How to select the best type of gear reducer

Thursday - 31/01/2019 11:40

How to select the best type of gear reducer

Gear reducers are an essential part of mechanical power transmission systems, being used for speed and torque conversion. Selecting the right one for an application is a fairly straightforward process.
The first step in selecting a gear reducer is to know the required torque and speed as well as the most suitable type of motor to use. Then, it can be determined if a gear reducer is needed in the particular case. If so, then the next step is to select the proper type and ratio.
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So what does a gear reducer do? The most common benefits are that they can reduce speed, multiply torque, and solve inertia mismatching issues. For instance, to take speed reduction, the amount needed will depend on the type of motor used. That’s because some motors can operate at low speeds (less than 1,000 rpm, for instance) without the need for a gear reducer, while others cannot.

For speed reduction the gear ratio is calculated using the equation:

where G is the gear ratio, and N is the angular velocity of the motor and the load.

The way gear reducers work is that they essentially multiply the output torque of the motor. So for instance, a small motor with a gear reducer may be less expensive and smaller than a larger motor without a gear reducer producing an equal amount of torque.

To calculate the required gear ratio when torque multiplication is needed, use the equation:

where G is the gear ratio, T is torque and e is efficiency.

Another function of gear reducers is to reduce the reflected load inertia to the motor by a factor of the square of the gear ratio. For most motion applications but especially high performance ones, the ideal condition is for the reflected load inertia to equal the motor inertia.

To calculate the gear ratio when inertia matching is the main concern, take the square root of the ratio of the load inertia to the motor inertia as in the equation:
G= sqroot [ JLOAD / JMOTOR ]

where G is the gear ratio and J is the inertia of the motor and load.

Gear reducers come in two basic classes or configurations with a few different types within each of these classes. The basic classes are right angle and parallel shaft. Within the right angle class of gear reducer there are different types including worm, bevel (straight and spiral) and hypoid.

As for the parallel shaft configuration, options include spur (both internal and external), helical, planetary, and harmonic gears. It’s also possible for a single gear reducer package to be composed of multiple stages that combine more than one type of gear reducer.

Typically, what determines the use of a right angle or parallel shaft gear reducer are the space restrictions of the application. Beyond space restrictions, there are other reasons that determine which configuration is most suitable. For instance, worm gears with ratios of 20:1 or higher are typically self-locking.

This might be desirable in applications that require the load to stay in place after the motor is turned off. However, worm gears are also much less efficient than other types of gears and require a larger motor to get the same amount of continuous output torque.

The following equation shows the effect of gear efficiency on motor horsepower:
HPMOTOR = (TLOAD * NMOTOR) / (1,008,400 * G * e)

where HP is horsepower, T is load torque, N is angular velocity of the load, G is the ratio of input to output speed, and e is the mechanism efficiency, which differs for different types of drive mechanisms.

For instance, a worm gear reducer with a 50 % efficiency rating requires a motor with a horsepower rating 1.6 times that of an 80 % efficient spur gear reducer with the same gear ratio.

On the other hand, bevel, helical, and spur gears are much more efficient but tend to produce more noise than worm gears. This may or may not be acceptable based on the specific application requirements.

Another point to consider is that high-precision positioning applications may not tolerate the inherent backlash of bevel, helical, spur and worm gears. Those applications frequently require low-backlash planetary or harmonic gear reducers.

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