Worm Gear Gearbox

[Peter Matteson]

Originally Posted by Max Lobovsky

Does anybody have any links to information for calculating worm gear strength?

Do you mean strength of the gear teeth or force that you can generate with a worm gear?

[Jizvonius]

Originally Posted by Max Lobovsky

Does anybody have any links to information for calculating worm gear strength?

Depending on where you get them, the company will supply that information. However, if they don't, and you know the material and the thickness of the teeth, you can probably get an estimate by calculating from yield stress in shear from the material properties (look these up). The number won't be exact but with a good safety factor (which you need anyway) you should be able to meet your design needs. Use a larger safety factor if you do the calculations yourself though (your number will probably be less accurate).

*make sure you get the correct material properties as different treatments can change properties(case hardening etc.)

[Max Lobovsky]

What are "starts"? The number of threads? I thought the number of threads was equivalent to the number of teeth so more threads is less reduction (and i would assume more efficient)

[Jizvonius]

Originally Posted by Max Lobovsky

What are "starts"? The number of threads? I thought the number of threads was equivalent to the number of teeth so more threads is less reduction (and i would assume more efficient)

If you look at the side of the worm, you should see the places where thread starts, that's a "start". The number of starts will change the gear ratio. i refer you to Paul's post.

Originally Posted by Paul Copioli

Remember, gear ratio for a worm gear is # of teeth on the gear divided by number of thread starts on the worm.

[Bacchus]

Yes, you are correct. I got just about everything back wards. In an attempt to make up for it, Here is a decent link that some pretty good info on worms... http://www.bostongear.com/pdf/gear_theory.pdf Pages 13 and 14

[Andy Baker]

Originally Posted by Max Lobovsky

What are "starts"? The number of threads? I thought the number of threads was equivalent to the number of teeth so more threads is less reduction (and i would assume more efficient)

Here is a product manual from Martin Spocket and Gear. On page 69 of this link, the worm gear has 1 start (a single thread). On page 70, it shows a worm gear with 2 starts (double thread). On page 71, a worm gear with 4 starts (quadruple thread) is shown.

On the shoulder joint of our elbow for last year's TechnoKat robot, we used a 40 tooth worm gear (Martin # W1240) and a single threaded worm gear (Martin # WG12).

This design worked well, except when the arm would bounce or hit something. The teeth on the worm gear would shear. If we ever need to use this sort gearing again for an arm, we will need to do two things: 1. use a gear with larger teeth 2. provide more of a counterbalance for this same sort of joint

Andy B.

[Max Lobovsky]

I definitley do not think I could do my own calculation for teeth strength. It's far more complicated then simply shear strength for that material. There are multiple engagement points at a given time, and even just calculating the area being sheared isn't simple. I need to know this for design purposes, so I can't wait to receive the part and any info it comes with. None of the gear supplier websites I have checked list the formula.

[Andy Baker]

Originally Posted by Max Lobovsky

I definitley do not think I could do my own calculation for teeth strength. It's far more complicated then simply shear strength for that material. There are multiple engagement points at a given time, and even just calculating the area being sheared isn't simple. I need to know this for design purposes, so I can't wait to receive the part and any info it comes with. None of the gear supplier websites I have checked list the formula.

Yes you can... well, you can do some rough calculations. Stock Drive Products has a very good technical catolog section where you can follow instructions to figure gear strength (page T78). These calculations can be used as ball park figures to get you close when choosing your gear pitch.

When it comes to dynamic and shock loading, the engineering gets waaay more complicated. For our applications in FIRST robots, I suggest using the gear strength calculations linked above at the Stock Drive website. Depending on what you will be doing with your gears, you can then put in a safety factor to absorb dynamic and shock situations.

Andy B.

[Max Lobovsky]

I've read SDP's technical section and I still cannot find any information for worm gears. Even if the gear always breaks before the worm, the calculations are not comparable because the number of meshing teeth is very different and worm gears often have a sort of concave curve in the tooth to increase mesh that further changes the mechanics.