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Basic knowledge of hydraulic press

What is a hydraulic press?

A hydraulic press is a machine that uses liquid as the working medium and is made according to Pascal’s principle to transfer energy to realize various processes. The hydraulic press generally consists of three parts: the machine (host), the power system, and the hydraulic control system.

The application of the hydraulic press

hydraulic press

Hydraulic presses use hydrostatic pressure to process metal, plastic, rubber, wood, powder, and other products, and are often used in pressing and pressing processes, such as forging, stamping, cold extrusion, straightening, bending (such as Plate Bending Machines, Bending Machines) ), flanging, sheet drawing, powder metallurgy, press fitting, etc. In addition to forging and forming, hydraulic presses can also be used for straightening, pressing, packing, pressing blocks, and pressing plates.

The working principle and classification of hydraulic press

The principal hydraulic press is a machine made of Pascal’s law that uses hydraulic pressure transmission, and there are many types. Of course, the use is also varied according to needs. For example, according to the type of liquid that transmits pressure, there are two types of water hydraulic presses and oil hydraulic presses.

The water-based liquid as the working medium is also called a water hydraulic press, and the oil as the working medium is also called an oil hydraulic press.

The total pressure generated by the water hydraulic press is large and is often used in forging and stamping. The forging hydraulic press is divided into die forging hydraulic press and free forging hydraulic press. Die-forging hydraulic presses use molds, while free-forging hydraulic presses do not use molds.

The specifications of hydraulic presses are generally expressed in terms of nominal working force (kN) or nominal tonnage (tons).

Forging hydraulic presses are mostly water-based hydraulic presses with high tonnage. In order to reduce the size of the equipment, large forging hydraulic presses often use higher pressures (about 35 MPa), and sometimes ultra-high pressures above 100 MPa are also used. Hydraulic presses for other purposes generally use a working pressure of 6 to 25 MPa.
The tonnage of the oil-based hydraulic press is lower than that of the water-based hydraulic press.

Working principle of hydraulic press

Pascal’s Principle

The working principle of hydraulic press. The areas of the large and small plungers are S2 and S1, respectively, and the forces acting on the plungers are F2 and F1, respectively. According to Pascal’s principle, the pressure of the closed liquid is equal everywhere, that is, F2/S2=F1/S1=p; F2=F1(S2/S1). Indicates the hydraulic gain. Like the mechanical gain, the force increases, but the work does not gain. Therefore, the moving distance of the large plunger is S1/S2 times the moving distance of the small plunger.

Fundamental

The basic principle is that the oil pump transports the hydraulic oil to the integrated cartridge valve block, and distributes the hydraulic oil to the upper cavity or lower cavity of the cylinder through each one-way valve and relief valve, and makes the cylinder move under the action of high-pressure oil.

Liquids obey Pascal’s law when they transmit pressure in a closed container.

Working medium of hydraulic press

The function of the working medium used in the hydraulic press is not only to transmit the pressure, but also to ensure that the working parts of the machine are sensitive, reliable, long-lived and less leaking.

5 basic requirements of hydraulic press for working medium

1. Have suitable fluidity and low compressibility to improve the efficiency of transmission;
2. Can prevent rust;
3. Have good lubricating performance;
4. Easy to seal;
5. Stable performance, long-term work without deterioration.

Three working media of hydraulic press

1. The hydraulic press initially used water as the working medium, and later switched to an emulsion made by adding a small amount of emulsified oil to the water to increase lubricity and reduce rust.
2. In the late 19th century, hydraulic presses using mineral oil as the working medium appeared. Oil has good lubricity, corrosion resistance and moderate viscosity, which is beneficial to improve the performance of hydraulic presses.
3. In the second half of the 20th century, a new type of water-based emulsion appeared, and its emulsification form was “water-in-oil” instead of the original “oil-in-water”.
   The external phase of “water-in-oil” emulsion is oil, its lubricity and anti-corrosion properties are close to oil, and its oil content is very small, and it is not easy to burn. However, the water-based emulsion is expensive, which limits its promotion.

The structure of the hydraulic press

Drive System

The drive system of the hydraulic press mainly has two types of pump direct drive and pump-accumulator drive.

Pump direct drive

The pump of this drive system provides high-pressure working liquid to the hydraulic cylinder, the distribution valve is used to change the direction of liquid supply, and the relief valve is used to adjust the limited pressure of the system, and at the same time play a role of safety relief.

The pump direct drive system has few links and simple structure. The pressure can be automatically increased or decreased according to the required working force, which reduces the power consumption. However, the capacity of the pump and its driving motor must be determined by the maximum working force and maximum working speed of the hydraulic press.
This type of drive system is mostly used in small and medium-sized hydraulic presses, as well as large-scale (such as 120,000 kN) free-forging hydraulic presses directly driven by pumps.

Pump-Accumulator Drive

There is one or a group of accumulators in the pump-accumulator drive system.
When the high-pressure working fluid supplied by the pump has a surplus, it is stored by the accumulator;
And when the supply is not enough, it will be supplemented by the accumulator.

With this system, the capacity of the pump and motor can be selected according to the average consumption of the high-pressure working fluid, but because the pressure of the working fluid is constant, the power consumption is large, and the system has many links and a complex structure.

The pump-accumulator drive system is mostly used for large hydraulic presses, or a set of drive system drives several hydraulic presses.

Structure type

Vertical and horizontal

Hydraulic presses are classified according to the direction of the force. There are two types of hydraulic presses: vertical and horizontal. Most hydraulic presses are vertical, and hydraulic presses for extrusion are mostly horizontal.

Structure type

Hydraulic presses include double-column, four-column, eight-column, welded frame and multi-layer steel belt winding frame, and medium and small vertical hydraulic presses also use C-frame type.

• The C-frame hydraulic press is open on three sides, which is easy to operate, but has poor rigidity.
• Welded frame hydraulic presses for stamping have good rigidity, open front and rear, but closed left and right.
• In the vertical four-column free forging hydraulic press with upper transmission, the oil cylinder is fixed in the upper beam, the plunger is rigidly connected with the movable beam, and the movable beam is guided by the column and moves up and down under the pressure of the working fluid. There is a worktable on the beam that can move back and forth. The upper anvil and the lower anvil are respectively installed under the movable beam and on the working table. The working force is borne by the frame composed of upper and lower beams and columns.
• Large and medium-sized free forging hydraulic presses driven by pump-accumulator often use three working cylinders to obtain three-level working force. Outside the working cylinder, there are also balance cylinders and return cylinders that exert upward force.

Four-column hydraulic press

The four-column hydraulic press is suitable for the pressing process of plastic materials. Such as powder product molding, plastic product molding, cold (hot) extrusion metal molding, sheet stretching, and processes such as transverse pressing, bending, turning through, and correction.

Four-column hydraulic press can be divided into four-column two-beam hydraulic press, four-column three-beam hydraulic press, four-column four-beam hydraulic press, etc.

Single arm hydraulic press (single column hydraulic press)

The working range can be expanded, and the three-sided space can be used to lengthen the stroke of the hydraulic cylinder (optional), the maximum retractable 260mm-800mm, and the working pressure can be preset; hydraulic system cooling device.

Gantry hydraulic press

The gantry hydraulic press can perform assembly, disassembly, straightening, calendering, stretching, bending, punching and other work on machine parts, truly realizing a multi-purpose machine. The working table of the gantry hydraulic press can move up and down, the size expands the opening and closing height of the machine, and it is more convenient to use.

Double column hydraulic press

The series of double-column hydraulic presses are suitable for press-fitting, bending and shaping, imprinting and indentation, flanging, punching and shallow stretching of small parts of various parts; forming of metal powder products and other processing processes.

The double-column hydraulic press adopts electric control, is equipped with jog and semi-automatic circulation, can maintain pressure and delay, and has good slider guidance, convenient operation, easy maintenance, economical and durable. According to the needs of users, functions such as thermal instrument, ejector cylinder, digital display of stroke, and counting can be added.

Application

Center Load Parts

The hydraulic press is especially suitable for bending, forming, flagging, and other processes of central load parts. The punching buffer device, it can also be used for punching and blanking processing. Preferred product.

Sheet metal parts

The hydraulic press is used for the processes of drawing, tumbling, bending, and stamping of sheet metal parts, and can also be used for general pressing processes. Devices such as punching buffers, punching, and moving worktables can be added according to user needs.

Pressing process

The hydraulic press can also be used for the pressing process of shaft parts, the calibration, crimping, and pressing process of profiles, and the bending, arguing, shaping, embossing, sleeve-forming, stretching and pressing processes of plastic materials of sheet parts, such as stamping, bending, Flanging, thin stretching, and other operations can also be engaged in calibration, press-fitting, plastic products, and powder products pressing and forming operations. It is also called a universal hydraulic press because of its wide range of applications.

Other

In addition to forging and forming, the three-beam four-column hydraulic press can also be used for straightening, pressing, packing, pressing blocks, and pressing plates.

Advantages of hydraulic presses

Compared with the traditional stamping process, the hydroforming process has obvious technical and economic advantages in reducing weight, reducing the number of parts and molds, improving stiffness and strength, and reducing production costs, and has been widely used in the industrial field, especially in the automotive industry. more and more applications.

Hydroforming techniques

In the automotive industry, aviation, aerospace and other fields, reducing structural mass to save energy during operation is a long-term goal that people pursue, and it is also one of the trends in the development of advanced manufacturing technology. Hydroforming is an advanced manufacturing technology to achieve lightweight structures.

Hydroforming is also known as “internal high pressure forming”. Its basic principle is to use the pipe as a blank, apply ultra-high pressure liquid inside the pipe, and apply axial thrust to both ends of the pipe blank to replenish the material. Under the combined action of the two external forces, the tube blank material undergoes plastic deformation, and finally adheres to the inner wall of the mold cavity to obtain a hollow part whose shape and precision meet the technical requirements.

5 advantages of hydroforming technology

For hollow variable-section structural parts, the traditional manufacturing process is to stamp and form two halves first, and then weld them into a whole, while hydroforming can integrally form hollow structural parts that vary along the cross-section of the component.
Compared with stamping and welding processes, hydroforming techniques and processes have the following main advantages:

1. Reduce mass and save material. For typical parts such as automobile engine brackets and radiator brackets, hydroforming parts are 20% to 40% lighter than stamping parts; for hollow stepped shaft parts, the weight can be reduced by 40% to 50%.
2. Reduce the number of parts and molds and reduce mold costs. Hydroforming parts usually only need one set of dies, while stamping parts mostly require multiple sets of dies. The number of hydroformed engine bracket parts has been reduced from 6 to 1, and the number of radiator bracket parts has been reduced from 17 to 10.
3. It can reduce the amount of welding for subsequent machining and assembly. Taking the radiator bracket as an example, the heat dissipation area is increased by 43%, the number of solder joints is reduced from 174 to 20, the number of processes is reduced from 13 to 6, and the productivity is increased by 66%.
4. Improve the strength and stiffness, especially the fatigue strength. For example, the stiffness of the hydroformed radiator bracket can be increased by 39% in the vertical direction and 50% in the horizontal direction.
5. Reduce production costs. According to the statistical analysis of the applied hydroformed parts, the production cost of hydroformed parts is 15% to 20% lower on average than that of stamping parts, and the mold cost is reduced by 20% to 30%.

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