The hydrostatic motor is an executive component that converts hydraulic energy into mechanical energy to perform work. In principle, it is reversible with a hydraulic pump and is basically similar in structure to a hydraulic pump. However, in practical applications, except for axial piston pumps and motors that can be used reversibly, others cannot. This is because hydraulic pumps and hydrostatic motors have different working conditions and therefore different performance requirements.
A hydrostatic motor generally needs to be able to reverse direction, so its internal structure is symmetrical. It requires a wide speed range and typically does not have self-priming capability, but needs a certain initial sealing performance to provide the necessary starting torque. Hydrostatic motors are usually classified into two main categories: high-speed and low-speed types.
Functions and Classifications
High-Speed Hydrostatic Motors
High-speed hydrostatic motors are defined as those with a rated speed above 500 r/min. The main types include gear-type, screw-type, vane-type, and axial piston-type motors.
These hydrostatic motors are characterized by high rotational speed, high power density, small moment of inertia, and small displacement. They offer convenient starting, braking, speed regulation, and reversing capabilities. However, their output torque is not large, usually ranging from tens to hundreds of Newton-meters.
In most cases, they cannot directly meet the torque and speed requirements of engineering loads and often require a reduction mechanism, which somewhat limits their applications despite the inherent advantages of a hydrostatic motor design.
Low-Speed Hydrostatic Motors
Low-speed hydrostatic motors have a rated speed below 500 r/min. These motors have large displacement and correspondingly larger volume. What makes the low-speed hydrostatic motor particularly valuable is its ability to output thousands to tens of thousands of Newton-meters of torque even at speeds as low as a few revolutions per minute.
This characteristic has earned them the common designation of "low-speed high-torque hydrostatic motors." The main types include multi-acting internal curve piston (ball) plug motors, crankshaft connecting rod types, and static pressure balanced radial piston motors.
The low-speed hydrostatic motor is suitable for direct connection and driving of loads, with short starting and acceleration times and excellent performance, making it widely used in engineering practice.
Classification of Hydrostatic Motors by Speed and Torque
High-Speed Range
500+ r/min, lower torque output
Low-Speed Range
<500 r/min, high torque output
Structure and Working Principle
Since the structure of a hydrostatic motor is similar to that of a hydraulic pump, this section will take the swash plate type fixed displacement axial piston motor as an example to introduce its working principle and structure.
Working Principle
Figure 3-31 shows a schematic diagram of the working principle of a swash plate type fixed displacement axial piston hydrostatic motor. When hydraulic oil is input into the oil inlet of the hydrostatic motor, the piston 3 corresponding to the oil inlet chamber of the valve plate 4 is pushed out by the hydraulic pressure and pressed against the swash plate 1.
The swash plate 1 generates a normal reaction force F on the piston 3. When F is orthogonally decomposed, the horizontal component force balances the hydraulic pressure, and the vertical component force is transmitted to the cylinder block 2 through the piston, thereby generating torque on the transmission shaft.
Since each piston is in a different position, the torque generated varies. The torque output by the hydrostatic motor is the sum of the instantaneous torques generated by each piston in the oil inlet chamber on the transmission shaft.
Figure 3-31: Working Principle of Swash Plate Hydrostatic Motor
Structure
Figure 3-32: Structure of Swash Plate Hydrostatic Motor
The typical structure of a swash plate type fixed displacement axial piston hydrostatic motor is completely interchangeable with that of the MCY14-1B series hydraulic pump. The structural characteristics of this hydrostatic motor are as follows:
- The cylinder block of the hydrostatic motor is divided into two sections: the front section is called the drum 4, and the rear section is called the cylinder block 7. The drum is connected to the transmission shaft through a flat key, and the cylinder block 7 is connected to the drum 4 through a drive pin 6. A spring 5 compensates for the axial clearance of the valve plate.
- Similarly, the piston is also divided into two parts: the part distributed in the drum 4 is called the push rod 9, and the part distributed in the cylinder block 7 is called the piston 10. The push rod 9 contacts the swash plate 2 under the action of the piston 10.
- Since the cylinder block 7 and the working piston 10 only bear axial force, while the push rod 9 and the drum 4 bear both axial and radial forces, the push rod 9 and the drum 4 can transmit torque effectively in this hydrostatic motor design.
- A thrust bearing 3 is installed between the swash plate 2 and the housing. During operation, due to the rigid contact between the push rod 9 and the swash plate 2, friction loss is reduced in the hydrostatic motor.
Main Performance Parameters
Pressure
- Working Pressure: The pressure difference between the inlet and outlet of the hydrostatic motor during actual operation. When the motor outlet is directly connected to the oil tank, the motor's inlet pressure can be considered as the motor's working pressure.
- Rated Pressure: The maximum pressure allowed in continuous use according to experimental standards when the hydrostatic motor is in normal working condition.
Displacement
The displacement of a hydrostatic motor refers to the volume of oil required to be input per revolution of the motor under ideal conditions with no leakage. It is calculated based on the geometric dimension changes of the working volume of the hydrostatic motor.
This parameter is critical in determining the relationship between flow rate, pressure, and output torque in a hydrostatic motor system.
Flow Rate
The flow rate of a hydrostatic motor is divided into theoretical flow rate and actual flow rate:
- The theoretical flow rate refers to the volume of oil that needs to be input per unit time for the change of its sealed volume under ideal conditions with no leakage in the hydrostatic motor. It equals the product of the displacement and rotational speed of the hydrostatic motor.
- The actual flow rate is the actual volume of oil that needs to be supplied to the hydrostatic motor to achieve a certain rotational speed, considering the inevitable leakage in real-world operation.
Additional Key Parameters
Torque
- Theoretical torque: Calculated based on pressure and displacement
- Actual torque: Theoretical torque minus losses due to mechanical friction
- Rated torque: Maximum continuous torque output at rated pressure
Speed
- No-load speed: Speed at rated pressure with no external load
- Rated speed: Optimal operating speed under rated conditions
- Maximum speed: Highest safe operating speed for the hydrostatic motor
Efficiency
- Volumetric efficiency: Ratio of actual flow to theoretical flow
- Mechanical efficiency: Ratio of actual torque to theoretical torque
- Overall efficiency: Product of volumetric and mechanical efficiency
Power
The power output of a hydrostatic motor is calculated as the product of torque and rotational speed, and is directly related to the hydraulic power input, which is determined by pressure and flow rate.
Power transmission efficiency is a critical factor in selecting the appropriate hydrostatic motor for specific applications.
Hydrostatic Motor vs. Hydraulic Pump
| Aspect | Hydrostatic Motor | Hydraulic Pump |
|---|---|---|
| Energy Conversion | Converts hydraulic energy to mechanical energy (torque and speed) | Converts mechanical energy to hydraulic energy (pressure and flow) |
| Primary Function | Actuator that does work on a load | Energy source that supplies hydraulic power |
| Efficiency Focus | Mechanical efficiency is prioritized | Volumetric efficiency is prioritized |
| Rotational Capability | Must be able to reverse direction; symmetric structure | Often unidirectional with specific rotation requirements |
| Port Configuration | Inlet, outlet, and separate leakage port | Typically only inlet and outlet (except axial piston pumps) |
| Efficiency Characteristics | Generally lower volumetric efficiency | Higher volumetric efficiency compared to motors |
| Speed Characteristics | Typically lower output speeds | Generally operate at higher rotational speeds |
Fundamental Similarities
- Reversibility in Principle: In theory, a hydrostatic motor and a hydraulic pump are reversible. If driven by an electric motor, it outputs hydraulic energy (pressure and flow), functioning as a hydraulic pump. If supplied with hydraulic oil, it outputs mechanical energy (torque and speed), becoming a hydrostatic motor.
- Structural Similarity: From a structural perspective, the two are similar, utilizing many common design principles and components in their construction.
- Working Principle: Both utilize changes in sealed working volume for oil suction and discharge. For hydraulic pumps, the working volume increases during oil suction and decreases when discharging high-pressure oil. For a hydrostatic motor, the working volume increases when high-pressure oil enters and decreases when low-pressure oil is discharged.
Hydrostatic Motor Selection Criteria
Selecting the appropriate hydrostatic motor for a specific application requires careful consideration of several key factors to ensure optimal performance, efficiency, and longevity:
Torque and Speed Requirements
First, determine the required torque and speed of the hydrostatic motor based on the load requirements. This forms the fundamental basis for selecting an appropriately sized hydrostatic motor.
Pressure and Displacement
Based on the load and speed requirements, determine the working pressure and displacement of the hydrostatic motor. These parameters are interdependent and must be balanced for optimal system performance.
Fixed vs. Variable Displacement
Determine whether a fixed displacement or variable displacement hydrostatic motor is appropriate based on the speed control requirements of the executive component. Variable displacement motors offer greater flexibility in speed regulation.
Reduction Mechanisms
When a hydrostatic motor cannot directly meet the load torque and speed requirements, consider configuring a reduction mechanism. This allows the use of a smaller hydrostatic motor while achieving the necessary output characteristics.
Efficiency Considerations
Evaluate the efficiency characteristics of the hydrostatic motor across the expected operating range. A hydrostatic motor that operates efficiently under actual working conditions will reduce energy consumption and operating costs.
Environmental Factors
Consider the operating environment, including temperature ranges, contamination levels, and potential exposure to moisture or corrosive substances, which can affect the selection of materials and seals for the hydrostatic motor.
Hydrostatic Motor Selection Flowchart
Proper selection ensures optimal performance, efficiency, and service life of your hydrostatic motor system