6 Bar Lift Breaking Gear Teeth

Hello, we are experiencing some issues with our 6 bar lifting mechanism. It has a 7:1 gear ratio geared for torque and powered by 6 motors. The lowest bar of the six bar is attached to the 84 tooth gear. The problem occurs when the lift reaches the top and can not go any higher. It appears that the motors continue running instead of stalling out when it reaches the top, and they break the teeth of the 84 tooth gear that can not move. Three of the motors on one side of the lift that power the six bar are hooked up to a power expander. The other three motors on the other side of the six bar that power the lift are hooked up into the cortex. Four drive motors are hooked up to the cortex.
We are also experiencing issues when the robot hangs. When the robot reaches the top of the pole and the six bar is at the lowest point, we have also broken the teeth of the 84 tooth gear. Please give us suggestions to help fix our problems. Please look at the pictures below. Thanks in advance.

Add a touch sensor at the top of your lift.

The lift has to hold stars and cubes when it gets to the top, so could the lift still carry the cubes and stars if a touch sensor is added?

yes, the touch sensor would just stop the motors from continuing to rotate.

Would the lift fall down due to the weight of the game objects?

yes, unless you use PID…our coach taught us how it works…I don’t really understand it though…

Can you post your code? Assuming it’s an if, else if, else statement, it’s an easy fix. But it is possible you’re doing something more complicated

My code or @Hydraulic_Pancake 's code?

I don’t have anything in my code that stops the lift at the top.

I’ll see if we still have our program, I’ll post it if we do.

We deleted that code with PID :confused:

Here is a link to a tutorial of how to use a touch sensor for RobotC.

Idk if this is a code for PID, but we’d make a little thing as simple as this to apply more power the farther away the bar is from a certain distance. We used the ability of the robot reading from top to bottom, so the robot would basically set the motor value to 0(Not truly, since there’s no wait and it’s being interrupted of the next code) then use the set motor value of 0 then add or subtract it by the potentiometer value:



How to change the angle:

motor[CMotor1]=motor[CMotor1]-((SensorValue[Potentiometer1]+100)/2); //The +100 increases the angle (Not in degrees) of the lift/catapult

I think that’s just P control then, which could be all you really need.

Thanks for the responses and the code. I wonder if the problem is that I have too much current going to the motors due to the power expander. However, the robot will not hang when the power expander is removed.

I doubt it, but anything is possible. I think that probably the reason the robot won’t hang is because of the use of the CORTEX not getting enough power because the majority of the power being used for the lift. I don’t think it’s the motors getting too much power. The expander basically grabs the signal from the CORTEX, and powers the motors by its own battery and not from the CORTEX.

I’ve seen it before. You need to spread out the forces a bit I think.

What is the weight of the robot?
How long is the arm?
Do you have rubber bands assisting the lift?

Do some basic calculations to see how much torque is being requested of those poor little motors and specifically from the 12 tooth to 84 tooth gear.

I would like some clarification. Is it just one 84 tooth gear on the 12 tooth gear? Or does the first picture of three gears accurately reflect the lifter’s design? I am taking the second picture as the complete lift part.

I think you may have to double up the width of the 12-84 tooth contact surface. The forces you will have calculated above are all being placed on the leading edge of that plastic gear. 6 motors would be about 42 inch-lb of torque applied there which is probably too much for the plastic. And then it will give. Spreading that force along a larger surface will reduce the forces per square inch (as small as that contact surface is). Grease will help somewhat but you are getting into the limits of the material properties of the plastic gear. It still has to push the gear, grease helps with the friction part, but if the force pushing on the gear tooth is too great for the material, you are still up a creek.

Weight: 15.8 lbs
Arm Length: 20 in
Rubber Bands: None

The second picture is the complete lift gearbox.

OK, you are on the hairy edge here for a 15.8 lb robot and what stresses you should subject a 393 gear to.

Not sure of the Delrin properties to get the real bending point of the material but from the pictures I think you definitely passed it. I would suggest adding some rubber bands if possible too as that reduces the force required to run through your gears. Making your lift be control more than having to be the lift will help a ton in the speed and reliability of your lift.

So the width of the gear is about the only thing you can play with to reduce the stress on the teeth. Or making a step in your design to not go from 12 tooth gear right to the 84 tooth model. But you still have the forces transmitted from a 36 tooth gear in the middle as well as the extra friction.

See slide 13 on page 3 of this first link to describe the Lewis equation. That shows when your teeth bend. Since your choice of materials is fixed, the choice of gears is fixed, the only thing you can change is the width of the contact surface. The width of the contact area is directly proportional to the force being applied. So double up your gears to reduce the stress. Try some lithium grease too to assist with other elements of friction force reduction.

Another nice handy lecture from a course at Notre Dame. Look at page 43 for some helpful tips.

Tip #1: “Reduction in the load, such as by increasing contact ratio, or altering other aspects of the system” I am suggesting tip #9, “use a wider effective face width”.

One other item of note, see how the breakage on the gear was not the full width? It appears the 12 tooth gear is hanging off the edge a bit and that decreases the contact area. Make sure they align/overlap as much as you can.

Thanks for the thorough response. I appreciate all the time you put into it. I’m trying to keep the space between the two c-channels as small as possible. Do you think combining one high-strength 84 tooth gear with a standard 84 tooth gear would work?