Can an Electric Actuator Work Without a Gearbox? A Comparison of Thrust, Speed, and Planetary Gearbox Calculations

In electric actuator applications, planetary gearboxes are primarily used to reduce motor speed and increase torque, enabling precise motion control and meeting high-load transmission requirements.

Advantages of Using a Planetary Gearbox in Electric Actuators

1. Increased Torque and Speed Reduction
   Electric actuators must convert a motor’s high speed and low torque into low speed and high torque to drive loads effectively. A planetary gearbox efficiently reduces motor speed while significantly increasing output torque to meet actuator requirements.
2. Compact and Space-Saving Design
   The unique structure of planetary gears allows loads to be evenly distributed, enabling high torque output while maintaining a compact size—especially beneficial for electric actuator designs with limited installation space.
3. High Efficiency and Precision
   Planetary gearboxes can achieve output efficiencies of up to 97% and feature zero or extremely low backlash, ensuring high-precision motion control, which is critical in automated production and precision industrial equipment.
4. Wide Range of Gear Ratios
   Through multi-stage configurations, planetary gearboxes offer a broad range of reduction ratios, providing greater flexibility to match different load and speed requirements.
5. Stable and Reliable Structure
   By evenly distributing loads across multiple planetary gears, the gearbox enhances structural stability and service life, ensuring long-term reliable operation of electric actuators.
6. Cost Reduction
   Integrating a gearbox allows designers to select smaller, more cost-effective motors and reduce additional system requirements, improving the overall cost efficiency of the electric actuator system.
7. Modular Design and Broad Applications
   Thanks to their compact structure, planetary gearboxes can be integrated with ball screws and motors to form highly modular electric actuators, making installation easier across various automation applications.

 

Analysis and Explanation

The following section presents a hypothetical electric actuator application scenario to evaluate the performance achieved by incorporating a planetary gearbox.

According to the THK catalog, when torque is applied to a screw, the resulting thrust force can be calculated as follows:

Formula 1 – Screw Thrust Force Calculation

Fa = 2 × π × η × T / (R × 10⁻³)

Fa: Thrust force (N)
T: Input torque (Nm)
R: Lead (mm)
η: Efficiency

 

Formula 2 – Screw Linear Speed Calculation

v = R × n / 60

v: Linear speed (mm/s)
R: Screw lead (mm)
n: Screw rotational speed (rpm)

 

Example

Assume a system using a 400 W servo motor, designed to generate a thrust force of 800 N with a target linear speed of approximately 300–400 mm/s. Available screw lead options are 5, 10, 15, 20, and 25 mm. The task is to determine which lead is suitable.

Step 1: Determine the Rated Torque of the Servo Motor

A 400 W servo motor has a rated torque of approximately 1.27 Nm.

Step 2: Efficiency Assumption

Assume an efficiency of 0.85.

Step 3: Calculate Thrust Force Using Formula 1

Step 4: Calculate Linear Speed Using Formula 2

Based on the results of Steps 3 and 4:

• A 5 mm lead screw meets the thrust requirement but does not achieve the required linear speed.
• 10 mm and 15 mm lead screws meet the speed requirement but fail to provide sufficient thrust.

 

Step 5: Calculation with a Planetary Gearbox

From the results of Step 5, two viable configurations are identified:

• A 20 mm lead screw combined with a 3:1 planetary gearbox achieves a thrust force of 1020 N and a linear speed of 333.3 mm/s.
• A 25 mm lead screw combined with a 3:1 planetary gearbox achieves a thrust force of 816 N and a linear speed of 416.7 mm/s.

Note: Final evaluation must also consider the allowable output force of the electric actuator, ensuring it does not exceed the actuator’s rated capacity.

 

What Screw Lead Is Required Without a Planetary Gearbox?

(Without considering actuator allowable output or standard lead values)

Based on the above analysis, a screw lead between 6 and 8 mm can meet the application requirements. However, whether such lead specifications are standard depends on individual screw manufacturers. In general, 5 mm and 10 mm leads are more common industry-standard options.

 

Conclusion

Planetary gearboxes offer compact size, high torque output, high efficiency, and a wide range of reduction ratios. By incorporating a planetary gearbox, designers gain greater flexibility in balancing thrust and speed for electric actuator applications.

Furthermore, the ability of planetary gearboxes to evenly distribute loads ensures smooth, precise, and reliable actuator motion, making them highly suitable for automated production lines, industrial machinery, and automotive applications.

With extensive experience in planetary gearbox design and application engineering, GearKo Taiwan provides professional selection support and customized solutions. For application evaluation or technical inquiries, please feel free to contact us.