Interesting speed reducer mechanism

[dtengineering]

Originally Posted by Qbranch

As with electrical PWM, though there is less power output (due to lower duty cycle) there is no loss, since instead of doing resistive (linear) control of the output, we're just engaging/disengaging from the power source in varying ratios... weather the power source be an input torque or an input voltage source.

-q

We may be describing the same thing, but in different ways. The analogy to PWM control is an excellent one. If a speed control is set up so that a motor draws 100 watts, then the load on the power source is 100 watts plus whatever the speed controller gives off in heat. If the duty cycle is increased so that the motor draws 200 watts, then the load on the power supply is just over 200 watts.

Electric watts (voltage x current) are the same as mechanical watts (rpm x torque), and power in must equal power out (plus heat loss due to inefficiencies).

So when driving our robot motors (we'll assume a constant voltage situation), when you use PWM to chop the average current to the motor you get a lower motor speed... but you also get a lower current draw on the battery. In this mechanism since you are lowering the speed of the output shaft, relative to the input shaft, it must also increase the torque on the output shaft relative to the input shaft in order to keep the power equations balanced.

Like I say... I think we're both kind of getting at the same thing from different ways, rather than debating a point here, and the PWM model is an excellent way of looking at this system for people familiar with speed controls.

Jason

[Alex.Norton]

I don't know, I think it is easier to simply say that If the tourque didn't increase as the speed decreased the box would start getting hot a lot faster than normal or would start vibrating a lot. Which ever way the power comes out if your talking 100 watts of lost power I would think that it would become pretty obvious.

From what I see I think the way that it effects to output speed is by changing the length of the arc that the output shaft moves through every time the lever engages the clutch. For higher output speeds it move the lever through a very large arc and for lower output speeds it moves the lever through a small arc.

Thats what I get from looking at the animation.

[Qbranch]

Originally Posted by Alex.Norton

I don't know, I think it is easier to simply say that If the tourque didn't increase as the speed decreased the box would start getting hot a lot faster than normal or would start vibrating a lot. Which ever way the power comes out if your talking 100 watts of lost power I would think that it would become pretty obvious.

When i was explaining this in PWM terms... i was referring to the fact that while the RMS power throughput of this transmission does decrease with the speed... however while a 'jerk' is taking place, the output shaft is connected to the input shaft, in a 1:1 configuration. The only power that is lost is due to friction, since the two shafts are either directly connected, or completely disconnected. This would cause the input shaft's load to drop near zero (constant voltage) while the two shafts are not 'jerking', and when 'jerking, the motor would take the full torque of the output.

It moves like a stepper motor... just varys the number of degrees of revolution of the input shaft are connected to the output shaft... kind of like periodically engaging/disengaging a clutch VERY RAPIDLY, making little tiny high frequency steps of the output shaft. It does not change the torque, it does not generate heat (besides friction).

</engineering soapbox>

-q

[Alex.Norton]

But the output shaft is directly connected to the input shaft constantly because the animation is only 1/4 of the actuall mechanism. The acutall mechanism inslude 4 sets of of levers so the output is constantly being provided with power.

The reason that the output tourque would increase is not because the length of the lever on the one way clutch is increasing but because you are pushing at it from a different angle. An angle that simulates a longer lever.

Also you can think of it in terms of the movement of the shafts. If there were just one cam then the output shaft would only move when the levers were moving in one direction and be stationary the rest of the time. The levers would be moving in the right direction while the input shaft was moving through 180 degrees of arc. No matter how big an arc the output moves through, the arc that the input shaft is pushing for is 180 degrees. During that time if the output moves through 90 degrees then the reduction is 1:2 and there is twice as much output torque as input torque (during the push). If you then change the arc so that for 180 degrees of imput you get 45 degrees of output I really don't care what happens when the output is stop there has to be a reduction of 1:4 and there has to be 4 times as much output torque during the push.

Yes when the output is stoped there is no output power or torque just like in a PWM. However, since there are four sets of levers the output shaft is never stoped since there is always one set of levers pushing on it.

[Madison]

As per the bullet points above the animation, the mechanism "delivers constant torque throughout the speed range." It doesn't increase torque as speed decreases because the output shaft's angular velocity is equal to the input when it is turning -- assuming they're of equal diameter. It's output is intermittent. Glancing at the animation suggests that arrangement rotates the output about 30* per control arm cam per input revolution. Four control arm cams, then, rotate the output 120* per input revolution; a reduction of 1:3. Adding more control arm cams or changing the geometry of the existing cams can vary this ratio. For the 2/3 of each input revolution that the output is not moving, the energy is being lost to friction (heat) within the one-way bearings.

Presumably, the attached lever has some role in adjusting the control arm cam geometry and varying the output speed as a result, but how it does so isn't clear from the animation.

[AdamHeard]

Definitely interesting...

Although, for robotics applications (not necessarily industrial) the downsides of it definitely outweigh the advantages. However, I can see the benefit of such a device in industry (After using a similar, much larger version for a fixture this summer).

It'd be nice if we had an explanation from an engineer who 100% knows how it works.

[Richard Wallace]

As Qbranch pointed out, this thing is a system of levers and clutches. By moving the fulcrum, the user adjusts the lever ratio and thereby swaps speed for torque. The manufacturer's "constant torque" spec refers to a safe operating area (SOA).

My boss, whose current title is VP Engineering, recalls specifying one of these units for a factory machine about 40 years ago, when he was a co-op. So it seems these have been around for quite a while. They must work well in some applications. But I don't think FRC robots would be one of them.

[Dick Linn]

I wouldn't be surprised if that thing (or a predecessor) was patented 100 or more years ago. Must be many steam-powered machines that have some such mechanism.

I can see this concept being useful to ratchet out a robot appendage or perform some similar task.

I did see a site with a laboratory stirring mechanism that uses this same reducer.

[Salik Syed]

Originally Posted by Richard

As Qbranch pointed out, this thing is a system of levers and clutches. By moving the fulcrum, the user adjusts the lever ratio and thereby swaps speed for torque. The manufacturer's "constant torque" spec refers to a safe operating area (SOA).

My boss, whose current title is VP Engineering, recalls specifying one of these units for a factory machine about 40 years ago, when he was a co-op. So it seems these have been around for quite a while. They must work well in some applications. But I don't think FRC robots would be one of them.

See that doesn't seem right. I don't think the lever distance has anything to do with swapping speed for torque but rather the frequency at which the shaft is engaged or disengaged. If this frequency is increased (by moving the lever farther back and forth) then we get higher RPM but the torque still remains the same.

Essentially all this really is is a fancy clutch mechanism.... it engages and disengages the output at different frequencies resulting in different speeds. However this does not change the torque characteristics of the system because during the time that the shaft IS engaged the motor is still in the same state had we just connected it directly to the output shaft.

I don't see the practical advantage over a traditional gear box, perhaps less motor load (because you allow the motor to free spin between cycles)
well... of course the fact that it is continuously variable

[Qbranch]

Originally Posted by Salik Syed

See that doesn't seem right. I don't think the lever distance has anything to do with swapping speed for torque but rather the frequency at which the shaft is engaged or disengaged.

Exactly what i've been trying to say!

-q