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

• 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

• 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

Industrial Robots

Electro-Mechanical Actuators

Heavy-Duty Machine Tools

Conclusion