Accounting for friction in calculations?

FIRST General Forum
1

How does one account for possible friction loss when designing a mechanism?

I am building an acquisition unit using the small dead axles design similar to this: https://www.chiefdelphi.com/t/how-do-these-254-mechanisms-work-bearing-retention/409093/14 .

I have 4 3D printed pulleys, 8 bearings (two in each pulley). 2 timing belts, one a 2:1 reduction the other a 1:1 ratio, so more than half pulley wrap on the one pulley and half pulley wrap on the 2 1:1 pulleys. Belts are properly tensioned. I also have 2 2" polycarbonate tubes 10 inches long to complete the acquisition.

No obvious friction such as pulleys scraping sides, bearings not centered, etc. The motor (non-FRC approved motor) is calculated to do the acquisition work assuming it isn’t using all of its power just to turn the rollers.

I am pulling 1.5 amps just spinning the motor and the one pulley connected by the 2:1 timing belt reduction. Adding one polycarbonate tube makes it jump to 2.5 amps. Everything connected together but not grabbing the game piece pulls 5 amps.

All of this means I am on the wrong side of the motor curve and not achieving the speed I want the acquisition to run.

I am not a mechanical engineer. I can put a bigger motor on it, but I am trying to understand why all the energy loss in the mechanism. Any thoughts on what I did wrong?

2

That doesn’t sound like an unreasonable jump in current draw. I don’t know what your motor is, but if 2.5A or 5A is not a lot and your motor is almost certainly extremely underspecced for what you are trying to do and will not do what you want.

Viscous and dry friction. Usually, its assumed to be negligible, and offset with a stronger motor, as friction is hard to know exactly before building the mechanism unless you are intentionally adding it because friction is sensitive to what particular bearing (not model, but individual physical bearing) you are using and tolerances.

3

Specifically, what are the specs of the motor you are using?

For example:

Most FRC-scale motors draw at least 1-2 amps even with zero load on them.

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