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An Overview of Planetary Roller Screw (PRS): Design, Manufacturing And Applications

Views: 1 Author: Site Editor Publish Time: 2025-11-14 Origin: Site

Planetary Roller Screw (PRS) is a high-precision, heavy-load mechanical transmission device that has gained widespread adoption in aerospace, high-end CNC machine tools, industrial robots, electro-mechanical actuators, and military equipment. Compared with traditional ball screws, PRS features superior load-carrying capacity, longer service life, and higher rigidity, making it ideal for precision transmission under extreme operating conditions. This article provides a comprehensive overview of PRS, covering its structural principles, key design aspects, manufacturing processes, and typical applications, serving as a practical reference for engineering professionals.

Core Structural Principles

Basic Components

Screw: A main shaft with helical grooves, typically featuring trapezoidal or circular arc threads.
Nut: Embeds planetary rollers or balls that mesh with the screw.
Planetary Rollers: Multiple rollers that perform planetary rolling motion between the screw and nut, enabling power transmission.
Retainer/Cage: Fixes the axial and circumferential positions of the rollers to ensure uniform force distribution.
End Caps: Prevents roller detachment and provides lubrication and sealing functions.

Operating Mechanism

When the screw rotates, the rollers undergo planetary rolling between the screw and nut, similar to planetary gear transmission. There are three main motion modes:

Screw rotation with nut linear motion (the most common application).
Nut rotation with screw linear motion (used in electro-mechanical actuators).
Dual-nut reverse motion (for high-rigidity, backlash-free transmission).Unlike ball screws that rely on point contact, PRS adopts line contact via rollers, which significantly enhances its rigidity and load-bearing capacity.

Key Design Considerations

Main Parameter Design

Lead (P): Selected based on speed and precision requirements, with a typical range of 1–20mm.
Nominal Diameter (D): Calculated according to load and rigidity demands, ranging from 10–100mm.
Number of Rollers (n): Limited by the nut inner diameter and roller size, usually 3–12.
Contact Angle (α): Typically 45° to balance axial and radial loads, with an adjustable range of 30°–60°.
Helix Angle (λ): Calculated as λ=arctan(P/(πD)), ranging from 2°–10°.

Strength and Rigidity Calculations

Dynamic Load Rating (C): Complies with ISO 3408-5 standard, calculated by the formula C=fc·(i·n)^0.7·dr^2.9·cos²α, where fc is the material factor, dr is the roller diameter, and i is the number of effectively loaded rollers.
Axial Rigidity (K): Calculated as K=(πEdr²)/(4L), with E (elastic modulus of steel) approximately 210GPa.
Critical Speed (nc): Determined to avoid resonance, related to the stiffness (K) of the system and the mass (m) of moving components.

Material Selection and Heat Treatment

Screw/Nut: High-carbon chromium steel (GCr15) quenched to HRC 58-62; 17-4PH stainless steel for corrosive environments.
Rollers: Silicon nitride (Si₃N₄) ceramic for high-speed, light-load applications; hardened SUJ2 bearing steel for general use.
Retainer: Polyetheretherketone (PEEK) for high-temperature resistance; copper alloy (bronze) for self-lubricating performance.

Precision Grades

Based on ISO 3408 standards, PRS precision grades are classified as follows:

P0: Repeat positioning error ≤5μm, suitable for ultra-precision machine tools.
P1: Repeat positioning error ≤10μm, used in aerospace actuators.
P3: Repeat positioning error ≤30μm, ideal for industrial robots.
P5: Repeat positioning error ≤50μm, applicable to general automation equipment.

Critical Manufacturing Processes

Thread Modification Technology

Profile modification: Reduces edge stress concentration.
Lead compensation: Compensates for thermal deformation errors.

Lubrication and Sealing

Lubrication methods: Grease lubrication (for lifelong maintenance-free operation) and oil mist lubrication (for high-speed working conditions).
Sealing designs: Labyrinth seal (for dust prevention) and magnetohydrodynamic seal (for vacuum environments).

Preloading and Backlash Elimination

Dual-nut preloading: Applies preload through adjusting shims or springs.
Single-nut variable lead: Self-preloading structure (e.g., SKF TorqueTube).

Advanced Manufacturing Technologies

Hard turning + grinding: Enables precision machining of screw threads.
Rolling forming: Enhances the surface hardness of rollers.
3D printing: Facilitates customized lightweight structures (e.g., titanium alloy nuts).

Typical Applications

Aerospace

Applied in aircraft control surface actuators and landing gear control systems, requiring resistance to extreme temperatures (-60°C~200°C) and shock/vibration.

Industrial Robots

Used in six-axis joints (replacing harmonic reducers) due to high rigidity, zero backlash, and long service life (over 1 million cycles).

Electro-Mechanical Actuators

Adopts the configuration of servo motor + planetary roller screw, as exemplified by the high-precision pressing mechanism in Tesla's automotive battery production lines.

Heavy-Duty Machine Tools

Meets the demands for large thrust (>50kN) and micron-level positioning accuracy.

Conclusion

With its outstanding advantages of high rigidity, heavy load capacity, and long service life, planetary roller screws are gradually replacing traditional ball screws and hydraulic systems. As material science and intelligent manufacturing technologies advance, PRS will play an increasingly crucial role in precision transmission, electrification, and intelligent equipment in the future.