3.1. The Magnitude of the Resulting Hydraulic Oil Pressure Is Affected by the Different Values of the Hydraulic Oil Pressures in the Hydraulic Cylinders and the Pressure in the Supply Pipe
The measurements were carried out under the same technical conditions. Tensometric force sensors 4 (
Figure 3) were calibrated with a load of known weight before the experimental tests were performed on the laboratory equipment (
Figure 5). At the beginning of the measurements, the piston rods were fully extended from the bodies of all hydraulic cylinders 3, whereby the tensometric force sensors 4 were loaded only by the weight of the freely suspended ropes 2. In
Figure 9, this state is designated as
(a).
The hydraulic oil was supplied under a pressure of p
p1 [Pa] by the manual hydraulic pump lever, with valves D and A open (see
Figure 4) and valves B and C closed, under the hydraulic cylinder piston 3
(1). The pressure p
p1 [Pa] of the hydraulic oil acting on the piston of the hydraulic cylinder 3
(1) pushed the piston rod into the body of the hydraulic cylinder 3
(1), thereby generating a pulling force F
R1 [N] of a certain magnitude in the steel rope 2, which was detected by the tensometric force sensor 4. In
Figure 9, this state is designated as
(b).
After valve A was closed (with valve D open and valves B and C closed), valve B was opened and hydraulic oil was fed into the space under the piston of hydraulic cylinder 3
(2) under a pressure of p
p2 ≠ p
p1 [Pa], then valve B was closed. The pulling force F
R2 [N] in the steel rope 2 detected by the tensometric force sensor 4 is directly proportional to the size of the applied pressure p
p2 [Pa] of the hydraulic oil under the piston of the hydraulic cylinder 3
(2). In
Figure 9 this state is designated as
(c).
With valve D and C open (and valves A and B closed), hydraulic oil was pumped under pressure p
p3 ≠ p
p2 ≠ p
p1 [Pa] into the space under the hydraulic cylinder piston 3
(3). When the pressure p
p3 [Pa] was reached in the space under the piston of the hydraulic cylinder 3
(3), valves C and D were simultaneously closed. The pulling force F
R3 [N] in the steel rope 2 detected by the tensometric force sensor 4 is directly proportional to the magnitude of the supplied pressure p
p3 [Pa] of hydraulic oil under the hydraulic cylinder piston 3
(3). In
Figure 9, this state is designated as
(d).
When valves A, B, C and D are closed, hydraulic oil pressure pp = pp3 [Pa] is present in the supply pipe.
With valve D closed and the hydraulic oil pressure in the supply pipe of the hydraulic circuit pp [Pa], valves A, B and C were gradually opened. Valve A was opened first. The different pressure values pp1 [Pa] and pp = pp3 [Pa] (with valves B, C and D closed) stabilized at the same pressure of pp1p [Pa]. The magnitude of the pressure pp1p [Pa] is dependent on whether the pressure pp1 [Pa] is less or greater than the pressure pp [Pa] and also on the volumes V1 [m3] and Vp [m3]. During experimental measurements, it was not possible to accurately measure the volumes of hydraulic oil in the spaces above the piston Vi [m3] for all hydraulic cylinders, nor the volume Vp [N] of hydraulic oil in the pipes.
Assuming that the volume of hydraulic oil under the piston V1 [m3] of the hydraulic cylinder 3(1) is the same as the amount of hydraulic oil in the pipe Vp [m3] of the laboratory equipment, then the resulting pressure is pp1p = 0.5· (pp1 + pp) [Pa]. If the volume of hydraulic oil under the piston of the hydraulic cylinder V1 [m3] is greater, less or equal to the volume of hydraulic oil in the hydraulic pipe Vp [m3] and if pressure pp1 = pp [Pa], then the resulting pressure is pp1p = pp1 [Pa].
The magnitude of the resulting pressure p
p1p [Pa] is greater than p
p1 [Pa] if p
p > p
p1 [Pa], see
Figure 10(a). The magnitude of pressure p
p1p [Pa] is less than p
p1 [Pa] if p
p < p
p1 [Pa], see
Figure 10(b).
In the next steps, first valve B and then valve C were opened (while valve D was closed). The pressures in the spaces above the pistons in all three hydraulic cylinders (when valves A, B, C are open) as well as the pressure in the supply pipe of the hydraulic circuit (when valve D is closed) stabilize at the same pressure p
pr [Pa], the theoretical magnitude of which can be described by the relation (6). In
Figure 9, this state is designated as
(e).
After valve D is opened, the hydraulic oil moves to the tank of the manual hydraulic pump, so that the pressure of the hydraulic oil in the supply pipe of the hydraulic circuit and in the space under the pistons of all hydraulic cylinders is zero (takes on the value of hydrostatic pressure). In
Figure 9, this condition is designated as
(f).
Table 1 presents different initial values of pulling forces F
Ri [N] in ropes 2, measured using tensometric force sensors 4, see
Figure 3, which were generated by the pressure of the oil supplied by the hydraulic pump into the space under the pistons of the hydraulic cylinders 3
(i). The measured pulling forces F
Ri [N] in ropes 2, during the gradual filling (first cylinder 3
(1) and last cylinder 3
(3)) of hydraulic oil into the space under the pistons of hydraulic cylinders 3
(i) of the laboratory equipment, were used to calculate pressures p
pi [Pa] of hydraulic oil under the piston of the hydraulic cylinder 3
(i).
The calculated pressures p
pi [Pa] were verified with the measured (using manometers, see
Figure 11) hydraulic oil pressures under the pistons of hydraulic cylinders 3
(i). The manometer detected pressures in the unit of bar = 1·10
5 Pa (0.1 MPa).
Figure 12(a) presents the values of the measured pulling forces, which were generated by the pressure of the hydraulic oil supplied under the pistons of the hydraulic cylinders during the gradual opening and closing of valves A, B and C while valve D is open, see
Figure 4.
Figure 12(b) shows the value of the detected force F
R1p [N] (in the rope above the hydraulic cylinder 3
(1)) which was measured by the tensometric force sensor (with valves B, C and D closed) at the moment when valve A was opened. At this moment, the pressure p
p1 [Pa] of the hydraulic oil under the piston of the hydraulic cylinder 3
(1) equalized with the pressure of the hydraulic oil in the pipe p
p [Pa] to the value of p
p1p [Pa], and the pulling force F
R1p was acting in the rope above the cylinder 3
(1) [N].
Table 2 lists the values of the forces detected in the ropes 2 by the tensometric force sensors 4 at the moment when valves A, B and C were gradually opened while valve D was left closed.
Figure 12(c) shows the values of the detected forces F
R1pR2 [N] (in the rope above the hydraulic cylinder 3
(1)) and F
R2R1p [N] (in the rope above the hydraulic cylinder 3
(2)), which were measured by the tensometric force sensors when valve B was opened (with valves C and D closed and valve A open). At this moment, the pressure p
p2 [Pa] of the hydraulic oil under the piston of the hydraulic cylinder 3
(2) equalized with the pressure of the hydraulic oil in the pipe p
p1p [Pa] (as well as in the space under the piston of the hydraulic cylinder 3
(1)) to the value p
p2p1p [Pa] and pulling forces F
R1pR2 [N] (in the rope above the hydraulic cylinder 3
(1)) and F
R2R1p [N] (in the rope above the hydraulic cylinder 3
(2)) were measured in the ropes.
Figure 12(d) shows the values of the detected forces in the ropes above the hydraulic cylinders 3
(i) that were measured by the tensometric force sensors at the moment when valve C was opened (with valve D closed and valves A and B open). At this moment, the pressure p
p3 [Pa] of the hydraulic oil under the piston of the hydraulic cylinder 3
(3) equalized with the pressure of the hydraulic oil in the pipe p
p2p1p [Pa] to the value p
p3p2p1p [Pa], and pulling forces F
R1pR2R3 [N] (in the rope above hydraulic cylinder 3
(1)), F
R2R1pR3 [N] (in the rope above hydraulic cylinder 3
(2)), and F
R3R1pR2 [N] (in the rope above hydraulic cylinder 3
(3)) were measured in the ropes.
By analyzing the individual phases of the measurements, see
Figure 12(b) to
Figure 12(d) and
Table 2, on the laboratory device (see
Figure 5) it can be observed that when valves A to C are gradually opened, the measured pulling forces in the ropes are not completely equalized. The pressure p
p1 [Pa] (pulling force F
R1 [N]), when opening valve A, should equalize to the same magnitude as the hydraulic oil pressure in the hydraulic pipe. The magnitude of pressure p
p1p [Pa] depends on the magnitudes of the volumes V
1 [m
3] and V
p [m
3] and the magnitudes of the initial pressures p
p1 [Pa] and p
p [Pa]. According to relation (7), it is possible to calculate the theoretical pressure p
p1p [Pa] under the piston of the hydraulic cylinder and the pressure in the pipe when valve A is open (and valves B, C and D are closed).
The volume of hydraulic oil in the hydraulic pipe V
p [m
3] is constant and does not change in the laboratory equipment during all experimental measurements. The volumes V
i [m
3] of hydraulic oil under the pistons of the hydraulic cylinders are different for each measurement performed. Their values depend on the magnitude of the supplied hydraulic oil pressure under the piston of the respective hydraulic cylinder (3, see
Figure 3), i.e., on how far the piston rod is inserted into the hydraulic cylinder body. Due to the different and unknown volumes V
i [m
3], relation (7) does not allow the calculation of the theoretical magnitude of the hydraulic oil pressures in the hydraulic circuit pipeline when opening valves A and B without knowledge of the volumes V
i [m
3].
3.2. The Magnitude of the Resulting Pressure of the Hydraulic Oil Affected by the Change in the Different Magnitudes of the Pulling Forces in the Ropes
The hydraulic pump (
Figure 11(c)) was used to supply hydraulic oil with a pressure of p
p [Pa] through the hydraulic pipe (when valves A, B and C of all three hydraulic cylinders 3
(i) are open, see
Figure 4) into the space under the pistons of hydraulic cylinders 3
(i). The pressure of the hydraulic oil p
pi [Pa] under the pistons of the hydraulic cylinders 3
(i) caused the piston rods to be inserted into the bodies of the hydraulic cylinders 3
(i) and generated the pulling forces in ropes F
Ri [N], whose magnitudes were detected by the tensometric force sensors 4 (
Figure 3) [
37]. After valve D was closed, the pressure of the hydraulic oil in the hydraulic pipe and the pressure under the pistons of the hydraulic cylinders 3
(i) was p
p [Pa], which can be expressed according to relation (4), provided that values of forces F
Ri [N], which were detected by tensometric force sensors, are known.
When valves A, B and C of the hydraulic cylinders 3
(i) are closed (provided that the valve D is closed) and the screws that mechanically attach the suspension nuts of the tensometric force sensors 4 (
Figure 3) to the steel frame of the laboratory equipment 1 are gradually tightened, the pulling forces F
Ri [N] in ropes 2 differed. The maximum pulling force F
Ds [N], which could be generated in the ropes 2, is given by the permissible load m
Ds = 250 kg (F
Ds = 2.45 kN) of the tensometric force sensor 4.
The magnitudes of pulling forces F
Ri [N] measured by tensometric force sensors 1, see
Figure 6, were recorded by modules 3 and 4 of the DEWESoft DS-NET measuring apparatus and displayed on the PC monitor 6 in the environment of DEWESoft
® X2 SP5 7, see
Table 3.
From the measured pulling forces FRi [N], the pressures ppi [Pa] in the hydraulic cylinders 3(i) were calculated according to relation (4) (using the known cross-section Shc [m2] of the hydraulic cylinder surface).
When valve A of hydraulic cylinder 3
(1) is opened (and when valves B, C and D are closed), pressure p
p1 [Pa] of hydraulic oil under the piston of hydraulic cylinder 3
(1) (at volume V
1 [m
3]) and pressure p
p [Pa] of hydraulic oil in the supply pipe (at volume V
p [m
3]) of the hydraulic circuit equalized to the pressure value p
p1p [Pa], see
Figure 13.
The pressure of the hydraulic oil p
i [Pa] in the spaces under the pistons of the hydraulic cylinders 3
(i) generates pulling forces F
Ri [N] in the i-th steel rope, and their magnitudes are detected by tensometric force sensors 4 (see
Figure 3). Hydraulic oil pressures p
i [Pa], as recorded in
Table 4, were calculated from the recorded values of pulling forces F
Ri [N] according to relation (4).
By analyzing
Figure 15(a) to
Figure 15(c), it can be observed that when valves A, B and C are gradually opened, the pulling forces (detected by tensometric force sensors [
36]) acting in the ropes, as well as the hydraulic oil pressures in spaces under the pistons of the hydraulic cylinders, did not equalize to the same value. The pulling forces in all three ropes equalized to approximately the same value only after valve D was opened, see
Figure 15(d).
The different values of pulling forces, see
Figure 16(c), can be explained by the fact that the spaces above the pistons of the hydraulic cylinders are interconnected by a closed and sealed hydraulic pipe. When the piston rods are inserted into the hydraulic cylinders, the contact surfaces of the seals (polyurethane) of the pistons with the inner surface of the hydraulic cylinders create friction between these contact surfaces, which acts as a resistance force when the piston rods are inserted into the hydraulic cylinders.