I am working on a drive gearbox that uses 3 775pros. On the Vexpro website, it says that the motor is primarily used for applications that don’t have long periods of stall. Have people used 775pro gearboxes as drive gearboxes without major issues? Do you need some sort of fan near the motors cooling them down?
I would recomend checking out PWNAGE’s gearboxes from the past. They have had some good success with them. Remember to keep the air vents exposed on the motor! You dont NEED a fan but it is a good idea to keep them cool. I would just zip tie or make a mount for a fan to be used on the back or side of the motor.
Not sure how effective fans would be for cooling motors to prevent a stall from burning them out. 775s burn out in just a few seconds in a stall condition, the amount of heat the generate in that short period is quite impressive.
775s do have internal fans, so as long as they don’t reach a stall condition, they generally self-cool fairly effectively.
That said, I have played around with the idea of using pneumatic air pressure to force lots of air through the vent ports of a 775 motor rapidly during a stall condition so that the forced air serves to make up for the lack of airflow from the internal motor fan (since the fan isn’t moving during a stall). Not sure if this is legal (technically, the motor wouldn’t be a pressurized component, and you’d basically just be venting the pneumatics into the motor) or if it would be practical, but it seems like something that would be interesting to try. In theory, this might also be an effective way to internally cool CIM-type motors if used in conjunction with vented mounting screws.
To answer the first part of your question, my team used a pair of 775s and Decimate gearboxes on each of our two 3-CIM ball shifters this year, along with a pair of MiniCIMs on each gearbox (8 total drive motors). Never had any problems with the motors (aside from some gearbox mounting issues that were later resolved with Loctite). Overall, I’d definately be open to using 775s in a drive again, though it’s worth noting that this game also didn’t lend itself to the heavy-defense situations that you’d generally worry about a stall, so if we end up with another open-field high-defense game (ala 2014), it might be a harder call to make.
If you were to set up a small stack of legal fans, and then 3D print a duct to fit to the vent port, you could probably get a similar effect. Or, better yet, use another 775Pro with a shrouded fan on it through a reducing duct…
I can’t say for sure how legal that would be from a safety point of view, or from a pneumatics point of view, though I do recall a past Q&A that would seem to allow such a thing (though it would need to be asked again to be valid for 2019).
There was a thread on this a while back. The conclusion at the time was “you need a pretty big fan to make a dent”.
We ran a single speed 775pro drive this year with 4 motors per gearbox. While machining the gearbox we made sure the motors could still get air by giving them a bit of a standoff. (you could also just machine openings in the plate) I’m sure current limiting helped also.
It would be cool to see the results of less motors! 
Here is a snip of what we did.
https://drive.google.com/file/d/1ZHmUpXGjEms8lu7Ldpp7K1Fdvy-653n9/view?usp=sharing
-Mat
I’d say “to make a dent in the middle of a match”. Teams like Stryke Force have made adapters for their shop vacs so they can bring the internals down to ambient temperature quickly between matches.
Consider VEX’s testing of the 775pro, especially the locked rotor stall test. As people will tell you, stalling at 12V is insta-death. It takes getting down to 6V before you can really get some breathing room, and even that has a pretty significant drop-off in torque over the time.
You do have to take care of 775-class motors more than you would a CIM or Mini CIM, but it’s not impossible. Designing to run the motors on the high-speed side of the power curve, planning to run with voltage and current limits (so get your programmers studying the Talon SRX) well below peak (how far below depends on weight, wheel size, and CoF), and ensuring you’ve got enough power after all that nerfing to be effective (which is why a lot of single-speed designs use a fourth motor).
Do some deep searching–I’m time-limited this morning, but there are some top-notch threads on “775s on drive”.
It might be different with the FRC-legal fans, but I actually tried something similar with an 80mm computer fan a few months back and found that if you try to reduce the air channel smaller than a certain point (less than about 50% the diameter of the fan), the airflow simply “leaks” back out around the fan blades and doesn’t go into the duct.
Now that said, I could see a fan mounted on the back of a 775 with a 3D printed duct that forces air through the rear vents as having some potential (perhaps in a “pulling” configuration), and it could augment the existing internal fan without interfering with it, though I still think pneumatics may be the only way to force enough air through fast enough to prevent (or delay) a stall burnout.
Axial fans like computer fans work best at moving air in low back pressure applications like ventilating a box. Centrifugal fans give higher air flow than axial fans in applications with higher back pressure like forcing air along a long heatsink with a tight shroud around it or through a motor like the 775’s. Centrifugal fans also tend to have a smaller outlet so there is less need to reduce the cross sectional area of the air path so there will be much less of the effects you are describing in your first paragraph.
The testing my colleagues did showed that at some point, increased airflow actually decreased the amount of heat removed from a heat sink. If I recall correctly, a high air speeds, the air molecules are not in contact with the heatsink for enough time to absorb the heat. You will need to find a way to install temperature sensors on your motor to get meaningful, quantitative results.
My team ran 8- 775 Pros on our drivetrain and got them warm, but never to the point of stalling/ destroying. We used current limiting to help protect the amount of current that was allowed through (acting as a protector).
On our robot cart (inspired by Stryke Force’s 2017 and 2018 cart), we installed 2- electric car radiator fans that would move a ton of air across the motors to cool them. A motor almost too hot to touch when placed on the cart was cold by the time we walked back to the pits.
The part of the motor that really needs to be cooled is the rotor. Moving air over the motor will mostly only cool the outside can of the motor. The rotor may still be hot when the can feels cool.
May, but testing has shown a fan blowing on the motor after hard running makes a meaningful impact. Maybe not enough to totally cool a CIM-based drivetrain between finals matches, but an 83% drop in cooling time for the air inside is significant in my book.
A 775-class drivetrain should be able to cool even faster, since it has less thermal mass, and I imagine big radiator fans cool a bit better than a 120mm KOP fan. (5402 used those in testing in 2017 mounted axially with respect to the CIMs, and they made a difference but didn’t reach to the outside edges of the cans so there was definitely meat left on the bone.)
So I suppose the lesson is “let the fan run beyond the time the outer can is cooled to ambient”, but how long is a matter for more instrumented testing (and then better data logging than Aren’s post, though I appreciate him sharing what he has).
We tried these heat sinks on our robot last year. We did not do any testing to measure the impact, but they did look pretty cool.
We still smoked a lot of motors, so take this with a grain of salt…
Last summer in our testing of our 775 pro chassis that we ran this competition season, we took some thermal images of the motors after high load activity. It’s apparent looking at the photos that the primary heating in the motors is occurring in the region of the brushes and brush arms. Active cooling will do little to protect these regions because of how quickly heating occurs and how little thermal mass the components in question have.
Instead of active cooling, I would strongly suggest considering imposing an artificial current limit in your design space. Since the standard in FRC drive design is to target a traction limited situation, make sure you cross your traction limit at a lower current. This means more gearing (larger reduction) and a lower limit for peak robot speed if you’re targeting a single speed drive. JVN’s calculator outputs the current information natively.
For reference we were geared for 21ft/s theoretical with imposed limits of 14 ft/s in auto and 18 ft/s in teleop with a full weight robot on black 6" hi-grip wheels in a drop center six wheel configuration with eight motors in the drive. Our theoretical traction limit occurred at ~40% rated stall torque. In changing traction materials for off season play we discovered that by increase our available traction our drive was no longer traction limited, because we were exceeding the capacity of the battery to supply the required current. It’s a new limit, and something we’re developing a method to instrument for testing purposes.
If you have any specific questions, please reach out. People seem to be afraid of these drive trains, and there are considerations, but the threshold for safe use of 775’s in the drive is not so daunting as often advertised.
I agree that active cooling is of limited utility during the match, though I would venture a fan setup for after the match is more valuable in bringing the motors down to ambient temperature.
I’ll ask a couple about your particular setup: How did you implement your speed limits? Did you replace any motors during the season, either due to failures or as preventative maintenance?
Speed limits were imposed based on encoder feedback, no other programmatic protections were in place.
We didn’t replace any motors through the course of the season on the practice or competition robot and haven’t observed any degradation in robot performance to indicate that the system needs service.
The testing described in the post linked does not show if the internal ambient air temperature rose again after the various forms of airflow were stopped. If the internal ambient temperature does not rise, the armature has truly been cooled. If the internal ambient does rise, the airflow has mainly flushed out the air that was heated.
I agree with Kevin’s post, after the linked post, that the internal ambient temperature is not a good proxy for the armature temperature. From the many thermal tests I have had to do at several of my day jobs, I have found that a gap of 1-3 mm due to a thermocouple that detached from a heatsink will read between 5 and 20 degrees C lower than an adjacent one that is properly attached.
Another way to keep a 775 pro drive train alive is traction limiting. You give the motors all the power they want but limit how much of it can go to the ground. This year we ran a 8 motor 4 module parallel swerve. The only thing that kept the drive motors alive was limiting the wheel traction. (I am talking about 1 module) 2 775s drive 1 4x1in andymark performance wheel wrapped in wedge top tread. At full power it will break the wheel free and spin it back off and it regains traction. This gives max acceleration and keeps motors alive for. We ran 3 completions and a lot of practice on the same 8 motors. And before you go yelling how do you push its swerve that goes 18 ft/s it doesn’t need to push. Ask anyone that watched 323s robot it is fast!
Nick
I know that team 1690 from Israel used a 775 chassis in 2018.
They didn’t really find a way around stalling, they just cut the power once the motors were stalled. Though as earlier mentioned, you can set current limits in a way that lets your motors stall without risking an RS775, instead of shutting the power off all together.
Overall I know that 1690 were pretty satisfied from their chassis, though during the season they were very susceptible to defense.
Could you please clarify how you did this?