Worm Gear
noun

  1. A gear consisting of a shaft with a spiral thread that engages with and drives a toothed wheel.

  2. The toothed wheel driven in this way.

Worm gears are an old style of gear, and a version of one of the six simple machines. Basically, a worm gear is a screw butted up against what looks like a standard spur gear with slightly angled and curved teeth. It changes the rotational movement by 90 degrees, and the plane of movement also changes due to the position of the worm on the worm wheel (or simply "the wheel"). They are typically comprised of a steel worm and a brass wheel.

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Figure 1. Worm wheel. Most worms (but not all) are at the bottom.

How They Work
An electric motor or engine applies rotational power via to the worm. The worm rotates against the wheel, and the screw face pushes on the teeth of the wheel. The wheel is pushed against the load.

Why to Use Worm Gears
There area few reasons why one would choose a worm gear over a standard gear.

The first one is the high reduction ratio. A worm gear can have a massive reduction ratio with little effort - all one must do is add circumference to the wheel. Thus you can use it to either greatly increase torque or greatly reduce speed. It will typically take multiple reductions of a conventional gearset to achieve the same reduction level of a single worm gear - meaning users of worm gears have fewer moving parts and fewer places for failure.

A second reason to use a worm gear is the inability to reverse the direction of power. Because of the friction between the worm and the wheel, it is virtually impossible for a wheel with force applied to it to start the worm moving. On a standard gear, the input and output can be turned independently once enough force is applied. This necessitates adding a backstop to a standard gearbox, further increasing the complication of the gear set.

Why Not to Use Worm Gears
There is one particularly glaring reason why one would not choose a worm gear over a standard gear: lubrication. The movement between the worm and the wheel gear faces is entirely sliding. There is no rolling component to the tooth contact or interaction. This makes them relatively difficult to lubricate. The lubricants required are usually very high viscosity (ISO 320 and greater) and thus are difficult to filter, and the lubricants required are typically specialized in what they do, requiring a product to be on-site specifically for that type of equipment.

Lubrication
The main problem with a worm gear is how it transfers power. It is a boon and a curse at the same time. The spiral motion allows huge amounts of reduction in a comparatively small amount of space for what is required if a standard helical gear were used. This spiral motion also causes an incredibly problematic condition to be the primary mode of power transfer. This is commonly known as sliding friction or sliding wear.

With a typical gear set the power is transferred at the peak load point on the tooth (known as the apex or pitchline), at least in a rolling wear condition. Sliding occurs on either side of the apex, but the velocity is relatively low.

With a worm gear, sliding motion is the only transfer of power. As the worm slides across the tooth of the wheel, it slowly rubs off the lubricant film, until there is no lubricant film left, and as a result, the worm rubs at the metal of the wheel in a boundary lubrication regime. When the worm surface leaves the wheel surface, it picks up more lubricant, and starts the process over again on the next revolution.

The rolling friction on a typical gear tooth requires little in the way of lubricant film to fill in the spaces and separate the two components. Because sliding occurs on either side of the gear tooth apex, a slightly higher viscosity of lubricant than is strictly needed for rolling wear is required to overcome that load. The sliding occurs at a relatively low velocity.

The worm on a worm set gear turns, and while turning, it crushes against the load that is imposed on the wheel. The only way to prevent the worm from touching the wheel is to have a film thickness large enough to not have the entire tooth surface wiped off before that part of the worm is out of the load zone.

This scenario requires a special kind of lubricant. Not only will it will have to be a relatively high viscosity lubricant (and the higher the load or temperature, the higher the viscosity must be), it must have some way to help overcome the sliding condition present.

Viscosity
Viscosity is the major factor in preventing the worm from touching the wheel in a worm gear set. While the load and size of gearing determines the required lubricant, an ISO 460 or ISO 680 is fairly common, and an ISO 1000 is not unheard of. If you've ever tried to filter this range of viscosity, you know it is problematic because it is likely that none of the filters or pumps you have on-site will be the proper size or rating to function properly. Therefore, you would likely need to get a specific pump and filter for this type of unit. A lubricant that viscous requires a slow operating pump to prevent the lubricant from activating the filter bypass. It will also require a large surface area filter to allow the lubricant to flow through.

Lubricant Types to Look For
One lubricant type commonly used with worm gears is mineral-based, compounded gear oils. There are no additives that can be put into a lubricant that can make it overcome sliding wear indefinitely, but the natural or synthetic fatty additive combination in compounded gear oils results in good lubricity, providing an extra measure of protection from metal-to-metal contact.

Another lubricant type commonly used with worm gears is mineral-based, industrial extreme pressure (EP) gear oils. There are some problems with this type of lubricant if you are using a worm gear with a yellow metal (brass) component. However, if you have relatively low operating temperatures or no yellow metal present on the gear tooth surfaces, this lubricant works well.

Polyalphaolefin (PAO) gear lubricants work well in worm gear applications because they naturally have good lubricity properties. With a PAO gear oil, it is necessary to watch the additive package, because these can have EP additives. A standard-duty antiwear (AW) fortified gear oil will typically be acceptable, but check that the properties are compatible with most metals. The author recommends to closely watch the wear metals in oil analysis testing to make sure that the AW package isn't so reactive as to cause significant leaching from the brass. The effect should be far less than what would be seen with EP even in a worst-case scenario for AW reactivity, but it can show up in metals testing. If you need a lubricant that can handle higher- or lower-than-typical temperatures, a suitable PAO-based product is likely available.

Polyalkylene glycols (PAG), a fourth type of lubricant, are becoming more common. These lubricants have excellent lubricity properties, and do not contain the waxes that cause low-temperature problems with many mineral lubricants, making them an excellent low-temperature choice. Caution must be taken when using PAG oils because they are not compatible with mineral oils, and some seals and paints.

Metallurgy of the Gears
The most common worm gears are made with a brass wheel and a steel worm. This is because the brass wheel is typically easier to replace than the worm itself. The wheel is made out of brass because it is designed to be sacrificial. In the event that the two surfaces come into contact, the worm is marginally safe from wear because the wheel is softer, and therefore, most of the wear occurs on the wheel. Oil analysis reports on this type of unit almost always show some level of copper and low levels of iron - as a result of the sacrificial wheel.

This brass wheel throws another problem into the lubrication equation for worm gears. If a sulfur-phosphorous EP gear oil is put into the sump of a worm gear with a brass wheel, and the temperature is high enough, the EP additive will activate. In normal steel gears, this activation produces a thin layer of oxidation on the surface that helps to protect the gear tooth from shock loads and other extreme mechanical conditions. On the brass surface however, the activation of the EP additive results in significant corrosion from the sulfur. In a short amount of time, you can lose a significant portion of the load surface of the wheel and cause major damage.

Other Materials
Some of the less common materials found in worm gear sets include:

Steel worm and steel worm wheel - This application does not have the EP complications of brass gearing, but there is no room for error built into a gearbox like this. Repairs on worm gear sets with this combination of metal are typically more costly and more time consuming than with a brass/steel worm gear set. This is because the material transfer associated with failure makes both the worm and the wheel unusable in the rebuild.

Brass worm and brass worm wheel - This application is most likely found in moderate to light load situations because the brass can only hold up to a lower amount of load. Lubricant selection on this metal combination is flexible because of the lighter load, but one must still consider the additive restrictions regarding EP because of the yellow metal.

Plastic on metal, on plastic, and other similar combinations - This is typically found in relatively light load applications, such as robotics and automotive components. The lubricant selection depends on the plastic in use, because many plastic varieties react to the hydrocarbons in regular lubricant, and thus will require silicon-based or other nonreactive lubricants.

In Conclusion
Although a worm gear will always have a few complications compared to a standard gear set, it can easily be an effective and reliable piece of equipment. With a little attention to setup and lubricant selection, worm gears can provide reliable service as well as any other type of gear set.

Related Reading 
Ray Thibault. "Lubrication of Worm Gears." Machinery Lubrication magazine. May-June 2001.