US 3154925 A
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Description (OCR text may contain errors)
Nov. 3, 1964 R. A. DE VITA 3,154,925
POWER PLANTS Filed Jan. 9, 1963 M pz 90 M United States Patent O 3,154,925 POWER PLANTS Raymond A. De Vita, Winchester, Mass., assignor to United Shoe Machinery Corporation, Boston, Mass., a corporation of New Jersey Filed Jan. 9, 1963, Ser. No. 250,362 3 Claims. (Cl. 60-97) This invention relates to improvements in power plants and more particularly to improvements in power plants of the multiple motor type having a pair of fluid pressure operated motors, which are to be operated simultaneously at equal speeds. The invention is adapted for use in cutting apparatus having a cross member arranged to be moved downward by motors attached at laterally spaced points on the cross member. However, there are many other circumstances where it is desirable to maintain equal speeds of movement of such motors. A common expedient for operating two fluid pressure operated motors at approximately equal speeds is to supply each motor independently from one of two similar pumps of the socalled positive displacement type, suitably rotary pumps of the gear or sliding vane type which pumps are driven at the same speed as by connection to a common prime mover. This expedient Works quite well as long as the two motors are loaded equally. However, if one motor is more lightly loaded than the other, it will tend to operate faster than the other motor because of the greater volumetric eficiency of the pump operating at the lower pressure.
Positive displacement pumps include such rotary pumps and reciprocating or piston pumps. Volumetric eiciency is defined for reciprocating pumps as 1 :L v Q+QL where Q is the actual volume of liquid discharged and QL-t-Q is the true piston or plunger displacement. Q'L includes all losses of capacity due to leakage past piston packing, stufiing boxes, and valves, and also that loss due to delayed closing of valves. All losses of capacity given in percentage of the displacement are referred to as slip (=l-ev). In new pumps the slip is of the order of 2%. Volumetric eiciency may be correspondingly defined for rotary pumps. (See Kents Mechanical Engineers Handbook, Twelfth Edition, Power Volume, Section 5, pages 72, 76 and 77.) At pages 5-76 of Kent it is stated: Rotary pumps comprise an exceedingly great variety of types (the number of patents run into thousands), all of which have several common characteristics. They are of the positive displacement type without valves and, except for leakage, can deliver a constant capacity against variable pressure. v
It is with this exception that the invention is concerned. Manufacturers commonly rate rotary pumps by stating the volume of fluid delivered per minute at a given r.p.m. at a nominal pressure which may be or 100 p.s.i. However, the manufacturers supply further data of capacity versus pressure. For example, a typical rotary pump of the sliding vane type is shown to provide at 1200 r.p.m.
with respect to the other, and valve means controlled by the sensing means for bleeding fluid from the leading motor. Such an expedient introduces considerable mechanical complication and is subject to large inaccuracies due to wear.
Accordingly, it is an object of the present invention to provide in a power plant comprising two uid pressure operated motors each individually supplied by one of two positive displacement pumps operated as substantially equal true displacement, of means for substantially equalizing the movements of the two motors under conditions of unequal loading thereof.
To this end and in accordance with a feature of the invention, a power plant system is provided comprising a pair of fluid pressure operated motors, a corresponding pair of positive-displacement pumps of similar type and rated capacity, means for operating said pumps at substantially equal displacement speeds, and means connecting each pump to one of said motors for directing fluid thereto under pressure, said connecting means comprising a fluid pressure piloted throttle valve, piloting means connected to the output of the pump supplying said valve for urging said valve open with a force proportional to the pump delivery pressure and second piloting means connected to the opposite motor for urging the valve toward its closed position with a force proportional to the pressure in the opposite motor. Y Other objects and advantages of the present invention will best be understood from the following description taken in connection with the accompanying drawings, in which:
FIG. 1 is a schematic diagram of apparatus embodying the present invention;
FIG. 2 is a section of a valve employed in the schematic of FIG. 1; and
FIG. 3 is a section of the valve shown in FIG. 2 with its valve spool in a different position.
Referring to FIG. 1, the illustrative apparatus comprises a pair of fluid pressure operated motors 10, 12, each comprising a piston 14 and a cylinder 16 which are relatively movable. For purposes of illustration the piston 14 is shown as fixed but it will be understood that either element of the motor may be fixed with relation to the other.
For supplying fluid under pressure to the motors 10 and 12, the fluid is drawn from a sump 20 through an intake line 22 by two positive displacement type rotary pumps 24 and 26. These pumps are arranged to be driven at equal displacement speeds, suitably by the connection of v each pump to one end of a common motor shaft of a 11.2 gals. per minute at 0 p.s.i., 10.9 gals. per minute at motor M. The pump 24 is connected to the motor 10 through a delivery line 30, a piloted throttle valve 32 and a feed line 34. Referring to FIG. 2, the valve 32 has a valve spool 40 piloted at each end through pilot passages 42 and 44. When the pressures in the passages 42 and 44 are equal, a bias spring 46 urgesfthe spool 40 into the position illustrated in FIG. 2 wherein it closes communication between a delivery passage 48 and a feed passage 50. TheV valve 32 has its delivery passage 48 connected to the line 30, its feed passage 50 connected to the line 34, and its pilot passage 42 connected through a pilot line 52 to a two position two-way solenoid valve54. The valve 54 opcrates when its solenoid is de-energized to exhaust the pilot line 52' to the sump Y2) and when its solenoid is energized to connect the pilot line 52 through a damping restrictor Y` check 5 6.to the delivery ,line 30.
4 VIn similar fashiom the pump 26 is connected to the motor 12v through a delivery line 60, a piloted throttle valve 62 and a feed line 64. The valve 62 which is similar to the valve 32 has its delivery passage 4S connected to the line 60, its feed passage 50 connected to the line 64 and its pilot passage 42- connected through a pilot line 72 to a two position two-way solenoid valve 74, which operates when de-energized to exhaust fluid from the pilot line 72 to the sump 2t) and when its solenoid is energized to connect the pilot line '72 through a damping restrictor check '76 to the delivery line 6l). The pilot passage 44 of the valve 32 is connected through a pilot line di) to the feed line 64 of the motor 12 to admit fluid therefrom to the passage 44. Likewise, the passage 44 of the valve 62 is connected through a pilot line 82 to the feed line 34 of the motor to admit fluid therefrom to the piloting passage 44 or the valve 62. A relief valve 84 is connected to the delivery line 3i) and a relief valve S6 is connected to the delivery line 69. For purposes of the present disclosure it may be assumed that the relief valves 34 and 35 are set to operate at a pressure above the normal working pressure of the motors lu and l2. Movement of the motor l@ is terminated upon de-energization of the solenoid valve 54. Means for returning the motor l@ to its original position may comprise a source of air under pressure, suitably an accumulator 9u connected to the cylinder 16 by a conduit 92, the air being supplied to the accumulator by a compressor (not shown) through a check valve 94 which will taire effect upon the motor lli upon the energization of a solenoid valve 96 for exhausting the pressure fluid from the cylinder 16 through an exhaust line 98 to the sump Ztl. A corresponding solenoid valve 199 is provided for the motor l2 for returning it by air pressure.
Operation Assuming that the motor M is running and that the pumps 24 and 2d are delivering fluid under relief pressure to the lines fait and et?, if the solenoid of the valve 54 is energized, pressure iiuid will be admitted from the delivery line Btl-,to the pilot line 52 thereby moving the spool 40 of the valve 32 into its open position admitting uid from the line 30 to the line 34. The spool l0 will be so positioned, however, only while the pressure in the line 30 is greater than the pressure in the line 8l) by the relatively small amount required to overcome the bias of the spring 46. Suitably the bias pressure may be, for example, about 25 p.s.i. When the valve 32 is open to its fullest extent as shown in PEG. 3, the pressure in the line 30 will tend to fall to substantially equal the pressure in the line 34. However, since the pressure in the line St) is substantially equal to the pressure in the motor 12, if the working pressure of the motor l2 plus the bias pressure is greater than that of the motor 19, the spool of the valve 32 will shortly be moved toward its closed position until the pressure in the delivery line 3? has risen sufficiently to bring about a balance of the piloting forces. In the same manner, the valve 62 will be closed down if the pressure in the line 32 plus the valve bias pressure exceeds the pressure in the line 60.
Thus where both solenoid valves 54 and 74 are energized simultaneously for operating the motors lil and 12 together, it Will be seen that apart from friction losses the back pressure on either pump is equal to the pressure in the motor supplied by the pump, or the pressure in the opposite motor plus the bias pressure, whichever is greater, and the back pressures on the pumps are equalized to within the bias pressure. Obviously, valve bias to insure valve closure is not needed when adequate motor pressure is maintained, as by means of the illustrative accumulator 90.
Accordingly, it will be seen that the pumps 24 and 26 will be operating at substantially equal delivery pressures even where the loading of the motors 10 and 12 is greatly mismatched.
The degree to which the pumps 24 and 26 should be matched will depend upon how closely the speeds of the motors 10 and 12 are to be matched. The pumps should be selected to give at their driven r.p.m. substantially equal volumes of iluid at the different Working pressures within the tolerances prescribed by the desired results. Suitably the pumps will be driven by one motor, although if they are driven by separate induction motors the difference in their r.p.m. within the rated capacity of the electric motors will be less than three tenths of one percent. To give a quantitative idea of pump matching problems and conditions, I have estimated that vane pumps operated at maximum pressure have slippage in the range of about 8 to about 18 percent, and that a random pair of pumps of the same type of a given manufacturers production run would have a worst case delivery mismatch of about 3 to 4% when operated at a given speed and pressure. If closer matching is needed, the pumps can be ordered from the manufacturer on the basis of specifications calling for the pumps to be provided in pairs matched as needed in volumetric etciency and capacity at given pressures, for example, at 500 and 1500 p.s.i.
Having thus described my invention, what I claim as new and desire to secure by Letters Patent of the United States is:
1. In Va power plant comprising a pair of hydraulic motors adapted to act simultaneously, a pump for each of said motors having inlet and outlet passages with the inlet passage connected to a fluid reservoir, a pair of throttle valves each controlling the supply of pressure fluid from a pump to a motor and having a valve element movable between an open position in which pressure uid is supplied to the motor and a closed position in which the supply of lluid is prevented, a pair of delivery lines for supplying fluid from each pump outlet, and means for operating the valve element of each throttle valve connected for response to the pressure of the fluid in the delivery passage of the valve and the pressure of the uid in the opposite hydraulic motor for urging the valve element to- Y ward its closed position irl-accordance with the liuid pressure in the opposite motor and to urge the valve element in the opposite direction in accordance with the pressure in its delivery passage. y
2. In a power plant comprising a pair of hydraulic t motors adapted to act simultaneously, a pair of positive displacement pumps of similar type and capacity having inlet and outlet passages with the inlet passages connected to a liuid reservoir, means for operating the pumps at subs tantially equal cyclic rates, a pair of piloted throttle valves having delivery and feed passages and a valve spool movable between an open position in which the delivery and feed passages are in communication and a closed position in which the delivery and feed passages are closed from one another, said valves having pilot passages adjacent opposite ends of the valve spool, a pair of delivery lines for supplying duid from each pump Vto the delivery passage of a throttle valve, a pair of feed lines connecting the feed passage of each throttle valve to a motor, and pilot lines connecting the pilot passages of each throttle valve to actuate its valve spool into its open position by pressure from the delivery line connected to the valve and into its closed position by pressure from the opposite hydraulic motor.
' 3. Apparatus as in claim 2 and additionally comprising bias means for urging the valve spool of each valve into its closed position when the fluid pressure on opposite ends of the valve spool is equal.
Ernst Apr. 30, 1935 Anderson et al Nov. 1, 196)