worm gear emergency

[ClicheCow]

Last night, nearing completion of our robot, we picked up a tetra with for the fourth time on the full Assembly. This was met by an unpleasant sound of the worm gear losing over 1/4 of its teeth, not a pleasant sound when you just spent 6 weeks of intensive work building it. We called our mechanical engineer over, who looked at it, and said that the shaft (a custom made part that took a day to machine) was crooked, and above that we realized that the worm was engaging the worm gear only 1/4 of the way down the teeth, creating some nasty forces on said teeth. So right now, we have days before ship, and need something fast. Re-machining the gear box is not an easy option at this stage, so we are hoping to find steel worm gears (not worms) which will take the forces better than the cast iron ones did. So far we have been unable to find any. If my memory is correct, they are pitch 12 18 tooth worm gears, but I'll check at the lab tonight and post the correct specs when i get home (the Internet at my school locks down on the weekends)

thank you, and if anyone does know where steel worm gears can be found: THANK YOU!

[Greg Needel]

try

www.sdp-si.com

and

http://www.mcmaster.com/ (page 956)

[dlavery]

If Stock Drive or McMaster don't have them, you can also try Boston Gear (lots and lots of stuff, but they don't always sell direct and may make you go through a distributor), Rush Gears (will custom make it and have it to you overnight - but you will pay for the service), W. M. Berg (they do have worms and worm gears), PIC Design (great source for some unique metric gears), or Motion Industries (great distributor of many power transmission parts). Hopefully, one of these should have what you need.

-dave

[Bill Gold]

The Martin Sprocket (www.martinsprocket.com) cast iron part numbers appear to be W1218 and WG12 on page G-69. Motion Industries and Applied Industrial Technology can get you Martin stuff. Out here on the west coast we can get them next day, if they're not stocked by the local outlet or another nearby. I'm not sure about the east coast, though. Best of luck on your search.

-Bill

[Andrew]

My experience with worms and worm gears is that you often have to replace them as a set. The lead angle on the worm+worm gear is usually a custom number.

So, be careful trying to mate a "Stock Drive" worm with a "Boston Gear" worm gear of the same pitch, leads, and number of teeth.

My advice, look at your pitch distance between the worm and worm gear, and buy a set with the same gear ratio and pitch distance.

The next issue, if you failed the original system because the shaft was deflecting (cast iron gears throwing teeth?), you will probably fail the steel system in "low cycle fatigue". In other words, if you passed the ultimate strength of cast iron, you will probably be at a significant fraction of the strength of steel that, after 20-50 cycles, you will fail the steel gear due to fatigue.

I'm assuming that your shaft was not doubly supported, over a short length. You might want to consider a redesign (not sure how much time you have) to get that shaft better supported and therefore remove the chance of failure.

Good luck and sorry that I only have cautions and not much positive,...

[ClicheCow]

Originally Posted by Andrew

My experience with worms and worm gears is that you often have to replace them as a set. The lead angle on the worm+worm gear is usually a custom number.

So, be careful trying to mate a "Stock Drive" worm with a "Boston Gear" worm gear of the same pitch, leads, and number of teeth.

My advice, look at your pitch distance between the worm and worm gear, and buy a set with the same gear ratio and pitch distance.

The next issue, if you failed the original system because the shaft was deflecting (cast iron gears throwing teeth?), you will probably fail the steel system in "low cycle fatigue". In other words, if you passed the ultimate strength of cast iron, you will probably be at a significant fraction of the strength of steel that, after 20-50 cycles, you will fail the steel gear due to fatigue.

I'm assuming that your shaft was not doubly supported, over a short length. You might want to consider a redesign (not sure how much time you have) to get that shaft better supported and therefore remove the chance of failure.

Good luck and sorry that I only have cautions and not much positive,...

The shaft was supported on both ends, with a thrust bearing on one over 3 inchs. We only think that it is bent, but our lathes aren't properly centered (as we discovered trying to test the shaft) so we can not confirm that. But then again, if the lathes are not centered, and they were used to machine the shaft, thats probably our problem.

Seeing as we ship tuesday, and will only be able to work from 9 - 12pm on monday (the alarms in our school will be turned on after then), we don't have time to re make our gear box. Buying multiple sets of steel gears that will only last 40-50 cycles seems much better than what we have now, which will last us less than one match, but I still have been unable to find one. We may have to take the cost and have one custom made unless we can find a better option.