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.
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 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.
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:
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.)
where
= 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 =
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.
References
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].
Bapat.S.M. & Desai.Y.M, 2009. Design Optimization of A 30 Ton Hydraulic Press Machine. International Journal for Research in Applied Science and Engineering Technology ( IJRASET ), pp. 24-30.
Bickers, E., 2016. Hydraulic Cylinders and Cushioning Devices. [Art] (SlidePlayer).
Coelhoa, P. G., Fariab, L. O. & Cardosoa, J. B., 2005. “Structural Analysis And Optimization Of Press. International Journal Of Machine Tools & Manufacture, Volume 45, pp. 1451-60.
Filters, B. B. W., 2018. Plastomatic RVT Pressure Relief Valve. [Art] (Big Brand Water Filters).
Fitzwater, L., Khalil, R. & Hunter, E., 2008. Topology Optimization Risk Reduction. Montreal. Canada, American Helicopter Society 64th Annual Forum.
H.N.Chauhan & M.P.Bambhania, 2013. Design & Analysis Of Frame Of 63 Ton Power Press Machine. Indian Journal Of Applied Research, Volume 3, pp. 285-88.
Parthiban, B., Eazhumali, P., S.Karthi & P.Kalimuthu, 2014. International Journal Of Research In Aeronautical And Mechanical Engineering. Design And Analysis Of C Type Hydraulic Press, Volume 2, pp. 47-56.
Prabaharan, M. & Amarnath, V., 2011. Structural optimization of 5ton hydraulic press and scrap baling press for cost reduction by topology. International Journal of Modeling and Optimization, 1(3), p. 185.
Sachas, N., 2009. Root cause failure analysis understanding Mechanical failure. Plant maintenance Recourses center home.
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