4-Beam Arm Questions

Our team has never used an arm before, so we were curious. How much weight could our robot lift if it had a 4-beam arm with the following specifications?:
4 1x2x35 steel C-Channels (with the vertical bars being smaller c-channels)
2 393 motors, with internal torque gearing, and a 1:7 gear ratio for torque

i used 2 for gateway but i had all aluminium and it still wouldnt hold the arm up with a 7:1 ratio

I’m just wondering how much weight it can lift.

Well since the motors have 13.5 in/lbs of torque, and you have 17.5 inch pieces of metal, you could lift 1.54 lbs with no gear ratio. with the 1:7, you could lift 10.8 lbs. This is excluding the metal, and i do this simply because it is easier to do the math without adding the metal in. Using rubber bands on your arm will also add to what you can lift, you could nearly double the amount if you did it right.

4886a was able to lift our metal/aluminum lift plus 7 game objects with no problem. We did this by placing our rubber bands and proving that the placement was better by using physics.

Adding on to this thread rather than making a new one, could anyone offer advice to prevent axle breaking/twisting? I have seen many examples of this at local tournaments, and since we will probably be making our first arm without a linear slide lift on it this year, I would like to have some idea of what to try before moving right into it.

For the axle question, there are a few things I would recommend. There is certainly more, but here are a few tips:

-always support the axle on both sides, with braced L or C channels.
-the thinner the arm tower, the more force the axle can take before bending or twisting
-as with the previous one, use the shortest axles possible
-check the direction the motors spin before attaching the arm; if one goes up and the other goes down, bad things happen
-use high strength gears, pinions, or double up on 84 tooth gears

In general
-use elastics. Be cautious while using elastics though so as to not raise your arm up in its lowest position. We’ve also snapped some axles by experimenting with the direction of the force of tension relative to the rotational radius (it was near 160 degrees).

Our robot could high-hang in Round-Up with a 4-bar and 2 393s with 2 269s geared 1:7, so you have a lot of force available with that gear ratio and a few high strength motors.

If you want to really keep the axle from twisting, do everything that RampanFang said, but also you can screw the 84-tooth gears to your arm, which will reduce the torque on the axle to 0.

Agreed, metal lock bars are not the way to go – they’re both worse for the axle, and a terrible terrible pain to remove.

u mean 7:1 not 1:7 for speed


aperture science


Richard P

Axle twisting is easy to avoid by proper design.

  • Only motor axles should be used to pass torque to the first gear/sprocket
  • The last (output) gear/sprocket should be bolted to the load,
    – Do not pass the torque to the axle, then use a lockbar to fasten load to axle.

Every axle supported by one bearing block on each side, since two points will fit on a line. When using 3 or more bearings on an axle to help keep it from flexing, it can be difficult to insure they stay co-linear.

Could you clarify what you mean by “Only motor axles should be used to pass torque to the first gear/sprocket” ?

He means that after your first powered gear (connected to the motor via an axle), axles should not be used to transfer any torque, rather just to hold the gears in place.

So you have motor, then axle, then first gear (12t), geared to second gear (84t), which is bolted to the arm.
As opposed to having motor, then axle, then first gear (12t), geared to second gear (84t), then axle, then the arm.
If you do the second way, then there is 7x more torque on the second axle than there is on the first axle in each case.

This explains a problem team 1581C was having this year. Their scissor lift used shafts to transfer torque into the lift. The shafts were attached to the bars of the lift using locking shaft bearings. They kept turning their shafts into screws that way, and no one could figure out why…

Thank you all for your input!