I know some teams I spoke to this season mentioned problems with worm gears breaking. What are the best ways to build a gearbox with a worm drive included? What are good material choices and types of gears? Both for arm applications, or for drive train applications as recently shown by 269.
If you have built a worm drive gearbox what were some problems you experienced, and what are some tips and tricks you would recommend?
1425 did a worm gearbox this past year with some good success.
Personally I would recommend against them. This is a personal recommendation because if it is not built perfectly you have tremendous problems on your hands.
I was also working on a worm drive. I was planning on using a small (no reduction) gearbox to transfer the power of 2 small cims to one 1/2 in keyed drive shaft where the worm will be
For any teams that have used worm gears for a drive train, did you find that other teams could backdrive the worm gear by pushing you hard enough? And how often if at all did the gears get damaged by overly aggressive pushing?
This past year was our 4th year of using a worm gear drive and as far as I know, we have not had any problems with the gearboxes themselves. We have modified them a number of times with success to reduce both their weight and size. A number of pictures of robots from past years can be found on the team website
To answer Noah’s question, I don’t recall our gears ever being damaged in any way.
Our drive base used worm gears with relative success this year, and we had no problems with them breaking, though we used brass gears which at first threw up brass shavings something awful, but eventually the gears became comfortable and deceased in their hurtful ways. (Had that continued any longer us on the electrical team would have cried.) On the subject of back-driveability, we were back-drivable, but only with great effort, this is because we used “beveled” (?) gears (again I’m electrical, so don’t quote me on this) which means the teeth on the non-worm-gear gear meshed nicely, and probably kept them from breaking.
Find one that’s already built! Here are some places I would recommend starting:
-http://www.applied.com/apps/commerce/catalog/catalog.do?e=2&c=1960
-http://www.chiefdelphi.com/forums/showthread.php?p=566568#post566568
-http://bostongear.com/products/enclosed/wgsr.html
I would NOT suggest buying from McMaster Carr. McMaster only sells cast iron worms, which will turn to dust after a few matches.
We used a worm gear on our arm - both to get the desired reduction and eliminate backlash. The housing consisted of a piece of 1/4" thick boxed aluminum, the spur gear was bronze, and the worm was made from stainless steel. The gears were purchased from Applied Technologies.
The first worm drive gearbox we built this year was too weak to handle the loads on our arm. The housing was made from 1/8" wall aluminum and the shafts were cold-rolled 3/8" steel. After a little testing the housing had become distorted and the output shaft was bent :ahh:
The second worm drive was beefier! We used 1/4" aluminum for the housing, and switched to 5/8" shafts made from 01 tool steel - which we had hardened.
Basically: make it strong and accurate and it will last forever. We built a spare and never had to use it.
We had our boxes custom machined, and used a worm gear from Boston Gear and a brass gear that gave us a 15:1 ratio. We could’ve used a 12 as well.
I’ll post some drawings on our boxes in a sec.
Alright, here is the gear we used was a
Boston Gear WORM, 12 DP, 14.5 PA, 1. PD, LH, STEEL/UNHARDENED
Part Number: DH1407LH
We used it with a 30 tooth brass gear.
As Elise previously stated, our worm-boxes have been super reliable and as far as I know, we have never had a problem with any of them.
Over the summer, we developed a worm gear driven shoulder, that ended up being modified for our robot this year. Our 14lb arm was lifted entirely by a single globe motor (that never burnt out, through 3 competitions) and a 30:1 worm gear reduction. We never had a problem with the gear breaking, or even wearing down, in fact the only problem was that it was too slow (a mind-bending 2.6 rpm!).
The first picture is the inventor rendering of the shoulder transmission (the arm attached to both ends of the shaft)
The second picture is the off-season project transmission (using a 20:1 reduction). Both gears were bought off of McMaster-Carr and are cast iron I believe. If you have any questions, feel free!
About 269’s gearboxes: We first used the worm for our drivetrain in 2004, we used it again in 2005 and now this year on our crab in 2007.
On each competition bot, these drives have NEVER had a problem. Not once has one failed, all we have had to do between matches on these boxes was to lubricate them.
They have proved effective and durable for our team and we look to continue using them in the future.
In regards to 1425’s Worm Drive - I have been the mechanical student leader this past season, and the drive train and gearboxes were almost entirely of my own design (minus some helpful advise from mentors).
We used a worm drive setup with 2 small CIM’s per side geared directly to the shaft of the center wheel, then chain driven out to the corners of a 6wd setup. (traditional 6wd, w/ center wheels lowered 1/8") There’s a couple good overhead shots of our practice robot’s drivebase here, for reference: http://www.putfile.com/pic.php?img=4690741
There were a few reasons we went with the worm gear setup. First it allowed us to get a complete reduction out of one set of gears, not multiple stages of reduction. It also relocated the motors to be alongside the outer chains and frame rails instead of taking up all the space in the middle of the robot. The worm drive is definitely more resistant to backdriving, and never once were we pushed without purposely allowing ourselves to be pushed (we didn’t exactly go around pushing ALL the other robots - with a single speed its always a matter of balancing speed and power to get a competitive mix, although more than once this season we pushed other robots sideways). The particular characteristics of our setup were not most ideally suited for anti-backdriving - instead the specs of the gears favored higher efficiency, but it was notably resistant to pushing. It was our first experience with both a custom frame and gearbox, outside of that provided in the kit - and plenty of lessons were learned as a result. However I can say that we never once had our drivetrain fail in competition - we attended 3 regionals and championships this year, totaling an estimated 50 rounds.
First I’ll describe the specifics of our setup, then I’ll get into some of the lessons learned and future tips.
The system had 2 small CIM’s facing one another, connected to either end of the shaft with the worm on it (visible in this picture http://www.chiefdelphi.com/media/photos/26656). That is a hardened steel quad thread worm driving a 60 tooth brass worm gear, for a total reduction of 15:1. The quad thread is MUCH more efficient than single thread worm gears, and a fair amount more efficient than double thread worm gears - but it is also much more uncommon. A quad thread is pretty much halfway between a worm gear and a helical gear, and it took three different suppliers before we found one with 2 gears in stock and willing to make us 4 more within a month w/o additional costs. The worm gear (the lower, larger one on the output shaft) is brass, again due to efficiency. This was our drivetrain, and we didn’t want to be loosing power frivolously - I actually ran through MANY equations months ago to estimate the efficiency of our setup, and came up with ~86%, which isn’t bad at all - especially compared to this year’s kit gearboxes, which yield ~72% efficiency. We were careful to use shaft couplers between the motors and worm shaft, to allow for misalignment, as well as tapered roller bearings to take the thrust load of the worm. We spec’d in .015" engagement slop between the gears, which entirely prevented any interference binding.
We did enough things right that this gearbox was a complete success this year, but there is plenty of room for improvement in the future. For one - don’t just design the system based on gear specs in catalogs - before you get too deep, contact a supplier to make sure you can get the gears for a reasonable price. As I mentioned, it took three suppliers to find the gears we needed, and this gearbox nearly didn’t happen as a result. The first supplier, whose catalog I had designed based on, told us upon calling that they had never manufactured that gear before and it was going to be over $350 apice - we eventually bought them from Boston gear for ~$110 ($75 for the brass worm gear, and $35 for the worm). Secondly - Make sure to take thrust loads into consideration - worm gears, unlike spur gears, generate thrust along the shaft of both the driving and driven sides. We supported the driving side with tapered bearings. We did not, however, adequately plan for our worm gear’s attachment to the output shaft, and all season long we were hassling with set screws when we should have broached our gear and gone with a keyway. After set screws in both of our practice robot’s transmissions failed, we eventually drilled and put a hardened bolt through the output shaft to secure the worm gear - not the ideal solution, but a decent fix. (this problem was not just due to thrust loading, but it was definitely made worse by it)
Outside of this, as long as you carefully design out what you wish to make and you have the means to purchase and fabricate all the necessary parts, then you should be fine going with a worm drive. However, as others have mentioned, there aren’t always easy fixes if the system fails - so keep this in mind. (we always had spare worm gears with us, just in case one ever failed) Also, if you don’t already have it - try and seek out experienced mentors to offer assistance and advice. Nearly all the design was my own, but without mentors peering over my work and making suggestions, it would not have turned out as well as it did.
Sorry for the long post, but I was trying to be thorough. Let me know if you have any additional/more specific questions - I’d be glad to give my input from experience.
- Jeff
WOW! Thanks for all the responses! Tha CAD files, photos, advice, and links are amazing!
I’m going to get my team to prototype either a drive train or a an arm with your advice about worm gears. It seems like it has many benefits that I never even thought about it.
Where did you get (or did you machine them yourself?) the wheel coupler for your direct drive?(the one that atatches the wheel to the output sprocket) I have looked in nemerous locations but could no find what I wanted (which is what you used).
That was custom machined.
Was the rest of your entire setup custom machined as well?(minus the actual gears)
Yes, correct. We had a local machine shop offer to teach some of our high schoolers how to operate their CNC mills. The sideplates could have been done on a manual, but the UHMW base was intricate enough for CNC use.