Help, our robot keeps breaking.

Today the middle axle on the left arm bent for the third time. The first time I didn’t see it happen. The second, I thought it was because when we replace the bent one, one arm was slightly raised relative to the other one due to a bump sensor that had been added and so the arms were out of alignment. This time, I was very careful to make sure the arms were in the same position when I re-seated the axles.
It works fine up to three sacks… then when the arm and spatula are fully extended and there are four sacks on it, the gears on the left pop and the axle bends.
One thing I noticed was that there seems to be an awful lot of play/slop on both sides, the first gear (off the motors) seems to turn one-third to one-half rotation before the arm starts to move, and the side that popped is worse.
(BTW, in the pictures, the metal pinion on the motor axle isn’t in position, I slid it over to the left; it engages the large gear on the middle shaft, but I couldn’t get it back in for the photo due the middle axle being bent so much.)
Any ideas?
arm closeup 2.jpg
arm tower.jpg

Are there the metal or plastic inserts on both sides of the gears?
and a good way to help avoid axles bending is to keep the distance between the bearing surfaces as short as possible, so it looks like you can put the right bearing on the inside of the towers and that would help stop it bending.

Last year, we also had the problem of our axles breaking…it was because we didn’t screw the gears to the metal, so you shuld try doing that. If you ever hear clicking while operating the arm, that’s the sound of the axles bending, so if the problem persists, stop whenever you hear the noise

It looks like the tower has no significant structural support holding the two sides together (I only see one beam attaching them, perhaps there is more that is not visible). This means that the axles themselves are acting as support. The tower needs to be structurally solid before you add any axles or this will happen. You may also wish to consider changing the order of the compound gearing, if there is room, to reduce the torque on that intermediate axle.

Hi Figures

Compliments on the robot. Your team have done a tidy job on the build.

Our team has seen the same phenomenon and discussed what you could possibly do. You are on the right track with your comment about the amount of play in the gears observed before the arm lifts being a problem. To prevent the axle from failing, you need to a) provide more support and b) reduce the load on it.

a) Provide more support to the axle

  • As Mark said, move all your tower bearings to the inside of the tower C channels (to shorten the unsupported axle length). You have used screws to hold the bearings which is good - periodically check they are tight as they will work loose over time.
  • As jpearman said, fit a pair (or more) of standoffs (one front and one back) between the tower C channels as high as possible up the tower (so they are just clear of the rotating gears and arm).

b) Reduce the load on the axle

  • If possible, shorten your arm length. Does it have to be the length it is to reach the height you want?
  • If possible, reduce the weight of the arm and spatula, especially the end of the arm/spatula. Remove any structure that is not vital and/or replace steel with aluminium or plastic.
  • Use rubber bands to assist the lifting of the arm - this will help a lot. Re-design if required to make this possible, because this is important. Your motors will appreciate this also.
  • Consider using a slower arm gear ratio on the tower.

If none of this help, then stick to lifting 3 sacks at a time :slight_smile:

Good luck, Paul

Thanks, everyone.

Today I will move the bearing blocks, reinforce the tower (there are only four standoffs holding them together, but there used to be a 5 hole c-channel running across connecting all four towers and I took it off to get the hack saw in there and it seemed sturdy enough without it when I was done cutting out the old bent axle…), add rubber bands, check the gear inserts, and see if we can lighten the arm.

I don’t think there is room for changing the order of gearing and our gears are bolted to the metal arm up top (that axle hasn’t bent yet, nor the bottom one).

Thanks again for your time; I’ll let you know how it works.

I can’t tell for sure but it looks like it might be possible for the arm to hit that 60 tooth gear on the bent axle. If this is the case it would definitely put more force on the axle than it is designed to hold. Once the axle had bent a little it might be much more prone to the gears skipping that you described.

If you haven’t already got one, add a physical limit to prevent this from occurring - just something down the end of the arm that hits the chassis and stops it going down far enough to hit the gear.

I made the changes suggested and it is working good now.

In addition to the great suggestions already given…

  1. Add in the white spacers on the shafts to keep the 12 tooth in place with the 60 tooth gear. That 12 tooth looks like it can slide about.
  2. Try shoulder screws on the bearing blocks of that axle attached to the motors. You can get some play with regular screws under really heavy load. It’ll fit if you bang them in.
  3. Does the plastic spatula need to be that deep? Looks pretty long when it’s sticking out. The longer it is the more torque will be exerted on the lift mechanism. We’ve seen scrunching of sacks even with spatula designs and are wondering about the long spatula design.
  4. PTFE/nylon washers where slidy-things meet non-slidy things. Once the white washers are added, your shaft collars may well be against a nylon washer. I see them on the lowest shaft but not the middle one (but it was not rubbing before since it never met another surface).

For the c-channel, did you go all the way across both towers or just on each other independently? Going all the way across will make left and right towers not flex independently as much. With such a big spatula you also have to be concerned about torque twisting along the axis coing at you in the back view pictures here. Put some sacks on the far left or right corner of the spatula to see what I mean. Sure looks like a good candidate for bending a shaft to me.

  1. Reduce the space inbetween your tower posts by moving your inner tower posts in one hole space. You have a lot of extra space that is allowing more of your arm shafts to go unsupported. Or as others have said, move your bearing blocks inside your arm towers. You want to make the lengths of shaft that go unsupported as small as possible in your arm tower because it’s going to be taking a lot of stress when you go to raise an arm full of sacks.

  2. Consider changing your gear ratio to make the spacing nice for your arm bars. I would switch the first 60T gear to a 36T gear, then swap the current 36T gear on that same shaft for a 12T gear. You’ll get an increase in torque & will be able to lift additional sacks. Add elastic to help lift your arm up if you haven’t already, this is essential.