Worms / Must the mating gear be helical 1

[ArcheJoe]

Hi All,

Forgive me for not knowing more about gears, however, my expertise is in construction engineering. This said I recently had an idea for a building system that I wanted to prototype. The scale at which I'm working is to small to use the hydraulic positioning system the full scale system would use, so I'm approximating the function with a gear system, which needs a lot of refining.

essentially I have two perpendicular drives. One drive is oriented along the x axis, the second axis along the z axis. The x axis drive is fitted with a worm, specifically I'm planning to use [URL unfurl="true"]https://sdp-si.com/eStore/Catalog/Group/175[/url] part number A 1C 5-N32.

The site provides me the following info
Pitch = 32
Worm Spec. 4.08degree RH, 1 lead / .438" pitch diameter
Pressure Angle 14.5 Degree
Face Width (FW) 0.6875
Overall Width 0.875"
Outside Dia. (O.D.) 0.5"
Hub Dia. 0.32"

Now, the problem I'm having is figuring out the geometry of the mating gear. I think it has to be a helical gear. but for my purpose it needs to be a big helical gear. The diameter is 388 mm. Now I'm pretty handy with parametric modeling, but I don't know how I'm supposed to combine the specifications of the worm in order to develop the geometry of its mate (dia 388mm). Furthermore is it possible to use a worm with a basic spur so I could save on 3d printing costs and instead laser cut it from plexi?

I've included a screen shot of what I'm trying to do below

http://www.radical-craft.com/gearSketchFromRhino.jpg

Any feedback would be greatly appreciated

 

[MikeHalloran]

You can mate a worm with a regular straight cut gear or rack of the same pitch and pressure angle, provided that you skew the worm axis by the worm's helix angle.

I.e., you still need a big gear, but it doesn't have to be helical.



Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

ArcheJoe-

The answer to your question is no, a conventional involute spur or helical gear will not mesh properly with that worm. The faces of typical worm gear tooth flanks are somewhat concave to better conform to the convex worm flanks.

If you need a proper 388mm worm gear digital model for rapid prototyping, I'd suggest you find someone capable of creating the model for you. There are lots of gear consultants that can do this for you quickly and relatively cheaply. Getting the prototype right on the first try will be much cheaper in the long run.

 

[MikeHalloran]

Allow me to resolve the apparent discrepancy.

Tbuelna is correct; mating a worm to a spur gear can't work.
... if you need long life and quiet operation and minimal lash.

For a prototype/throwaway development mule, it may be good enough, depending on loads and operating conditions so far not revealed.



Mike Halloran
Pembroke Pines, FL, USA

 

[tbuelna]

MikeHalloran,

There is an orthogonal axis thread form that mates perfectly with an external spur or helical gear. It's the grinding wheel shape used in generating type finish grinding of external spur or helical gears.

Not the same as a cylindrical worm.

 

[gearcutter]

Worms mating with spur gears is more common than you think.
I've seen many examples through the years that I've had to reverse engineer.
Situations where worms mate with helical gears is even more common.


Worm & Helical Gear Set by ronvol, on Flickr

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[mfgenggear]

HAHA typically it is more common to cut a worm gear with a hob to match the worm.
the helix of the worm gear must match the worms helix.
then the set has to be pattern to make sure it is correct at the right center distance.
I threw another monkey wrench in the works.

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[gearcutter]

Here's a model of a relatively large set that I had to reverse engineer.
The set was out a German made wire coiling machine. The machine ran 12 strands so there were 12 of these sets for each machine in the plant.
Pitch is 4.233 module, 1 start worm, 66 teeth in gear, gear OD = 288.65mm.


Worm & Helical Gear 02 by ronvol, on Flickr

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[gearcutter]

..........and here's an extreme example, pretty much a spiral set.
10 start worm, 6 teeth gear, 2.25 module.


Worm & Helical 03 by ronvol, on Flickr

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[mfgenggear]

gearcutter

it will work but I for see issues with such a set. I guess it depends on the application.
it can not be pattern very well. right?

Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[gearcutter]

If you're interested, here's a video of one of our machines cutting a 15 start worm that mates with a 15 teeth helical pinion shaft.
At 64deg helix angle, not really the sort of thing to be doing in a hobbing machine.
Parts are out of another German machine, this one made wire netting.

http://youtu.be/xNE3uSn3MwE

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[gearcutter]

mfgenggear - the contact pattern is what you would expect to see from a spiral (crossed helical) set.......similar to the pattern of an unloaded spiral bevel set but with a much narrower contact zone.
'Horses for courses' I guess.


Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[mfgenggear]

Gearcutter

I believe what your saying is true, but at my place we always pattern worm gears with worms.


Mfgenggear
if it can be built it can be calculated.
if it can be calculated it can be built.

 

[MikeHalloran]

ArcheJoe, there's another option available.

Buy a spare worm (or two) and use your Dremel tool to turn it into a hob.

Then hob the worm wheel from a plain blank.
Brass will probably work better than Plexiglass.
... but Plexi is possible, if the hob teeth are razor sharp.
Use kerosene as a cutting lubricant.



Mike Halloran
Pembroke Pines, FL, USA

 

[ArcheJoe]

Thank you everyone for the feedback, I will definitely be looking to a gear consultant as the full the scale project comes online, I've decided to take the short route and use SolidWorks for now and just 3d print the pieces to make it easier to include ducts for lubrication. I look forward to having some photos to post when the prototype is completed. Thanks for the help!

 

[tbuelna]

gearcutter- I checked my copy of Dudley and it agrees with your comment about a crossed-axis helical gearset being essentially the same thing as a non-enveloping worm gearset.

ArcherJoe- Look forward to seeing photos of the finished product.

 

[gearcutter]

tbuelna - If you're interested; here's a link to a video showing a simulation of the unloaded contact pattern of one of the worm/helical images shown above. As you'll see; an extremely narrow contact zone.

http://youtu.be/qhwAgdCHic0

Ron Volmershausen
Brunkerville Engineering
Newcastle Australia

http://www.aussieweb.com.au/email.aspx?id=1194181

 

[tbuelna]

gearcutter- Nice animation.

The benefit of a "non-enveloping" worm gearset would be less sensitivity to the position of the worm relative to the face center of the gear, versus an "enveloping" type of worm gearset. The downside, as you point out, would be significantly reduced load capacity.

 

[mfgenggear]

tbuelna

thank you I was trying to say that.

 

[tbuelna]

mfgenggear-

This was an interesting topic of discussion for me. I do not know a whole lot about worm gears, and I learned a thing or two from this thread.