Worm Gears

[Keith Parsloe]

Are worm gear ratios calculated the same as spur gears ie: 20/40 teeth = 2 to 1 ratio ?.

 

[TonyM]

The ratios are calculated by the number of 'starts' on the worm and the number of teeth on the wormwheel. A worm can have just one 'start' giving very high ratios and high torque. On these with a low number of starts the worm must be the driver .

 

[minh-thanh]

The ratios are calculated by the number of 'starts' on the worm and the number of teeth on the wormwheel.

It means :
ratio = 1/n
1 revolution of the shaft of the worm
n = the number of teeth on the worm wheel
Right ?

 

[Jasonb]

For timing gears you can take it as you say 20T and 40T will give 1:2 ratio. And that is how off the self gears will be sold by tooth count

What can be varied is the angle of the teeth, off the shelf ones will all be ground with the same angle so your camshaft gear can get bulks. But by altering teh angles you can get the same ratio and end up with the two gears the same overall diameter which is quite common on sideshaft engines

 

[timo_gross]

Something like this, the lower gears both have 16 teeth. The Upper ones 32 teeth. The 16 tooth gear on the left has 60° helix angle, the upper one 30° helix angle.
Theoredically all that works. At some point angles are so weird that the pair only works with one side as the drive, because it locks.
A typical one start worm is ( if I believe the computer program) just a helical gear with one tooth and a helix angle that is steep einough that the tooth is forming a spiral. (helix?).

with that tricks the distance between the shafts can be changed. All four gears could be cut with the same cutting tool.

Greetings Timo

 

[TonyM]

These are two types of gear being discussed. Worm and wormwheel and crossed helical. Both of which are used on 90 degree shafts. Although Timo may well be correct in assuming the OP meant crossed helical as this is an engine forum, he did ask about worms.
Some general info here on the two types. Differences Between Worm and Helical Gear - Premium Transmission.

 

[timo_gross]

These are two types of gear being discussed. Worm and wormwheel and crossed helical. Both of which are used on 90 degree shafts. Although Timo may well be correct in assuming the OP meant crossed helical as this is an engine forum, he did ask about worms.
Some general info here on the two types. Differences Between Worm and Helical Gear - Premium Transmission.

Yes, actually what I am thinking is. A worm appears to be "just another" helical gear, because my computer behaves like it. If I enter 1 tooth and 84° helix angle I get this.

The definition from the link seems to contradict, but they assume that helical gears stay below 30° helix angle, and that would not allow for the crossed helical gear pair. If the difference is as big as I made it in the first example with 30° and 60° that the pair is probably not functioing very well when the gear with the lower helix angle is the driver.

Happy experimenting!

 

[PeterDRG]

If anyone is interested, i have Step files for a worm and matching 60t gear. I ducked out of cutting them for a dividing head for my Hobbymat.

 

[timo_gross]

If anyone is interested, i have Step files for a worm and matching 60t gear. I ducked out of cutting them for a dividing head for my Hobbymat.

I am curious:
How did you "make" them? (the files) Problem is always the wormwheel. If a helical gear is "good enough" things become easier to some extend.
How big are the gears and what features inside?

Greetings Timo

 

[PeterDRG]

Hi Timo,
A friend did them for me. He used Fusion 360 and bought in a special add on. Id originally asked him for just the worm but he found it easier to do the two. The gear is 60mm in dia.

 

[timo_gross]

Hi Timo,
A friend did them for me. He used Fusion 360 and bought in a special add on. Id originally asked him for just the worm but he found it easier to do the two. The gear is 60mm in dia.

Looks like pockets for "nuts", did you try to 3d print it?

 

[PeterDRG]

Yes, It has printed fine.

 

[gbritnell]

While it's true that worms and helical share a similarity in that that the tooth profile is wound around the axial shaft that's where the similarity ends. A helical gear has it's teeth angled from the axis of the shaft while a worm has it's teeth angled from an angle which is at right angles to the axis. Yes you could say that the angle could be the compliment of the axis angle but in looking at them they present a totally different picture.
A pair of helical gears can run parallel to each other or at right angles to each other (or at some weird angle if needed) whereas a worm and gear set will only operate at right angles to each other.
A worm is calculated by the dimensional advance of a tooth making 1 revolution. This dimension is then used to calculate the amount of rotation of the worm wheel by it's number of teeth. A worm of a given D.P. will operate a worm wheel/gear of any number of teeth as long as the D.P. is the same

 

[Keith Parsloe]

Boy, getting a bit too technical for my old brain. I'll probably just go online when I need a set for my next project.
Kpar

 

[Jasonb]

If you say what your next project is it would help us understand what gears you want.

Talk of tooth numbers and 2:1 ratio suggests timing gears

Worm would tend to be used for making a rotary table or steering on say a traction engine model

 

[GreenTwin]

McMaster Carr has a variety of gear types available, and they also have 3D models of these gears that can be downloaded, if you are into 3D modeling.

Here is their main gear page:
https://www.mcmaster.com/products/gears/
And here is the worm gear page:
https://www.mcmaster.com/products/gears/metal-worms-and-worm-gears/
I found a lot of good information about gears and gear ratios/diameters, etc. on the McMaster Carr site.

The one set of gears that I did not find on the McMaster-Carr or any other site is a set of helical gears that are the same diameter, but with a 2:1 speed ratio.
Luckily JasonB saved me with his very nice 3D modeled version, which I was able to 3D print and verify that they meshed and worked perfectly.

The way I use the McMaster-Carr page is to look for a gear pitch that is about the size I want, and then pick a gear ratio, and number of teeth.
Then work backwards and find a worm that will run with that gear (I am not sure exactly how to do this, but someone here does).

The worm and gear have to be ordered separately.

Hope this helps.

Pat J.
.

 

[timo_gross]

While it's true that worms and helical share a similarity in that that the tooth profile is wound around the axial shaft that's where the similarity ends. A helical gear has it's teeth angled from the axis of the shaft while a worm has it's teeth angled from an angle which is at right angles to the axis. Yes you could say that the angle could be the compliment of the axis angle but in looking at them they present a totally different picture.
A pair of helical gears can run parallel to each other or at right angles to each other (or at some weird angle if needed) whereas a worm and gear set will only operate at right angles to each other.
A worm is calculated by the dimensional advance of a tooth making 1 revolution. This dimension is then used to calculate the amount of rotation of the worm wheel by it's number of teeth. A worm of a given D.P. will operate a worm wheel/gear of any number of teeth as long as the D.P. is the same

I think we are basically on the same page, concerning the naming. Some clever person(s) made software, same rules and formulas seem to work out to make both types of objects.
1 tooth 84° helix angle, gear width 20 mm. Computer: "It is a single tooth helical gear with a 84° helix angle"
Human: "It is a 20 mm long worm with 11.57 mm outside diameter"

Timo: "It is same thing!"

Everybody else: "Computer/Timo are nuts!" ;-) haha.

Greetings Timo

 

[methuselah1]

have a look at "Pittler" on "lathes.co.uk". If the helix angle is over about 3-4 degrees, you need to divide the diameter of a corresponding spur gear by the cosine of the helix angle. I wrote a lengthy article for Tony, years ago, detailing the calculations for worms and wheels. The information for worms is what you need to look at. The big problem is usually generating the required lead- Ask my Dad!

Andrew UK

 

[gbritnell]

When building the model of the Galion road grader I had to make all the gears for it because they were all odd sizes. This necessitated making all the cutters. The 2 gear boxes for elevating the blade required worms and wheels. I started with my size requirements and drew a gear that would fit inside the gear box. I then calculated the number of teeth that would fit onto that gear. With that as a starting point I calculated the pitch of the gear and used that to match up to the available threads I could cut with my lathe. I had to keep adjusting the numbers until I could cut the right pitch screw on the lathe. The O.D. of the worm wasn't critical but the pitch was. So now I had the specs for the gear and the pitch of the worm. I adjusted the worm diameter until it matched the center distance needed keeping in mind that the O.D. of the worm couldn't get too large because it had to fit into the depth of the gear box cavity.
I made a cutter the diameter and pitch of the worm. I mounted my gear into my dividing head and tilted it to match the lead angle of the thread I was using. I then cut the gear teeth. A special lathe tool had to be ground to match the pitch of the gear for cutting the worm. It required a large clearance angle on the front face to clear the tooth while threading.
The pictures show the cutter, the setup with the dividing head tilted, the finished gears and worms.
I forgot to mention that 2 sets of worms and gears were required because one was left hand and the other right hand.

 

[ajoeiam]

When building the model of the Galion road grader I had to make all the gears for it because they were all odd sizes. This necessitated making all the cutters. The 2 gear boxes for elevating the blade required worms and wheels. I started with my size requirements and drew a gear that would fit inside the gear box. I then calculated the number of teeth that would fit onto that gear. With that as a starting point I calculated the pitch of the gear and used that to match up to the available threads I could cut with my lathe. I had to keep adjusting the numbers until I could cut the right pitch screw on the lathe. The O.D. of the worm wasn't critical but the pitch was. So now I had the specs for the gear and the pitch of the worm. I adjusted the worm diameter until it matched the center distance needed keeping in mind that the O.D. of the worm couldn't get too large because it had to fit into the depth of the gear box cavity.
I made a cutter the diameter and pitch of the worm. I mounted my gear into my dividing head and tilted it to match the lead angle of the thread I was using. I then cut the gear teeth. A special lathe tool had to be ground to match the pitch of the gear for cutting the worm. It required a large clearance angle on the front face to clear the tooth while threading.
The pictures show the cutter, the setup with the dividing head tilted, the finished gears and worms.
I forgot to mention that 2 sets of worms and gears were required because one was left hand and the other right hand.

What was the declination angle in pic 2453?