Conveyor belts, industrial machinery, automation systems and various types of lifting equipment use worm gears for speed reduction and when space is very limited. However, these specialist gears need the right lubrication to ensure they can function effectively and to extend their life span.
Worm gears are specifically designed to transmit motion and power between nonintersecting shafts, such as shafts at right angles to each other. As worm gears work differently to other gears, they require specialist lubrication.
There are two components in these types of gears: the worm and the worm wheel or gear. The worm is a special form of helical gear with teeth that are called threads (it looks a bit like a screw). While the worm is usually made of a hard, wear-resistant steel, the worm wheel should be made from a good bearing bronze. However, bronze is highly sensitive to the active sulphur that is found in most EP gear oils.
Lubrication Engineers (LE) South Africa’s National Marketing Manager Gavin Ford explains that when the bronze reacts with sulphur, the gears can get etched. This is why LE recommends avoiding lubricants that include active sulphur. “It’s really important to choose an oil that adheres to the metal surfaces (whether the worm is above or below the wheel), provides an excellent film strength and prevents surfaces from bonding to each other from the sliding action,” says Ford.
Lubricating different types of worm gears
Worm gears come in three main variations, and each requires specific lubrication. The first is a cylindrical or non-throated worm gear, which is similar to a rack and pinion and has straight teeth. Ford suggests this type of gear be lubricated with a high-quality gear oil, such as LE’s Almasol Vari-Purpose Gear Lubricants.
Second is the single enveloping/throated worm gear. This has a helical cut with a parallel-sided gear mating with a concave curved worm wheel. For these types of worm gears it’s best to use a specialist worm gear lubricant, like LE’s Almasol Worm Gear Lubricants, or a high-quality gear oil.
The third type is the double enveloping (double throated) worm gear. It also has a helical cut but has a curvature of both gears that increases the surface of contact between the gears. “This can be very useful for power applications, but also generates the most heat from the sliding motion,” says Ford. The constant friction in this type means only specialist worm gear lubricants should be used.
Ford says the manufacturer’s specifications can also offer helpful guidance on the right product for a specific gear.
Understanding worm gears
Unlike other gears, the worm can easily turn the gear, but the gear cannot turn the worm. This is because the angle on the worm is so shallow that when the gear tries to spin it, the friction between the gear and the worm holds the worm in place. This feature is useful for machines such as conveyor systems, in which the locking feature can act as a brake for the conveyor when the motor is not turning. “However, this static action can cause massive pressures on the film of oil, often rupturing the film and allowing metal-to-metal contact,” explains Ford. Preventing this is where lubrication becomes so critical.
The action of the worm is similar to the action of a screw on a nut. Due to the wedge-like action of the worm thread on the gear tooth, the set is relatively quiet in operation. It also provides a very wide range of speed reduction. “High gear ratios can be achieved, often over 300:1. However, at high ratios the efficiency can be as low as 40–50%,” explains Ford. “The worm drives by its threads sliding into contact with the teeth of the worm wheel. This constant sliding action generates heat due to friction and therefore adequate cooling and superior lubrication must be provided for gears working at high sliding velocity and load.”