General Explanation about Speed Reducers


Speed reducers are mechanical devices generally used for two purposes. The primary use is to multiply the amount of torque generated by an input power source to increase the amount of usable work. They also reduce the input power source speed to achieve desired output speeds.

The selection and integration of speed reducers entails much more than simply picking one out of a catalog. In most cases the maximum torque, speeds, and radial loads published cannot be used simultaneously. Proper service factors must be applied to accommodate a wide range of dynamic applications. And, once the appropriate speed reducer is selected, proper installation and maintenance are the keys to maximizing life.


The wide variety of mechanical speed reducing devices includes pulleys, sprockets, gears, and friction drives. There are also electrical products that can change the motor speed. This discussion will focus on enclosed-drive speed reducers, also known as gear drives and gearboxes, which have two main configurations: in-line and right angle. Each can be achieved using different types of gearing. In-line models are commonly made up of helical or spur gears, planetary gears, cycloidal mechanisms, or harmonic wave generators. Planetary designs generally provide the highest torque in the smallest package. Cycloidal and harmonic drives offer compact designs in higher ratios, while helical and spur reducers are generally the most economical. All are fairly efficient.

Right angle designs are typically made with worm gearing or bevel gearing, though hybrid drives are also available. Worm gears are perhaps the most costeffective reduction solution, but usually have a minimum 5:1 ratio and lose considerable efficiency as ratios go higher. Bevel reducers are very efficient but have an effective speed reduction upper limit of 6:1. The type of application dictates which speed reducer design will best satisfy the requirements.

Before choosing any reducer, specifications must be collected to properly size and install the unit: torque, speed, horsepower, reducer efficiency, service factor, mounting position, connection variables, and life required. In some applications the amount of backlash, transmission error, torsional rigidity, and moment of inertia are also important.


In spite of well-engineered designs and intense selection analysis, speed reducers are subject to wear and eventual failure. To maximize life, proper maintenance procedures should be established. The most important element is the routine oil change. Oil and grease molecules break down under the extreme pressure of mating gear teeth under load. The shearing effect of gears cutting through the oil and high temperatures inside the box also contribute to oil breakdown. When oil and grease lose their lubricating properties, reducer wear rapidly follows.

Although a certain number of failures can be attributed to problems with materials or workmanship, usually operating conditions or application dynamics are to blame when a speed reducer fails prematurely. When overload causes failure, it is usually because the original design criteria has changed, the proper service factors were not applied, the oil was not changed, or there was a major shock load. When bearings fail it is usually the result of excess overhand load, shaft misalignment, or too much heat. When seals fail, it is most likely because something came between the seal and shaft, or because they were painted, dried out, and became brittle.


When a selected speed reducer or drive package is just too expensive for the application parameters, a different approach should be considered. Because all speed reducer types come in a variety of quality levels, it should be determined how much gearbox is really needed. However, you usually get what you pay for. There are several other “tricks” that can be implemented to shave costs.

• One is to consider changing the mating drive components. Although most speed reducers can handle higher torque at lower speeds, the relationship is not linear. Instead of trying to get an entire speed reduction out of the gearbox and therefore the entire torque multiplication, a smaller reducer can be selected by getting some of the ratio and torque from less expensive pulleys or gears. For example, instead of choosing a 10:1 speed reducer driving a 1:1 belt, select a 5:1 reducer driving a 2:1 belt. The gearbox only has to do half the work.

• When at first glance it seems that a larger reducer is required to handle excess overhang load, consider changing the pulley size. Radial forces can be reduced in direct proportion to increases in pulley, sprocket, or gear diameters. When accelerating and decelerating inertial loads, the torque required is also in direct proportion to the time required to achieve the desired speeds. Altering the acceleration profiles can lead to reduced torque requirements. Instead of sizing reducers to handle all torque resulting from estops or machine jams, install brakes to assist in deceleration and torque limiters to protect against extreme shocks. Savings up to 50% can be realized.

• Some motor manufacturers offer designs that provide full torque to 3,600 rpm. By doubling the reducer ratio, the same output torque and speed can be achieved with a less expensive motor. Lowering operating expenses can also reduce costs. When high speeds are required, consider oil circulation systems or external cooling options. Although initially more expensive maintenance and down time will be reduced and gearbox life will be extended.

• Use higher efficiency speed reducers. For example, select a helical or worm design instead of a standard worm reducer. Although the unit may cost 20 to 30 percent more, the high efficiency may allow for a smaller motor and result in less power consumption for payback in only a few months.

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