US2344133A - Fluid system control - Google Patents

Description

March 14, 1944. DAVIS, JR 2,344,133

FLUID SYSTEM CONTROL Filed June 9, 1941 2 Sheets-Sheet 1 INVENTOR: ARCHIBALD H. DAVIS JR.

ATTORNEYS,

Fl G.

March 1944- A. H. DAVIS, JR 2,344,133

FLUID SYSTEM CONTROL Filed June 9, 1941 2 Sheets-Sheet 2 &

l 6- 5 I I INVENTOR:

ARCHIBALD H. VIS JR.

ATTORNEYS.

Patented Mar. 14, 1944 FLUID SYSTEM CONTROL Archibald B. Davis, Jr., Moline, Ill., assignor to American Machine and Metals, Inc., New York, N. Y., a corporation of Delaware Application June 9, 1941, Serial No. 397,170

7 Claims.

This invention relates to hydraulic driven testing apparatus in which the movable platen is to move toward or away from a fixed head at a uniform rate.

This requires that the rate of motion of the ram be uniform within closer limits than is required in most other hydraulic equipment. This uniformity of motion must be maintained even though the resistance of the specimen and the load on the machine vary sharply.

Though the invention is primarily applicable to universal testing machines the invention is not limited thereto but includes all hydraulic plunger applications in which uniformity of motion is to be seemed despite variable resistance.

In the practice of this invention a cylinder is provided with a metering feed device giving a constant rate of liquid feed and a secondary supply from which measured additional liquid may be discharged into the system depending upon the load on the machine and the position of the ram in the cylinder.

The additional liquid may be discharged into the cylinder manually or, preferably, automatically as the pressure rises.

The amount discharged may be made to vary not only with the pressure change of the liquid but also with the amount of oil already in the cylinder of the testing machine.

At the beginning of the stroke, before the specimen creates much resistance, the pressure on the ram, and therefore the pressure on the oil, that is fed to the cylinder is low. As the resistance of the specimen increases, so will the pressure on the oil, and in commercial practice this pressure may rise to 3000# per square inch or more.

Various devices have been provided, the object of which is to feed the liquid, usually oil, at a constant metered rate regardless of the counter pressure of the ram. Applicant's earlier application on a Fluid system control filed March 22, 1941, Serial No. 384,692, shows such a control designed to feed liquid to the cylinder at a uniform rate.

Tests with machines equipped with metering controls that feed the oil at an absolutely uniform rate show that the rate of feed of the table decreases as the load is applied. The reason for this is that when the ram moves forward under low pressure, the space underneath the rain has been filled with oil at low pressure. Even though, under the increased resistance, the metering device delivers additional 011 already compressed at the same rate as before some of that oil will be used to make up for the reduction in volume of theoil already in the cylinder due to the compression of the oil under the wide range of pressure used.

The tests have shown that, to secure uniform motion of the ram we must feed at a faster rate than normal when the pressure in the system is rising and at a slower rate than normal if the pressure is falling.

- In a standard universal testing machine, with a piston elevated 6" from the bottom of the cylinder. the volume of oil required to compress the 300 cubic inches of oil under the piston to 3000# is approximately .33 cubic inch. This means that with an oil feed of .50 cubic inch per minute (corresponding to .01" per minute desired feed), and a pecimen that loads the machine to full rating in one minute the .50 cubic inch of oil is divided into .33 cubic inch for compression of oil and .17 cubic inch for movement of the piston. The feed would be only .17/50:.0034" per minute instead of the .01 feed desired.

The difliculty cannot be eliminated by recalibrating the metering system so that an opening of the control valve that is marked .010'. per minute actually passes oil enough to both cover that movement and to offset oil compression.

For if the next specimen loaded itself in two minutes instead of one minute th oil compression would be overcorrected, and if it took ten minutes the necessary correction would be only .033 cubic inch per minute so that one valve, for the same setting, would give us a feed of .010" for a one-minute loading and .016" per minute for a two-minute loading time.

The invention provides a second supply of liquid from which enough is discharged into the already metered supply to the cylinder to make up for the compression of the oil previously fed to the cylinder at the lower pressures.

This recognizes that the additional amount of oil is a function of the compressibility of the oil, of the volume of oil under the plunger at the time the test is to be made, and of the rise of pressure during the test.

The device may be manually operated at the right time or it may be made to automatically discharge the proper amount of oil into the cylinder as the pressure rises.

The object of the invention is to provide a universal testing machine or the like in which A further object is to provide a supplementary storage reservoir from which an amount of liquid approximately equal to that needed to offset the compression of the oil is automatically discharged into the cylinder.

A further object is to provide a device which will automatically correct for the compression of oil in the ram of the testing machine taking into account both the rate of change in pressure and the volume of the oil in the cylinder at the working range.

This compensator is intended for use in connection with a precalibrated rate of oil feed. If we had a slow speed and a uniform load over wide range, such as an extruding operation, the precalibrated speed would be maintained and require no correction. However, when, as in normal testing operations, the early portion of the stroke is merely to take up slack and apply the initial load and then the pressure rises up to the end of the test, the precalibrated speed will not be maintained during the latter, and the important part of the test, because some of the, oil being supplied instead of being used to move the ram is used to make up for the compression of the oil which has already been supplied.

Figure 1 is a diagrammatic arrangement, partly in section, of a universal hydraulic testing machine equipped with a control constructed as disclosed in the above cited prior application and with the additional oil compression compensator which is the subject of this invention.

Figure 2 is an enlarged elevation, partly in section, of the oil compensator.

Figure 3 is a section of the compensator taken along line 3-3 of Figure 2.

The invention comprises a source of power generally indicated at I, a pump generally indicated at 6, drawing liquid from a reservoir 8, and feeding through the device 23 designed to give a uniform flow for any given setting of the valve 52 to the cylinder of the machine generally indicated at 20. The machine is equipped with means for preventing leakage along the cylinder walls which do not impose frictional resistance in the manner explained in applicants earlier application, Serial No. 334,333, filed May 10, 1940. This fluid packing is applied through the groove generally indicated at H. The plunger I9 of the testing machine exerts pressure upon a specimen 50 in a universal machine of conventional type I00.

A cylinder generally indicated as MI in Figure 2 contains liquid, which may be discharged through line I04 when the pressure under the ram overcomes a calibrated spring I20. This amount also takes into account the height to which the plunger has moved. This is accomplished by manually setting handle I35 in proper relation to scale I40.

When the test is completed and the pressure is released the spring I20 will automatically refill cylinder |I preparatory to the next test to be performed.

I is normally a motor shaft driving a multi-. cylinder plunger pump 0 which draws liquid in through a check valve 1 and discharges through a check valve I2 liquid drawn from reservoir I into line I3. It is understood that oil under adequate pressure and in adequate volume may be supplied to main I2 in any manner desired as the pump does not form any part'of the invention. Liquid under pressure is supplied through lines I4 and I0 to the fluid packing groove I1 .I28 slides along arm I35.

referred to in the previous application on fluid packing.

Liquid under pressure is also supplied through line I5 to a pressure controller generally indicated at 22 which provides a uniform pressure differential across the manually adjustable controller valve 52 which feeds into a. line 55 to space 20 in cylinder I8. The construction of this pressure reducer is shown in detail in Figure 1 of the fluid control application #384,692, previously referred to. The uniform pressure differential across valve 52 is obtained by controlling the discharge through bypass 42 which reduces the pump pressure to a predetermined amount above the pressure in line 55. This control is automatically accomplished by the pressure control at 22. The discharged oil passes into line 22 which collects the leakage from groove H and discharges through I0 into tank 0.

Summarizing, the pressure in lines I4, I5, I5, and fluid packing groove I! will always be slightly above the pressure in line 55 and space 20. This amount will be just suflicient to give the desired rate of uniform discharge across the contraction imposed at valve 52. Through line I02 which connects with feed line I5, space 98 below the piston is under the same pressure as the supply side of valve 52. Normally this piston is in the position shown resting upon the piston extension 95. The upper side of this piston, space 91, discharges through line I04 into line 55. Accordingly, whenever piston I02 moves upwards a quantity of oil proportional to the amount of movement of piston I02 is discharged through line I04 into 55. This is in addition to that supplied through the metering valve 52.

The unit pressure on the under side of the piston I02 is higher than the unit pressure on the upper side of the piston. Also the area of the upper side is smaller than the area of the lower side by the area of stem I08. Accordingly there will be a differential pressure tending to move piston I02 and its stem I08 upwards. This is resisted by pad II8 of a swinging lever II5 which is pivoted at I I1.

This frame carries a slot II5, substantially an arc concentric with pivot I30 when the pad III is in its lowest position. On pivot I30 an arm I35 may be swung by handle I30. A carriage It carries a roller I25 on a stud N9, the roller traveling in the 'arcuate sIOtIIB of lever H5. The carriage I20 is urged toward pivot I30 by a calibrated spring I28 which engages the carriage at I21 and the end piece of the lever, I29, at I3I. This spring is selected of such length and such resistance to elongate as is required. The initial stress will Just offset the unbalance of forces on piston I02, just described, tending to lift the piston before load is applied.

Assume that the lever I25 has been swung to the extreme left'and the plunger I5 is at the bottom of the cylinder. The end of the lever moves along an arcuat scale I40 which is fastened by screws I4I to studs I42 upon the panel generally indicated at 09. The spring, which is shown of a fixed length but one end of which may be fastened adjustably to permit varying its effective length is under a tension Just sufficient to prevent the rise of plunger I08 when the pressure under the ram of the machine is merely that corresponding to the weight of the table and its accessories being lifted. There will be no discharge from space 01 into line I04. As the testing proceeds and the ram meets rei 2,s44,1as

sistance the pressure in line I04 will increase and the pressure in I03 will increase correspondingly. The diflerence between the unit pressures on the two sides or the piston has already been counterbalanced by the initial tension of spring I28. The increased pressure in line I03 acting over the difference in area between the two sides of the piston is not counterbalanced however and therefore the piston I02 will rise, somewhat increasing the tension on spring I08. This small movement of plunger I02 has discharged a small quantity of oil through line I04 to oil'set the compression of the oil already in the cylinder by the increased pressure at which the oil is now being delivered to the cylinder. It will be seen that the amount liquid discharged from space 01 is directly proportional to the increased pressure in the cylinder.

The purpose of lever I35 permitting the movement of roller I I5 in slot H6 is to give an increased movement to pad II! and therefore an increased movement of plunger I02 for a given pressure in the cylinder, it-desired, by moving the handle I36 a small distance to the right. Thus the amount of oil discharged can be increased and we can determine experimentally the point of the end of the lever on the scale I40 that will correct for the compression of th 011, not only in the cylinder but in the piping system when the piston I9 is at the bottom. It is clear that it the piston is part way up there will be more oil under the piston, more oil required to make the compressibility correction. Accordingly lever I35 will be moved further to the right to a point on the scale I40 which may be experimentally determined that will give the increased discharge of oil through I04 to make the correction.

In the arrangement shown in the drawing it is necessary to manually move lever I35 by scale I40 to correspond to the position of platen I50 as indicated by pointer I5I on vertical scale I52.

Since the position of lever I35 is related to the position 02! platen I50 on top of piston I9 it may be moved thereby by the use of a, suitable system of levers and connecting rods so that the position of lever I35 will automatically correspond to the position of the platen.

In the ordinary test where the position of the piston corresponds to the adjustment oi the platen to the specimen length, and the actual test extends over only a short vertical range, this procedure may be simplified by moving the lever I35 but once to the position corresponding to that at which the actual test load is applied.

If however the load increases over a considerable range of the stroke of plunger I 3 lever I35 should b moved over the corresponding range of the scale in the same manner as it would had it been connected mechanically to the platen.

When the loadis released from the specimen and the pressure in line I04 drops the extended spring I28 will automatically restore the parts to the condition shown in Figure 2 and therefore space 91 will draw in the same amount of oil discharged duringthe load application. Thus, at the beginning of each new test piston I02 is always in its lower position as shown. The device 50# per square inch, and the loss of liquid in a well fitted piston and plunger under such low pressure diflerential is negligible. Line I05 and groove I 06 form a fluid packing of the same type shown in the earlier application and the loss of oil due to the high pressure drop between grooves I00 and I0! represents only the return of an indefinite and unimportant quantity of liquid from the unmetered supply line I05 through I01 back to the sump.

It will be understood that the parts are so designed that when the plunger of the testing machine is at maximum load and at maximum stroke and lever I35 moves toward the right the movement of piston 102 will not quite equal the length of stroke permitted by cylinder I M and that for all other conditions of testing the stroke of piston I02 will be less.

I claim:

1. A hydraulic system comprising; in combination, a source of liquid supply, a pressure chamber, a conduit leading from the source of supply to the pressure chamber, a metering device in said conduit, a liquid container for adding additional liquid to the pressure chamber to offset the loss of volume of liquid in the chamber when the pressure increases, the container including a difl'erential piston, a connection from the source of pressure to one end of the container tending to drive the difierential piston forward, a connection from the opposite side of the difl'erentiai piston leading from the container to the chamber and means offering increasing resistance to the movement of the difierential piston so that the volume of liquid ejected corresponds to the movement of the differential piston and the change of pressure in the system.

2. In combination, a cylinder, a plunger movable therein, a source of supply of fluid under pressure, a conduit leading from the fluid supply to the cylinder, a metering valve in the conduit, a pressure controller ahead of the metering valve, a discharge opening in the conduit contherefore corrects for the decompression of the oil as pressure is released, for instance, because of passing the elastic limit of the specimen, as well as for the compression as pressure is increased.

It will be evident that in attempting to correct for inaccuracies of the order of a fraction of a cubic inch or fluid there must be no leakage withtrolled by the pressure controller for diverting a part of the fluid supply, the pressure controller being adapted to maintain a constant differential across the metering valve by varying the proportion oi. the fluid supply that is diverted; a supply chamber ror additional fluid, a floating piston therein, a connection from one side of the metering valve to one end of the supply chamber, a connection from the other side of the metering valve to the other side of the supply chamber, a spring stressed by the movement of the piston in the chamber, the spring permitting further movement of the piston and further discharge of additional fluid into the cylinder only as the pressure on one side of the metering valve increases.

3. A hydraulic system comprising, in combination, an expansible chamber, a source of liquid under pressure, a conduit leading from the source to the chamber, a metering device in said conduit, means for compensating for the change of volume of the liquid already metered due to changes in the pressure on the liquid in the chamber by adding or withdrawing a measured quantity of additional liquid to the chamber comprising a in the correcting device. Accordingly, to prevent tank, a diflerential piston therein, conduits connecting one side of the piston to one side or the metering device and the other side oi the piston 'to the other side of the metering device, the pressures upon the diilerential piston areas creating a force that varies with the pressure in the chamher and tends to move the piston, means including'a spring progressively stressed as the piston tion, an expansible chamber, a source of liquid under pressure, a conduit from the source to the chamber, a metering device in said conduit, means for compensating for the change of volume of the liquid already metered due to changes under pressure, a conduit from the source to the chamber, a meterinr device in said conduit,

, means for compensating for the change 01 volume oi. the liquid already metered due to changes in the pressure of the liquid in the chamber by adding or withdrawing a measured quantity of additional liquid ,to the chamber comprising, a

- tank of liquid, a diflerential piston therein, conin the pressure of the liquid in the chamber by adding. or withdrawing a measured quantity of additional liquid to thelcham-ber comprising a tank of liquid, a differential piston therein, conduits connecting one side of the piston to one side of the meter and the other side of the piston to the other side of the meter, the pressures upon the diflerential piston areas creating a force that varies with the pressure in the chamber, a base on which the tank is mounted, a lever pivoted on the base, one end bearing against the piston, a cam surface on the lever, an arm pivoted on the base, a slider bearing on the cam that is free to move along the arm, a spring "urging the slider against the cam and thereby exerting pressure on the plunger whereby the quantity of fluid delivered from the tank for a given pressure change in the chamber can 'be varied by setting the arm.

5. A- hydraulic system comprising, in combination, an expansible chamber, a source of liquid duits connecting one side oi! the pistonjro one side of the meter and the other side 0! the piston to the other side of .the meter, the pressures upon the differential piston areas creating a force that varies with the pressure in the chamber, a base on which the tank is mounted, a lever pivoted on the base, one end bearing against the piston, a cam surface on the lever, an arm pivoted on the base, a slider bearing on the cam that is free to move along the arm, a spring urging the slider against the cam and thereby exerting pressure on the plunger, a scale on the base over which the arm moves calibrated for the approximate volume or the chamber in use whereby the quantity of fluid delivered from the tank for a given pressure change in the chamber can be varied according to the size of the expansi-ble chamber by setting the arm.

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