Basic Information of hydraulic motors

May 04, 2023

Basic Information of hydraulic motors

1、 Hydraulic motor torque and speed

The working pressure of the motor p: the actual pressure of the motor inlet oil

The working pressure difference of the motor Δ p: The difference between the motor's inlet pressure and outlet pressure. Namely, Δ P=p-p out, usually set p out=0

be Δ p=p

The actual flow rate of the motor: The flow rate at the motor inlet.

Considering leakage, the theoretical flow rate of the motor is: qt=q· η V

In the equation: η V - volumetric efficiency of the motor

The output speed of the motor is equal to the ratio of the theoretical flow rate qt to the displacement V (displacement per revolution), i.e

n= qt/V= q· η V /V

The actual output torque of the motor shall consider the influence of mechanical efficiency, namely

T=Tt· η m

If the outlet pressure of the motor is zero, the inlet working pressure is p, and the displacement is V, then the theoretical output torque of the motor is:

The actual output torque of the motor is:

2. High-speed hydraulic motor

·Generally speaking, motors with a rated speed above 500r/min belong to high-speed motors, while motors with a rated speed below 500r/min belong to low-speed motors.

·The basic forms of high-speed hydraulic motors include gear type, blade type, and axial piston type

·Main features: High rotational speed, a small moment of inertia, easy to start, brake, adjust speed, and change direction.

·Usually, the output torque of high-speed motors is not large, and the minimum stable speed is high, which can only meet the high-speed and small torque working conditions.

Working principle of plunger motor

When pressure oil is input into the hydraulic motor, the plunger in the pressure chamber is pushed out and pressed against the swashplate. The swashplate generates a reaction force on the plunger, which can be decomposed into axial and perpendicular components. Among them, the component force perpendicular to the axis generates a torque that causes the cylinder to rotate.

3. Low-speed high torque hydraulic motor

· Low-speed and high-torque hydraulic motors are relative to high-speed motors, and usually, these types of motors are radial piston types in structural form.

·Features: Low minimum speed, approximately 5-10r/min, high output torque, up to tens of thousands of N · m; Large radial size and large moment of inertia.

·Usually, it can be directly connected to the working mechanism without the need for a reduction device, greatly simplifying the transmission structure.

· Three basic forms of low-speed and high-torque hydraulic motors are crank-connecting rod motors, static balance motors, and multi-acting internal curve motors.

Low-speed and high torque hydraulic motor with crank connecting rod

·The following diagram shows the working principle of a crank connecting rod hydraulic motor.

The motor is composed of a shell 1. connecting rod 3. piston assembly 2. crankshaft 4. and flow distribution shaft 5. Five cylinder bodies are evenly arranged radially along the circumference of the shell, forming a star-shaped shell (non-rotating); The cylinder body is equipped with a piston which is connected to the connecting rod through a ball joint. The large end of the connecting rod is made into a saddle-shaped cylindrical tile surface and tightly adheres to the eccentric wheel of the crankshaft, with its center being O1. The eccentricity OO1=e between it and the crankshaft rotation center O. The distribution shaft 5 of the hydraulic motor is connected to the crankshaft 4 through a cross key and rotates with the crankshaft. The pressure oil of the motor passes through the distribution shaft channel and is distributed to the corresponding piston cylinder by the distribution shaft.

In the figure, the cavities ①, ②, and ③ of the oil cylinder are filled with pressure oil, and the piston is subjected to the action of pressure oil; According to the motion principle of the crank connecting rod mechanism, the plunger under the action of oil pressure exerts a force N on the center O1 of the eccentric wheel through the connecting rod, pushing the crankshaft to rotate around the rotation center O and outputting speed and torque externally. The other piston cylinders are connected to the oil discharge window; If the inlet and outlet ports are reversed, the hydraulic motor will also rotate in the opposite direction.

Multi-acting internal curve motor

·There are many structural forms of multi-acting internal curve hydraulic motors, including axial rotation and shell rotation hydraulic motors in terms of usage.

·From the perspective of internal structure, there can be various forms of force transmission and the structure of plunger components, but the main working process of hydraulic motors is the same.

The structural principle of multi-acting internal curve motor

The hydraulic motor is composed of main components such as stator 1, rotor 2, distribution shaft 4, and plunger group 3. The inner wall of stator 1 is composed of several evenly distributed and identical curved surfaces. Each curved surface of the same shape can be divided into symmetrical sides, with the side that allows the plunger pair to extend outward called the oil inlet working section and the side that retracts the plunger pair called the oil outlet working section.

The number of times each plunger reciprocates in each revolution of the hydraulic motor is equal to the number of stator surfaces X, which is called the number of actions of the hydraulic motor. There are Z plunger cylinder holes on the cylinder block, and there is a flow distribution window at the bottom of each cylinder hole, which is connected to the flow distribution hole that matches its central flow distribution shaft. There are oil inlet and return ports in the middle of the distribution shaft, and the position of its distribution window corresponds to the position of the oil inlet and return working sections on the guide rail surface.

The oil is distributed to the bottom oil chamber of the plunger in the working section through the oil distribution window on the oil distribution shaft. The pressure oil causes the roller of the plunger group to press against the surface of the guide rail. At the contact point, the guide rail generates a normal reaction force N on the roller, which is perpendicular to the surface of the guide rail and passes through the center of the roller. This force can be divided into two components: component P along the axial direction of the plunger and component T perpendicular to the axial direction of the plunger. It is transmitted to the cylinder body through the side of the crossbeam, Generating torque on the cylinder block. Make the cylinder block drive the load to rotate.

The spindle rotates once and the plunger reciprocates X times (shown in the figure as 6 times). Therefore, with the same number of plunger diameters and strokes, its output torque increases by a multiple of the number of applications compared to a single-acting plunger motor. That's six times. In addition to single-row column plugs, they can also be made into double or triple-row column plugs, making it easy to meet the requirements of large displacement and small size.

If the inlet and outlet ports of the hydraulic motor are switched, the hydraulic motor will reverse. If the drive shaft is fixed, the stator, distribution shaft, and casing will rotate, commonly referred to as the casing rotation condition, becoming a wheel motor.

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