Select figure to enlarge.

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.

Speed reducer categories

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.

The torque, speed, horsepower relationship

The amount of torque needed is perhaps the most important criteria, as this translates to the amount of work the speed reducer must perform. Although in simple applications determining the torque can be relatively straightforward, it can be difficult in complex machinery. Inertia, friction, and gravity – the physical phenomena that tend to resist motion – have to be identified so that enough torque can be generated to overcome them. Considering coefficients of friction and the acceleration and deceleration of inertial masses is important when calculating required torque. A more detailed discussion of these elements can be found in the Machinery’s Handbook, Motion System Design Handbook, and other machine design publications.

A typical reducer selection chart shows the relationship of torque capacity to speed. Lower speeds allow for higher torque capacity.
Select figure to enlarge.

A shortcut in finding an existing machine’s required torque is to take amperage readings from the motor by identifying the current draw. Then calculations can be made to find the required horsepower. Finally, by using a standard torque formula and considering the various ratios, the ultimate torque value can be realized.

Once the required power is identified, the service factor, must be considered to properly size the unit. The service factor takes into account other operational parameters, including length of work days, numbers of starts and stops, load characteristics, and power sources. Most reducers are rated for a maximum torque at a given number of lifetime hours. The limiting factor in these ratings is not the gear or shaft strength, but the bearing life. Because bearings must support the separation forces of the gears under load, loading less than the maximum rating increases gearbox life. Conversely, by increasing the load variables as highlighted above, a decrease in gearbox life will result. Therefore, to arrive at the effective torque requirement, the appropriate service factors need to be applied.

At this stage the speed reducer and motor can be selected. Typically a main power source such as a motor or engine has been selected, which operates at a specific speed. Arriving at the correct speed reducer ratio and the resulting torque multiplication is merely a matter of dividing the motor speed by the driven element speed. Then, the proper motor size can be found by plugging in the various factors and values in a standard horsepower formula.

Continue on page 2