Views: 2534 Author: Site Editor Publish Time: 2020-06-09 Origin: Site
In an attempt to alleviate the problem of the dearth of equipment in our laboratories in most of our higher institutions, a 30-ton hydraulic press was designed, constructed and tested using locally sourced materials. The principal parameters of the design included the maximum load (300 kN), the distance the load resistance has to move (piston stroke, 150 mm), the system pressure, the cylinder area (piston diameter = 100 mm) and the volume flow rate of the working fluid. The major components of the press designed includes the cylinder and piston arrangement , the frame and the hydraulic circuit. The machine was tested for performance with a load of 10 kN provided by two compression springs of constant 9 N/mm each arranged in parallel between the upper and lower platens and was found to be satisfactory. a steel bolt fastened at the bottom plate of a hydraulic press, is subjected to high impact forces. this bolt has a major diameter of 14 mm and pitch of 2mm. it is 300 mm long and nut carries an impact energy of 4500 n-mm. bolt used is shown in figure 1b. thread is cut for the full 14 mm diameter. using the dfm principles design a better screw which can reduce the root area stress to 245 mpa from standard root area stress of 290 mpa. show the calculations.
The development of engineering over the years has been the study of finding ever more efficient and convenient means of pushing and pulling, rotating, thrusting and controlling load, ranging from a few kilograms to thousands of tons. Presses are widely used to achieve this.
Presses, as defined by Lange, are pressure exerting machine tools. They can be classified into three principal categories as: hydraulic presses which operate on the principles of hydrostatic pressure, screw presses which use power screws to transmit power and mechanical presses which utilize kinematic linkage of elements to transmit power.
In hydraulic press, the force generation, transmission and amplification are achieved using fluid under pressure. The liquid system exhibits the characteristics of a solid and provides a very positive and rigid medium of power transmission and amplification. In a simple application, a smaller piston transfers fluid under high pressure to a cylinder having a larger piston area, thus amplifying the force. There is easy transmissibility of large amount of energy with practically unlimited force amplification. It has also a very low inertia effect.
A typical hydraulic press consists of a pump which provides the motive power for the fluid, the fluid itself which is the medium of power transmission through hydraulic pipes and connectors, control devices and the hydraulic motor which converts the hydraulic energy into useful work at the point of load resistance.
The main advantages of hydraulic presses over other types of presses are that they provide a more positive response to changes in input pressure, the force and pressure can accurately be controlled, and the entire magnitude of the force is available during the entire working stroke of the ram travel. Hydraulic presses are preferred when very large nominal force is required.
The hydraulic press is an invaluable equipment in the workshop and laboratories especially for press fitting operations and for the deformation of materials such as in metal forming processes and material testing for strength. A look at the workshop in Nigeria reveals that all such machines are imported into the country. Therefore, it is intended here to design and manufacture a press, which is low cost and hydraulically operated using locally sourced materials. This will not only help to recover the monies lost in the form of foreign exchange, but will enhance the level of our local technology in the exploitation of hydraulic fluid power transmission.
Fluid power systems are designed by objective. The primary problem to be solved in designing the system is transposing the desired performance of the system into system hydraulic pressure.
Fig. 1. Schematic diagram of the hydraulic press. volume flow rate and matching these characteristics with an available input to the system to sustain operation.
The principal parameters of the design included the maximum load (300 kN), the distance the load resistance has to move (piston stroke,150 mm), the system pressure, the cylinder area (piston diameter = 100 mm) and the volume flow rate of the working fluid. The critical components that require design included the hydraulic cylinder, the frame, the hydraulic circuit (Fig. 1).
2.1.Component Design
● Hydraulic Cylinder:
Hydraulic cylinders are tubular in structure in which a piston slides when hydraulic fluid is admitted into it. The design requirement includes the minimum wall thickness of the cylinder, the end cover plate, the flange thickness and the specification and selection of number and sizes of bolts. The output force required from a hydraulic cylinder and the hydraulic pressure available for this purpose determine the area and bore of the cylinder and the minimum wall thickness.
● Cylinder End-Cover Plate:
The thickness T, of the end-cover-plate, which is supported at the circumference by bolts and subjected to an internal pressure uniformly distributed over the area, is given by Eq. (2) from Khurmi and Gupta (1997), as: T = KD(P/δt) 1/2, (2) where: D = Diameter of end cover plate (m), 0.1; K = Coefficient depending upon the material of plate, 0.4, from Khurmi and Gupta (1997); P = Internal fluid pressure (N/m2 ), 38.2; δt = Allowable design stress of cover. plate material, 480 N/m2 ; from which the thickness of the plate was obtained to be 0.0118 m.
● Bolt:
The cylinder cover may be secured by means of bolts or studs. The possible arrangement for securing the cover with bolts is shown in Fig. 2. In order to find the correct size and number of bolts, n, to be used, the following Eq. (3) was used as adopted from Khurmi and Gupta (1997): (πDi 2 /4)P = (πdc 2 /4)δtbn, (3) where; P = Internal fluid pressure (N/m2 ); Di = Internal diameter of cylinder (m); dc = Core diameter of bolt (m), 16 × 10-3 m; δtb = Permissible tensile strength of the bolt.
If the size of the bolt is known, then, the number of bolts can be computed and vice versa. However, if the value of n as obtained. above is odd or a fraction, then the next higher even number is adopted. The number of bolts was computed to be 3.108, hence four bolts were chosen. The tightness of the joint between the cylinder and the end-cover-plate depends on the circumferential pitch, Dp, of the bolt, which was obtained as 0.0191 m from Eq. (4): Dp = Di + 2t +3Dc, (4) where: t = thickness of cylinder wall (m), 17 × 10-3.
● Cylinder Flange:
The design of cylinder flange is essentially to obtain the minimum thickness tf, of the flange, which may be determined from bending consideration. There are two forces in action here, one due to fluid pressure and the other that tends to separate the flange due to sealing which has to be resisted by the stress produced in the bolts. The force trying to separate the flange was computed to be 58.72 kN from Eq. (5): F = (π/4)D1 2 P, (5) where: D1 = outside diameter of seal, 134 × 10-3 m.
● Flange Thickness Determination:
The thickness of the flange, tf may be obtained by considering the bending of the flange about section A-A being the section along which the flange is weakest in bending (Fig. 3). This bending is brought about due to the force in two bolts and the fluid pressure inside the cylinder.
Therefore, Eq. (6) gave a flange thickness of 0.0528 m: tf = (6M)/(bδf), (6) where: b = Width of flange atsection A-A, 22.2×10-3 m; δf = Shear stress of flange material, 480N/m2 ; M = Resultant bending moment, 5,144.78 Nm.
● Piston:
The required piston rod column size necessary to sustain applied load and which is in alignment with the center line of the cylinder bore is influenced by the strength of the rod material, the force applied to the rod column in compression, the mounting situation of the cylinder itself and the stroke over which the load is to be applied.
The procedure for computing piston rod column size and cylinder lengths under end thrust condition was accomplished using the procedure suggested by Sullivan. By this the size of the piston rod of diameter not less than 0.09 m was considered adequate for the design.
● Selection of Seals:
Seals are used to prevent internal and external leakages in the system under varying operating conditions of pressure and speed. Static seal selected uses the groove and ring principle to affect a seal. The groove dimension is calculated such that the Oring selected is to be compressed 15-30% in one direction and equal to 70-80% of the free cross-sectional diameter. The problem in the selection of static seal is to specify the groove such that an O-ring can be compressed in one direction and expanded in another, Therefore; a grove dimension of 4 mm × 3 mm was specified for the seal.
The frame provides mounting points and maintains proper relative positions of the units and parts mounted on it over the period of service under all specified working conditions. It also provides general rigidity of the machine (Acherkan 1973). The design consideration is that of direct tension imposed on the pillars. Other frame members such as the platens (as in our case) are subjected to simple bending stresses.
● Platen:
The upper and lower platens provide point of direct contact with the object being compressed. Hence, they are subjected to pure bending stress due to an equal and opposite couple acting in the same longitudinal plane. The design consideration is essentially for bending and consists primarily upon the determination of the largest value of the bending moment (M) and shear force (V) created in the beam which was found to be 45 kN/m and 150 kN, respectively. These were computed using the procedure adopted.
● Section Modulus:
The values of V and M obtained facilitate the computation of the modulus of section of the platens. This gives the minimum depth (thickness) d, and was computed to be 0.048 m from Eq. (7): d = [(6M)/(δb)]1/2, (7) where; M = Maximum bending moment, 45 kN/m; b = 600 × 10-3 m; δ = 480 × 106 N/m2 .
2.3.Pump
The initial parameter in the design is to estimate the maximum fluid discharge pressure required at the cylinder and a factor is then added to account for frictional loss in the system. This was obtained to be 47.16 × 106 N/m2 .
The pumping action is actuated by a lever system. The actual length of the lever was obtained to be 0.8 m. This was calculated by assuming a maximum theoretical effort and taking moment about the fulcrum.
200 mm × 70 mm U-channel section steel was obtained locally from the structural steel vendor and two 200 × 400 × 40 mm steel plates were obtained from scrap yard in Benin City, Nigeria. After determining the main dimensions of the critical sections from design, two 2,800 mm sections were cut off the steel using power hacksaw in the workshop from which the frame was fabricated. A Φ150 mm with Φ90 mm internal diameter pipe was also obtained from the scrap yard and was bored and lapped to Φ100 mm on the Lathe. Also a Φ70 mm and 15 mm thick tubular mild steel pipe was also obtained which was turned at one end to Φ60 mm to house the seal and the seal housing. The piston and cylinder were assembled and mounted on the base of the frame with bolts which were previously welded together. A guide bar made from a steel pipe was also provided to enable the straight vertical motion of the platen. The platens were produced from the steel plate and two holes of Φ20 mm were drilled at both ends for the guide bar passage. The lower platen was assembled on the top of the piston and held in position by a recess machined on it. A calibration ring was also produced from a 10 mm thick mild steel plate and was placed between the upper platen and the press cross bar as shown in Fig. 1.
3.1.Performance Test Result
It is a normal practice to subject engineer-ing products to test(s) after manufacture. This is a significant step in the manufacturing process. Under tests the product is checked to see if functional requirements are satisfied, identify manufacturing problems, ascertain economic viability, etc.
Testing is therefore employed to prove the effectiveness of the product. For the hydraulic press, test for leakages is the most significant test. The test commenced with the initial priming of the pump. After which the fluid was pumped. This was carried out under no-load condition. The machine was left to stand in this position for two hours.
The machine was then subjected to a load of 10 kN provided by two compression springs of constant 9 N/mm each arranged in parallel between the platens. The springs were then compressed axially to a length of 100 mm. This arrangement was left to stand for two hrs and was observed for leakages. Leakage in the system was not indicated as the lower platen did not fall from its initial position.
A 30-ton hydraulic press was designed, manufactured, and calibrated. The machine was tested to ensure conformability to design objectives and serviceability. The machine was found to be satisfactory at a test load of 10 kN. Further testing to the design load is yet to be carried out.
5.Failure Analysis
5.1 Overview
To analyze the failure of the main cylinder of the four-column hydraulic press, the following issues deserve attention:
●In-depth analysis of the hydraulic system diagram, combined with the relevant electromagnet action table and related circuit diagrams, work out the complete working mechanism of the circuit, and at the same time, correctly understand the circuit design intent and ideas, the technical measures taken, and the related background.
●Correspond to the working principle diagram of the hydraulic press and the actual object, to form a specific impression, the pipeline in the hydraulic circuit, the schematic diagram is often very different from the actual object. When possible, make clear the relationship between the collusion between the valve holes on the valve plate and the barrier resistance. These factors are closely related to the circuit inspection.
●Refer to relevant books and materials to find the basis for judging the characteristics of hydraulic devices, and then judge them.
●According to relevant web pages, books, and equipment instruction manuals, explore the failure mechanism and related analytical testing methods.
●Analysis of No Holding Pressure in Master Cylinder
As shown in the figure, the main cylinder of the four-column hydraulic machine uses a liquid filling valve to achieve a rapid downward movement. The main cylinder often does not maintain pressure. This machine has pressure retention requirements, and generally requires a pressure drop of <2 to 3 MPa within 10 minutes.
Analysis: If the main cylinder does not maintain pressure, it must be a pressure oil leak. From the schematic analysis, it is related to the oil circuit, and there are no more than 5 components that cause leakage.
●Pipes and joints: stress, poor welding, cracks, etc .;
● Holding pressure check valve: poor sealing;
●Filling valve body: poor seal or loose valve seat;
●Filling valve control oil push rod: slightly longer, lift and unload the small spool
●Master cylinder piston (guide bush): The seal ring is damaged.
Exclusion method: According to the analysis results, check and exclude from simple to complex, from outside to inside.
First check the piping and joints (from simple to complex, from outside to inside), and perform initial welding for poor welding and cracks. It is best to remove the O-ring seals at the joints and heat the bends with oxygen welding to turn red, Lightly put on the nut, wait for cooling and setting before assembly.
If there are no defects in the pipelines and joints, check the pressure-preserving check valve (from the outside and inside), remove the check valve plug, polish its seal line, grind it with the valve seat, clean it and assemble it.
After checking the check valve, if the main cylinder still cannot maintain the pressure, check the control valve of the filling valve (from the outside and inside), remove the control oil rod and block the control oil to check whether the pressure is maintained ; If it is impossible to maintain the pressure to confirm whether the putter is long, sand the end of the putter. After checking the push rod, the pressure cannot be maintained. The filling valve should be checked. The main purpose is to check whether the seal line and the seat ring are loose. Polish or grind or reassemble the seat ring.
After the filling valve is checked, the pressure cannot be maintained, and the main cylinder seal ring can be determined to be damaged, and it can be removed and replaced.
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