Design And Analysis Of Hydraulic Press System
The hydraulic press was invented by John Brahmah as one of the first pieces of machinery that used hydraulics as its prme mover. It consisted of a plunger pump attached inside to a large cylinder and a ram. This press found wide use in England because it provided means of applying large forces in industries.
Today, hydraulic power is used to operate tools and mechineries of varying sizes. In a garage, a mechanics uses hydraulic jack to raise the end of an automobile, dentists and barbers use hydraulic power to lift their chairs to a convenient working height. Hydraulic doorstops keep heavy doors from slamming and hydraulic brakes have been standard equipment for automobiles. Automobiles are equipped with automatic transmissions and power steerings, both of which are applications of hydraulic power. Construction workers depend upon hydraulic power for the operation of various components of their equipment, for example, the blade of a bulldozer is operated by hydraulic power.
Hydraulic systems in engineering applications are composed of pumps, motors, pipes, pistons, valves, filters, and accumulators that use nearly incompressible fluids. Such systems are found in machine tools, earth-moving equipment, power transmissions and aircraft control surface servomechanisms. Almost all hydraulic systems are equipped with a pressure regulator to maintain the working pressure at a predetermined level.
Hydraulic press finds uses in mass production involving the cold working of metals, usually in the form of thin sheet or strip. Press working is one of the extensively employed methods of fabricating parts of intricate shapes with thin walls. Press working processes apply large forces for a short time interval which results in cutting or shaping the sheet metal. Since, press working does not involve heating of the parts, close tolerances and high surface finish can be obtained on the part. The unit cost of labor for operating the press is fairly low since presses can produce components at fast rates.
- Neville Saches: Author focused on the causes of failure and gives the brief introductions about the ductile fracture, fatigue failure and stress concentration. Paper also discussed load responsible for failure i.e., internal pressure, bending, torsion or a combination (Sachas, 2009).
- E. Uys, K. Jarmai, J. Farkas: Cost effective design of a multipurpose hoisting device that can be easily mounted on regular farm vehicle was attempted. The optimization is constrained by local and global buckling and fatigue conditionsand is solved by means of the leap-frog algorithm (Uys, et al., 2003).
- Sinha and Murarka: Authors represented a 3-D complex structure and found that an exact analytical method of stress and deformation analysis is time-consuming. A simplified plane stress FEM model for a hydraulic press structure was identified for its analysis. Certain significant guidelines have been obtained for the design of press frames which resulted in savings in computational time, core memory requirement and cost of analysis (Sinha & Murarka, 1988).
- Muni Prabaharan and V.Amarnath: Topology optimization is applied on various components of scrap baling. Topology optimization results in a better product design along with being cost effective (Prabaharan & Amarnath, 2011).
- LeRoy Fitzwater, Richard Khalil, Ethan Hunter: Topology optimization method outlined herein convinces to re-think the way aircraft structure is designed. Topology optimization requires further development to achieve effective use and to aid in producing quality designs (Fitzwater, et al., 2008).
- M.Bapat and Dessai Yusufali: Authors have investigated on a 30 ton hydraulic forming press machine. Analysis of the frame structure in terms of its material, geometry and stressed induced was performed. Hydraulic press continuously deals with the stress that may be compressive or tensile because press machine always works under impact load condition. Different components of the machine are subjected to different types of loading condition and are analyzed by using ANSYS. Weight optimization of press frame and upper head reduces thickness of the frame structure and material (Bapat.S.M. & Desai.Y.M, 2009).
- Parthiban: Author conducted study on C-type hydraulic press structure and cylinder. It is studied that the structural design of the frame depended on the pressing force which determined for the required rigidity, the dimensions of dies influencing the size of the tool area, work area accessibility that determined by the shape of the press frame. The frame and cylinder are modeled by using modeling CATIA (Parthiban, et al., 2014).
- Pedro G. Coelhoaet: Authors conducted a study on structural analysis and optimization of hydraulic press brakes. Based on the model of the bending process in press brakes defined by Timoshenko theory of beams, workpiece bending errors have been derived by considering the influence of dimensions, shape and initial deformation of the structural components . Shape optimization and dimensional optimization is performed to achieve uniform bending angles over entire length and optimal initial deformation decreasing the bending error (Coelhoa, et al., 2005).
- N.Chauhan and M.P.Bambhania: Authors shows that, FE Analysis has been applied on a frame of 63 ton power press machine.. It is said that Instead of sharp corner of the c – frame, fillet is provided to reduce failure in the structure. Amount of fillet depends on load condition experienced by frame and it is analyzed by using FEM Tool. It is concluded that simulation software is the powerful tool for prediction of plate required for a given load. This analysis given the result of reduction in thickness of plate of frame structure, material saving and as well as cost benefits (H.N.Chauhan & M.P.Bambhania, 2013).
Principles of hydraulics: Hydraulics is the science of transmitting force and/or motion through the medium of a confined liquid and is based on Pascal’s law. In a hydraulic device, power is transmitted by pushing on a confined liquid. The transfer of energy takes place because a quantity of liquid is subject to pressure.
Hydraulic System: A hydraulic system confines a liquid in such a way that it uses the Pascal’s law to transmit power and do work. The oil reservoir serves as a storehouse and filters, strainers, and magnetic plugs removes harmful impurities that clog passages and damage parts. Heat exchanges are used to keep the oil temperature within safe limits and prevent deterioration of the oil.
Working principle of Hydraulic press
Working of Hydraulic Press: Since the hydraulic press works on the basis of Pascal’s Law, its working is similar to the one of the hydraulic system. A hydraulic press consists of cylinders, pistons, pressure relief valves, hydraulic pipes etc. The system comprises of two cylinders; the fluid is poured in the cylinder having a small diameter. The piston in this cylinder is pushed to compresses the fluid in it and flows through a pipe into the larger cylinder. The pressure is exerted on the larger cylinder and the piston in cylinder. The force applied on the fluids by the smaller cylinder results in a larger force in the larger cylinder. Hydraulic presses are used in industries where a large pressure is required for compressing metals into sheets.
Hydraulic Fluid The hydraulic fluid has four primary purposes: (1) To transmit power, (2) To lubricate moving parts, (3) To seal clearance between parts, and (4) To cool or dissipate heat
Double acting cylinder: Double acting cylinders have two opposing effective areas which are of the same or different sizes. They are fitted with 2 pipe ports which are isolated from each other. By feeding fluid via ports “A” or “B” (Figure 1), the piston may transfer pulling and pushing forces in both stoke directions. This type of cylinder may be found in nearly all types of application.
Pressure relief valve: Pressure relief (Figure 2) valves are used in hydraulic system to limit the system pressure to a specific set level. If this set level is reached, the pressure relief valve is activated and feeds the excess flow from the system back to the tank. This valve is always arranged as a bypass valve. The pressure relief valve is also known as a safety valve.
This study is aimed to design a hydraulic press in Simulink using hydraulic blocks and analyze various systems characteristics like piston position, piston velocity, flow rate delivered by pump and fluid power wasted through relief valve when the press is bottomed out.
It is assumed that press simulated here develops a maximum pressure of 95e5 Pa inside cylinders. The fluid is delivered through a fixed displacement pump with nominal pressure of 100e5 Pa and relief valve is set at 95e5 +/- 5e5 Pa. Power source for pump is a constant angular velocity motor running at 188 rpm. A 4-way valve is used to connect cylinders with the pump. The motion of 4-way valve is controlled through a servo-valve actuator whose input is shown in Figure 4. The valve is gradually opened till 2 secs and is held at full open for a total of 3 secs and is then gradually returned to partial opening in reverse direction to enable piston to reach the initial position.
Components of Hydraulic Press
The whole system is simulated for a total of 10 secs and various parameters are analyzed as shown in proceeding sections.
Based on the waste fluid that flows through relief valve as shown in Figure 8, it is concluded that fluid power is wasted when the press is required to hold the bottomed-out configuration for an extended period of time. To optimize the press and reduce the wastage of fluid power, following modifications are proposed:
- Replacement of fixed displacement pump with a variable displacement, pressure compensated pump.
- Installation of closed feedback and control system to drive the servo-actuator.
It is assumed that efficiency of motor driving the pump is not of concern. The volumetric efficiency of pump is 0.92 and overall efficiency is 0.8. The internal diameter of pipes is 0.02m and each segment of pipe is 3m in length with internal roughness height of 1.5e-5m. The resistance to fluid flow offered by flow rate sensors is negligible. Connection of piston rod with translational sensor is ideal. The mass that piston rod acts against in both cases is same and equal to 100kg with translational spring constant of 15000N/m and damping constant of 2N/(m/s). The gain of servo-actuator is also kept at 50 in both cases.
The following chapter gives a comparison of various characteristics obtained from analysis of conventional and modified hydraulic press.
First we give the circuit diagram followed by block diagram, actuating signal to servo, piston velocity, piston position, flow delivered by pump and flow wasted through relief valve in conventional press followed by same features of proposed modified press.
As can be seen from the diagrams. The rod velocity is much more stable due to a feedback and control system which makes the rod position follow the actuation signal more closely. The fluid flow required from the pump is also reduced and wastage through relief valve is almost 2.5 times less. This results in saving in fluid power which means less energy expenditure in the motor and less part wear down.
Data from Simulink is first extracted to workspace by ‘to workspace’ block. The imported data from conventional and proposed press design is combined with time step from simulation. This data is then extracted in form a nx3 array in excel file.
According to the ASME Process Piping Code (B31.3) (ASME, n.d.)
= internal pressure
= wall thickness
= material’s tensile strength
= outer diameter
= wall thickness coefficient (B31.3-2002, Table 304.1.1) (ASME, n.d.)
= material and pipe construction quality factor (B31.3-2002, Table A-1A) (ASME, n.d.)
Maximum internal pressure =
Material tensile strength =
Development of case study
Outer diameter (m) =
(Temp assumed to be less than 900F)
Therefore, the minimum thickness of pipe is calculated to be
Here, , hence the assumption of is valid.
Calculating hoop stress induced in material,
Therefore, hoop stress generated is smaller that tensile strength of material. Hence the design is safe.
The variable displacement, pressure compensated pump used in the proposed design has a built in relief valve that regulates the pressure inside the system. An extra relief valve is installed in the pipes to act as a fail-safe feature. In case the pump malfunctions and continues to deliver fluid exceeding the pressure rating of system, the extra relief valve installed in the system with drain the fluid to sump.
The work presented here describes feedback control system for hydraulic press actuation. The effect of these modifications were analyzed in Simulink and the results were presented here. It was concluded that the pump auto-regulates the fluid being pumped to cylinder thus reducing fluid power loss. The control system modifies the input to actuator with feedback from rod position to match the required positions as closely as possible. This reduces the vibrations in rod and further minimizes fluid power wastage.
The reduction in fluid power wastage flowing through relief valve to sump is clearly reduced as seen from Figure 8 and Figure 13. The loss is minimized by a factor of 2.5. Further improvement to fluid power waste minimization is expected by optimizing control system gain and is subject of future investigations.
In the literature about controlling and optimizing the fluid power and hydraulic press, which is presented in the literature section of this report, it was observed that a similar feedback and control system was employed to control the rpm of motor attached to the pump. This was made possible through a PID controller circuit which took input from position as well as velocity of piston rod. This input was compared with a predetermined position and velocity values, against time, and used to generate an error signal which regulated the motor speed. The net effect was the optimization of on duty time of motor and reduction in electrical power usage. Such a modification to hydraulic press cannot be included in hydraulic circuit modification and hence was not considered in this report. Instead, use of variable displacement, pressure compensated pump and feedback & control system to optimize valve opening was considered and analyzed in this report.
ASME, n.d. ASME Process Piping Code. [Online]
Available at: https://www.asme.org/products/codes-standards/b313-2016-process-piping
[Accessed 04 04 2018].
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