Chuck Russell, Senior Principal Engineer, Dodge Gearing, Baldor
When a piece of equipment needs slow input speed and high torque, it is common to use an electric motor coupled to an industrial gear reducer to provide the desired speed and torque. The gear reducer multiplies the output torque coming from the motor and reduces the motor output speed by the same ratio. There are many different kinds of industrial gear reducers from which to choose. Internally, those gear reducers may use worm, helical, or bevel gears to increase the torque and reduce the output speed. One of the most popular types of gear reducers is based on worm gears.
Torque vs Purchase Price
When a customer selects a gear reducer, one of the most important decision criteria is the relationship between the cost of the gear reducer and the amount of torque generated from the output shaft. Worm reducers are the leaders when it comes to the cost vs torque comparison.A principal factor driving the cost competitiveness of worm reducers relates to the overall gear reducer ratio -- the number of input shaft rotations that results in one rotation of the output shaft. Most worm gear reducer manufacturers offer ratios of up to 60:1 in a single stage of gearing, and a few offer even higher ratios.With only one stage of gearing, the related multiple shafts, bearings, spacers, and other hardware that would be required in a multi-stage reducer are not required. It is typical for a 60:1 helical or helical-bevel reducer to have 3 stages of gearing vs the one stage of gearing in a worm reducer. The gears, bearings, and shafts are the most expensive parts in a complete gear reducer. With fewer of these components, worm reducers cost less than other types of gear reducers.
Availability and Commonality
Most domestic worm gear reducer manufacturers stock a substantial number of reducers in their warehouses. In addition to readily available inventory, there is also dimensional commonality between the major domestic worm reducer manufacturers on critical mounting dimensions. Those critical dimensions include:
- the output shaft diameter and length;
- mounting hole size and location in the housing;
- output shaft height;
- overall reducer housing height.
The gear ratios offered have also been standardized among manufacturers. Some manufacturers have increased the output and input torque ratings of their products through improved gear designs along with the use of high performance materials and synthetic lubricants. Those increases in input and output torque make it possible to easily replace an older reducer with a new reducer from a different manufacturer. The increased ratings provide a larger service factor and longer life than the reducer being replaced.
Mounting Options and Features
The worm gear in a worm reducer runs at an angle of 90 deg to the input worm shaft. Since the worm gear is mounted on the output shaft and the shaft is 90 deg relative to the input, both ends of the output shaft are available for use in an application. Available common output shaft configurations include single and double extended solid shafts along with hollow output shafts.The use of hollow output shafts is becoming more and more common in the domestic U.S. market as it offers several advantages over a solid output shaft. An equipment OEM can extend its drive shaft a relatively short distance and mount a hollow bore worm reducer on the extended shaft. That mounting method eliminates the need for chains, belts, or other equipment to transmit torque from a solid shaft reducer to the application. Guarding is simplified, and the maintenance normally required for chains and belts is eliminated.Several manufacturers offer a sealed for life reducer that does not require a vent plug or oil level changes for different mounting positions. Those features allow mounting in multiple positions without the need to relocate the breather or add lubricating oil. Due to the popularity of worm reducers and the multitude of different applications, they are available in standard and food industry washdown duty packages, offer C-face inputs for standard NEMA motors, and have multiple options, such as mounting flanges and bases.One aspect of worm gearing to avoid is the self-locking feature that some gear ratios may theoretically offer. Due to the many variables that affect the reducer’s ability to self-lock, most manufacturers recommend use of a brake motor or other means to keep the reducer from being back driven, as required.
Long Life
Industrial worm gear reducers use gearing consisting of a hardened worm running with a worm gear manufactured from a softer material. The softer material is a bronze alloy selected with wear in mind. Unlike hardened helical or bevel gear sets, a worm gear set is designed to “wear-in” over time.Some people think that the wearing of the worm gear will result in a shorter life than other types of gearing. However, wearing of the bronze worm gear drops off substantially after the wear-in period is over.When worm gear manufacturers select a bronze material, they balance the resistance of the bronze to wearing out over time against the ability to wear-in. During the wearing-in process, the temperature of the worm gear will temporarily increase. A softer bronze will wear-in more quickly with lower break-in temperatures, but it will not last as long. A harder bronze will last longer but will generate higher break-in temperatures. A harder bronze is the best compromise between wear-in and wear-out. When properly sized to the application, worm gear reducers last at least as long as other types of gearing.
Noise Advantages
All types of gear reducers create a certain level of noise depending on the type of gearing used, the accuracy of the gearing, the quality level of the manufacturing processes, and the product design. Worm gear reducers have multiple advantages in the noise arena.The gear teeth in worm reducers operate by sliding into contact with each other. That sliding action avoids the abrupt tooth to tooth contact that can occur with helical or bevel gearing. In addition to the benefit of sliding into engagement, well-designed worm reducers have more teeth in contact in the gear mesh compared to a helical or bevel reducer, which results in lower noise levels.Also, worm reducers typically have only one set of gears, and that one set produces less noise than the multiple sets of gears required in helical and bevel reducers. The lesser number of bearings also helps to keep the noise level lower in worm reducers.
Conclusion
Overall, gear reducers with worm gears offer numerous advantages over helical and bevel gearing. Worm gear reducers have a low cost-versus-torque ratio, and domestic models are readily available and interchangeable. The 90-degree mounting of a worm gear reducer, along with the hollow output shaft and sealed-for-life options, make worm gear reducers suitable for a host of applications. Also, the long life and reduced noise levels of gear reducers with worm gearing have contributed to their popularity.
