Characteristics of hydraulic transmission and classification of pressure levels

First, the characteristics of hydraulic transmission:

1. Advantages:

(1) Light weight per unit power. That is: under the condition of the same power, the hydraulic actuator is small in size, light in weight and compact in structure.

(2) The layout is flexible and convenient. Various components of hydraulic transmission can be conveniently and flexibly arranged as required.

(3) The hydraulic device works relatively smoothly. Due to its light weight, small inertia and fast response, the hydraulic device is easy to realize quick start, braking and frequent reversing.

(4) It is easy to operate and control, and it can realize a wide range of stepless speed regulation (the speed regulation range is 2000:1), and it can also be adjusted in the process of running.

(5) Mineral oil is generally used as the working medium, the relative moving surface can be self-lubricated, and the service life is long.

(6) It is easy to realize linear motion. Using hydraulic transmission to achieve linear motion is simpler than mechanical transmission.

(7) It is easy to realize the automation of the machine and easy to realize the overload protection. When the electro-hydraulic joint control or even the computer control is adopted, the large load, high precision and remote automatic control can be realized.

(8) Standardization, serialization and generalization of hydraulic components are realized, which is convenient for design, manufacture and use.

2. Disadvantages:

(1) Hydraulic transmission cannot guarantee fixed ratio transmission, which is caused by the compressibility and leakage of hydraulic oil.

(2) The working performance is easily affected by temperature changes, so it is not suitable to work under very high or very low temperature conditions.

(3) The transmission efficiency is low. The resistance loss and leakage of the liquid flow are large, so the efficiency is low. If not handled properly, leaks not only contaminate the site, but can also cause fire and explosion accidents.

(4) In order to reduce leakage, hydraulic components have high requirements on manufacturing precision, so their cost is high and they are more sensitive to oil pollution.

(5) Difficulty in fault diagnosis. Hydraulic components and systems are prone to system failures due to hydraulic oil contamination and other reasons, and failures are difficult to diagnose.

2. Classification of pressure levels (JB824-66):

Low pressure 0~2.5 MPa ( 0~25Kgf/cm^2);

Medium pressure >2.5~8.0 MPa (>25~80Kgf/cm^2);

Medium and high pressure>8.0~16MPa (>80~160Kgf/cm^2);

High pressure>16~32MPa (>160~320Kgf/cm^2);

Ultra high pressure>32MPa (>320Kgf/cm^2)

According to the different structure of the web part of the paper machine, the paper machine can be divided into: long web type, round web type, clip web type, stacked web type and with finishing roller type.

The paper machine can be regarded as a kind of linkage machine composed of several equipments. Usually it can be divided into two parts: wet end and cadre. The wet part includes the pulp conveying equipment, the mesh part and the pressing part; the cadres include the drying part, the calender and the paper coiler.

After sizing, sizing, filling and purification, the pulp with the properties suitable for paper making enters the pulp conveying equipment of the paper machine at the concentration of 0.3% - 1.3%. Then, the paper flows evenly and stably to the net surface of the forming net after being distributed and leveled by the pulp distributor and the headbox. After the pulp is fed into the net near the center line of the chest roll, it is gradually filtered and dehydrated to form a continuous wet web. The net table is usually equipped with table roll, vacuum water suction tank, couch roll and other forming dehydration elements. When the wet web is dehydrated to a certain degree of dryness (usually about 20%), it can be peeled from the mesh and sent to the pressing part for further dehydration.

The press section of paper machine is composed of several groups of roller press. The wet web is dragged by the press blanket and dehydrated by mechanical extrusion between the press rolls. In order to maintain the good dehydration performance of the press blanket, the press upper roller is equipped with a blanket washing device. After pressing, the dryness of wet web can reach about 40%.

The wet web is further dehydrated on the paper machine, usually by heating and evaporation. The drying section of paper machine usually consists of many drying cylinders heated from the inside by steam. The drying cylinder is covered with dry blanket (or canvas, dry net), which aims to press the web tightly onto the cylinder surface, improve the heat transfer efficiency and improve the surface quality of the web.

After drying, a calender consisting of 6-8 rollers is usually used to improve the surface quality of the web.