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Worm 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.
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 50°F (28°C) over ambient temperatures while worm gear temperatures typically rise 90°F (50°C) over ambient.
The following factors affect worm gear efficiency:
Due 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 190°F (88°C) 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 1 relates ISO viscosity grade with the AGMA 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.
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 180°F (82°C). 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 worm’s type, size, speed and operating 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.
EP mineral gear oils are used more extensively in worm gears. Under conditions of high pressure and 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 180°F (82°C).
Typical commercial oils: Shell Omala, Texaco Meropa, Exxon Spartan EP, Mobilgear 634 and 636, and Chevron EP Gear Oil.
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 don’t 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.
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.
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 the laboratory 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.
A 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 214°F (101°C). A PAO synthetic fluid was put in the gearboxes. This resulted in an average temperature decrease of nearly 20°F (-11°C). All gearboxes were converted to the synthetic. After 18 months of operation at higher loads and speeds than previously, no gearbox failures occurred.
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 180°F (82°C) because they often reduce temperatures 20°F or more (11°C 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.