Worm - Worm Gear as a Speed Increaser 1

[takoot]

I know it's not common nor recommended, but I've seen it done before -if you open the head box of a portable grinder/cutter you'll see the wormgear being used as a driver and the large lead angle worm as the driven.

In my application, power is minor -less than 0.05 HP, but the speed increase of around 20:1 is essential. I appreciate if somebody can tell me where I can buy those.

 

[EnglishMuffin]

Are you sure those aren't skew gears ?

 

[takoot]

The ones that I saw were worm and wormgear. I was surprised, and if the grinder wasn't of a reputable make, I would have thought it's a bad practice.

 

[Mandrake22]

They could be worms. Keep in mind that at about a 40:1 ratio, worm gears can't be "back driven". Your 20:1 ratio should be doable.
Here is a starting place:

http://www.wmberg.com/prgears.html

 

[sreid]

At 0.05 HP could you consider a high speed motor with no gear increaser?

 

[takoot]

Mandrake, Sreid,

Thanks for your replys. I've checked wmberg, all what they have is low lead angle and hence can't be back-driven.

As for the high speed motor; it doesn't work for my application as there's no motor. The high speed is not a target by itself. We need to trasform a low speed high torque hand driven motion to a "higher" speed motion resisted by a fly wheel.

Thanks again for your contribution.

 

[JStephen]

Checking a catalog and the Bosch site, I find that the right-angle hand grinders have no-load speeds of 6,000-10,000 RPM. These are the large grinders used by welders, etc. That speed range doesn't sound like it has been run through a big reduction or increase from the motor. The Bosch site shows "spiral bevel bears", I believe.

I have seen worm-drive saws, but the worm drive is used to reduce the speed, not increase it. Layout is similar to the grinders.

 

[drawoh]

takoot,

You can run helical gears at 90 degrees apart. Are you sure that is not what you saw?

I would worry about the efficiency of your speed increaser. Is there no motor that runs at the speed you want? Try searching for spindle motors/drives.

An alternate speed increaser would be a two stage spur gear or belt drive. A 4:1 ratio and a 5:1 ratio together would be more efficient than any worm drive.

JHG

 

[notnats]

Worm gears have been used for a long time by Alfa Laval and other centrifuge manufacturers to increase drive speed from a 4 pole motor to 7,000 - 10,000 rpm with motors from .75 kw -30 kw, but this is only a 4:1 or 5:1 ratio.

Similarly Peugeot used a worm drive back axle with ratios of 4:1 or 5:1 up till about 1975 with no problems when the wheels were driving the engine. With good hydrodynamic lubrication conditions, an hourglass shaped worm and several teeth in contact, the drive can be very smooth with high efficiency.

However a 20:1 ratio would probably need 2 stage gearing to keep the helix angle high. With a very small motor, lube oil sliding viscosity losses might be high.

Jeff

 

[takoot]

Thanks notnats, any idea where I can buy those high lead angle worm drives - sizes will have to be really small now I'm going with 2 stage gearing, and the space is tight.

 

[byrdj]

Have you looked at any of these sites to see if they have the size you need
"

http://www.schafergear.com/chicagogear/gear_catalog/worm_gears.html"

"

http://www.calsmall.ne.jp/KHKCat_en/tocf_KHK.htm"

"

http://www.ohiogear.com/"

 

[notnats]

The small gears from Chicago Gear (see Byrdj's links) look about the size for a .05HP (37W) application. A 4 start worm driven by a 20 tooth wheel will have a 5:1 ratio.

I would check their suitability for the application with the manufacturer.

But at 20:1 your final output shaft will be running at 70,000 rpm if your drive motor is a 2 pole induction motor at 60hz. If this is what you are looking for you will have some lubrication and bearing problems to consider. The whole 37W might turn into heat.

It may be that something like a 100mm dia rubber wheel driving a 5mm dia shaft might be more suitable, but I don't have any experience of this type of drive.

Jeff

 

[takoot]

byrdj, notnats,

Thank you for your valuable contributions. The mechanism will be driven by hand, hence the high rpm is not a concern.

 

[amorrison]

I have read that it is good practice to assume at most 50% power efficiency with worm gears - unless you know for sure otherwise. Small usually means even less efficiency.

An average person can put out 100W continously - BUT does this apply to arms alone (ie. no leg power)? - I don't know. So getting a 37W steady output via arms alone could be a problem if the coversion efficiency really stinks.

 

[amorrison]

Talk to your grandpappy – maybe he used something like this.

http://www.whitemountainexports.com/other_tool3.html

From a google search using - hand tool grinder

 

[sreid]

Can harmonic drives or cyclo reducers be back driven for a "gear step-up"?

 

[drawoh]

sreid,

Apparently, Harmonic drives can be back driven. I have been told that they are used this way. The smallest ratio of a harmonic drive is around 60:1, so it will not work in takoot's application.

I am trying to wrap myself around the input torque. This can be surprisingly high if you did not sit down and work out the effect of the speed increase. Inertia increases by the square of the gear ratio.

My gut feeling about this is that this mechanism should be made as efficient as possible.

JHG

 

[notnats]

A well designed and lubricated worm drive can be >95% efficient. I would be suprised if off the shelf miniature gear sets come into this category. One of the gear company sites emphasised the importance of accurate positioning of the gears. The whole assembly would need to be in a housing which enabled the gears to dip into a light oil to give good hydrodynamic lubrication. It probably wouldn't work if run dry.

Jeff

 

[drawoh]

nonats,

According to my machine design book, worm gears are very efficient at lead angles between thirty and sixty degrees. The whole process is massively affected by friction, not surprisingly.

By messing with the worm diameter, I suppose you can get just about any ratio you want out of an efficient worm drive. Unfortunately, most high ratio worm drives are meant for people like me who want something that _cannot_ be back driven. You could make a custom worm gear set, but this is getting as expensive as all the other options being thrown around here.

I suspect that an off-the-shelf two-stage spur gear drive will do the job and cost the least money. As you noted, we get around all sorts of workmanship issues by buying a packaged drive.

JHG