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MDP Hydraulics
The five-star internal curve radial piston hydraulic motor (often simply referred to as "five-star motor" or "internal curve motor") is a low-speed high-torque hydraulic actuating component. Its core feature lies in the adoption of internal curve guide rails and a radially arranged piston-roller assembly. The motor is driven by high-pressure oil to push the piston. The rollers at the end of the piston roll along a fixed internal curve track, converting the liquid pressure directly into torque for the rotation of the drive shaft. Due to its outstanding advantages such as high torque, low speed, smooth operation, and high startup efficiency, it is particularly suitable for transmission systems that require heavy loads, low speeds, and direct drive. It is a core driving component in fields such as construction machinery, mining equipment, and ship deck machinery.
High-pressure oil passes through the distribution ports on the valve shaft (or valve disc) and is distributed into the cylinder cavities at specific phase angles.
The piston in the intake zone extends outward under the oil pressure, with its top roller tightly pressing against the fixed five-pointed star (or polygonal) inner curved track inside the cylinder.
The curved surface of the inner track forces the roller (along with the piston) to generate a tangential component force perpendicular to the piston axis. This tangential force acts on the cylinder body (or is directly transmitted to the output shaft through the crossbeam and connecting rod), generating driving torque.
As the output shaft rotates, the distribution mechanism rotates synchronously, distributing the high-pressure oil successively to the next piston that enters the intake zone, while connecting the completed working cylinder cavity with the return oil path. The piston retracts under the action of the track curved surface. Multiple pistons continuously alternate in oil intake and oil discharge at different phase angles on the track, thereby driving the output shaft to rotate continuously and smoothly.
The speed of the motor can be adjusted by changing the size of the input flow; the forward and reverse rotation of the motor can be achieved by changing the direction of the oil intake and return.
This is the core component of the motor. It is usually a five-star or multi-star shaped cam ring, with its curve precisely designed (such as an equal acceleration - constant speed - equal deceleration curve), ensuring uniform and smooth torque output and enabling multiple strokes, thereby allowing each piston to perform work multiple times within a single rotation, significantly increasing torque density and low-speed stability.
The pistons are radially arranged in the cylinder and have rollers at the ends. The rollers have rolling friction with the inner curve track, resulting in low friction loss, high mechanical efficiency, and long lifespan. The number of pistons is usually an even number (such as 10), evenly distributed.
It is usually an oil distribution shaft or end face oil distribution disc. It rotates synchronously with the output shaft, precisely distributing the pressure oil to the cylinder cavities in the intake phase and drawing the return oil out from the cylinder cavities in the discharge phase. The accuracy of the flow distribution mechanism directly affects the volumetric efficiency and low-speed performance of the motor.
In some designs, the tangential force generated by the pistons is directly transmitted through the crossbeam or connecting rod to the eccentric wheel of the output shaft. This structure enables the output shaft to only bear torque and not radial force, resulting in longer bearing lifespan.
The housing structure is sturdy and is used to support the inner curve track and bearings. The output shaft is usually supported by large-sized heavy-duty bearings to withstand huge radial and axial loads.
Some models offer a through shaft interface, allowing the connection of other hydraulic components (such as an oil replenishment pump or another motor) on the same shaft line.
Product Specifications:
• Extremely high torque density: The multi-action inner curve design enables a large single rotational displacement, allowing for the generation of significant torque within a compact volume. The power density is far superior to that of gear motors and vane motors.
• Outstanding low-speed stability: Multiple plungers operate alternately in sequence, with a high number of strokes and minimal torque fluctuations. It can run smoothly at extremely low speeds (even below 1 rpm), without any "slipping" phenomenon.
• High starting torque: The mechanical efficiency of starting is high, and the starting torque can reach over 90% of the theoretical torque. This is particularly suitable for heavy-load starting conditions.
• Direct drive capability: The torque is sufficiently strong to directly drive the wheels, drums, track sprockets, etc., eliminating the need for a large mechanical gearbox, simplifying the system and enhancing the transmission efficiency.
• Excellent variable adaptability: Through the servo variable mechanism, some models can achieve stepless or step-variable functions (by changing the number of effective plungers or the number of operations), thereby expanding the speed regulation range.
• High rigidity and impact resistance: The structure is sturdy, the internal oil compression volume is small, the speed stiffness is good, it responds quickly to load changes, and can withstand strong impact loads.
• Long lifespan and high reliability: The main friction pair (roller - track) is in a rolling contact, resulting in minimal wear. The robust design of the bearings and housing ensures durability in harsh environments.
• Construction machinery: Traveling drives and rotation mechanisms for excavators and cranes; Drum drives for concrete mixing and transportation vehicles; Vibration wheel drives for rollers.
• Mining and port machinery: Mining winches and conveyor drives; Large vehicle drives for port gantry cranes and ship loaders.
• Ship and Ocean Engineering: Direct drive for deck machinery such as anchor winches, winches, steering gears, and propellers.
• Agricultural and forestry machinery: The walking drive for combine harvesters, the feeding mechanism for wood harvesting machines.
• Special vehicles: Wheel drive for heavy-duty transport vehicles, leveling mechanism for wind power installation vehicles.
• Industrial equipment: Drive for large-torque rotary worktables, mixers, and winding machines.
1. Selection steps:
Determine the load torque and speed: Calculate the maximum working torque and speed range required for the driven load.
Calculate the required displacement: Based on the maximum working pressure of the system and the required torque, use the torque formula to calculate the required displacement. Displacement (L/r) ≈ (Torque Nm × 62.8) / Pressure bar.
Verify the speed range: Ensure that the required operating speed is within the range permitted by the motor.
Select the installation and connection method: Based on the structure of the host, choose the flange type, hub type or shell rotation type; determine the shaft extension form (bevel gear, flat key) and the oil port connection (thread, flange).
Consider auxiliary functions: whether parking brake, speed sensor, oil drain port, universal joint drive, etc. are required options.
Alignment and Support: The output shaft of the motor and the load shaft must be precisely aligned. It is recommended to use a flexible coupling. Ensure that there is sufficient support at the load end to avoid additional bending torque being applied to the motor bearings.
Oil drain pipe connection: The separate oil drain port (oil return port of the housing) must be directly and smoothly connected to the oil tank using a sufficiently large oil pipe, and the back pressure should not exceed the specified value of the product (typically < 0.05 MPa). This is crucial for protecting the shaft seal.
First startup: Before starting, the shell should be filled with clean hydraulic oil.
• Oil cleanliness: This is the lifeline that ensures the lifespan of the valve train and the piston train. The cleanliness of the system oil should be at least at NAS 1638 grade 8 or ISO 4406 grades 19/17/14 or higher. High-quality oil filters must be used.
• Lubricating oil and oil temperature: Use anti-wear hydraulic oil with a high viscosity index (such as VG46 or VG68). The normal operating oil temperature should be maintained between 30°C and 70°C.
Back pressure control: The return oil path must maintain a certain back pressure (typically 0.5 - 1 MPa) to prevent "loose contact" shock when the roller detaches from the track surface, but the back pressure should not be too high.
• Regular maintenance: Periodically check the tightness of the connecting bolts and for any leakage at the shaft seal. According to the manufacturer's recommended schedule (such as every 2000 working hours), inspect the oil quality and replace the filter element. Regularly monitor the noise, temperature rise and output performance of the motor.
• Fault Diagnosis: Common faults include insufficient output power, unstable rotational speed, leakage, etc. During the troubleshooting process, it is necessary to first check whether the system pressure and flow are normal, whether the oil is clean, and whether the oil discharge pipeline is unobstructed. For internal wear, professional personnel need to conduct disassembly inspection.
