HYDRAULIC PNEUMATIC POWER TRANSFER UNIT

Abstract: A hydraulic to pneumatic power transfer unit which converts a high pressure oil flow into a flow of high pressure air by means of three stages of piston pumps. The first stage includes a pair of linked pistons (11,14) of substantially equal size, both supplying air to a second stage piston (15) which is directly linked to a third stage piston (18) so that the pair of second and third stage pistons work alternately with the pair of first stage pistons. A linkage between each pair of pistons actuates spool valves (40,41) which control the motion of the other pair of pistons. A second embodiment includes an internal oil reservoir (34) and a swash-plate pump (70) that may be powered by a rotating shaft instead of a flow of oil. The pulses of oil produced by the swash-plate pump directly control the piston pairs without the need for spool valves.

Description: This invention relates generally to pumps and more specifically to piston type fluid intensifiers in which one fluid, for example hydraulic oil, is used to increase the pressure of a second fluid, for example, air. Fluid intensifiers may be of various configurations and are used in many types of industrial devices. For example, the second, pumped fluid may be the same as, or may even be a portion of, the first, powering fluid as is the case in the well-known "water rams" used in less developed areas to supply water under pressure from a stream. Alternatively, the second fluid may be similar to the first but without intermixing as in oil driven fuel transfer pumps. The fluids may even be of quite different types, i.e. one may be a gas and the other a liquid, as in air driven oil pumps or hydraulically driven air compressors. It is this latter configuration which is of most interest in the present invention. Compressed air is becoming more useful in many industrial applications, but one of the most demanding applications is in modern aircraft in which air is used for environmental support systems and for pneumatic control systems. Air is usually supplied to such systems, and for other uses, by bleeding a small amount of air from the compressor stages of the gas turbine propulsion engines or auxiliary power units. However, many modern gas turbines are designed so that very little excess air is available for such use even though sufficient power is available to drive a separate pump. Prior art air pressure intensifiers have several problems which limit their life and/or reliability. Such intensifiers, or air compressors, are generally multi-stage, positive displacement types in which several pistons of graduated sizes (i.e. stages) are mechanically driven by a crankshaft or Scotch yoke mechanism. The first stage piston, and its check valves, tend to be quite large, resulting in high inertia forces when running at high speeds. These high inertia forces tend to cause early failures, particularly in the check valves. On the other hand, the last stage pistons are small but highly loaded from the pressure of the compressed air. This high face load, when combined with side forces from the crank or Scotch yoke, causes excessive bearing stresses on the side of the pistons resulting in rapid wear of the sealing parts. Typically, commercially available units have a mean time between failure of only about 500 to 1500 hours. Furthermore, the geometry of a crankshaft driven unit results in a great deal of wasted space which is only partly eliminated in the Scotch yoke design. Thus it should be apparent that there is a need in the art for an improved air pressure intensifier. A cursory search of the available prior art shows the following US patents related to the general subject matter of the present invention: 2,293,097; 2,296,647; 2,508,298; 2,864,313; 3,059,433; 3,200,596; 3,809,502; 4,212,597; 4,523,895; and 4478556. In particular the disclosures of US patent numbers 3,407,601 and 3,916,931 illustrate and describe some of the complexities and problems of such equipment. US 4478556 discloses a three stage gas compressor having two interconnecting compression units operated by hydraulic pistons. The first unit comprises a central section flanked by two lateral sections incorporating the first and third stages and operated by an alternating piston. The second unit is similar but each section has a smaller respective diameter. It is an object of the present invention to provide a new and improved pneumatic intensifier of relatively simple design and low cost having a minimum of moving parts and seals so as to reduce or eliminate maintenance and/or adjustments. Another object of the present invention is to provide a hydraulically operated piston-type pneumatic intensifier in a compact unit which wastes less space and has less side loads on the pistons than crankshaft operated units. A further object of the present invention is to provide a fluid intensifier which may be operated at high speeds and high pressures without undue inertia loads on the components. According to the invention, there is provided power transfer unit which comprises: two first stage chambers for a gas such as air; a pair of first stage pistons respectively located for reciprocation within the first stage chambers the first stage pistons being interconnected; a second stage chamber for the gas with a second stage piston located for reciprocation therein; a third stage chamber for the gas with a third stage piston located for reciprocation therein; the second and third stage pistons being interconnected; the two first stage chambers being in fluid communication with the second stage chamber and the second stage chamber being in fluid communication with the third stage chamber; and a hydraulic fluid system engaging the pistons to drive the two first stage pistons alternately with the second and third stage pistons whereby gas is transferred from the first stage chambers to the second stage chamber and from the second stage chamber to the third stage chamber with increasing pressure from stage to stage; characterised in that the pistons and cylinders are contained within a block-like housing; the first stage chambers and piston are on a common, longitudinal axis, the pistons being interconnected by a first link; the second and third stage chambers and the corresponding pistons are located along a common longitudinal axis, the second and third stage pistons being interconnected by a second link; and in that the - hydraulic fluid system comprises a swash plate hydraulic pump having at least four pumping cylinders and adapted to be driven by a rotating shaft, and a series of conduits for transferring pressurised oil from each pumping cylinder directly to a corresponding one of the air pistons to cause the pistons to move under the influence of hydraulic pressure. Preferably the first fluid is air whereby the unit comprises a pneumatic intensifier. The system preferably includes valve means in communication with the conduits and an oil reservoir, the valve means being arranged to maintain oil pressure in the conduits at a level between a selected high pressure level and a selected low pressure level by allowing oil to flow to and from the reservoir as necessary. Preferably, the two common longitudinal axes are parallel so that all of the pistons lie in a common plane. Preferably, the pair of first stage air pistons are substantially of the same size as each other and larger than the second stage air piston which itself is larger than the third stage air piston. The preferred intensifier, or hydraulically driven air compressor, features three stages of graduated sized air cylinders, each of which contains an air compressing piston moved linearly by hydraulic pressure. This arrangement takes up less space than a crank-shaft drive and eliminates side loads on the air pistons. Instead of a single large first stage air piston, however, two smaller pistons are in sequence thereby resulting in a total of four pistons in this three stage air intensifier. The use of smaller pistons and their associated valves in the first stage reduces inertia loads at high speeds. The four pistons are arranged in pairs and each pair is connected by a lightly loaded link which functions to retract one of the pair on a suction stroke while the other of the pairs is on its compressing stroke. When the pistons are in any intermediate position, friction prevents valve motion. The valve logic is established by connecting passageways in the intensifier housing and operates to cause the piston pairs to stroke alternately as will be described in more detail below. Thus, each complete cycle of the intensifier causes air to be compressed in three stages by a series of four hydraulically operated pistons which automatically reciprocate in a predetermined sequence. For example, movement of the first stage pair of pistons near the end of their stroke causes the nearby spool valve to change position so that hydraulic oil is ported to the second and third stage pair of pistons to begin their stroke. Near the end of their stroke, that pair of pistons causes its nearby spool valve to change position and port oil back to the first stage pair of pistons. This sequence is continuously repeated during operation of the compressor so that a flow of high pressure hydraulic oil is mechanically converted into a flow of high pressure air. With this arrangement the flow of oil can be interrupted at any point in the cycle but resumed later at that same point when oil pressure is restored. Another advantage of this invention is that the level of pressure of the output air stream may be designed to be either higher or lower than the pressure of the input oil stream depending on the size selected for each piston assembly. The concepts set forth above have been described with reference to an oil powered air intensifier. However, the same concepts are equally applicable to air powered oil intensifiers. The motive force for the pistons is provided by a swash-plate pump. This pump has at least four cylinders and is driven by a rotating shaft to produce discrete pulses of high pressure oil from each cylinder. By selectively connecting each pumping cylinder to one of the air compressor pistons, the harmonic motion of the pump is transferred to the pistons. However, to insure that an adequate supply of oil is available at all times an internal reservoir (preferably pressurised) and associated pressure relief valves are provided in the system as discussed in more detail below. Thus, it should be apparent that the present invention provides an improved pneumatic intensifier in various configurations suitable to meet the requirements of a particular installation. The invention may be carried into practice in various ways and some embodiments will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a cross section through a fluid intensifier which is outside the scope of the present invention but which shows many of its operative characteristics, the Figure showing the condition where the first stage reciprocating pistons are in the middle of a working cycle while the second and third stage piston assembly is at rest; Figure 2 is a view similar to Figure 1 showing the next cycle where the second stage piston is working while the first stage pistons are at rest; Figure 3 is a similar view showing a later step in the cycle where the first stage pistons are again working while the second and third stage assembly is at rest; Figure 4 is a similar view showing the final step in the cycle where the third stage piston is working; and Figure 5 is another similar view of a fluid intensifier in accordance with the present invention. Referring to Figure 1, the apparatus invention includes a block-like housing 10 containing an upper pair of reciprocating pistons 11-14 and a lower pair of reciprocating pistons 15-18 which co-operate to move air from two upper chambers 19,39 to a lower left chamber 20 through a pipe 21. A check valve 22 prevents air in the chamber 19 from flowing back out through the air inlet while a a check valve 23 allows-air to enter into the upper left chamber 39. This part of the cycle is the fist stage of air compression. During the second stage of air compression, as shown in Figure 2, air in the lower left chamber 20 is compressed further by the piston 15 and forced through a check valve 28 into a pipe 29. Check valves 24,25 in the entrances to the pipe 21 prevent backflow of air into the upper chambers 19,39. As shown in Figure 3, air from the pipe 29 enters a lower right chamber 30 where it is further compressed by the piston 18 and forced through a check valve 31 into a high pressure delivery pipe 32 thus completing the third stage of compression. Preferably, the air transfer pipes 21, 29 have intercoolers 36,37 for dissipating some of the heat caused by compression. The air compressing pistons 11, 14, 15, 18 are moved by the force of high pressure hydraulic oil which, in this embodiment, is controlled by two sliding spool valves 40,41 whose motion is shown in Figure 4. The upper spool valve 40 controls the action of the lower pair of air pistons 15,18 and the lower spool valve controls the action of the upper pair of pistons 11,14 as explained in more detail below. Figure 4 also illustrates various means for sealing the reciprocating pistons such as metallic rings 45, elastomeric O-rings 46, or high pressure seals 47. The operation of the apparatus is best explained by following the flow of hydraulic oil throughout one complete cycle starting with the first of the three stages of air compression. Referring back to Figure 1, high pressure hydraulic oil from any suitable source (not shown) flows from the main supply passage 50 into the interior of the housing 10. A branch passage 51 leads upwards into a chamber 52 on the right side of the upper spool valve (40). At this point in the cycle, the chamber 52 is in communication with a passage 54 which leads down to an oil chamber 55 behind the third stage air piston 18. High pressure oil in this chamber 55 exerts a force on the air piston 18 holding it in a dwell mode at the end of its stroke. At the same time, high pressure oil flows from the main supply line 50 through the hollow interior of the lower spool valve 41 to a chamber 58 on its left. This chamber 58 is now in communication with a passage 59 leading upwards to an oil chamber 60 behind the upper right-hand air piston 11. High pressure oil thus flows upwards into the chamber 60 exerting pressure on the air piston 11 and moving it to the right. As previously mentioned, the air piston 11 forces air out of the chamber 19 past the check valve 24 into the pipe 21. At the same time air is drawn in through the left air inlet 26 past the check valve 23 and into the chamber 39 by the piston 14 which is also forced to move towards the right since it is attached to the piston 11 by a link 12. Oil in the chamber 65 behind the piston 14 is allowed to flow through a passage 64 to the oil return area 49. Figure 2 illustrates a later stage in the cycle after the piston 11 has been forced all the way to the right and is in a dwell mode. During the last portion of its movement, a tang 13 on the link 12 contacts and moves the upper spool valve 40 to the right so that the passage 54, which had previously been in communication with the high pressure oil supply, is now in communication with the oil return area 48. Now, oil flows from the high pressure supply line 50, up through the branch 51 and through the hollow interior of the upper spool valve 40 into the chamber 53 on its left. That chamber 53 is now in communication with a passage 56 which leads the oil down into the chamber 57 behind the second stage air piston 15 forcing it to move to the left thereby compressing the air in the lower left air chamber 20. After the piston 15 has moved all the way to the left, to the end of its stroke, Figure 3 shows that a tang 17 on the link 16 connecting the lower pair of pistons 15 and 16 has contacted and moved the lower spool valve 41 to the left. At this point, high pressure oil from the main supply line 50 flows into a chamber 63 on the right side of the lower spool valve 41 from which it can flow upwards through the passage 64 to a chamber 65 behind the upper left hand air piston 14 thereby moving it, and the attached piston 11, to the left. This movement compresses the air in the chamber 39 and forces it through the check valve 25 into the air pipe 21 while, at the same time, drawing fresh air into the right-hand chamber 19 through the check valve 22. The third stage of air compression and the next step of the cycle, shown in Figure 4, occur as the top pair of air pistons 11 and 14 move to the end of their stroke. During the last portion of their movement, the tank 13 on the link 12 connecting the pair of pistons contacts and moves the upper spool valve 40 to the left. Now, high pressure oil from the main supply pipe 50 flows up the branch 51 to the chamber 52 on the right hand side of the upper spool valve 40. Since the chamber 52 is now in communication with the passage 54, the oil flows through the passage 54 into the chamber 55 behind the third stage air piston 18 moving it to the right. This movement compresses the air in the right hand lower air chamber 30 and forces it out past the check valve 31 into the high pressure air delivery pipe 32. Again, during the last portion of movement of the lower piston pair 15 and 18, the tank 17 on their connecting link 16 contacts and moves the lower spool valve 41 to the right. Thus, the apparatus returns to the configuration shown in Figure 1 and the entire cycle repeats. Turning now the Figure 5, a special type of hydraulic pump 70 has been added to the system. The pump 70 is known in the art as a swash-plate pump which functions to convert mechanical energy from a rotating shaft into a flow of hydraulic fluid by means of several individual pumping cylinders operating in sequence. Such pumps are well known in the art (see, for example, US Patent No.4,620,475) and need no detailed description here. However, the piping arrangement of the present invention differs from that commonly used in the art. Typically, when such a pump is normally used, the oil input from each of the several pumping cylinders is combined into one delivery pipe so that the sequential pulse of oil from each cylinder is smoothed out to form a steady stream of fluid. In contrast, the present invention uses each individual pulse of oil to move one of the air compressing pistons of the basic invention. The sequential nature of these pulses eliminates the need for the two spool valves 40,41 and also simplifies the fluid passageways as explained in more detail below. The pump 70 has six individual pumping cylinders 71-76 which operated by an angled swash plate attached to a rotating shaft (not shown). Each of the pump cylinders 71-76 is connected to a conduit 66-69 which leads to one of the air compressing pistons. For example, the pump cylinder 71 is connected to the conduit 66 which is in communication with the chamber 65 behind the air piston 14. Thus, as the swash plate is rotated, the hydraulic fluid in some of the pumping cylinders, for example 71, is being forced out of the cylinder, through its associated conduit, and into the chamber behind one of the air compressing pistons 14 moving it on a compression stroke. At the same time, hydraulic fluid in those pumping cylinders which are diametrically opposite, for example 74, is being sucked into the cylinder from its conduit and associated chamber 60 behind the corresponding air piston 11 moving on an intake stroke. As the swash plate continues to rotate, the hydraulic fluid in an adjacent pumping cylinder, for example 72, will be forced through its associated conduit 67 into a chamber 55 behind another one of the air compressing pistons 18 while the diametrically opposite pumping cylinder, for example 75, will receive fluid through a conduit 69 from the chamber 57 behind the corresponding air piston 15. Thus, each of the air compressing pistons is moved in sequence by fluid from the pump cylinders. In the event that the volume of fluid provided by a single pumping cylinder e.g. 72 is not sufficient to move its associated air piston 18 far enough to complete its stroke, then an adjacent pumping cylinder 73 may be connected to the same conduit so that its volume may be added to the chamber 55 without disrupting the sequence of operation. Since it is not practical to match exactly the volume of each fluid chamber to one, or even two, of the pumping cylinders, the present invention contemplates the use of a reservoir 38 and stroke compensation 81,88 in the hydraulic circuit as follows. Each of the conduits, for example 66, is connected to two pressure relief valves 81,82 which are themselves connected through an oil make up line 80 to the reservoir 38. One of the two valves is a high pressure relief valve 81 while the other is a low pressure relief valve 82. When the pumping cylinder 71 has supplied sufficient fluid to fill the chamber 65 completely and thereby move the piston 14 to the end of its stroke, any further rotation of the swash plate will cause the fluid pressure in the conduits 66 to increase and open the high pressure relief valve 81 so that excess fluid escapes to the reservoir. Later in the cycle when all the fluid has been returned from the chamber 65 to the pumping cylinder 71 during its suction stroke, any additional fluid needed to fill the cylinder 71 is supplied from the reservoir 38 through the low pressure relief valve 82. It is preferred that the fluid pressure in the conduits not be permitted to become negative (i.e. Below atmospheric) so the invention contemplates pressurising the reservoir. One method to accomplish this is to supply air from the first stage of compression (i.e. from pipe 21) through a bleed pipe 33 to a chamber 34 behind a movable piston 35 in the oil reservoir. Therefore, if at any time during operation, one of the air compressing pistons bottoms out during the return stroke, and thus ceases to supply fluid to the pump, the pressure in its associated conduit will fall below the reservoir pressure and fluid will be added to the circuit through the low pressure relief valve to restore synchronisation so that the cycle will continue to repeat.

Claims: A power transfer unit which comprises: two first stage chambers (19,39) for a gas such as air; a pair of first stage pistons (11,14) respectively located for reciprocation within the first stage chambers (19,39), the first stage pistons (11,14) being interconnected; a second stage chamber (20) for the gas with a second stage piston (15) located for reciprocation therein; a third stage chamber (30) for the gas with a third stage piston (18) located for reciprocation therein; the second and third stage pistons (15,18) being interconnected; the two first stage chambers (19,39) being in fluid communication (21) with the second stage chamber (20) and the second stage chamber (20) being in fluid communication with the third stage chamber (30); and a hydraulic fluid system engaging the pistons (11,14,15,18) to drive the two first stage pistons (11,14) alternately with the second and third stage pistons (15,18) whereby gas is transferred from the first stage chambers (19,39) to the second stage chamber (20) and from the second stage chamber (20) to the third stage chamber (30) with increasing pressure from stage to stage; characterised in that the pistons and cylinders are contained within a block-like housing (10); the first stage chambers and piston are on a common longitudinal axis, the pistons (11,14) being interconnected by a first link (12); the second and third stage chambers (20,30) and the corresponding pistons (15,18) are located along a common longitudinal axis, the second and third stage pistons (15,18) being interconnected by a second link (16); and in that the hydraulic fluid system comprises a swash plate hydraulic pump (70) having at least four pumping cylinders (71,76) and adapted to be driven by a rotating shaft, and a series of conduits (66,69) for transferring pressurised oil from each pumping cylinder directly to a corresponding one of the air pistons to cause the pistons to move under the influence of hydraulic pressure. A unit as claimed in Claim 1, characterised in that the first fluid is air whereby the unit comprises a hydraulically powered pneumatic intensifier. A unit as claimed in Claim 1 or Claim 2, characterised by valve means (81-88) in communication with the conduits and an oil reservoir (34), the valve means being arranged to maintain oil pressure in the conduits at a level between a selected high pressure level and a selected low pressure level by allowing oil to flow to and from the reservoir as necessary. A unit as claimed in Claim 3, characterised in that the reservoir (34) is located within the block-like housing (10) and is arranged to be pressurised by a movable piston (35) acting under the influence of gas from the first stage. A unit as claimed in any preceding Claim, characterised in that the two common longitudinal axes are parallel so that all of the pistons lie in common plate. A unit as claimed in any preceding Claim, characterised in that the pair of first stage air pistons (11,14) are substantially of the same size as each other and larger than the second stage air piston (15) which itself is larger than the third stage air piston (18). A unit as claimed in any preceding Claim, characterised by intercooler means for cooling gas heated by compression, the intercooler means being located in the flow path of the gas from the second stage.

Inventors: GAZZERA, RAYMOND WILLIAM
Applicants: ALLIED-SIGNAL INC.
Application year: 1987
Main classification: F04B9
IPC codes: F04B25, F04B9