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Worm Gearing – The Underrated Performer in Motion Control
Worm Gearing
Worm gearing has many characteristics ideal for power transmission and motion control applications. Improvements in lubrication, gear set design and machining accuracy have improved efficiency. These improvements, combined with numerous additional advantages, continue to make worm gearing suitable for challenging drive applications across a variety of industries.
Key Advantages of Worm Gearing
Efficiency Considerations
Rising energy costs have increased engineering emphasis on the efficiency of drive systems. Luckily, worm gearing efficiency has increased through a variety of technical improvements.
Efficiency across all types of gear technologies is impacted by losses at the gear mesh, bearings, seals and lubricant. Gearing involves sliding and rolling contact and requires rotation of a mass in an oil bath. Both characteristics generate losses, which can be minimized.
Worm gearbox efficiencies will vary with speed, ratio, load, ambient temperature, duty cycle, lubricant viscosity and break-in period. Accurate comparisons depend on the use of realistic efficiencies (for all gearboxes considered) and proper size selection.
Using “rough approximations” of efficiencies can completely distort the comparison. Realistic values should be used, which are best determined by comprehensive dynamometer or field evaluation. Dynamometer testing is a reasonably simple procedure for evaluation under these various conditions. Field testing in a particular application is more realistic but can be challenging to set up and control.
The comparative evaluations of realistic efficiencies often show that the advantages of using a worm gearbox far outweigh slight losses in efficiency. Even if small efficiency losses are important, it’s possible to use a configuration with a planetary input to provide higher efficiency while retaining worm gear drive advantages.
Helical vs. Worm Gearing Systems
A helical-bevel system has a slight advantage if efficiency is the only consideration for selection. Efficiency is weighed against cost, durability and other considerations. In general, spur or helical gearing is more efficient than worm gearing per gear stage. This difference is notable for ratios above 15:1 but becomes less noticeable in ratios below 15:1, which are more commonplace for servo motor applications. The smoothness, low noise, higher shock load capacity, higher single stage ratio and more compact size of worm gearing is equal, if not more advantageous than efficiency alone.
For those applications requiring high efficiency, the advantages of both systems can be obtained by combining both types of gears. When worm gearing is used in conjunction with a helical, spur or planetary gear primary stage, higher drive train efficiency can be achieved without loss of basic worm gear advantages.
As an example of the advantages of this approach, consider an application requiring a 50:1 ratio. To obtain this ratio, all-helical gearing will require three gear sets and four sets of bearings. Typically, the estimated efficiency of this helical gearbox will be about 90% under full load at 1750 rpm input. Shock load capacity of this helical gearbox will be 200%. In contrast, a helical-worm gear system with the same ratio will only require two gear sets and 2-3 sets of bearings. This helical-worm gear system will achieve the 50:1 ratio by combining a 5:1 ratio helical primary with a 10:1 worm gear secondary. In the 4-8” center distance range, the resulting unit will be 88% efficient and will have a shock load capacity of300%.
Impact of Lubrication
Lubricant selection and lubrication procedures play important roles in maximizing efficiency. Most worm gear manufacturers provide approved lists of lubricants for their products. The gearbox manufacturer knows from testing and experience which lubricants work best. The use of an unapproved “gear oil” just because it happens to be stocked may considerably impair efficiency and even lead to premature failure. Loads, speeds, duty cycle and operating temperatures are important factors in selecting the best lubricant. The lubricants listed by the gearbox manufacturer are usually suitable for a range of typical operating conditions. If any of these conditions are exceeded, an alternate lubricant may be recommended for best efficiency.
For example, in a lightly loaded, low temperature or high-speed application, efficiency can be improved by using a lighter viscosity lubricant. The lighter viscosity makes it easier for all moving parts to move through the lubricant. Conversely, if high loading, high temperatures or low speeds exist, then a high viscosity lubricant may be more appropriate. Viscosity will drop as the lubricant heats up, and worm gears operating under high loads or low speeds may benefit from a heavier lubricant. Extreme operating conditions should always be reviewed with the manufacturer, who understands the impact of various lubrication products.
Tooth Profile
As mentioned above, 300% shock load capability is one of the key advantages that separates worm gearing from other technologies. This is made possible through the use of globoidal gearing. Globoidal gearing includes a worm and gear set that fully envelopes against each other, resulting in up to eight times more tooth contact than cylindrical worm gearing, which provides 1-1.5 gear teeth in contact with the worm.
Self-Locking Ability
The self-locking properties of worm gearing can also be beneficial in many applications. Self-locking means the gear cannot back drive the worm. Self-locking can occur when the assembly is in either a static or a dynamic state, although it is more common when the worm gear is static. Ability to back drive is directly correlated with helix angle and efficiency. In theory, as long as the coefficient of friction between the gear and the worm is larger than the tangent of the worm’s helix angle, the worm gear is considered self-locking and will not back drive. This typically occurs in ratios above 40:1.
The static coefficient of friction generally depends on the materials of the two components and any lubrication between them. But in real-world use, other factors such as the condition of the surfaces or the presence of external vibrations can reduce the static coefficient of friction. Dynamic friction is lower than static friction, so dynamic self-locking is less likely to occur than static self-locking for a worm gear with the same ratio, tooth profile and operating conditions. In addition to the considerations mentioned above, the coefficient of dynamic friction is also impacted by the worm gear’s speed of rotation and the lubrication’s behavior under dynamic conditions.
Self-locking is particularly useful in applications that require lifting and holding loads. However, it is strongly recommended that a brake be used on the servomotor to ensure that a worm gear will not back drive, rather than relying on the worm gear’s theoretical ability to self-lock.
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