RS-395 safe loading

[asid61]

Originally Posted by orangemoore

So your savings overall would be 1lb?
It really doesn't sound like it would be worth it.

Well it's basically a free pound, so why not?
I think I'll design for both for now, and try them out as needed.

[Jared Russell]

Steering motors can be loaded in lots of unexpected ways. Trying to turn when your wheel is up against an obstacle or imperfection in the floor...when dynamics shift most of your normal force onto one or two wheels...on slopes...during intermittent bouncing contact with the floor...

Also, the 300-series motors have not been used nearly as often in FRC as the 500- and 700- series motors. There is less (not none, but less) collective community experience in knowing their limits and endurance under FRC loads. Spec sheets seldom tell the whole story (especially when you are talking about a fairly generic trade sized motor from China).

Given the severity of losing a steering motor, my personal inclination would be to accept the 1 lb penalty and look for opportunities to save weight elsewhere.

EDIT: Also, out of curiosity, what method did you use to calculate the load on each motor in a steering application?

[asid61]

Originally Posted by Jared Russell

Steering motors can be loaded in lots of unexpected ways. Trying to turn when your wheel is up against an obstacle or imperfection in the floor...when dynamics shift most of your normal force onto one or two wheels...on slopes...during intermittent bouncing contact with the floor...

Also, the 300-series motors have not been used nearly as often in FRC as the 500- and 700- series motors. There is less (not none, but less) collective community experience in knowing their limits and endurance under FRC loads. Spec sheets seldom tell the whole story (especially when you are talking about a fairly generic trade sized motor from China).

Given the severity of losing a steering motor, my personal inclination would be to accept the 1 lb penalty and look for opportunities to save weight elsewhere.

EDIT: Also, out of curiosity, what method did you use to calculate the load on each motor in a steering application?

I used the JVN calc "rotary mechanism" tab. To calculate load I used
wheel width * 1/4 bot weight * CoF. In my case that was 0.75*37.5*1.3, or about 36lbs. Then I added a ~1.5x factor of safety and decided on 50lbs as a good estimate for loading.

Based on what I'm seeing here I guess I'll just suck it up and use a 500. Kind of unfortunate, but I'll take what I can get.

EDIT: Whoops, that calculation is wrong. The load on the wheel is equal to 150lbs/4 wheels * 1.3 CoF, or 48lbs. The lever arm is 1/2 the wheel width, or 3/8".

[MichaelBick]

Originally Posted by asid61

I used the JVN calc "rotary mechanism" tab. To calculate load I used
wheel width * 1/4 bot weight * CoF. In my case that was 0.75*37.5*1.3, or about 36lbs. Then I added a ~1.5x factor of safety and decided on 50lbs as a good estimate for loading.

Based on what I'm seeing here I guess I'll just suck it up and use a 500. Kind of unfortunate, but I'll take what I can get.

1/4 bot weight per wheel is a best case scenario. Expect to routinely get pushed onto one or two wheels during pushing matches and having more weight on certain wheels when accelerating. 1/2 or 3/4 * bot weight seems like a far safer number.

Drivetrain is a terrible place to try to skimp out on. I want my drive to be as reliable as possible. I'd be especially concerned with the intensity of pushing matches and the small breaks between matches during eliminations, where reliability is the aspect that differentiates between teams the most.

In my mind, reliability trumps weight in this situation given that 540s are already pushing limits because they aren't very well suited to the needs of your steering motor.

[Gdeaver]

Yes, 1640 used RS395 motors 1 year. Decision was based on motor availability. The 395's performed OK that year but we did burn up a few. That summer active cooling for them was explored. Heat was a problem. Since then RS540's have been used with great success. We also added a quick change feature to the Banesbot trans and motor mount. the motor, pinion and base plate can be removed and replaced without total transmission removal. 2014 was the most abusive year for our swerve. The 540's held up but were routinely hot after matches. The 300's would not have worked in that game. Do not under estimate the amount of power needed for future games. RS540's are probably the minimum. 2015 put absolutely no stress on our swerves. 300 would have worked.