Viscosity of helical gears in worm gear reducers

　　 Viscosity of helical gears in worm gear reducers

　　Viscosity is the primary factor in preventing contact between the worm and the wheel in a worm‑gear assembly. While the gear’s load and size determine the required lubricant, ISO 460 or ISO 680 are quite common, and ISO 1000 is by no means unheard of. If you’ve ever tried to filter a lubricant within this viscosity range, you’ll know it can be problematic, as on‑site filters or pumps often lack the appropriate size or rating to operate effectively.

　　Therefore, you may need to equip this type of equipment with a dedicated pump and filter. Lubricants with such high viscosity require low‑speed pumps to prevent the lubricant from triggering the filter bypass. They also necessitate filters with large surface areas to ensure adequate flow.

　　 Types of lubricants to be aware of

　　A commonly used lubricant for worm gears is a mineral-based composite gear oil. No additive can enable a lubricant to indefinitely prevent sliding wear; however, the combination of natural or synthetic fatty additives in composite gear oils provides excellent lubricity and offers additional protection against metal-to-metal contact.

　　Another commonly used lubricant for worm gears is a mineral‑based, extreme‑pressure (EP) gear oil. If your worm gear includes brass components, this type of lubricant may present certain issues. However, if your operating temperatures are relatively low or the gear teeth do not contain brass, this lubricant performs very well.

　　Polyalphaolefin (PAO) gear lubricants are ideally suited for worm‑gear applications due to their inherently excellent lubricating properties. When using PAO gear oils, care must be taken with the additive package, as it may contain extreme‑pressure (EP) additives. Standard load‑carrying (AW) enhanced gear oils are generally acceptable, but it is essential to verify that their performance is compatible with most metals.

　　The author recommends closely monitoring wear metals in oil analysis to ensure that the AW additive package does not become overly reactive, leading to significant brass leaching. Even under conditions of minimal AW reactivity, its impact should be far less than that of EP additives; however, it may still be detectable in metal‑wear testing. If you require a lubricant capable of operating at temperatures above or below the typical range, there may be suitable PAO‑based products available.

　　Polyalkylene glycols (PAG) are a fourth type of lubricant that is becoming increasingly common. These lubricants offer excellent lubricity and, unlike many mineral-based oils, do not contain waxes that can cause low‑temperature issues, making them an outstanding choice for cold‑weather applications. However, caution is required when using PAG oils, as they are incompatible with mineral oils, certain seals, and some paints.

　 　Worm Gear Metallurgy

　　The most common worm gears are made of 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 of brass because its design is sacrificial.

　　If two surfaces are in contact, worm‑gear wear is relatively mild, since the worm wheel is softer and thus undergoes most of the wear. Oil analysis reports for such systems almost invariably reveal some copper and low levels of iron—hallmarks of a sacrificial worm wheel.

　　This brass worm wheel introduces another issue into the lubrication equation for worm gears. If sulfur-phosphorus extreme-pressure (EP) gear oil is introduced into a worm gear housing containing a brass worm wheel, and the temperature is sufficiently high, the EP additive will become activated. In conventional steel gears, this activation forms a thin oxide layer on the surface, helping to protect the gear teeth from impact loads and other severe mechanical conditions.

　　However, on brass surfaces, the activation of extreme-pressure additives can lead to severe sulfur‑induced corrosion. Within a short period, large portions of the wheel’s load‑bearing surface may be compromised, resulting in serious damage.