Worm Basics – Part 2

Continuing our discussion of the “least you need to know” about worm gearing:

5. Efficiency is highly variable for worms. If it is critical to your application, you have to test.

6. Worm gears require careful attention during assembly. If you do not get the gear centered on the worm it will not perform as designed. If you are using a double-enveloping — sometimes called “hourglass” worms — the axial positioning of the worm is critical as well. No matter how precisely the parts are manufactured, they will be quickly ruined if improperly assembled. They are not as fussy as bevel gears, but contact patterns under load must be checked.

7. Most worm designs feature a “hard” worm turning a “soft” gear. That soft gear is usually a bronze alloy, although iron is sometimes used to reduce costs. This results in noticeable pitting in the gear after a relatively short period of service. Modern eyes are calibrated to hard pinions running with hard gears, so the sight of “potholes” on those shiny tooth flanks can cause a panic unless someone investigates the situation and assures them the sets are “wearing in” — not “wearing out.” Less than 40 years ago, mechanics were accustomed to pitting since most American-built helical speed reducers used through-hardened materials.

8. Worm gears usually required much higher viscosity lubricants than similar-sized helical, spur, or bevel gearboxes. Do not think you will solve an efficiency problem by using a thinner oil. Follow the recommendations of the equipment suppliers or well-known lubricant vendors. Anti-pitting and phosphorus-containing additives must be carefully chosen lest they react with the gear material and accelerate the damage to the tooth flanks.

9. Worms are common in precision motion applications, but do not confuse low backlash alone with precision. Reduced backlash worm sets are manufactured to tighter tolerances. Simply adjusting the center distance with eccentric carriers is not the same thing, although it may be sufficient in some applications.

Oddly, the first gears I learned about were worms. My employer decided to add worm boxes to their product offerings and took a short cut by importing the parts from the U.K. I was assigned to revise bills of material and look at drawings to make sure the new numbers were correct. On the drawings was a pre-printed bit of advice: IF IN DOUBT, ASK! Good advice then and now. If you need gear info that you can count on, use the Gear Technology online archives.

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