Worm gearing is a proven and economical solution for applications that require high speed reductions in limited space, and with very smooth and quiet operation. Worm gears have inherent self-locking ability depending on design and ratio. Lubrication is very important for the life of the worm gear set and efficiency can be improved by using enveloping worm sets.
A worm gear assembly resembles a single threaded screw that turns a modified spur gear with slightly angled and curved teeth. Worm gears can be fitted with either a right-, left-hand, or hollow output (drive) shaft. This right angle gearing type is used when a large speed reduction or a large torque increase is required in a limited amount of space. Figure 1 shows a single thread (or single start) worm and a forty tooth worm gear resulting in a 40:1 ratio. The ratio is equal to the number of gear teeth divided by the number of starts/threads on the worm. A comparable spur gear set with a ratio of 40:1 would require at least two stages of gearing. Worm gears can achieve ratios of more than 300:1.
Worms can be made with multiple threads/starts as shown in Figure 2. The pitch of the thread remains constant while the lead of the thread increases. In these examples, the ratios relate to 40:1, 20:1, and 13.333:1 respectively.
Worm gear sets can be self-locking: the worm can drive the gear, but due to the inherent friction the gear cannot turn (back-drive) the worm. Typically only in ratios above 30:1. This self-locking action is reduced with wear, and should never be used as the primary braking mechanism of the application.
The worm gear is usually bronze and the worm is steel, or hardened steel. The bronze component is designed to wear out before the worm because it is easier to replace.
Proper lubrication is particularly important with a worm gear set. While turning, the worm pushes against the load imposed on the worm gear. This results in sliding friction as compared to spur gearing that creates mostly rolling friction. The best way to minimize friction and metal-to-metal wear between the worm and worm gear is to use a viscous, high temperature compound gear lubricant (ISO 400 to 1000) with additives. While they prolong life and enhance performance, no lubricant additive can indefinitely prevent or overcome sliding wear.
Enveloping Worm Gears
An enveloping worm gear set should be considered for applications that require very accurate positioning, high efficiency, and minimal backlash. In the enveloping worm gear assembly, the contour of the gear teeth, worm threads, or both are modified to increase its surface contact. Enveloping worm gear sets are less common and more expensive to manufacture.
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Copyright Bodine Electric Company © 04/2016. All rights reserved.
Large distribution warehouses operated by wholesalers, retailers, or large manufacturers have turned to Automated Guided Vehicles (AGVs) to keep up with ever-increasing demand for faster and more economical deliveries. A major original equipment manufacturer (OEM) for AGVs contacted Bodine Electric Company to help develop two new custom gearmotors for their latest AGV. Bodine has since built, tested and shipped over 50,000 gearmotors to this customer.
The OEM’s specifications required two gearmotors, one of them had to lift up to 1,000 lbs (with substantial peak loads). The AGV’s chassis had already been finalized, and left only limited space for the new gearmotors. The gearmotors were required to operate almost continuously for five years, under worst-case environmental conditions, and would be subjected to extreme vibration and shock.
Almost every part of these new gearmotors was engineered to match the customer’s extensive list of requirements.
- Low-voltage brushless DC motors paired with all-new, highly efficient gearboxes prolong the AGV battery life and minimize downtime
- Gearbox designed to simplify the assembly process of the AGV
- Custom output shaft assembly designed for extremely heavy loads
- Feedback device tracked the position of the drive shaft
- 1024 PPR encoder provided servo feedback to control the gearmotors
- “Military Style” plug and screw connections
- Temperature sensors monitor gearmotor performance and to prevent overloads
- Manual over-ride in the event of a power failure
With Bodine’s help, these sophisticated robotic vehicles have been performing flawlessly all over the world. Bodine not only developed two entirely new gearmotors for the application, they also helped navigate the difficult third-party approval process. Bodine engineers extensive experience in motion control made them an ideal partner in the development of this new product, which in turn is making warehouse jobs easier, more productive, and cost-efficient.
Bodine Electric engineers bring over 110 years of application engineering and problem solving experience to a wide range of applications in industries as diverse as medical, packaging, industrial automation, and solar powered outdoors equipment. We look forward to working with you on your next FHP gearmotor design challenge.
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Copyright Bodine Electric Company © 03/2016. All rights reserved.
Posted in Application Tips, Company News, Custom Solutions, Engineering Talk | Tags: AGVs, Automated Guided Vehicles, Autonomous Robotic Vehicle, Bodine Electric Company, Brushless DC Gearmotor, Factory Automation, Fractional Horsepower, Gearmotors, Industrial Automation, Low-Voltage BLDC Motor, Self-driving Factory Robots, Self-driving Vehicles, Variable Speed Gearmotors, Warehousing Robotics
What is a gearmotor?
A gearmotor consists of a small motor designed specifically with an integrated gear reducer (gearhead). The end shield on the drive end of the motor provides a dual function. The side facing the motor provides the armature/rotor bearing support and a sealing provision through which the integral rotor or armature shaft pinion passes. The other side provides multiple bearing supports for the gearing itself and a sealing and fastening provision for the gearhousing. This construction eliminates the guess work of sizing a motor and gear reducer separately.
Why use gearmotors?
Gearmotors offer advantages over separate motor and reducer combinations. The gearhead acts as a torque multiplier, allowing a small motor to generate higher torque. Integral pinions that are ground or hobbed on the armature or rotor shaft reduce noise. And shared castings result in fewer parts and “near perfect” alignment of the rotor pinion and gear train. There is a minimum risk for lubricant leakage because of “O-ring” and lip seal construction, and the compact design provides better lubrication control for various mounting configurations. Gearmotors also eliminate the need for motor/gearhead couplings, and eliminate possible bearing misalignments.
What are the features and benefits of various types of gearing?
Parallel shaft: “Parallel shaft” refers to gear trains where the axis of the gear shaft is parallel to the motor shaft axis. They usually employ spur and/or helical gearing. Spur gearing is easier to manufacture and less expensive than helical gears. However, because of the greater overlapping or “load-sharing,” the transmission of power is usually smoother and quieter with helical than with spur gearing.
Right Angle: In right angle gear trains, the axis of the output shaft is 90 degrees to the motor shaft axis. Right angle gearmotors are used frequently in applications where space restricts the use of parallel shaft gearmotor of comparable torque. Worm gearing is the most common gear type used in this kind of gearmotor. Worm gearing can be self-locking; meaning the friction torque to the output shaft prohibits the gearbox to drive backwards. Worm gears have higher resistance to shock loads compared with spur and helical gears. In addition, the sliding tooth action of worm gears offers minimal noise. However, they are more difficult to lubricate and less efficient.
Planetary (center shaft): A Planetary gearhead is comprised of a sun gear, a ring gear, and a planet carrier assembly that uses three to five planet gears. The sun gear is generally the motor pinion, and the ring gear is the housing. The planet gears share the load, revolving around the sun gear. The output shaft typically attaches to the planet carrier of the output stage. A key benefit of planetary gearmotors is their compact size and high torque density. They provide up to twice the torque of a same-size parallel shaft gearmotor. Coaxial center shaft arrangements eliminate any output shaft offset for easier installation.
What types of motors are commonly available?
AC Induction Motors: In an AC induction motor or gearmotor, the stator winding sets up a magnetic field, which reacts with the current-carrying conductors of the rotor to produce rotational torque. The essential operating characteristics of AC induction motors and gearmotors will vary according to:
- Winding types (split-phase, capacitor run, shaded-pole, three-phase, etc.)
- Number of poles and phases, the line frequency, and the voltage of the power source
Permanent Magnet DC Motors: PMDC motors generate torque directly from DC power through internal commutation, stationary magnets and a rotating armature. They use carbon brushes that press against a commutator attached to the armature. As the armature turns, the brushes create contact with different segments of the commutator changing the current path through the winding. The rotation of the armature is a result of the interaction between the magnetic field from the armature and the stationary permanent magnet field.
Brushless DC Motors: BLDC speed-torque curves are very similar to those of PMDC motors. However, the magnets on BLDC motors rotate, and their windings are stationary. The other major construction difference is the means for switching winding phases on and off. Operation of a BLDC motor is similar to a PMDC motor except that the winding phases are switched on and off electronically by means of a control device. The control “knows” when to switch the windings because of feedback received from the rotor position Hall Effect sensors.
Copyright Bodine Electric Company © 03/2016. All rights reserved.