A query about worms & wheels

[lemelman]

The project I'm working on requires a 1:22 worm & wheel to transmit a little more than 60Nm torque at about 4RPM.
From what I've read, it seems that I need Mod2 or 12.7DP gearing to fit into the space available. Because the price of commercial items exceed my budget I've decided to make my own - by making a hob at the same settings as the worm, and using that to cut the wheel. The worm will be steel and the wheel, bronze or brass.
It appears that the Mod2 requirement is to satisfy the need for teeth strong enough to transmit the required torque. Since steel is rather stronger than bronze it seems to me that thick wheel teeth coupled with thinner worm teeth would be a better combination than teeth of equal thickness. Since I'm making a hob, could I not make the gaps bigger than the teeth, so that the teeth of the wheel turn out to be thicker than the gaps?
Or is my thinking wrong?

 

[tel]

No, if you make the gaps bigger the teeth will be smaller

 

[lee9966]

I think he means making the gaps on the hob bigger, which will result in bigger teeth, right?

 

[lemelman]

I think he means making the gaps on the hob bigger, which will result in bigger teeth, right?

Yes, that's what I mean.
The bigger gaps on the hob, and hence the worm, because they will be cut at the same settings, will result in thicker (or bigger) teeth on the wheel.

 

[tel]

Well, I suppose that would work, but I can't really see much advantage to it

 

[Ken I]

I guess you can do it - I've done the opposite - making the worm teeth "thicker" to accomodate wear and eliminate backlash (jukebox restoration project).

If you are making your own hob you can also suit yourself as to pitch - when working with standard DP and Module gears the worm pitch is some fraction of Pi - refered to as Pi pitch - normally done using 22/7 changewheels but I have seen some lathes with this as a selectable option.

If you are making your own hob you can make it any standard metric or imperial pitch and make your wormwheel to suit - the axle centres will be a result of this selection process - not possible when working between existing axle centres.

Regards,
Ken

 

[lemelman]

Well, I suppose that would work, but I can't really see much advantage to it

The advantage I'm trying to obtain is strength.
According to the literature I've managed to find, a Mod2 worm/wheel is able to transmit around 60Nm torque as a result of the teeth size.
My thinking is that thicker teeth are able to transmit more torque, and this normally means a bigger Mod. A 22 tooth Mod2 wheel has an overall diameter of 48mm. A 22 tooth Mod2.5 wheel diam is 60mm - I've only got space for 48mm.

 

[tel]

Fair enough, as far as it goes BUT to strengthen the teeth on the one you will need to weaken the teeth on the other resulting in no net gain?

 

[Ken I]

Tel, I see his point - if the bronze wormwheel is say 50% the tensile strength of the steel worm then by upping the tooth thickness of the wormwheel to double the thickness of the worm - then both have the same optimised strength.

(approximately - there are other issues such as bending moments)

Ken

 

[lemelman]

Tel, I see his point - if the bronze wormwheel is say 50% the tensile strength of the steel worm then by upping the tooth thickness of the wormwheel to double the thickness of the worm - then both have the same optimised strength.

(approximately - there are other issues such as bending moments)

Ken

Exactly. Not only is steel stronger than bronze, but if you think about the part of the worm in contact with the wheel, then the "teeth" of the worm are effectively supported at each end, whereas the teeth of the wheel are on their own.

 

[tel]

Yes, I can see the argument, but I'm not really convinced of any real advantage, if there was the commercial boys would have jumped on it before this. Also, I suspect that the wear rate of the steel worm would be higher than that of the bronze wheel and if the worm teeth are thinner to start with .....

 

[lemelman]

Yes, I can see the argument, but I'm not really convinced of any real advantage, if there was the commercial boys would have jumped on it before this. Also, I suspect that the wear rate of the steel worm would be higher than that of the bronze wheel and if the worm teeth are thinner to start with .....

What I didn't mention was that the 4RPM is intermittent and no more than 20 seconds at a time, so torque transmission is the major consideration, with wear a minor also-ran.
It's actually for a powered reclining mechanism on a wheelchair, and it mustn't break. I can tolerate gradual wear but a broken tooth would be a disaster.

 

[cfellows]

The project I'm working on requires a 1:22 worm & wheel to transmit a little more than 60Nm torque at about 4RPM.
From what I've read, it seems that I need Mod2 or 12.7DP gearing to fit into the space available. Because the price of commercial items exceed my budget I've decided to make my own - by making a hob at the same settings as the worm, and using that to cut the wheel. The worm will be steel and the wheel, bronze or brass.
It appears that the Mod2 requirement is to satisfy the need for teeth strong enough to transmit the required torque. Since steel is rather stronger than bronze it seems to me that thick wheel teeth coupled with thinner worm teeth would be a better combination than teeth of equal thickness. Since I'm making a hob, could I not make the gaps bigger than the teeth, so that the teeth of the wheel turn out to be thicker than the gaps?
Or is my thinking wrong?

I think what you're asking is, "If I make the gaps in the hob bigger, will that result in thicker teeth in the gear that I plan to cut with it?"

The answer is yes, but I don't see the advantage of doing that.