In marine engineering, the anchoring system plays a critical role in ensuring vessel stability during port arrivals, where ships often await berths, undergo inspections, or avoid adverse weather. Traditional anchor handling equipment, such as electric, hydraulic, or electro-hydraulic windlasses, is used to drop anchors and maintain position. However, under the influence of wind, waves, and currents, anchor chains can experience significant tensile forces, leading to inadequate braking between the chain wheel, drive shaft, and drum. This can result in unintended chain release, increasing the vessel’s movement range and raising risks of grounding or collisions. To address this, we propose a novel self-locking mechanism based on worm gears, which enhances braking reliability by leveraging the self-locking properties of worm gear systems. Our design incorporates dual worm gears and a PLC monitoring system for precise control over chain extension and retraction, improving safety and efficiency in marine operations.
Traditional worm gear reducers consist of key components like the worm, worm wheel, shafts, bearings, and housing. These systems transmit power between non-intersecting shafts, with the worm acting as the driving element and the worm wheel as the driven element, achieving high reduction ratios. The self-locking condition for worm gears depends on the lead angle of the worm being less than the friction angle at the contact surface, expressed as $\phi < \rho$. This principle is analogous to an object on an inclined plane not sliding down when the gravitational component parallel to the plane is less than the frictional resistance, formulated as $G \cdot \sin \phi < G \cdot \cos \phi \cdot \mu$, where $\phi$ is the lead angle, $\mu$ is the coefficient of friction, and $G$ represents the load. For instance, with a friction coefficient of 0.6, the lead angle must be below $3^\circ 29’11”$ to ensure self-locking. However, practical challenges arise due to friction losses, lubrication effects, and vibrational impacts, which can alter the effective friction angle and compromise self-locking performance. In marine environments, these issues are exacerbated by variable operating conditions, necessitating a more robust design.
Our new marine worm gear self-locking device is designed to overcome these limitations by integrating a dual-worm configuration and automated control systems. The design requirements were derived from industry standards, such as the GB/T 549-2017 specification for anchor chains, which mandates a maximum load capacity of 476 kN. Key design criteria include: (1) using a single-start worm with a module close to 15 to ensure proper self-locking engagement; (2) implementing a PLC-controlled lubrication system to minimize maintenance; and (3) selecting an electric motor with a power rating between 30 kW and 50 kW for optimal performance. The device comprises three main subsystems: a lubrication mechanism, a worm gear transmission机构, and a PLC-based self-locking unit. The lubrication system features an activated carbon罐 to absorb excess oil vapors, enhancing啮合 efficiency, while sensors monitor oil levels and trigger precise spraying via nozzles. The transmission机构 uses a synchronizer and electromagnetic clutch to switch between single-stage and multi-stage input, adjusting the reduction ratio dynamically. For example, during anchor retrieval, the PLC processes signals from position and speed sensors to control the clutch and synchronizer, achieving ratios up to 80 for efficient operation. In self-locking mode, the worm wheel becomes the driving element, and the dual worm gears provide enhanced braking, with the PLC ensuring rapid disengagement upon reaching safe limits.
The core components of our worm gear self-locking device include electromagnetic clutches, synchronizers, PLC control modules, and various sensors. The electromagnetic clutch, of the fixed-coil type, facilitates quick engagement and disengagement between shafts based on PLC commands, enabling seamless transitions between power transmission and braking. The synchronizer, an inertial type, ensures smooth gear shifts by equalizing speeds between mating components, reducing wear and vibration. The PLC module, equipped with a central processor, memory, and I/O modules, processes inputs from oil level, speed, and position sensors to output control signals for actuators like clutches and lubricant valves. Sensors play a vital role: oil sensors detect hydraulic changes, position sensors track anchor movement, and speed sensors monitor worm gear rotation, all contributing to real-time adjustments. This integration allows for precise control over chain length, with the PLC capable of detecting increments as small as 1.822 meters (one fathom), ensuring accurate positioning in port areas.
For the selection and design of worm gears, we conducted detailed calculations based on load requirements. Using the AM3-24 anchor chain standard, which has a tensile load of 332 kN and a mass per unit length of 12.61 kg/m, we considered a typical setup with six chain sections of 27.5 meters each and an anchor mass of 3.2 kg. The torque on the worm wheel connection shaft was determined as the product of the chain’s tangential stress and the effective lever arm, resulting in $T_1 = 55,110 \, \text{N·mm}$. Material selection was critical for durability: the worm is made from 20Cr steel, surface-hardened to 58–63 HRC, while the worm wheel uses sand-cast tin bronze ZCuSn5Pb5Zn5 for its excellent wear resistance. The design process involved verifying contact and bending stresses, as well as worm stiffness. For instance, the center distance $a$ was calculated using the formula:
$$a \geq \sqrt[3]{K_A \cdot T_2 \cdot \left( \frac{Z_E \cdot Z_\rho}{[\sigma_H]} \right)^2}$$
where $K_A = 1.4$ is the application factor, $Z_E = 150 \, \text{MPa}^{1/2}$ is the elasticity coefficient for steel-bronze pairs, $Z_\rho = 2.8$ is the contact coefficient, and $[\sigma_H] = 200 \, \text{MPa}$ is the allowable contact stress. With $T_2 = T_1 \cdot \eta \cdot i_{12} = 55,110 \times 0.65 \times 50 = 1,791,075 \, \text{N·mm}$ (where $\eta = 0.65$ is the efficiency and $i_{12} = 50$ is the transmission ratio), the minimum center distance was 211.63 mm. After iterative calculations, we selected a module $m = 8 \, \text{mm}$, worm diameter factor $q = 10$, and actual center distance $a = 270 \, \text{mm}$, with a lead angle $\gamma = \arctan(1 / 17.5) \approx 3.27^\circ$, which satisfies the self-locking condition $\gamma < \rho$ for typical friction angles. Bending stress was verified using:
$$\sigma_F = \frac{1.53 \cdot K_A \cdot T_2}{d_1 \cdot d_2 \cdot m \cdot \cos \gamma} \cdot Y_{Fa2}$$
where $d_1 = 140 \, \text{mm}$ and $d_2 = 400 \, \text{mm}$ are the worm and wheel diameters, and $Y_{Fa2} = 2.35$ is the form factor for 50 teeth. The result, $\sigma_F = 17.2 \, \text{MPa}$, is below the allowable $[\sigma_F] = 32 \, \text{MPa}$. Worm stiffness was checked by calculating the deflection $Y$ under tangential and radial loads, ensuring it remains within the permissible limit of $0.14 \, \text{mm}$.
Motor selection was based on power requirements, with the minimum power $P$ estimated as:
$$P = \frac{n_1 \cdot T_1}{9550}$$
where $n_1 = 1000 \, \text{r/min}$ is the input speed, yielding $P = 5.8 \, \text{kW}$. We chose a Y160M-6 motor with a rated power of 7.5 kW and speed of 970 r/min to accommodate operational variations. The table below summarizes key parameters for the worm gear design:
| Parameter | Value | Unit |
|---|---|---|
| Module (m) | 8 | mm |
| Worm Diameter (d₁) | 140 | mm |
| Wheel Diameter (d₂) | 400 | mm |
| Center Distance (a) | 270 | mm |
| Lead Angle (γ) | 3.27 | degrees |
| Transmission Ratio (i₁₂) | 50 | – |
| Allowable Contact Stress ([σ_H]) | 200 | MPa |
| Allowable Bending Stress ([σ_F]) | 32 | MPa |
In conclusion, our novel marine worm gear self-locking device addresses the shortcomings of traditional systems by incorporating dual worm gears and advanced PLC control. The design ensures reliable braking under high loads, with dynamic adjustment of transmission ratios and automated lubrication. While this study focuses on theoretical development, future work will involve experimental validation, optimization of worm gear arrangements, and refinement of PLC algorithms for enhanced reliability in real-world marine applications. The integration of worm gears in this context highlights their versatility in providing efficient power transmission and self-locking capabilities, making them indispensable in modern anchoring systems.