Torque spec does not match gear ratio

Technical Technical Discussion
#1

Hello,
We are trying to build a climber using only off the shelf parts and have run into an issue regarding the torque generated by a motor on a PG71 gearbox.
The motor specification (am-0912) reads a stall torque of 60.64 oz-in and a no load RPM of 16000. The assembly of the 9015 and PG71 (am-2084) has a specified stall torque of 6.25 ft-lbf and a No load free speed of 216rpm.

The problem is that these numbers seem to have different ratios.

16000/216 = 74 which is close to the 71:1 gear ratio but 0.316 ft-lbf / 6.25 ft-lbf = 19.8 which is no where near the 71:1. However the power output of the transmission plus motor assembly is stated to be the same at the motor alone, 179 Watts but after using a linear relationship for torque v. rpm I get a maximum power of 47.8W. This is illustrated by the graph here https://www.desmos.com/calculator/w7q7m7wgp6
Am I missing something or are the values reported incorrectly?
Thank you in advance.

#2

Send an email to support at andymark dot com or call the number listed on their website. They should be able to clarify the numbers for you (or update the website if it happens to be a mistake).

#3

This is because the the PG71 is not very strong; it can’t withstand a motor stall.
If you want something more robust, try using a Versaplanetary.

#4

On the product page for just the gearbox (am-0939), it lists the maximum torque as 39ftlbs before failure (failure mode being the output shaft shearing), which doesn’t explain the 6.25ftlbs number. At 71 * 0.316 ftlbs = 22.436 ftlbs of stall torque, the PG71 should be able to handle the 9015 stalling. But we still don’t know where the 6.25 ft*lbs number came from, so further clarification from AM would be the safe bet.

Just for comparison, it’s pretty close to the same max torque as a 3/8" hex VP (42 ft*lbs), which makes sense given the size of the output shaft. Obviously, bigger output shafts will be stronger, possibly to the point where they are not the failure mode.

#5

Those numbers are suspiciously close to the specs of a PG27 with the RS775 motor. I wonder if they were copied from the wrong place.

#6

When applying a gearbox to a motor, there are a few factors that come into play.
#1, the output speed will reflect a precise multiple of the motor speed, but the motor free speed will be reduced due to drag in the gearbox. Different models use different factors for this, but if you take the speed and gear ratio, you can calculate the actual speed of the motor. If you look at the motors power curve and/or use the current from the PDP, you can figure out what the power draw is of just the gearbox drag (or windage).
#2, Efficiency. Every element in a gearbox causes the gearbox to loose efficiency. Torque of a gearbox is typically torque of the motor x ratio x efficiency-drag or windage factor. With that high of a ratio, I would suspect it is multiple planetary sets. Depending on gear quality, loading, grease, carrier stiffness… efficiencies of planetaris can vary a lot. If you assume 90% per stage, then the overall efficiency is 81%. If the single stage is at 80%, then a two stage would be 64% efficient. Yikes! Remember, that energy is going somewhere, and often it is heat…

#7

The numbers in OP would indicate the gearbox is only 27% efficient. As the PG71 is three stages, that works out to an average of 65% per stage. :ahh: I doubt the PG71 is THAT inefficient.

#8

xxx…removed due to bad analysis…xxx

#9

For the motor+gearbox?

If the motor is capable of only 179 mechanical Watts max at its output shaft, you’re not going to get 179 mechanical Watts at the output shaft of a 3-stage gearbox attached to that motor.

But that still doesn’t explain this:

motor only:

60.64	oz_in
16000	rpm

242560	oz_in*rpm/4
179.36	watts

motor+gearbox:

216	rpm
6.25	ft_lb
1200	oz_in

64800	oz_in*rpm/4
47.92	Watts
#10

AM makes some great products. A lot of them. But in the case of a climber (and frankly almost every FIRST application these days), my motor search starts and ends with the 775Pro.

As to a gearbox, a VersaPlanetary with a nice 1/2" hex and about 150:1 with a 2" diameter drum is going to raise your robot in about 3-4 seconds drawing ~30A.

If you want to climb in under 2 seconds, we switched to a dual 775Pro input to a 75:1 VersaPlanetary (still 30A per motor).

There is no way you’re going to get that sort of climb times using an AM motor, even if you get 179W at the output shaft of the gearbox (rather than at the motor shaft).

The 1/2" shaft is easily extended to a longer 1/2" hex shaft with an off the shelf coupler from Vex and then you can support that shaft with “standard” hex bearing (available from several sources – and you should "simply support the shaft, i.e. support each end with a bearing – no need to get fancy with cantilevered shafts. Remember your robot will be hanging 3 feet in the air) and couple it to your winch drum with “standard” FIRST hubs (from several sources).

If you are feeling lazy, you can buy gussets to hold the VP and the bearing. Easy peasy lemon squeezy.

We were planning on building it from COTS part between the draft and the first quarterfinal match if we needed to cheesecake our way to a 3 climb alliance for the playoffs.

Free advice. Worth every penny.

Dr. Joe J.

#11

FYI, the live axle colson wheel hub imo makes a better hex coupler than the plastic hex coupler… and it’s cheaper too!

#12

I have had good success with this coupler. $20 but solid performance.

Edit. I may have misunderstood. If you mean shaft coupling then, I like the 4008 from Vexpro. If you mean coupling the shaft to the winch drum then there are many choices. I like the 500 Hex Hub (am-0096a) $11 – an oldie but a goodie…

Dr. Joe J.

#13

Joe,

I think Adam was referring to the hub below

#14

Hi everyone,

I looked at the numbers on our 3 PG gearboxes with 9015 motor and agree with most of you that they don’t make sense, so we fixed them!

We have updated the pages in question to reflect realistic conservative estimates for the properties of these gearmotors.

Sorry for the confusion.