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.
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.
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…
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.
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…