Free rotating coaxial drive question

[ekapalka]

I'm trying to make something similar to a coaxial swerve module, however the module needs to spin freely (as in there's no positional control). If I remember correctly, when the wheel of a coaxial swerve drive is slowed down or stopped, the motor driving the wheel tries to turn the whole module by the interaction of the bevel gears. Can anyone think of a way to allow the module to spin freely without putting the motor in the rotating module and using slip rings? Thanks!

[MoistRobot]

Originally Posted by ekapalka

Can anyone think of a way to allow the module to spin freely without putting the motor in the rotating module and using slip rings? Thanks!

¯\_(ツ)_/¯

[asid61]

Originally Posted by ekapalka

I'm trying to make something similar to a coaxial swerve module, however the module needs to spin freely (as in there's no positional control). If I remember correctly, when the wheel of a coaxial swerve drive is slowed down or stopped, the motor driving the wheel tries to turn the whole module by the interaction of the bevel gears. Can anyone think of a way to allow the module to spin freely without putting the motor in the rotating module and using slip rings? Thanks!

Wouldn't you simply be making a driven caster at that point?
The torque that the driving motor puts on the bevel gears is small if you do a 1st stage bevel gear reduction. Couple that with good bearings and friction and that might be enough.
You could also use 2 sets of counter-rotating bevels in the caster box (lower part of the module), although that would be harder to set up mechanically.

What kind of loads/what size motor are we looking at here? The slip ring solution isn't half bad if this is for something like a motorized suitcase.

[ekapalka]

Originally Posted by asid61

Wouldn't you simply be making a driven caster at that point?

That's exactly what it is

The torque that the driving motor puts on the bevel gears is small if you do a 1st stage bevel gear reduction. Couple that with good bearings and friction and that might be enough.

That's what I've been thinking I might resort to.

You could also use 2 sets of counter-rotating bevels in the caster box (lower part of the module), although that would be harder to set up mechanically.

I think I understand this but could you elaborate on how it would work? Like two side-by-side shafts going into the lower part of the module to drive the wheel? I've been thinking about that but haven't worked out how it would work

What kind of loads/what size motor are we looking at here? The slip ring solution isn't half bad if this is for something like a motorized suitcase.

Two of these would have to support the weight of a ~250lb machine. The reason I'd like to do it this way is so that I can power both wheels from one motor that's larger than reasonable to put in the rotating module. Thank you for your suggestions!

[GeeTwo]

Off the top of my head, I can think of a few ways to drive the casters without transmitting steering torque:

• Slip rings. Models designed for windmills should be adequate and reasonably inexpensive (discarded)
• Counter-rotation (already discussed)
• Battery AND Motor on the caster module (control wirelessly)
• Pneumatic rotary actuators. Run the air in through a swivel down the axis. This option may also save you weight on gearing, and is insensitive to stall/low speeds. I've never done anything like this, just an idea.

[asid61]

Originally Posted by ekapalka

I think I understand this but could you elaborate on how it would work? Like two side-by-side shafts going into the lower part of the module to drive the wheel? I've been thinking about that but haven't worked out how it would work

Not exactly. So if you look at the Team221 Revolution Pro, you'll notice that there is 1 vertical bevel gear that's coaxial with the rotation of the module, and 1 horizontal bevel gear.
Essentially, instead of using just 1 horizontal bevel gear, if you use 2 on opposite sides of the vertical bevel, the vertical bevel gear applies force in two opposite directions and won't cause the module to rotate. Granted, this gives you the issue of having the two horizontal bevel gears rotating in opposite directions so you'd have to cantilever them, but it would guarantee (in theory) that nothing moves when you don't want it to.

[GeeTwo]

Originally Posted by asid61

Not exactly. So if you look at the Team221 Revolution Pro, you'll notice that there is 1 vertical bevel gear that's coaxial with the rotation of the module, and 1 horizontal bevel gear.
Essentially, instead of using just 1 horizontal bevel gear, if you use 2 on opposite sides of the vertical bevel, the vertical bevel gear applies force in two opposite directions and won't cause the module to rotate. Granted, this gives you the issue of having the two horizontal bevel gears rotating in opposite directions so you'd have to cantilever them, but it would guarantee (in theory) that nothing moves when you don't want it to.

On this, I was thinking something a bit different. That is, there would be two concentric shafts, counterrotating at the same speed. The outer shaft would end above the wheel axle, with a bevel gear with downward-facing teeth. The inner shaft would end the same distance below the wheel axle, with the teeth facing up. The output bevel gear would engage both bevel gears on the shaft.
Actually, to make this a proper caster and to allow for two wheels, one on each side of the drive shaft, the gears would not be true bevels, but would allow an offset angle so that the wheel axle is behind that shaft.
If you have the capability to make or buy such gears, it might work.

[asid61]

Originally Posted by GeeTwo

On this, I was thinking something a bit different. That is, there would be two concentric shafts, counterrotating at the same speed. The outer shaft would end above the wheel axle, with a bevel gear with downward-facing teeth. The inner shaft would end the same distance below the wheel axle, with the teeth facing up. The output bevel gear would engage both bevel gears on the shaft.
Actually, to make this a proper caster and to allow for two wheels, one on each side of the drive shaft, the gears would not be true bevels, but would allow an offset angle so that the wheel axle is behind that shaft.
If you have the capability to make or buy such gears, it might work.

Interesting. You could use two hypoid bevel gears for that, at huge expense though.

[ekapalka]

Originally Posted by GeeTwo

On this, I was thinking something a bit different. That is, there would be two concentric shafts, counterrotating at the same speed. The outer shaft would end above the wheel axle, with a bevel gear with downward-facing teeth. The inner shaft would end the same distance below the wheel axle, with the teeth facing up. The output bevel gear would engage both bevel gears on the shaft.
Actually, to make this a proper caster and to allow for two wheels, one on each side of the drive shaft, the gears would not be true bevels, but would allow an offset angle so that the wheel axle is behind that shaft.
If you have the capability to make or buy such gears, it might work.

Oh man that doesn't sound easy to implement but it makes sense to me. Thank you!