Optimization of Manufacturing Processes by Reducing the Costs of Tools and Equipment on Hydraulically Operated High-Pressure Technological Lines

.


Introduction
There are known two solutions for generating high pressure in hydraulic drive systems: an expensive one, based on pumps and equipment for adjusting / controlling high-pressure hydraulic parameters, and a cheaper one, based on pumps and equipment for adjusting / controlling low-pressure hydraulic parameters, plus hydraulic pressure intensifiers. For example, Fig. 1 shows the two drive solutions for a hydraulic cylinder with a load of 700 bar. In the hydraulic drive diagram [1] shown in Fig. 1-left, motor M drives high-pressure pump (1), to direct hydraulic oil, at a pressure of 700 bar, indicated on pressure gauge (2), limited by valve (3), via directional control valve (5), switched to the field of parallel arrows, to the cylinder rod chamber. The cylinder piston chamber discharges into the tank via return filter (8), and the hydraulic cylinder moves to the right with a load of 700 bar.
One can move the same hydraulic cylinder to the right, with a load of 700 bar, according to the hydraulic drive diagram [1] shown in Fig. 1-right, where low-pressure pump (1), pressure relief valve (3) and directional control valve (5) operate at 200 bar, indicated on pressure gauge (2). Return filter (8) remains, while additional low-pressure filter (4) and pressure intensifier (6) appear; the latter is fed via connecting fitting P, from the primary side, by pump (1), and on the outlet of the secondary side, it delivers hydraulic oil at 700 bar in the cylinder rod chamber. Due to the pulsating mode of operation of the pressure intensifier, one uses this drive diagram for short displacements under load of the hydraulic cylinders or with the purpose of achieving and maintaining the load at the stroke end.

Structure and operation of an oscillating hydraulic pressure intensifier
An oscillating hydraulic pressure intensifier (minibooster) is connected in the primary side to a low-pressure pumping unit [2]: port IN connects to the pump outlet; port R connects to the tank. It consumes low pressure (at high flow rate) in the primary side, which it converts at port H, in the secondary side, to high pressure (at low flow rate) for a double or single acting hydraulic cylinder, Fig. 2.  There are four phases in the operation of a minibooster: 1. The oil enters port IN and passes through valves KV1, KV2, DV directly to port H. Pump flow bypasses pistons LP+HP; a hydraulic cylinder connected to port H moves fast forward; 2. When the cylinder encounters resistance, valves KV2 and DV close. The pump pressurizes Vol. 1, the pistons move downwards, valve BV1 discharges Vol. 2 to the tank, via Vol. 3 (oil suction in the secondary side of the minibooster); 3. When the pistons have moved fully downwards, hydraulic pilot channel 1 is pressurized, and bistable directional control valve BV1 changes its position. Pump flow is driven to Vol. 2, the pistons move upwards, delivering a low flow rate to the cylinder at high pressure (oil discharge from the secondary side of the minibooster); 4. When high-pressure piston HP has been moved fully upwards, hydraulic pilot channel 1 connects to the tank, and valve BV1 switches to its start position. The cycle is repeated until the required high pressure is set at port H. Switching 4/3 directional control valve CV on the hydraulic connection field on the right (R to the pump and IN to the tank), channel 3 for hydraulic control of valve DV is pressurized and the high-pressure circuit connected to port H is discharged to the tank.

Test bench for the pumping unit equipped with minibooster
To demonstrate that high-pressure pumping units, consisting of low-pressure pumping modules and oscillating hydraulic pressure intensifier [3], can also be used for displacement of a hydraulic cylinder with constant load over the entire stroke, the authors have built the experimental bench [4] shown in Fig. 3.   Fig. 3. Experimental test bench for the pumping unit equipped with minibooster.
The structure of the bench and the pumping unit in the figure above is as follows: ─ 4: Control and data acquisition unit, acquiring data from the transducers for: acceleration measured in the direction of displacement of the minibooster pistons (Acc1); acceleration measured in the direction of displacement of hydraulic cylinders (Acc2); minibooster primary side pump pressure (p1); load cylinder pressure (p2); filling pump pressure (p3); minibooster primary side input flow rate (Q1); load cylinder flow rate (Q2); stroke (incorporated into the load cylinder).

Excerpt from the nomenclature of experimental tests
The results of the tests carried out for two values of the load of the test cylinder are presented, namely: for 800 bar, equivalent to a resistive force of the load cylinder of 91x103N, and for 700 bar, equivalent to a resistive force of 80x103N. Experimental tests [5,6] have been performed in the following conditions: on the test bench shown in Fig. 3 the opening pressure of the normally closed valve, which the low-pressure pumping unit is equipped with, has been set to 160 bar; the opening pressure of the safety valve of the load cylinder filling pump has been set to 19 bar; the experimental tests have been performed on the advance stroke of the test cylinder; the data acquisition has been done as follows: with a speed of 200 samples / s and a duration of 44 s, for the 800 bar test -a duration of 20 s, for the 700 bar test; on a segment of 27.6 mm, for the 800 bar testa segment of 169.6 mm, for the 700 bar test, out of the total 257 mm stroke of the test cylinder.   respectively. At higher loads, there is a smaller variation in pressure, because the pump pressure valve in the minibooster primary side, set to 160 bar, opens when in its secondary side a pressure of 160x5=800 bar is achieved.
In Fig. 5-bottom one can notice that the time variation of the pressure in the load cylinder is less at the load of 800 bar compared to the one at the load of 700 bar, for the same reason: the pumping unit pressure regulating valve has been set to 160 bar for both sets of measurements (800 bar load and 700 bar load).  Due to the low flow rate of the test cylinder at a load of 800 bar, its displacement, Fig.  7-top, has a small slope compared to that for a load of 700 bar. In the detail of Fig. 7bottom one can notice the deviations from the linearity of the displacement of hydraulic cylinders.

Conclusions
It has been shown experimentally that the displacement of a high-pressure hydraulic cylinder with a small size and constant load over the entire stroke can be done safely by resorting to a cheaper solution: cylinder supply with low-pressure pumping unit, equipped with oscillating hydraulic pressure intensifier (minibooster).
The pulsating mode of operation of the minibooster, caused by the alternating symmetrical displacement of the pistons and the spool of the bistable directional control valve, with a frequency of 0.01 ... 20 Hz, does not induce dangerous shocks that would affect the mechanical strength of the hydraulic cylinder and its drive system.
The deviations from linearity of the displacement are small, and the speed of displacement at high loads increases if the normally closed valve of the pumping unit is set to an opening pressure higher than that corresponding to the load.