Deep space exploration represents a critical strategic endeavor in scientific advancement, and Mars probes serve as essential platforms for such missions. The optical payload turret on a Mars probe is responsible for orienting solar panels, optical detection instruments, and signal transmission devices. Given the constraints of launch vehicle capacity, minimizing the mass of the probe is paramount. Additionally, the harsh Martian environment, characterized by extreme temperature fluctuations and limited energy sources, imposes stringent requirements on the turret’s drive mechanism. This mechanism must achieve high precision, large reduction ratios, and operate without backlash to ensure accurate positioning. Traditional solutions, such as direct stepper motor drives or multi-stage gear systems, often lead to excessive mass or complexity. In contrast, a single-stage worm gear drive offers a compact and efficient solution for high-ratio speed reduction. However, conventional worm gears exhibit backlash due to design, manufacturing, and thermal effects, which can compromise precision. This paper presents the design of a dual-section backlash-free worm gear system tailored for the optical payload turret of a Mars probe. The system incorporates a unique split-worm design with adjustable shims and springs to eliminate backlash, while utilizing lightweight materials like polyimide and titanium alloy to reduce mass and enable self-lubrication. We detail the structural design, material selection, parameter optimization, and validation through finite element analysis and environmental testing, demonstrating that the proposed worm gear meets the demanding requirements of space applications.
The dual-section backlash-free worm gear system operates on the principle of axial preload to maintain continuous contact between the worm and worm gear teeth, thereby eliminating backlash. The system comprises a nut, springs, a dual-section worm, adjustment shims, and the worm gear. The dual-section worm consists of a solid worm shaft and a hollow worm segment connected by a expansion sleeve, ensuring synchronized rotation while allowing relative axial movement. Preload is applied via springs at both ends of the worm, and the intermediate adjustment shims control the axial displacement of the worm segments. By adjusting the nut and modifying the shim thickness, the worm segments are displaced axially to ensure tight meshing with the worm gear teeth. This configuration compensates for manufacturing tolerances, assembly errors, and thermal-induced dimensional changes, enabling precise bidirectional motion without backlash. The drive motor and angle sensor are mounted on the dual-section worm, forming a closed-loop control system that achieves accurate angular positioning of the turret. The large reduction ratio (80:1) is achieved in a single stage, making the system compact and lightweight. The elimination of backlash is critical for maintaining positioning accuracy under varying thermal conditions, as the Martian surface experiences temperatures ranging from -95°C to 70°C.
The design of the worm gear system focuses on material selection to minimize mass while ensuring mechanical strength and self-lubrication properties. Common aerospace materials, such as carbon fiber composites, ceramic matrix composites, titanium alloys, and engineering plastics, were evaluated. Polyimide (YS20) was selected for the worm gear due to its low density, high strength-to-weight ratio, and excellent self-lubricating characteristics, which reduce friction and energy consumption. Titanium alloy (TC4) was chosen for the worm for its high specific strength and stiffness, as well as good thermal stability. The material properties are summarized in Table 1. To enhance the self-lubrication of the worm gear, carbon content was incorporated into the polyimide, further reducing friction coefficients and improving efficiency in the absence of external lubrication.
| Property | Polyimide YS20 | Titanium Alloy TC4 | Beryllium | Aluminum LY12CZ | Magnesium MB5 |
|---|---|---|---|---|---|
| Density (g/cm³) | 1.44–1.53 | 4.51 | 1.85 | 2.8 | 1.8 |
| Elastic Modulus (GPa) | 200 | 110 | 303 | 68–72 | 45 |
| Tensile Strength (MPa) | 130 | ≥895 | 243 | 390–441 | 255 |
| Thermal Conductivity (W/m·K) | 0.1–0.35 | 7.955 | 216 | 159 | 134 |
| Thermal Expansion Coefficient (10⁻⁶/K) | 3 | 8.6 | 11.3 | 21.6 | 23 |
The worm gear parameters were designed based on requirements for a transmission ratio of 80:1, operating speeds of 2–100 rpm, and a starting torque of 15 mN·m. Design considerations included tooth contact strength, bending strength, and deflection. The worm gear geometry was optimized using software tools, and the key parameters are listed in Table 2. The worm gear has 80 teeth, while the worm has a single thread, achieving the desired reduction. The center distance is set at 73 mm, with a module of 1.6 mm. The lead angle of the worm is 5°4’47”, and the lead is 5.024 mm. The worm gear’s pitch diameter is 128 mm, and the worm’s pitch diameter is 18 mm. The total mass of the worm gear is 68.56 g, and the worm weighs 60.5 g, meeting the lightweight objective. Strength verification using KissSoft software indicated a bending safety factor of 15.6 and a wear safety factor of 9.7, with a transmission efficiency of 33.8%. These values confirm that the worm gear design satisfies mechanical performance criteria while minimizing mass.
| Parameter | Worm Gear | Worm | Parameter | Worm Gear | Worm |
|---|---|---|---|---|---|
| Center Distance (mm) | 73 | 73 | Lead Angle | 5°4’47” | 5°4’47” |
| Module (mm) | 1.6 | 1.6 | Worm Lead (mm) | — | 5.024 |
| Number of Teeth | 80 | 1 | Pitch Diameter (mm) | 128 | 18 |
| Tip Diameter (mm) | 130.56 | 20.56 | Dedendum (mm) | 1.6 | 1.6 |
| Addendum (mm) | 1.28 | 1.28 | Face Width (mm) | 35 | 15 |
| Root Arc Radius (mm) | 10.6 | — | Tip Arc Radius (mm) | 7.72 | — |
| Pitch Arc Radius (mm) | 9 | — | Outer Diameter (mm) | 133.76 | — |
The adjustment shim and spring parameters were designed to compensate for thermal expansion and ensure zero backlash across the operating temperature range. The nominal backlash at room temperature (25°C) is 0.3 mm, and the initial shim thickness is set to 0.5 mm. Thermal analysis was conducted to determine the change in backlash due to temperature variations from -95°C to 70°C. The change in worm gear tooth thickness, Δt_g, and worm tooth thickness, Δt_w, can be calculated using the thermal expansion coefficients:
$$ \Delta t_g = \alpha_g \cdot L_g \cdot \Delta T $$
$$ \Delta t_w = \alpha_w \cdot L_w \cdot \Delta T $$
where α_g and α_w are the thermal expansion coefficients of polyimide and titanium alloy, respectively, L_g and L_w are the characteristic lengths, and ΔT is the temperature change. For the low-temperature case (25°C to -95°C, ΔT = -120°C), the worm gear tooth shrinkage is 7.878 μm, and the worm shrinkage is 4.163 μm. The housing, made of aluminum silicon carbide, contracts by 8.55 μm, resulting in a net backlash increase of 3.491 μm. For the high-temperature case (25°C to 70°C, ΔT = 45°C), the worm gear expansion is 3.545 μm, and the worm expansion is 1.873 μm, while the housing expansion increases backlash by 3.848 μm, leading to a net backlash decrease of 1.57 μm. To achieve zero backlash under all conditions, the shim must be ground down by 3.491 μm at room temperature to preload the system, compensating for low-temperature expansion.
The spring preload force must exceed the axial force generated during operation. The worm gear load torque is 0.5 N·m, resulting in a tangential force F_t = 7.8 N. The corresponding axial force on the worm is also 7.8 N. A spring with a wire diameter of 0.8 mm, free length of 15 mm, and compressed length of 10 mm provides a preload force of 10 N, sufficient to prevent displacement. The friction coefficient between polyimide and titanium alloy ranges from 0.1 to 0.12, yielding a friction force of 1.2 N and a friction torque of 10.62 mN·m, which is below the starting torque requirement. To minimize friction and improve response, the worm surface was finished to reduce roughness.
Finite element analysis (FEA) was performed to evaluate the thermal performance of the worm gear system under Martian conditions. The model assigned polyimide properties to the worm gear and titanium alloy to the worm, with natural convection heat transfer (coefficient of 20 W/m²·K) and ambient temperature cycles from 25°C to 65°C, -60°C, and back to 65°C over 5400 seconds. The simulation results showed that the temperature distribution across the worm gear and worm followed the environmental cycles, with maximum and minimum temperatures within the operational range. The thermal-induced deformations were within acceptable limits, confirming that the design maintains structural integrity and meshing accuracy under thermal cycling. The FEA results validate the suitability of the material selection and geometric design for space applications.
Experimental validation included run-in testing, thermal environment testing, and mechanical environment testing to simulate Martian conditions. The run-in testing procedure involved initial operation to improve tooth contact and adjust for zero backlash. The process included measuring starting torque, running at low speed, and iteratively adjusting shims until backlash was eliminated. The thermal tests were conducted in a KM1 high-vacuum chamber, with temperature cycles from -75°C to +70°C at pressures as low as 600 Pa. The worm gear system was operated under load (0.5 N·m) at 100 rpm for intervals, accumulating less than 3×10⁵ cycles. The mechanical tests involved sinusoidal and random vibrations on a V8-440 shaker, simulating launch and surface conditions. The sinusoidal vibration profile covered frequencies from 5–100 Hz at 0.5 g, and random vibrations followed a specified power spectral density. After testing, the worm gear was disassembled and inspected, showing no damage or wear, and performance metrics met all requirements. These tests demonstrate the robustness and reliability of the dual-section backlash-free worm gear for the Mars probe optical payload turret.
In conclusion, the dual-section backlash-free worm gear system designed for the optical payload turret of a Mars probe addresses the challenges of mass reduction, high precision, and operation in extreme environments. The use of polyimide for the worm gear and titanium alloy for the worm, combined with an adjustable shim mechanism, ensures zero backlash and efficient performance. Finite element analysis and comprehensive testing confirm that the system withstands thermal and mechanical stresses, making it suitable for space missions. This design represents a significant advancement in worm gear technology for aerospace applications, contributing to the success of deep space exploration endeavors.