By Mitchell Machelski, Worm Gear Product Manager
The worm gear reducer has many characteristics ideal for power transmission applications. Efficiency improvements coupled with realistic evaluations combined with other advantages continue to make the worm gear reducer a suitable solution to a variety of equipment drive problems.
Escalating energy costs have increased engineer’s emphasis on the efficiency of drives and drive systems. Luckily, worm-gearing efficiency has increased through a variety of technical improvements. Gearing involves sliding and rolling friction and requires rotation of a mass in an oil bath. Both characteristics generate some losses, which can be minimized. These gains combined with the advantages of lower initial cost and higher overload capacity are powerful arguments for continued widespread use of worm gear reducers. What’s more, are the comparative evaluation of realistic efficiencies and other characteristics of competitive gear-reducer systems. Such comparisons often show that the advantages of using a worm gear reducer far outweigh any slight loss of efficiency. Even if small efficiency losses are important, there is the possibility of using a reducer with a helical, spur or planetary worm-gearing set to provide higher efficiencies while retaining worm gear drive advantages. Of course, accurate comparisons are dependent on the use of realistic efficiencies (for all reducers considered) and proper size selection. Using rough approximations of efficiencies can totally distort the comparison.
Advantages of worm gearing
The worm gear reducer has many characteristics ideal for power transmission applications. The advantages can include:
• Self locking (ability to stop and hold load) and overhauling (ability to operate in reverse).
• Supports very high overhung and thrust loads.
• Quiet (Cone worm gear noise level of 65 dB compared to 85 db for helical bevel).
• Support shock loads (300% for Cone Drive versus 100% for helical bevel).
• Consistent size with increase in ratio.
• Higher torque at low speeds.
• Capable of zero backlash.
• Cone Drive ratios are exact.
• High level of repeatability.
Efficiency use considerations
High efficiency combined with other advantages will continue to make the worm gear reducer a suitable solution to a variety of equipment drive problems. The worm gear reducer has been a traditional mainstay and workhorse of the mechanical power transmission industry. This position may seem to have been threatened by increasing emphasis on energy efficiency; however, there are other inherent advantages of the worm gear drive. In many systems, these advantages (including overload protection, compact size, low cost, smooth performance, and long life) are as important as or even more important than efficiency.
Any type of gear reducer efficiency is affected by losses at the gear mesh, in the bearings, in lubricant windage and at the oil seal lips. Worm gear reducer efficiency will vary with speed, load, ratio, ambient temperature, operating temperature, operating cycle, type of lubricant, lubricant viscosity and reducer break-in. Realistic values are best determined by comprehensive dynamometer or field-testing. Dynamometer testing is a reasonably simple procedure for accurate and comprehensive evaluation under these various conditions. Field-testing is even more realistic for a particular application, but is more difficult to set up and control.
Most worm gear manufacturers have made great strides in gear set design, materials and manufacturing. These improvements, together with advanced lubricants, make for considerable improvements in efficiencies. Through these improvements, there was an increase in the efficiency of double enveloping worm gear reducers by three to eight percentage points. This increase in efficiency resulted from thousands of hours of testing manufacturing processes, materials, and lubricants. It has made the worm gear reducer more attractive for today’s demands.
Worm gear/helical gear reducers
An all-helical system has a slight advantage, if efficiency is the only consideration. Efficiency is weighed against cost, durability and other considerations. In general, spur or helical gearing is slightly more efficient than worm gearing per gear stage (see chart on front page). However, the quietness, smoothness, higher overload capacity, higher ratio per gear set, greater simplicity and more compact size of worm gearing can be equally advantageous.
For those applications requiring higher efficiency, the advantages of both systems can be obtained by combining the two gearing types. When worm gearing is used with a helical, spur or planetary gear primary or secondary, higher overall drive train efficiencies can be achieved without loss of basic worm gear advantages. As an example of the advantages of this approach consider an application requiring a 40:1 ratio. To obtain this ratio, all-helical gearing will require three gear sets and four attendant sets of bearings. Typically, the estimated efficiency of this helical gearbox will be about 91% under full load at 1750-rpm input. Overload capacity of this helical gearbox will be 200% of the unity service factor rating (catalog rating). In contrast, a helical-worm gear system for the same ratio will only require two gear sets and two or three sets of bearings. (Reduction to two sets of bearings is possible by mounting the helical pinion on the motor shaft.) This helical-worm gear system will achieve the 40:1 ratio by combining a 4:l ratio helical primary with a 10:l worm gear secondary. In the 4- to 8-in. center distance range, the resulting reducer will be 88% efficient and will have an overload capacity equal to 300% of the unity service factor rating.
Lubricant and efficiency
The lubricant and lubrication procedures play surprisingly important roles in achieving maximum efficiency. Most worm gear manufacturers furnish approved lists of lubricants for their reducers. The reducer manufacturer knows from testing and experience which lubricants work best in his products. The use of some other “gear oil” may substantially impair efficiency and even lead to reducer failure. Selecting an unapproved “gear oil” just because it happens to be in stock can lead to serious problems. Loads, speeds, duty cycle and expected operating temperatures are important factors in selecting the best lubricant. The lubricants listed by the gear reducer manufacturer are usually suitable for a range of typical operating conditions. If any of these conditions will be extreme in the application, then an alternate lubricant may be recommended for best efficiency. For example in a lightly loaded, low temperature or very high-speed application, efficiency may 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 loads, high temperatures or very low speeds exist, then a higher than normal viscosity may be 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 be reviewed with the manufacturer who knows the capabilities and limitations of each lubricant and the operating conditions affecting them.
Filed Under: Design World articles, Gears • gearheads • speed reducers, Mechanical