Split Thread: New AndyMark am-4252 gearmotor for ventilator applications

Curiously for the first day of the fourth month, AndyMark added a new offering today - a 4 stage sport gearbox with 1 PG motor!

That’s a NeveRest motor. Less powerful than the PG motor, but incredibly tolerant of abuse in FTC applications.

The 3-stage 100:1 57 Sport is normally rated for 140 ft-lb; the 192:1 version with a NeveRest is putting out 30 ft-lb.

Put them together, and you’ve got something that will run happily for a long, long time.

Oh. That’s what I thought at first glance, but when I read the description, I assumed it was a 775-5 with an included encoder.

Added: It looks like these gear motors could be siblings. The PG-188’s key specs are:

Gearbox Reduction: 188:1
No Load Free Speed: 28 RPM
Stall Torque: 33 ft-lbf

This one’s are:

[Gearbox Reduction: 192:1]
No Load Free Speed: 30 RPM
Stall Torque: 30 ft-lbs

I’m not on the inside at AndyMark (who is right now? /s) but I imagine that’s the point. Especially since the 57 Sport also ships with the face-mount spacer installed, which would give two 10-32 holes in identical locations (one on each corner where the bolt head sticks out on the Sport).

I agree. I just never realized before that the PG and the NR had such similar peak power capabilities; I always thought there was a factor of 3 or so between them, but perhaps I had that understanding from the 9015. It just seems weird to me that with all the holes in the AM line*, they have had two motors for years with the same peak power and less than 15% difference in free speed vs stall torque.

* It’s definitely a challenge to find an oddball gear ratio in the AM infrastructure, while Vex simply produces every even-number-tooth gear over a wide span. Whenever I’ve been making choices, AM has always won with my OCD of steel over aluminum and I found something “close enough” in the AM lineup. If they’d’ve had every even size, it’d been a no-brainer.

Added 18 hours later after reading @Drew4564’s post below: I see where I got the factor of three impression. AM’s specs on the NeveRest are 6600 RPM free speed and 8.75 oz-in stall torque. RS775-5 are 5700 RPM and 35 oz-in. Using the approximation that peak power = free speed * stall torque / 4, these numbers indicate that the RS775-5 has 3.45x the peak power of the NeveRest. Unless the sport is massively more efficient than the old PG gearbox, something’s fishy here.
(Free speed in radians/sec and stall torque in N m, of course)

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It’s worth noting that the description says this:

This custom gearmotor is a high quality, fully assembled solution for the powertrain of breathing assistance devices. We developed this gearmotor in conjunction with BaneBots after we saw extremely high demand for our PG188 Gearmotor.

So it sounds like it’s mainly for people looking to prototype (or manufacture, assuming they know what they’re doing) ventilators following the MIT design, in the absence of the original PG188s. It might even be a product to keep around after this is all over, if there’s a market for FTC legal high reduction gearmotors.

This is correct, with the increased demand for the PG188 used in the MIT designed E-vent we sold out very quickly. More are on order from our supplier, but in the meantime we came up with a solution with components that are in stock in significant quantities to support the sudden demand for a drop in solution.

For high reductions on a NeveRest we would usually recommend the NeveRest Sport, but we needed a solution that was as similar as possible to the PG188 to avoid problems with designs that were already in production.


I’m working with a group at the University of Maine to spec out and build 10 of the MIT E-Vents and spent some time looking at the NeveRest as a possible option - they’re easy to work with, they fit the price range, and they have an encoder. However, the published power spec of 14W (and VEX’s testing putting it 25W) both suggest that it won’t output the 26W they calculated/36W they measured in testing, so we went with a different option.

Am I missing something and the the NeveRest is actually a suitable option for the ventilator system?

I assume you are referring to the conclusion on this page:

…we observed a peak current of under 3 A at 12V or 36 W.

To “observe” current I assume they are looking at the display on a benchtop power supply which is reading the INPUT power to the motor (or output of the supply).

Generally (unless a motor manufacturer is trying to be deceptive) motors are rated in OUTPUT power which is how much mechanical power they can deliver to a system. This number will always be lower than the input power because of efficiency.

Since a NeveRest at or near stall draws about 10A, at 12V by the same logic I could say it was a (10*12) =120W motor since it has the capability to demand 120W out of a power supply.

We obviously do not rate the motor “120W” because you can’t get 120W out of it into your device. It is rated by measuring the physical output torque and RPM and using this formula to calculate power.

When designing this gearbox as a stand in for the PG188 we cared about two things as they relate to output. Free Speed RPM at 12V (So that it would have the same operating speed (not squeeze too fast, etc.) and torque at stall. Stall is the easiest point to measure torque because it can be done with a simple force gauge (or fish scale if you are desperate) and a bar of known length (hence ft*lbs). Since torque is linear from free speed to stall, if both free speed and stall torque are similar it can be assumed that toque at any RPM is similar, meaning the gearmotors perform similarly.

Our tests showed that the free speed and stall torque of the PG188 and the NeveRest 192:1 were both within 10% of each other meaning that they will perform similarly if put in the same application.

Reading over the way they derived 26W they apply a very extreme efficiency factor of 50% in two different places thereby making the claim that the system is only 25% efficient. The power they calculate to “do the thing” is only 6.3W but due to the efficiency terms added in they conclude 26W.
To prove that these inefficiencies are not all truly there you can look at the VEX motor data (Click to expand below the graph) for the motor they used in that very test. If their power supply was showing 36W that means that the motor was only putting 14W of mechanical power into the system, meaning it was at WORST 45% efficient, not the 25% that the calculations assume.


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