How to Select the Gear Ratio of a Gear Reducer

　　 How to select the gear ratio of a gear reducer:

　　This is the first part of a series of articles on gear reducers. In this first installment, we will discuss how to select an appropriate gear ratio for a gear reducer.

　　Gear reducers are a critical component of mechanical power transmission systems, used to convert speed and torque. Selecting the appropriate reducer for a given application is a relatively straightforward process. The first step in choosing a gear reducer is to determine the required torque and speed, as well as the suitable motor type. Next, you can assess whether a gear reducer is necessary for the specific application. If so, the next step is to select the appropriate type and reduction ratio.

　　 Gear

　　Sectional view of a gear motor, showing the internal worm gear drive.

　　So what is the function of a gearbox? The most common benefits are that they can reduce speed, increase torque, and address inertia mismatch. For example, the amount of deceleration required depends on the type of motor being used. This is because some motors can operate at low speeds—such as below 1,000 rpm—without a gearbox, while others cannot.

　　 To reduce the speed, the gear ratio is calculated using the following formula:

　　G = N_motor / N_load

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

　　The operating principle of gear reducers is that they essentially multiply the motor’s output torque by a factor. For example, a small motor equipped with a gear reducer may be cheaper and more compact than a larger motor without one, yet deliver the same torque.

　　 When an increase in torque is required, calculate the necessary gear ratio using the following formula:

　　G = T load / [T motor × gear ratio]

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

　　Another function of a gear reducer is to reduce the load inertia reflected back to the motor by a factor equal to the square of the gear ratio. For most motion‑control applications, particularly high‑performance ones, it is ideal for the reflected load inertia to match the motor’s inertia.

　　When inertial matching is the primary consideration, to calculate the gear ratio, take the square root of the ratio of the load inertia to the motor inertia, as shown in the following equation:

　　G = Square root [ J load / J motor]

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

　　In Part 2   In this section, we will introduce the different types of gear reducers and explain how to select a high-quality one.