2084 Mecanums with Worm Drive

[Ether]

Originally Posted by Tristan Lall

mecanums being no better than 70% efficient in the forward direction (inherently due to the rollers)

If you are using the word "efficiency" in the usual sense of power_out/power_in, the only loss in efficiency of mecanum wheels compared to standard wheels in the forward direction would be due to roller motion caused by roller axial free play or compliance of the flooring surface or the roller material or flexing of the vehicle. I don't think this amounts to a 30% loss, but I don't have any test data to prove it. In the ideal case (no roller axial free play, no roller bearing friction, no squeezing/stretching of the carpet or vehicle), mecanums have the same efficiency in the forward direction as standard wheels.

[Tristan Lall]

Originally Posted by Ether

If you are using the word "efficiency" in the usual sense of power_out/power_in, the only loss in efficiency of mecanum wheels compared to standard wheels in the forward direction would be due to roller motion caused by roller axial free play or compliance of the flooring surface or the roller material or flexing of the vehicle.

I should probably have been more precise: that's not an efficiency loss in the wheel, but only about 70.7% of the torque available at the wheel can be used for forward motion, because of the sideways component of the wheel's friction force. (I suppose this assumes equal frictional coefficients in all directions.)

Edit: The reference to "forward motion" only applies in the limiting case where the wheel is about to slip, and even then, the relationship above is not quite right. (See below.)

[Kevin Sevcik]

Originally Posted by Tristan Lall

I should probably have been more precise: that's not an efficiency loss in the wheel, but only about 70.7% of the torque available at the wheel can be used for forward motion, because of the sideways component of the wheel's friction force. (I suppose this assumes equal frictional coefficients in all directions.)

That's a friction dependent limit, though. If your wheels aren't slipping, then you're getting 100% of the gearbox's output torque applied to forward motion. Thus, your "efficiency"/pushing force calculation above is bunk. The drivetrain's forward pushing ability is limited to EITHER 80% of the theoretical output torque of the gearbox or 70.7% of the available friction force, as calculated from mu and the normal force. The two limits are unrelated, so multiplying those two numbers together is meaningless.

[Tristan Lall]

Originally Posted by Kevin Sevcik

That's a friction dependent limit, though. If your wheels aren't slipping, then you're getting 100% of the gearbox's output torque applied to forward motion. Thus, your "efficiency"/pushing force calculation above is bunk. The drivetrain's forward pushing ability is limited to EITHER 80% of the theoretical output torque of the gearbox or 70.7% of the available friction force, as calculated from mu and the normal force. The two limits are unrelated, so multiplying those two numbers together is meaningless.

I will retract that expression—you're right, that's not valid for the case where the wheels are not slipping, which is most of driving around.

I was mistakenly conflating it with the limiting torque for mecanum wheel slip in forward motion, at which point you've got all of the available torque at the wheel spinning it, but not all of the frictional force in the longitudinal direction that you'd expect with a conventional wheel. (This is relevant when you're figuring out if your motors will stall before your wheels slip.) And even in that case, as Ether and Kevin noted, it's better expressed as a mechanical efficiency and a geometry constraint. They end up multiplied (or more accurately, divided) in the equation, but they're not part of the same quantities.