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What is a Hydraulic Press?
A hydraulic press is a mechanical device that uses the static pressure of a liquid, as defined by Pascal‘s principle, to shape, deform, and configure various types of metals, plastics, rubber, and wood. The mechanism of a hydraulic press consists of a mainframe, power system, and controls.
Pascal‘s principle states that when pressure is applied to a confined liquid, a pressure change occurs in the liquid. For a hydraulic press, the pressure in a liquid is applied by a piston that works like a pump to create mechanical force.
Chapter Two: How a Hydraulic Press Machine Works
The process of a hydraulic press begins when a hydraulic fluid is forced into a double acting piston. The compressive force inside the small hydraulic cylinder pushes the fluid into a larger cylinder where more force and pressure are applied. The movement of the larger piston forces the fluid back into the smaller piston cylinder.
The passage of the fluids between the two pistons creates increased pressure that produces mechanical force to drive the anvil on a hydraulic press downward onto a workpiece that is deformed to produce a desired shape. Once the movement is complete, the pressure is released and the anvil returns to its original position.
How a Hydraulic Press Works
The components of a hydraulic press include two cylinders, two pipes, and two pistons. One of the cylinders is the ram and the other is the plunger; they are connected by a chamber that is filled with hydraulic fluid.
Ram
In many hydraulic systems, several rams are used, the number of which depends on the working load. Multiple small rams are preferred over a single larger one to allow for better control over the thrust force. Fluid is supplied to the ram by a pump and hydraulic accumulator that works between the rams and the pump.
Accumulator
The accumulator stores hydraulic pressure as a fluid, which is released when required. The configuration of a hydraulic accumulator is a cylinder with a piston that is spring loaded or pneumatically pressurized. The pump continuously pumps hydraulic fluid into the accumulator to keep the pressure in it constant. The inlet of the accumulator is attached to the pump, while the outlet is attached to the machine.
Without the accumulator, the pump would have to be continually running. The accumulator helps avoid this by serving as a storage container for the energy needed to operate the machine.
Pump
The three types of hydraulic pumps are vane, gear, and piston with piston pumps being the most commonly used. The pumps for a hydraulic press are a positive displacement pump, which means they deliver a constant amount of fluid with each pumping cycle. The positive displacement pump can be fixed or variable with a fixed pump working at a constant speed while a variable pump changes speeds and can be reversed.
The reason that piston pumps are best suited for hydraulic presses is their ability to perform in high pressure hydraulic systems. The pump operates at large volumetric levels due to its low fluid leakage. The types of piston pumps include axial, bent axis, and radial.
Cylinders
The number of cylinders in a single column hydraulic press varies according to its design. The function of the cylinders is to generate the compressive force that drives the anvil and die. In a two cylinder design, the diameter of the cylinder that carries the ram is larger, while the diameter of the second cylinder that carries the plunger is smaller. Cylinders are metal pipes with two ports for the input and output of hydraulic fluid.
The cylinders are connected by a pipe containing hydraulic fluid. When the plunger in the small cylinder applies pressure to the hydraulic fluid through a downward action, the created pressure is applied to the ram. The action in the smaller cylinder by the downward movement of the plunger is that of a mechanical actuator used to produce unidirectional force by a unidirectional stroke.
Hydraulic Press Process
The mechanical results of the pressure created in the hydraulic system can be seen in the animation below where a plane slug is placed under the anvil, and pressure created by the cylinders drives the ram that drives the anvil down into the slug and forces it into the die.
Types of Hydraulic Presses
Hydraulic presses play a major role in the fabrication, assembly, and production of components for machinery and parts for commercial and industrial products. The differences between the types of hydraulic presses are determined by several factors, which include their frame and the metals used to manufacture them.
The popular use of hydraulic presses in manufacturing is due to their ability to apply substantial compressive force to billets to flatten, shape, straighten, stamp, and bend the billets into designs and various forms. The process of hydraulic presses, with the use of a variety of dies, can be changed and customized to fit a wide range of manufacturing requirements.
Types of Hydraulic Presses
H Frame Hydraulic Press
With an H frame (two column) hydraulic press, the frame, press cylinder, pump, and bolster are in the shape of an "H." The uses for H frame hydraulic presses machine include work in repair shops, maintenance buildings, and production assembly lines. They have a hand pump for low volume applications or air and electrical pumps where consistent operation is required. The amount of available force in an H frame depends on the size of its cylinder.
C Frame Hydraulic Press
Single column (C frame) hydraulic presses have a body frame in the shape of the letter "C" with a single arm structure. They have excellent rigidity, guide performance, speed, and exceptional precision. They are ideal for small operations and require limited floor space.
Four Column Hydraulic Press
Four column hydraulic presses can apply substantial force to any size work piece. They can have a single or two cylinder design depending on the requirements of the manufacturing process. Four column presses have a central control system with semi-automatic cycling and adjustable pressure and compression speed. The punching and working pressure are adjusted in accordance with the needs of the stroke range.
Horizontal Hydraulic Press
There are operations where it is difficult to shape a component or part using a conventional vertical press because the part is too long or short to load vertically. Those types of workpieces are shaped by a horizontal press where pressure is applied horizontally.
With a horizontal press, there are two platens with one being fixed while the other is movable. Pressure is applied horizontally between the platens. This type of press has a protective mechanism on the hydraulic system to avoid overload, and it can have different tools attached.
Hydraulic Wheel Press
A horizontal wheel hydraulic press is used for mounting and unmounting wheels, bearings, gears, and sheaves onto and off of shafts, axles, rolls, or armatures. They are used for the correction of shaft parts and pressing of shaft sleeve parts. A wide range of industries use hydraulic horizontal wheel presses as part of their production process. Examples include the automotive, motor, electronics, mining, and home appliance industries where precision press mounting is necessary.
Straightening Hydraulic Press
When large, long shafts from oil rigs, cars, trucks, and aircraft need to be straightened, the ideal method is a hydraulic straightening press that slowly controls the straightening process, resulting in accurate and fully functional parts. Hydraulic straightening presses can be used to straighten shafts, plates, and large weldments.
Straightening hydraulic presses for weldments straighten the workpiece prior to it being welded. The process for working with shafts is a little more difficult since they come in different diameters and lengths.
Straightening hydraulic presses can have a fixed head or moving gantry.
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