As I’ve seen this done many different ways, I decided to start this thread to discuss the advantages and disadvantages of different methods of attaching the drill motor (without gearbox) to a custom transmission.
My own experience has been interesting. The drill motor mount we used was on MVRT 115’s swivel drive this year. Made from 3/4 inch solid Lexan, the drill motor is pressed in, and pins in the Lexan that go into the holes in the front of the drill motor to keep it from rotating. Not that it would, it was quite a snug press. Then, this Lexan “adapter” was screwed onto the swivel drive, making it actually really quick and easy to replace drill motors, provided you had a press (we used a vice).
All in all it was a good idea, nice and light, easy to maintain, never really failed. However, under unknown circumstances to me, one of these Lexan pieces broke. I highly doubt it was from running the motor, because it broke in the pits. Anyway, we had aluminum replacements, so we popped those on. Personally, I feel that these Lexan things were really good for their weight and size.
However, as I said earlier, this thread is for discussion of different methods, so end of my blabbing.
But… here’s how I’ve seen or think it could be done:
Thing that goes around it, screws down to clamp onto the motor
I would like to correct Veselin there, first off its not even concerned that the material was lexan because one of the students had picked it off from a machine shop scrap. Secondly it wasn’t very durable, first competition it worked ok but there were many issues like couple times the motors died or had to be replaced because the cable housing on the back of the motor came out. So we had to take the motor out from the block and had to put back the motor in plenty of times. Which caused it to pop out easily and it was real sloppy. Second competition one of the modules the plastic piece cracked in half so our mentor had to get spare part made out of aluminum and had three set screws to hold it in place. This solution seemed to work well.
All in all it was a good idea, nice and light, easy to maintain, never really failed. However, under unknown circumstances to me, one of these Lexan pieces broke. I highly doubt it was from running the motor, because it broke in the pits. Anyway, we had aluminum replacements, so we popped those on. Personally, I feel that these Lexan things were really good for their weight and size.
Its not a good idea to press fit a motor into pretty much anything. The aluminum block with setscrews held it in place though.
I believe 968 and 45 did it by carefully using a 10-32 or Metric 5 roll tap to thread the holes in the front. They said you only get a couple threads, but apparently it never failed.
716 has a way of doing it that you can see in the pics of their transmission on here.
188 not only used 10-32 screws in the front of the drill (you do only get 1 to 1.5 threads), but also machined an aluminum collar (bolted to the gearbox) to surround the drill. The collars had two machine screws which were tightened to press on the two circular recesses on the side of the drill. Those drills weren’t going anywhere. (I’m a little bit concerned about using just the 10-32s–that seems like a dangerous proposition if the motor was struck from behind for any reason.)
This (attached image) is what Tristan speaks of. For all you visual learners, it’s the metal collar on the drill (which is the bottom motor) in the image. The collar is simply a big cylindrical piece of aluminum stock we hollowed out to fit a Bosch motor. You can see the press screws that sort of stick out. As a result, those collars (which we used last year as well), in conjunction with the normal 10-32s hold the drills in place perfectly, and we haven’t had any problems with them. The CIMs are a different story though; they have come a bit loose before, but we’ll fix that next year.
Actually, I would like to correct Akshay on this. The Lexan was quite durable I would say. It’s harder than polycarb. After asking around, I got the general idea that the plastic mount broke because the swivel drive was being assembled incorrectly, and the motor wasn’t aligned when it was attempted to be screwed on.
Second, there is nothing wrong with pressing a motor into plastic. If done correctly, you can press and un-press successfully every time, without damaging either part. Obviously this is difficult if you are using a vice and aren’t all that good at it.
AANNYWAY…
The holes on the drill motor on the front face have always been anoying to me. They look like the holes on the fischer price motor, but are not tapped, or tappable. I was wondering if any team has ever used rivets to hold the motor onto a plate, without anything that actually goes around the motor. I am guessing it would work, because the rivet would just expand on the inside of the motor, locking it in place. The only problem I would see is if the motor died, you would have to drill out the rivets.
By the way, here’s a pic of the Lexan mounts we used.
The solution we developed was to make a mounting plate that fit against the face of the motor, with a 0.695" hole for the drive pinion and pinion bearing bump to rest inside. Two 6-32 screws are put in the mounting plate to act as pins that stick into the two round holes on the face of the motor, to prevent it from rotating. Two 1/4" x 1/2" slots are opened up in the mounting plate to allow free airflow through to the ventilation slots in the motor face (more on this below). Finally, two “J-hooks” are made from 1/4" drill rod. The J-hooks screw into the mounting plate, and lock into the little square notches at the back of the body of the motor. The J-hooks keep the motor flush against the mounting plate, and prevent it from translating backwards. You can see how this works in the lower right corner of this image.
The thing that we liked about this approach is that it requires no modifications to the motors, and no special efforts to prep the motors for mounting. Swapping a motors takes about one minute - just long enough to loosen the two J-hooks, pull the motor backwards off the pins, put the new one in place, and tighten up everything.
I would offer one comment regarding the Lexan “press-fit” motor mounts described above. As shown in the drawings the motor mounts cover and occlude the arced slots in the face of the motor (they may actually be built differently, but the drawings are the only references shown here). The basic design behind the Lexan motor mounts is pretty slick. But these slots are ventilation intake slots, and they must remain unobstructed to allow proper airflow through the motor. If they are covered, the motor will build up heat, and you will run the risk of a thermal overload (resulting in the power leads becomming unsoldered, bearings seizing up, rotor colliding with the case, and other bad things). It is strongly recommended that you add ventilation slots to the motor mount that will allow airflow through the slots in the face of the motor.
I think Team 25’s solution was pretty elegant: they clamped the motor with a plate on the back (commutator) end which pressed the motor axially against their gearbox. A pin in the gearbox face located the end of motor to prevent rotation under torque reaction. I think they milled a shallow bore in the gearbox faceplate which was a slip fit on the end of the motor to locate the output shaft center.
Their setup allows rapid motor changes - just remove the two screws holding the back plate to release it - and the aluminum gearbox faceplate conducts some heat away. Plus the entire motor body is exposed for air cooling.
Can’t say I’m a big fan of Lexan (aka polycarbonate) for applications like this. It may be very tough, but it isn’t very dimensionally stable and it doesn’t conduct heat well, either.
This is exaclty what we did, and it seemed to work great. Make sure to use a roll tap vs. a regular cutting tap. You do not want the metal shavings to get into the motor. As far as this way being weak, I would have to argue that anything more is not necessary if the gearbox is protected. The gearbox should never see any forces from the side or back (if it does you probably have bigger issues to worry about) Another difference is that we carried the load on the end of the shaft using a bearing, meaning the only way to get the motor off is the pull it out axially and tear the threads off. A hit from the side should be okay, because alot of the force will be distributed through the shaft and into the bearing, making it less likely to put side loads on these threads.
The plate we made uses 4mm pins to keep the gearbox from rotating. We macined holes for air flow and hold the motor from the back, similiar to Dave Lavery’s method, but include a cross piece to keep the motor in-line axially.
The great part about engineering is the many ways there are to do the same thing! The great part about FIRST is the sharing of ideas from all over the world!
We had a mounting plate through which the BDM (Bosch Drill Motor) boss fit. There were two dowel pins (8 mm I think) pressed into this plate and which fit into the 8 mm holes in the BDM. This kept the motor from translating axially towards the gearbox and rotating.
We attached an aluminum tube around the motor and which screwed into the gear box plate. This tube had a slot milled into the cylindrical part and a muffin fan mounted onto it. This provided a heat sink around the motor and forced cooling.
We then had a back plate which screwed into the back end of the cylindrical shroud over the back boss of the BDM. This back plate pushed the BDM against the gearbox plate.
We’ve used this design for the past two years.
We had one problem when the back plate screws backed out. This allowed the BDM to come partially free and rotate when it backed off the dowel pins. The plastic brush holders broke causing a short which fried a speed controller. Now we loctite those screws and inspect them frequently.
However, we have had NO difficulties with BDMs heating up. We have also had no mechanical damage to the weak parts of the BDM.
Maybe I’ll post a white paper with drawings this summer.
We did a similar design for the Fischer Price motor. Alas, we did smoke one of these in competition this year. But that required a pretty elaborate set of circumstances that should occur very rarely.
But that required a pretty elaborate set of circumstances that should occur very rarely.