Helical Gear with Worm 2

[Snowmann]

I'm currently designing a 90 degree worm gearbox. Center distance (2.066&quot and (approximate 10:1) ratio is known. I'd like to use a P/M helical gear with the worm instead of a hobbed bronze or brass worm gear. I cannot find much information with this combination. How do I spec out a helical gear to mate with a worm?

The worm will be rolled onto an input shaft. I'll likely use 7/8" C1045. What diametral pitch's can be rolled to this shaft?

On a sidenote, when should a hunting tooth -not- be used?

Any help would be appreciated.

 

[plasgears]

The trend in worm gear systems is to provide a hard worm running against a resilient gear. A rolled worm wants to run against either bronze or plastic gears.

A notable exception was a molded nylon worm running against a molded acetal gear. It worked remarkably well.

 

[Snowmann]

Thanks, but the loading dictates that I definitely cannot use a plastic gear. This is not a lightly loaded gearcase.

 

[EnglishMuffin]

A non enveloping worm meshing with a helical gear is really just an extreme form of crossed-axis helical gear pair, and can be treated as such from a design point of view - all the kinematic and geometric formulae for helical gears will apply. You must of course ensure that the worm tooth is of the correct involute form. The contact conditions will not be good.
Don't know much about rolling.
There is no major disadvantage in using a hunting tooth that I know of, although people argue about the advantages. If you have a cyclic load that always occurs at the same angular location, then a hunting tooth is highly beneficial, but in general the merits of a hunting tooth are somewhat controversial. In the early days of gearing, when teeth were less accurate than they are today, it was probably more important. I believe the wooden cogs used in windmills always had hunting teeth, and the practice may have stemmed from there. Of course, if you need a ratio which happens to be factorable, you can't use a hunting tooth, which requires that the numbers of teeth be relatively prime. And if you happened to have a mechanism which relied on some sort of device which compensated for pitch error, you might not want a hunting tooth because it would vastly complicate the compensation algorithm.

 

[Spurs]

Snowmann

Helical gears are commonly driven by clylindrical worms such as the type you are considering.

Since you are planning on using a PM gear, you may be limited on what you can do in terms of the Helix Angle of the Gear. Very few PM houses are willing to tool a helical gear especially if the helix angles are much greater than 10 degrees. This is mainly due to the type of tooling and equipment limitations. If you do go ahead, I would highly recommend using a PM house who specializes in gears, not a PM house who also does gears!

There are other options in your design that you may want to consider. For example; instead of using a helical gear, you could have a worm drive a spur gear at some other angle than a 90 degree shaft angle. You may want to consider this to keep the cost of the PM gear down.

In terms of English Muffin's comments on using an involute worm, I disagree with the notion that the worm tooth must be involute shape. Straight sided worms (in the normal plane) are used sucessfully every day in countless applications. This is primarily due to the fact that the contact pattern in this type of system is point contact, instead of over a surface. The possible extra cost involved with producing an involute worm instead of a straight sided worm is not worth it.

You must also be made aware that efficiency in such systems of of significance. Efficiencies in the 30% to 70% range are not uncommon, so in your case, the output torque would be 10 x input torque x efficiency.

Should you need more help, follow this link as this company are experts in this particular field.

http://members.rogers.com/webgear/home.htm

 

[EnglishMuffin]

I certainly did not mean to imply by my comment that you can't use a straight sided worm. In the US, that is the most common type. Perhaps it was a bad choice of words - I merely meant that since worms and wormwheels are not standardized to the same extent as normal gearing, and a lot of custom tooling is used, you just have to make sure that the thread form adequately corresponds to the helical gear that you are using, in regard to pitch, root clearance etc. Perhaps I should just have said "correct form", although strictly speaking the profile of a straight sided rack is an extreme form of involute with an infinite base circle diameter.