How to Choose a Gearbox and a Motor?

To obtain a well-matched gear motor, start by choosing a gearbox that will provide the right speed and torque output for the application, and then select the appropriate motor to meet output performance. Once the motor has been selected, a calculation needs to be made for the actual speed, torque and efficiency. Assuming the Motor and Gearbox Quick Reference Guides have been followed to make some basic motor/gearbox type selections, use the vendor data for those components to make the calculations. More often than not, the whole process may require several iterations to determine the optimal gear motor combination.

How to choose a gearbox and a motor

Method 1 Process – select a gearbox, select a motor, and ensure matching interfaces.

Step 1: Determine application requirements

Determine the required speed and torque for the application. Evaluate optimum operational speed and then determine what torque is required to meet the needed performance. This information is gathered as part of the Application Checklist.

Step 2: Choose a gearbox

Given the application performance requirements, the designer must start gathering information in order to select a proper gearbox. This step often requires the designer to collect many datasheets from various suppliers to determine which design will best handle the output torque required. The amount of torque produced from the gearbox is dependent on the requirements for selecting a gearbox.

Requirements for Selecting a Gearbox:

  • Life Curve
  • Efficiency Curve
  • Thermal Capacity
  • Maximum input speed

The gearbox specifications will be used to determine whether a gearbox’s output torque is limited by thermal or mechanical constraints.

Step 3: Calculate motor speed and torque, select gearbox ratio

Now that a gearbox has been selected, the application’s specification for performance, along with the gearbox performance data can be used to determine the proper gearbox ratio and the required performance of the motor. The motor speed and motor torque equations are used to calculate the “perfect match” motor output.

  • Motor Speed = Gearbox Output Speed x Ratio
  • Efficiency = Look For Output Torque On Vendor Gearbox Efficiency Curve
  • Motor Torque = Gearbox Output Torque/Ration x Efficiency

Both the motor speed and motor torque equations are impacted by the gearbox ratio. When it comes time to select a motor, the results of the equations could lead to the selection of a larger, slower, motor than is needed for the application. By simply increasing the gearbox ratio, the motor size can be reduced. As the design process unfolds, it becomes clear that in order to select the gearbox ratio the designer must have some understanding of what motor outputs are generally acceptable.

Step 4: Selecting a motor

Following the selection of the gear ratio and the calculation of required motor performance, it is time to dig into the datasheets once again to see what motor best fits the needs of the application. There are several key requirements to gather and review.

Requirements for Selecting a Motor:

  • Nameplate speed and torque of the motor
  • Efficiency / Torque Curve
  • Speed / Torque curve
  • Breakdown torque (applicable to AC motors only)
  • Pull-up torque (applicable to AC motors only)

Step 5: Calculate speed, torque, and efficiency of the gear motor

Now that the motor has been selected, the actual outputs of the gear motor must be calculated. Starting with the required motor torque from step 3, the corresponding motor speed from the motor speed/torque curve must be gathered from the motor vendor datasheet. Using this motor speed, the actual application gear motor speed can be calculated by dividing the motor speed by the gear ratio.

Step 6: Compare multiple gear ratios

The designer will need to iterate through steps 2 through 5 in order to compare multiple designs with different gear ratios. Once a sufficient number of designs have been analyzed, the gear motor can be selected.

Step 7: Analyze gearbox strength

Following the selection of a design that meets the output performance, an analysis of the gearbox must be made to ensure that it will meet the life requirements by reviewing vendor data. If the selected gearbox does not match the requirements for yield strength and continuous loading, the designer must repeat steps 2-5.

Step 8: Review thermal characteristics of the gearbox

Finally, the gearbox should be evaluated and the vendor data reviewed to ensure that it will not overheat in the application. If there are any concerns the designer must once again iterate steps 2-5 in the design process.

Step 9: Mechanical interface

After selecting the motor and gearbox for the application, the interface must be designed to rigidly couple the components together. Care should be taken in order to ensure the interface provides proper alignment, packaging and sealing to meet application needs.

There are many types of geared motors, read the above to understand the basic information on selecting a geared motor. ATO DC worm gear motors and 75W- 2kW planetary gear motors are a collection of motors and gearboxes.

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