A Continuing Discussion on Worm Gears

I was pleased to see Aaron Black's article on worm gearing in the September 2007 issue of Practicing Oil Analysis.

I would like to add a few comments that might be helpful for clarification and provide a little background. As per the original article and to avoid confusion, the entire gear set shall be referred to as the "worm set", the steel driving gear as "the worm" and the bronze-driven gear as the "wormwheel" or "wormgear". AGMA standards 6022 and 6034 refer to worms and wormgears. British Standard 721 refers to worms and wormwheels.

Generally, the yellow metal of the wormwheel is referred to as bronze rather than brass, even though in many cases the material actually has a brass composition rather than a true bronze composition. This is due at least in part to the predominance of SAE 65 gear bronze as the material of choice for wormwheels in the last century.

Newer materials have since been applied, but SAE 65 or UNS C90700 often remains the material of choice. Other bronzes or copper alloys are listed in ASTM B427, Standard Specification for Gear Bronze Alloy Castings. Note that ASTM B427-93a was withdrawn in 2002, and the standard apparently was later replaced by ASTM B427-02.


Results of Rubbing
As Black wrote in the wormgear article, the steel worm thread rubs on the flank of the bronze wheel gear tooth. Bronze gear teeth rubbing on case-hardened steel worm threads provides strength and a lubricity not found in other metal combinations. The British Standard, BS 721, Specification for Worm Gearing, 1963, did allow for several metal combinations such as steel worms on gray cast iron wheels, but only for lightly loaded gear sets operating at very light loads.

The American Gear Manufacturers Association (AGMA) Standard 6034, Practice for Enclosed Cylindrical Wormgear Speed Reducers and Gearmotors, does not rate materials other than steel worms operating on bronze wheel gears.

The concept that a worm set may be self-locking has unfortunately been disproved, and can lead to a potentially dangerous situation. Wormgear sets may have trouble driving the worm by the wormwheel depending on the lead angle of the worm thread, but even a set that will not drive the worm by torque applied to the wormgear may backdrive when vibration or lubrication are present. If an assembly must be locked to prevent rotation, a brake should always be used because the worm set may backdrive without any warning.

The fact that the worm set will provide a large reduction in a small package with large increases in torque may be partially offset by the efficiency of the wormgear set, which tends to drop as the speed of the worm set decreases. This also results in a thermal limitation on worm sets due to the heat generated by the sliding losses.

Proper adjustment of the worm thread and wheel gear tooth mesh will aid in providing a lubricant film between the surfaces.

EP Additives
In agreement with Black's remarks concerning extreme pressure additive packages with yellow metal gears, in the presence of heat, the sulfur-phosphorous extreme pressure (EP) additives will attack the brass or bronze material.

Lastly, the temperature is usually thought of as the oil bulk temperature. However, the temperature of the film between the wheel gear tooth flank and the worm thread is also critical and can initiate corrosive action even when the oil bulk temperature is relatively low. AGMA Standard 9005, Industrial Gear Lubrication, provides lubricant recommendations for most types of gearing, including worm and wormgear sets.

With regard to using steel worms with steel wheel gears, only lightly loaded low-speed applications can succeed. The same is true of brass on brass. The tendency for adhesive wear is much greater when the two materials of the individual pair are closer in chemistry, and to a lesser extent, hardness and heat treatment.

The author also discussed the problem of filtering a gear lubricant that must meet the high viscosity requirement in this application. There are some things that can be done to mitigate this problem. Use an oil that is hydrophobic or one that sheds water readily, allowing for gravity separation of water droplets from the oil and minimal emulsification. This will allow the water to be withdrawn either periodically or automatically from the oil sump or reservoir.

The lubricant can also be run through a separator or oil purifier to remove both entrained water and particulate contaminants. This is practical only for plants with large numbers of drives or for drives with large quantities of expensive lubricants such as synthetics. Removal of particle and water contaminants will prolong the operating life of both the gear set and the lubricant, and can be verified with oil analysis. 

Richard L. Dornfeld
staff engineer, Walker Process Equipment
a division of McNish Corporation

Editor's Note

Aaron Black's original article, "The Ins and Outs of Worm Gears," can be read online at http://www.practicingoilanalysis.com/article_detail.asp?articleid=1080.

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