A planetary roller screw is a precision mechanical transmission device that converts rotary motion into linear motion. It uses multiple threaded rollers to transfer loads between a screw and a nut. Compared with a ball screw, it generally has more load-bearing contact points, allowing it to provide high axial load capacity and stiffness within a compact structure.
In UAV systems, a planetary roller screw can serve as the main transmission component in an electric linear actuator. It may be considered for mechanisms that require relatively high axial thrust and positioning accuracy. Its suitability depends on the actuator's thrust, stroke, speed, weight, and operating environment.
Main Components
A standard planetary roller screw mainly consists of a screw, nut, threaded rollers, a cage, and synchronizing gears.
Screw: The screw is commonly used as the rotary input. Its external surface has multiple threads that engage with the rollers and transfer rotary motion through the assembly.
Nut: The nut has internal threads that match the rollers. Together with the screw, it forms the rolling contact system. Depending on the mechanism design, either the nut or the screw can serve as the linear output.
Threaded rollers: Multiple rollers are arranged evenly around the screw. Their external threads engage with both the screw and the nut. They are the main load-bearing and motion-transmission elements.
Cage: The cage positions and guides the rollers, maintains their spacing, and prevents abnormal displacement during operation.
Synchronizing gears: In common standard designs, gears are fitted to both ends of each roller. These gears engage with internal ring gears at the ends of the nut to coordinate roller motion and keep the roller axes parallel to the screw axis.
Different planetary roller screw designs may use different thread profiles, roller-guiding arrangements, and output configurations.
Working Principle
A planetary roller screw operates through the threaded engagement and rolling contact between the screw, rollers, and nut.
In a common standard configuration, the operating sequence is as follows:
Rotary input: A servo motor drives the screw through a coupling or another transmission component.
Roller rotation and revolution: As the screw rotates, the rollers roll between the screw and the nut. Each roller rotates around its own axis while also moving around the screw axis.
Axial movement of the nut: The continuous engagement between the rollers, screw, and nut converts the screw's rotation into linear movement of the nut along the axis.
Motion synchronization: The cage and synchronizing gears guide the rollers and maintain their relative positions, reducing skewing and abnormal contact during operation.
This mechanism converts the motor's rotary output into linear thrust. The nut is the output member in a standard design, while an inverted configuration can use the screw as the linear output.
Planetary Roller Screws vs. Ball Screws
Planetary roller screws and ball screws are both rolling-element screw drives, but they use different load-bearing elements and contact arrangements.
| Comparison | Planetary roller screw | Ball screw |
|---|---|---|
| Load-bearing elements | Threaded rollers | Balls |
| Contact type | Rolling contact between roller threads and the screw and nut threads | Rolling contact between balls and raceways |
| Load-bearing contact points | Generally provides multiple contact regions | Depends on ball quantity, size, and arrangement |
| Axial load capacity | Generally higher under comparable size and properly designed conditions | Depends on the specific size and configuration |
| Axial stiffness | Generally high; depends on structure, preload, and load | Depends on the balls, raceways, supports, and preload |
| Maintenance | Requires lubrication based on load, speed, stroke, and environment | Also requires lubrication and maintenance based on operating conditions |
A planetary roller screw is not necessarily more efficient or longer-lasting than a ball screw under every operating condition. Actual performance depends on lead, contact geometry, preload, manufacturing accuracy, lubrication, and operating load.
Advantages in UAV Applications
Electric actuators used in UAVs must deliver the required thrust and positioning performance within strict weight and installation-space limits. Planetary roller screws offer high load capacity and a compact design, making them an option for some electric actuators that require substantial axial thrust and stiffness.
High axial load capacity: Multiple rollers share the transmitted load, making the mechanism suitable for actuators that require relatively high linear thrust.
High structural stiffness: A properly designed preload arrangement can reduce axial clearance and improve transmission stiffness and position retention.
Suitable for high-load electric actuators: A planetary roller screw can be combined with a servo motor, reduction mechanism, and position sensors to form an electric linear actuator.
Flexible mechanical layout: Standard, inverted, and other configurations can be selected according to the available installation space and required stroke.
Potential UAV applications include landing gear actuation, retractable onboard equipment, payload deployment mechanisms, and other electric actuators that require substantial linear thrust. However, not every UAV needs a planetary roller screw. For mechanisms with lower loads, strict weight limits, or cost constraints, ball screws, electric push rods, and other transmission solutions may be more suitable.
Parameters to Consider During Selection
Selecting a planetary roller screw requires more than comparing rated thrust or transmission efficiency. The complete operating conditions of the actuator must be considered.
Axial load: Define the working load, maximum load, and possible impact loads. Check the load capacity of the screw, rollers, nut, and support structure.
Lead and rotational speed: The lead determines the theoretical axial displacement per revolution. It also affects output speed, drive torque, and transmission efficiency.
Stroke length: Determine the screw length according to the maximum actuator stroke. Check critical speed, column stability, and installation space.
Preload and axial clearance: Preload can reduce axial clearance and increase stiffness. Excessive preload, however, may increase friction, heat generation, and drive torque.
Positioning accuracy and repeatability: These depend on thread manufacturing accuracy, assembly tolerances, support bearings, encoders, and the control system.
Weight and installation dimensions: UAV designs are sensitive to weight and available space. The total actuator weight and mounting interfaces must be considered during selection.
Operating environment: Temperature, humidity, dust, vibration, and impact can all affect transmission performance and service life.
Maintenance and Lubrication
A planetary roller screw is a precision rolling transmission mechanism. It requires suitable lubrication and clean operating conditions. The lubricant type, quantity, and replenishment interval must be determined according to the specific product and its operating conditions. A single grease grade or fixed maintenance interval should not be applied to every model.
Routine maintenance should cover the following areas:
Lubrication condition: Check whether the lubricant is sufficient and whether it has dried out, become contaminated, or changed color abnormally. Replenish or replace it according to the manufacturer's instructions.
Rollers and thread surfaces: Inspect for abnormal wear, pitting, scratches, and contact damage.
Axial clearance and operating condition: Check axial clearance, operating noise, vibration, and temperature rise at regular intervals to identify possible wear or assembly problems.
Supports and connecting components: Inspect bearings, couplings, mounting seats, and fasteners to maintain correct alignment throughout the transmission system.
Stroke limits: Do not exceed the designed travel range, as this may expose the rollers, nut, and other internal components to abnormal loads.
Inspection intervals should be determined according to operating frequency, load, stroke, speed, and environmental conditions. For UAV actuators, condition checks should also be included in pre-flight inspections and scheduled maintenance.
Custom Precision Machining for UAV Components
Planetary roller screw actuators may include screw support blocks, nut mounting seats, coupling adapters, actuator housings, and other precision structural components. These parts must meet the required dimensional, tolerance, and assembly specifications. Hansheng Automation provides custom CNC precision machining based on customer drawings, supporting both prototype parts and small-batch production for UAV actuators and other automation equipment components.
