True Motor Power

So the VEX website says that the stall torque on the vex motor is 1.67 Nm
The Vex Wiki says that a motor’s PTC will trip after 1.8 A for 7.1 seconds and the motor torque speed graph shows that at 1.8 amps the motor puts out .565 N
So what kind of torque can I expect out of the motor that can be sustained for a constant 105 seconds?

The PTC thermistor used in the motor is guaranteed to trip at 1.8 A or greater in 25 degrees C ambient air. It is guaranteed to never trip with 0.9 A in 25 degrees C ambient air. Somewhere in between those two numbers, likely on the lower end, will be a value that can be consistently held for 105 seconds.

Do you need that torque at stall, or that torque with the motor turning?

torque with motor turning

That is easier to deal with than at stall.

At 0.89 A, torque is listed at 2.22 in-lb or 0.25 N-m while turning 85 RPM with stock gearing. This is the level at which the motor will never trip. However, for 105 seconds continuous, you may be able to get away with 1.35 A with 3.7 in-lb or 0.41 N-m, turning at 75 RPM.

If you need more torque than that, you will likely need more motors, gear them down to trade RPM for torque, or both.

What mechanism do you need this for where the motor will run constantly?

@MetaphysicalEngineer A very active catapult

I get it now. Depending on intended launching power and reload speed, you may need several motors to pull that off reliably. However, depending on the speed of your intake (including driving to and lining up with game pieces), the catapult may not need to constantly run. Even a couple seconds of rest can allow the PTCs to cool, so only run the catapult like that if your intake can keep up.

I was planning on using more than one:P
the motors have to be able to hold the catapult in place to load the next stars

Oh, so they will have to deal with stall loads. That changes everything, since these motors are very weak at holding loads at stall. You may find they can reload the catapult over and over just fine, but then will trip out quickly when asked to hold it in the loaded position.

My team needed to find ways around that problem, as other design choices limited the number of motors available to each mechanism.

@MetaphysicalEngineer So, theoretically, if I did all the calculations for my arm and rubberbands, and I found the load to be less than 2.4 N with 6 motors, would I still stall quickly?

“Stall” is whenever the motors are supplying torque, but are not turning, such as when holding a catapult in the loaded position against the tension in the rubber bands. Stalled motors can consume large amounts of current and all power delivered to the motors turns into heat, which can cause them to trip out rather quickly.

I can’t say for sure if they would last the entire match or not if they spent a significant amount of time stalled during each catapult cycle. Once one motor trips, the load is spread to the others, forcing them to work harder and trip out sooner, causing a cascade of failure. You are free to try your idea to see if it will last. Report back if it works or not so people who read this thread in the future can learn something.

Ways around the motors tripping include gearing down the motors further, adjusting the catapult geometry to reduce the holding force when in the down position, or using some type of latch such that you do not need any motor power at all to hold the catapult in the down position.