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Lubrication of Worm Gears

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Noria CorporationTags:gear lubricationWorm gears operate under difficult conditions, presenting unique lubrication demands. They serve as

speed reducers in many different industries and applications. This article addresses how effectively

lubricated worm gears affect worm gear performance.

Worm gears are used in various industries and machinery applications. They are unique in their ability to

achieve large speed reductions in a compact space. They can transmit high loads at high-speed ratios.

Ratios of 20:1 up to 60:1 and higher are normally achieved.

There are three major types of worm gears:

1. Non-throated - a helical gear with a straight worm. Tooth contact is a single moving point on the worm

drive. This leads to high unit loads and wear.

2. Single-throated - has concave helical teeth which wrap around the worm. This leads to line contact,

permitting higher loads without excessive wear.

3. Double-throated - called a cone or hourglass. It has concave teeth both on the worm and helical gear.This increases from line contact area permitting increased loading and lower wear.

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Worm drives are inefficient because the gears experience sliding rather than rolling contacts, leading to

operating temperatures much higher than other gear types. Spur gears normally operate at 50F (28C)

over ambient temperatures while worm gear temperatures typically rise 90F (50C) over ambient.

The following factors affect worm gear efficiency:

Lead angle of the worm

Sliding speed

Lubricant

Surface quality

Installation conditions

Worm Gear LubricationDue to the sideway sliding motion in worm gears, it is difficult to maintain a hydrodynamic oil wedge. This

results in gears operating under boundary lubrication conditions. Also, high operating temperatures that

approach 190F (88C) and higher usually require oils with an ISO VG of 460 (AGMA Class 7) and

higher. They also require good thermal and oxidative stability.Table 1relates ISO viscosity grade with theAGMA Classification System.

The types of oils most commonly used to lubricate worm gears are compounded mineral oils, EP mineral

gear oils and synthetics. Each has its own unique characteristics and all three types are used

successfully.

Worm Gear Lube Types

Compounded Gear Oils

These lubricants have been used extensively in worm gears with great success in a wide variety of

applications. Compounded gear oil is a mineral basestock with normal rust and oxidation inhibitors that is

blended with four to six percent acidless tallow or synthetic fatty acid (the compounding agent). The

surface-active compounding agent gives these products excellent lubricity and prevents sliding wear in

worm gears.

Many OEMs recommend compounded gear oils. Compounded oils were initially used as steam cylinder

lubricants because of their ability to adhere to cylinder walls in the presence of steam. The temperature

limitation of compounded oils is approximately 180F (82C). Because compounded lubricants are difficult

to use out of this temperature range, they are often replaced with EP gear oils for consolidation purposes.

Most applications normally use an AGMA Class 7 or 8 compounded oil (ISO VG 460 and 680). In some

cases, an 8A (1000 VG) is used. The viscosity selection depends on the worms type, size, speed andoperating temperature. Refer to the OEM for specific viscosity recommendations.

Typical commercial oils: Mobil 600W Cylinder and Super Cylinder Oil, Texaco Vanguard 460 and 680,

Exxon Cylesstic 460 and 680, and Chevron Cylinder Oils 460 and 680.

Extreme Pressure (EP) Gear Oils

EP mineral gear oils are used more extensively in worm gears. Under conditions of high pressure and

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temperature, the EP (antiscuff) additive reacts with the metal surface to form a soft, slippery chemical

layer which prevents severe wear and welding. Previously, there was a concern that sulfur-phosphorous

EP additives would react with the bronze gear. However, new EP additive technology used by most of the

major lubricant suppliers has reduced the corrosive attack by utilizing nonactive sulfur. EP lubricants work

particularly well when shock loading occurs. EP gear oils also protect steel gears better than

compounded gear oils.

Typical recommendations are for both AGMA 7 and 8 viscosity grades. Like compounded gear oils, EP

gear oils limit operating temperatures to under 180F (82C).

Typical commercial oils: Shell Omala, Texaco Meropa, Exxon Spartan EP, Mobilgear 634 and 636, and

Chevron EP Gear Oil.

Synthetic Worm Gear Oils

Two major types of synthetic gear oils have been used successfully in challenging conditions with worm

gears: polyalphaolefins and polyalkelene glycols.

Polyalphaolefins (PAOs) are the most common type of synthetic lubricant. They have good high and low

temperature properties and are compatible with most mineral oils. Unlike some synthetics, PAOs dont

attack paints or seals. Most formulations contain a small amount of organic ester or antiwear mineral that

improves boundary lubrication conditions. Products that contain EP additives are also available. There

are no major disadvantages, other than cost, when using PAOs as worm gear lubricants.

Typical commercial oils: Chevron Tegra, Texaco Pinnacle, Exxon Teresstic SHP, Mobil SHC, Shell

Hyperia and Royal Purple Synergy.

Polyalkylene glycols (PAGs) are excellent for worm gear applications. They possess superior lubricity

properties and have good low and high temperature properties. The Viscosity Index of PAGs is higher

than most synthetics, approaching 280. Therefore, a lower initial viscosity grade can be used minimizing

internal friction resulting in improved worm gear efficiency. Most PAGs contain antiwear properties but

there are no formulations with EP additives.

Besides cost, a major disadvantage of polyalkylene glycols is their incompatibility with other fluids. They

also attack paints, seals and polycarbonate sight glasses.

Typical commercial oils: Shell Tivela and Mobil Glygoyle HE.

Case History 1

Laboratory tests were conducted on worm gears operating at a 20:1 gear ratio at 1,750 rpm.

Compounded and EP ISO 460 oils were evaluated for efficiency, then compared to both EP and non-EP

PAO ISO 460 oil. The EP PAO had a 1.2 percent efficiency improvement over the EP mineral oil. The

non-EP PAO had only a 0.9 percent improvement over the compounded gear oil. The viscosity at the

operating temperature was much higher for the synthetic because of its higher VI. By lowering the

viscosity of the PAO to 320 to more closely match the viscosity at the operating temperature, the

efficiency over the compounded gear oil was 3.5 percent.

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The final stage was to run an actual field trial at a utility on a coal pulverizer. The pulverizer was tested at

various load levels. It had a 22:1 reduction ratio. Controlled experiments were completed by measuring

electric power consumption comparing an EP ISO 320 gear oil with an ISO non-EP PAO. The same

viscosity was chosen for both the synthetic and non-synthetic oils because the operating temperatures

were low; therefore, the viscosity at the operating temperature was similar for both oils. Savings of 9.8

percent, 8.7 percent and 8.5 percent were achieved at different loadings. The difference between thelaboratory and field results is explained because the worm in the field exhibited more sliding friction.

A published paper demonstrated efficiency improvements in worm gears using a PAO synthetic fluid. The

paper was entitled Improvements in Industrial Gearing Efficiency through Application of Synthetic

Lubricants, by Blahey, Hakala, Swett, Straiton and Juves of ExxonMobil.

Case History 2A major beverage can producer in Texas was experiencing an average of four failures per year at a cost

of $12,000 per event in actual repairs and downtime. The company runs 11 double-throated worm gears

which operate at reduction ratios of 20:1 and 60:1. The gear units run at high speeds and loadings and

are lubricated with compounded AGMA 7 gear oil.

Temperatures on the gearboxes started to rise, reaching 214F (101C). A PAO synthetic fluid was put in

the gearboxes. This resulted in an average temperature decrease of nearly 20F (-11C). All gearboxes

were converted to the synthetic. After 18 months of operation at higher loads and speeds than previously,

no gearbox failures occurred.

Conclusion

Worm gears operate under difficult conditions and present unique lubrication demands.

Compounded gear oils with viscosities of ISO 460 to 680 are still used extensively.

An increasing number of EP mineral oils are used for consolidation purposes.

Synthetics have been used successfully. Both PAOs and PAGs produce excellent results.

Synthetics should be used when temperatures reach 180F (82C) because they often reduce

temperatures 20F or more (11C or more). They are also more tolerant of high temperatures.

Consider converting to synthetics when gear boxes have small reservoirs. The cost is low and the

benefits are high.