Minimal Worm Expertise

Worm gearing has long been a big part of the mechanical world. If you work in the gear trade, “civilians” will rightly expect you to know things about it. Very few of you will ever have to design or make a new set of worms, but most of you will encounter them in the machines you use to make other gears or that are commonly used in general life.

A good example of this is an e-mail I got from a racing buddy who was asked by his favorite local bakery to repair a sixty-year-old dough mixing machine. The bakery’s owner seemed surprised that parts were not on the shelf somewhere — and even more shocked that custom made replacements would take months to obtain and cost thousands of dollars. With a bit of guidance from me they were able to adapt an off-the-shelf “generic” worm gear speed reducer and restore the mixer to its kringle-making duties.

So, you may not make them, but you won’t be able to avoid them either. What do you absolutely have to know about worms to get by — or to at least point the questioner in the right direction?

Here is my list:

1. Worm gears are among the most compact ways to achieve a high reduction ratio. A single stage of worms can easily provide a 100:1 speed reduction; other gear types would typically need three or four stages to reach that level of reduction. Less stages, less parts, less cost.

2. Worm and worm gear outside diameter do not vary much with ratio changes. In other types of gears, increased reduction ratio results in a smaller pinion diameter and a larger gear diameter. For worms this is not the case, i.e. — another aspect of the compactness feature.

3. The worm and the gear are the same “hand.” Except for crossed helicals and internal helicals, a right-hand pinion drives a left-hand gear or vice-versa; not so with worms. The “handing” affects rotation direction so you have to keep your wits about you in analyzing the system.

4. Do not rely on “self-locking” claims. Much has been written about worms not “back driving.” Some of what is written has landed people in court when the mechanism “slipped” due to wear, vibration, temperature, or lubrication characteristics. My standard response to questions about self-locking is “If you want it to self-lock, it won’t — if you don’t want it to self-lock, it will.” You need a real safety device if people and/or product have to be protected. For other applications, a serious test and development program is required.

More worm information next time…

About Charles D. Schultz 678 Articles
Charles D. Schultz is President of Beyta Gear Service and one of Gear Technology's technical editors.

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