On Subsystem 2 of our robot, we are interested in utilizing both the 2011 FP and one (or more) Banebot RS775 motor(s) in tandem. For this task, I’m looking at gearing these motors together; using the 2011 FP and Banebot motor spec sheets, matching the free speeds of these motors works out almost perfectly* using a 15t pinion on the FP and 24t pinion on the RS775, both driving the same 60t cluster gear.
I’m looking to do this using 32DP 20° gears. AndyMark sells a 15t FP pinion, and I’m currently looking at buying spur gear stock from SDP/SI for the 24t and 60t gears. So here’s my questions:
What is the diameter of the Banebot RS775 shaft? Their website lists “0.2 in (5mm)”. Can someone with a micrometer check the actual dimension of the 775 motor shaft and verify whether imperial or metric is actual dimension? (Unfortunately, I don’t have access to a RS775 for a couple of days). This is important because…
We’re looking to press the machined 24t pinion onto the Banebot RS775 shaft. While we’ve removed and pressed pre-made pinions onto FP motors before, this is the first time we’re going to machine our own pinion out of gear stock. Are there any special considerations to take? Would a reamer sized for a 0.0002-0.0005" interference fit** with the Banebot shaft be sufficient for holding the pinion in place?
Thanks for your help.
Within 7.5 rpm (fraction of 1%) of each other!
** If shaft OD is actually Ø5mm, I’m looking at buying McMaster #8803a378.
I would think that a .0002-.0005 interference fit would work just fine.
Your issue may come in actually achieving that tolerance with a reamer. At .0002 tolerance you’re looking at a ton of factors that will influence your final size. Reamer length, initial hole diameter, spindle/chuck runout, tailstock misalignment, toolholder type, lubrication type, speeds, feeds, etc can all make or break you. Honing or boring would work best, but aren’t really practical for FRC use.
You can find some good information here about choosing the right reamer and pilot drill size. The minimum tolerance they recommend is .0005.
Your best bet may be to try to ream a few sample holes in steel just to see what you need to do get the right sized hole in there, without ruining gears.
The RS775 shaft is definitely 5mm. The motor mounting holes are M4. The shaft has a negative tolerance as most of our motors have a shaft diameter of 4.95 mm.
We have successfully taken the FP pinion from the AndyMark Fisher Price transmissions and modified them for use on the RS775. We also have modified the AM Planetary motor plate to receive the RS775 for use in the AM Planetary. It works great.
We are match drilling and reaming every pinion to every shaft. We had to purchase a new reamer. I will find out which one we are using today and get back to you.
Art, when reaming on the lathe, make sure your tailstock is aligned horizontally with turning axis. It wouldn’t be a bad idea to check vertical height of the tailstock as well. Slow speed and sufficient cutting oil. In addition, if you don’t have a collet chuck to fit the OD of the gear, you can make an adapter bushing slightly undersize to the gear OD (push fit) and cut a slot from the external diameter into the interior bore. Then you can hold it in your 3-jaw chuck without damaging the gear teeth. This will also serve to help face the gears when you cut them from the gear stock.
It really works to drill and ream the FP pinion? It’s not too hard?
Yep. we used this method on three different AM FP transmission pinions, even the ones we thought were too hard. They were actually only case hardened and the core was soft.
Here’s one (actually two) more questions: SDP/SI has 32DP 20° 24t pinion wire available in either brass or carbon steel. Which material would be preferable for a pinion? If carbon steel is preferable, would we be able to case harden it with either heat+surface-hardening compound or oxyacetylene flame hardening? Is this needed? (I tried looking up the material properties, but “carbon steel” is pretty generic…) Searching revealed two goodposts from Dave on these heat-treating processes.
While machining press-fit pinions may be new for us this year, I’m sure we can have fully functional gearboxes sooner than the 5-10 business day delay -plus shipping- currently plaguing the P60 gearboxes.
I just pressed a pinion onto an RS775 shaft, I drilled out the gear with a 0.196" drill bit as I measured a 0.198" diameter on the shaft. It seems to have worked out well.
Anyone have an opinion of plopping an AM planetary into a 3-stage GEM-500 and driving it with the 775 with a drilled out and pressed on FP pinion? And then maybe go 3 or 4:1 in sprockets and chains to an arm with say 75 ft-lbs of torque at the arm joint. Doing the same with a P60 worries me a little.
I reckon it’d work mighty-fine, and be a good deal stronger than a P60, due to the all-steel 20DP gears. Anyone agree? I take it’s rather easy to modify the AM planetary motor plate to accept the 775?
My intuition says that it’ll work fine. 75lb-ft at the arm joint? o.O remember that the RS775 (or any motor) won’t put out “stall torque” for any appreciable amount of time, it’s breaker will trip.
