors in tip top shape and we still get a couple ft/sec at the end of the arm.