Reciprocating drive apparatus and method

[0001] This application is directed to a reciprocating drive apparatus for converting rotary motion into linear motion, or vice versa.

[0002] Reciprocating drives for converting between rotary and linear motion may be used, for example, to drive the piston of a pump in various different types of machine. Air pumps are commonly used for ventilators in the medical field, but there are major challenges inherent in using a reciprocating drive to actuate such pumps. For example, such pumps, which are required to deliver clean air, cannot use lubricants between the piston and cylinder. They must also be sufficiently small to fit into the equipment. This means that any offset from a true linear motion is more critical and can cause side loads which will eventually lead to failure of the pump.

[0003] U.S. Pat. No. 5,762,480 describes one reciprocating machine which has been used to drive an air pump. This machine uses a four bar linkage mechanism to convert the rotary motion of the drive motor into a linear movement to operate the pump. This does not deliver a true linear motion, due to the nature of the linkage mechanism, and this results in side load to the seal diaphragms and the pump cylinder or cylinders. This could eventually result in air leakage and component failure.

[0004] It is an object of the present invention to provide a new and improved reciprocating drive apparatus.

[0005] According to one aspect of the present invention, a reciprocating drive apparatus is provided, which comprises a rotational drive, at least one rotary member or crank shaft having a first portion linked to the rotational drive for rotation about an axis of rotation, and a second portion offset from the axis of rotation for rotating in a circular path about the axis of rotation, a fixed linear bearing rail, a linear bearing slide member slidably mounted for reciprocal movement along the guide rail between opposite end positions, and a link arm having a first end rotatably linked to the second portion of the rotary member and a second end rotatably linked to the linear slide member, whereby rotation of the second portion of the rotary member about the circular path forces the linear slide member to slide back and forth between the end positions.

[0006] Since the slide member is constrained to move along a linear bearing rail, side loading on a member to be driven by the slide member is reduced or eliminated.

[0007] In an exemplary embodiment, the slide member is linked to the piston of a pump via a piston rod for driving the pump back and forth in its cylinder. The piston rod passes through a close tolerance diaphragm seal into the cylinder, where it is connected to the piston. The linear bearing slide is constrained within the bearing rail so that it moves back and forth along a linear path, so that any potential side load on the piston rod as a result of the rotational drive is reduced or eliminated. This also reduces the risk of potential damage to the diaphragm seal and cylinder as a result of side loads.

[0008] The components of the apparatus, including the pump, may be contained in a single housing. The apparatus may be used, for example, in a medical ventilator, an oxygen concentrator, a suction pump for medical and other applications, or for a pump in a vehicle such as an automobile or airplane.

[0009] In one embodiment of the invention, two pumps are included and each pump has a piston with a drive shaft, one of the drive shafts being connected to one end of the slide member and the other drive shaft being connected to the other end of the slide member, so that one piston is in the advanced position while the other is retracted, and vice versa. In another embodiment, the rotational drive or motor may have two oppositely directed drive shafts with a first rotary member connected to one drive shaft and a second rotary member connected to the other drive shaft, and first and second linear slide members each linked to a respective rotary member by a respective link arm. Each slide member may be linked to one or two pistons, such that a maximum of four pumps may be operated by the apparatus.

[0010] In an exemplary embodiment of the invention, the linear guide rail and slide member together comprise a linear bearing set, and the guide rail is fixed to the outer housing with bolts or the like to hold it stationary and resist any movement of the rail. The linear slide member may have a groove or channel for slidably engaging over the rail, with suitable interengaging formations between the rail and slide member to hold the slide member on the rail.

[0011] According to another aspect of the present invention, a method of converting rotational motion to linear motion is provided, which comprises the steps of: connecting the output of a rotational drive to one end of a crank shaft, rotatably linking the opposite end of the crank shaft to one end of a link member, rotatably linking the opposite end of the link member to a slide member slidably mounted on a linear bearing rail for movement back and forth in a linear path, whereby rotation of the crank shaft through one complete revolution drives the slide member back and forth between opposite end positions, and connecting the slide member to an object to be driven back and forth in a linear path.

[0012] The slide member may be connected to a piston rod which extends through a sealed opening into a cylinder of a pump or the like, and is connected to a piston slidably mounted in the cylinder. Opposite ends of the slide member may each be connected to a piston rod, each piston rod extending into a respective pump cylinder for driving a piston back and forth in the cylinder. The rotational drive may be connected to two oppositely directed crank shafts, each crank shaft being rotatably linked to a respective slide member which is slidably mounted on a respective linear bearing slide, and each slide may be connected to a piston rod extending into a respective pump cylinder for actuating a piston in the respective cylinder, to provide a two piston pump arrangement. If each slide is connected to two oppositely directed piston rods extending into two pump cylinders for actuating the pistons in the respective cylinders, a four piston pump arrangement is provided.

[0013] According to another aspect of the present invention, a reciprocating pump assembly is provided, which comprises a housing, at least one cylinder in the housing, a piston slidably mounted in the cylinder, a piston rod extending from the piston out of a first end of the cylinder, a linear slide rail secured in the housing adjacent the first end of the cylinder to extend in a direction parallel to the direction of travel of the piston and piston rod, a slide member slidably mounted on the slide rail in alignment with the axial direction of travel of the piston and rigidly secured to the piston rod, a rotational drive in the housing having a crank shaft extending towards the slide rail and slide member, and a link member having one end rotatably connected to the crank shaft and the other end rotatably linked to the slide member, whereby rotation of the crank shaft forces the slide member to move back and forth along the slide rail between opposite end positions and to actuate the piston to slide back and forth in the cylinder.

[0014] In an exemplary embodiment of the invention, a second piston and cylinder are provided in the housing, and the rotational drive has a second crank shaft extending in the opposite direction to the first crank shaft. The second piston has a piston rod extending out of a first end of the cylinder. A second slide rail is rigidly mounted in the housing to extend in a direction parallel to the axial direction of movement of the piston, and a second slide member is slidably mounted on the rail and rigidly connected to the second drive member. A second link member is rotatably linked at one end to the second crank shaft and at the other end to the second slide member. Rotation of the two crank shafts will simultaneously cause both pistons to reciprocate in their respective cylinders.

[0015] The pump assembly may be provided with four cylinders each with a piston slidably mounted in the cylinder. In this case, the third and fourth cylinders will be aligned with the first and second cylinders, respectively, on the opposite side of the first and second slide rails from the first and second cylinders. Piston rods extending form the third and fourth cylinders are linked to the opposite ends of the first and second slide members from the first and second piston rods. Rotation of the two crank shafts in this case will simultaneously cause all four pistons to reciprocate in their respective chambers.

[0016] The reciprocating drive apparatus of this invention uses a linear bearing slide rail and slide member in order to convert rotary motion to linear motion with substantially no side loads. When used in the pump assembly of this invention, it can deliver linear motion to one or more pistons. The piston rods extend through diaphragm seals into the respective cylinders, and the lack of any substantial side loads with this apparatus will extend the lifetime of the seals. The linear drive which has little or no side loading also allows the piston to be operated more easily and smoothly without lubrication, as is required for pumps in medical applications.

[0028] FIGS. 1 to 5 of the drawings illustrate a reciprocating drive apparatus 10 according to an exemplary embodiment of the invention in a four piston pump assembly 20. It will be understood that the reciprocating drive apparatus of this invention may alternatively be used to drive the piston of a single piston pump in other applications, or may be used to convert rotational to linear motion, or vice versa, for other possible purposes. However, in the exemplary embodiment of FIGS. 1 to 5, the apparatus 10 drives four pistons 11A, 11B, 11C and 11D of a four piston pump in a single outer housing 12, and the pump assembly may be designed to pump any fluid, but is particularly intended for use in pumping gases such as air or oxygen in medical applications.

[0029] Each piston 11A-D slides in a respective cylinder 13A-D in the housing and is linked to the drive apparatus by a piston rod 15A-D. The pistons and cylinders are arranged in aligned, opposing pairs on opposite sides of the drive apparatus, with one pair being aligned along drive axis 16 and the second pair being aligned along drive axis 17.

[0030] The reciprocating drive apparatus 10 is mounted in the central region of housing 12, between two opposing pairs of pumps. The apparatus 10 includes a rotational drive or motor 14 in a central chamber 23 with crank shafts or drive shafts 18, 19 projecting from opposite ends of motor 14. A pair of chambers 21,22 are located on opposite sides of the central drive chamber 23, and the crank shafts 18,19 project through opposite end caps 24,25 and rotary bearings 26,28 into the respective chambers 21,22. A linear bearing comprising a linear bearing slide rail 30, 32 and an associated linear bearing slide member 44, 45 is mounted in each chamber. Each slide rail 30,32 is rigidly mounted in each chamber 21,22 facing the crank shaft by means of a respective rigid mounting plate 34,35 which is secured to the outer surface of housing 12 by bolts 36,38. The slide rails are rigidly secured to the inner end faces of the respective mounting plates by bolts 39. The mounting plates each have an outer mounting flange which is bolted to an outer face of the housing, as illustrated in FIG. 4, and a mounting block which extends inwardly into the respective chamber 21,22 to provide a mounting surface for the respective slide rail. This mounting arrangement is extremely strong and secures the linear slide rails or bearing rails rigidly to the housing with no freedom of movement.

[0031] A connecting link or lever 40,42 is rotatably secured to the offset end of the respective crank shaft 18,19 at one end, and is rotatably linked to the respective linear bearing slide member 44,45 at its opposite end. Each linear bearing may be of the type used in sliding drawers and the like. The engagement between each slide rail 30,32 and the respective slide member 44,45 is illustrated in more detail in FIG. 4A. Each slide member is generally C-shaped to form a channel or groove 46 which engages over the slide rail, with rounded ribs 48 running along the side walls of the channel for engagement in smooth linear raceways or grooves 50 on the opposite sides of the slide rail 30, 32. This arrangement vertically constrains the respective slide members to run in a linear path along the linear bearing rails or slide rails. The linear bearing rail may alternatively be of the type having ball channels in which balls are moved, with the slide member having corresponding ball channels which cooperate with the ball channels on the rail to sandwich a plurality of balls between the ball channels, with end caps to secure the balls in the channels.

[0032] In an exemplary embodiment of the invention, the linear bearing sets 30, 44 and 32, 45 each comprise a miniature linear bearing. One example of a suitable miniature linear bearing is the C-sleeve Linear Way ML type, Part No. MLC5C2R55HS2 manufactured by IKO Nippon Thompson Co. of Tokyo, Japan.

[0033] A separate connecting plate 52, 54 is bolted to the outer surface of each slide member 44, 45, and the respective connecting link is rotatably connected to one end of the respective connecting plate 52, 54 on its outer face by pivot pin 55, with a snap ring 56 on the outer end of the pin to keep the bearing in place, as illustrated in FIG. 2. A similar snap ring 58 is mounted on the outer end of pivot pin 59 which rotatably connects the outer end of the respective crank shaft 18,19 to the respective connecting link. The respective pivot pins are rotatably connected to the connecting links via ball bearings or rotary bearings 57, as seen in the schematic illustrations of FIGS. 7 to 9, for smooth operation.

[0034] The first connecting plate 52 is rigidly connected at one end to the first piston rod 15A via a first set screw 60, and the opposite end of the plate is rigidly connected to the end of the second, aligned piston rod 15B via a second set screw 60. Similarly, the second connecting plate 54 is connected to the ends of the aligned piston rods 15C and 15D by set screws 61. This arrangement ensures that the piston rods will follow the movement of the linear slide member 44, 45. Each slide rail 30,32 is secured in the housing so as to extend parallel to the axes 16,17 of movement of the two aligned pairs of pistons, and the connecting plates 52 and 54 have central longitudinal axes which are aligned with the respective axes 16,17. This ensures that movement of the slide members 44,45 along the slide rails will simultaneously move the piston rods in a linear path along the axes 16,17.

[0035] Although the linear bearing slide member and connecting plate are separate elements which are bolted together in the illustrated embodiment, they may alternatively be formed from one solid block. In either case, the slide member is elongate in the direction of the reciprocating axis, and has an inner channel portion forming part of a linear bearing which engages over the linear bearing rail and an outer, solid portion which is aligned with the longitudinal axis of travel of the piston or pistons.

[0036] Each piston rod 15 extends through a diaphragm seal 62 into the respective cylinder 13 and is fixed to the respective piston. The internal diameter of each seal is arranged such that the piston can pass through the seal with its outer surface in sealing engagement with the seal, so that no fluid can leak past the seal. The opposite end of each cylinder is sealed by an end cap 64. A glass tube 65 of Pyrex® or the like is secured in each cylinder and is retained in position against a shoulder in the cylinder by a retaining ring 66. The respective pistons travel in the glass tube which has a smooth surface to avoid the need for lubricants. The pistons may be made of graphite to further ensure a smooth sliding motion. As best illustrated in FIG. 3, the chambers on each side of the respective pistons 11 each have a one way inlet check valve 68 on one side for fluid intake and a one way outlet check valve 70 on the other side for fluid delivery. In the case of an air pump for medical uses, the reciprocating linear motion of the four pistons generates air and delivers air via the one way check valves. For example, as the piston 11A moves in a direction towards the end cap 64, air in the lower chamber will be forced out of the chamber via the outlet check valve 70 in that chamber and air will simultaneously be drawn into the upper chamber via its inlet check valve 68, ready for the next cycle.

[0037] Operation of the reciprocating drive apparatus 10 will now be described in more detail. As the crank shafts 18,19 rotate, the associated connecting lever or link 40,42 will urge the connecting plate 52,54 and attached slide members 44,45 up and down (as viewed in FIGS. 1 and 2). The slide members are constrained to move in linear paths along the two linear bearing slide rails 30,32. This movement will simultaneously urge the associated pistons to reciprocate back and forth in the respective cylinders, with one complete cycle back and forth for each 360 degree rotation of the associated crank shaft. The two crank shafts are offset to coordinate air flow through the respective exhaust valves, so that a constant air supply is provided. FIG. 2 illustrates the relative positions of the four pistons at one point in the cycle. In this position, the two pistons 11A and 11B are at a central position in their travel in the respective cylinders. The piston 11C is at the outer end of its travel, with air drawn into the upper chamber and delivered out of the lower chamber. The upper piston 11D is at the innermost point in its travel, closest to the drive chamber 22, with air drawn into the outer chamber adjacent end cap 64 and air delivered out of the inner chamber adjacent diaphragm seal 62.

[0038] FIGS. 1 to 5 illustrate a four piston pump. FIG. 6 illustrates an alternative two piston pump assembly 80 which may be adequate in some applications. In this embodiment the upper two pistons and associated cylinders of the first embodiment are eliminated, along with the associated upper portion of housing 12. This embodiment is otherwise identical to that of FIGS. 1 to 5 and like reference numerals are used for like parts as appropriate. In this embodiment, each connecting plate 52,54 is connected to only one piston rod, 15A and 15C, respectively, at one of its ends. The operation is otherwise identical to that of the first embodiment.

[0039] In FIG. 6, the reciprocating drive apparatus 60 is mounted in the upper portion of reduced size housing 82 in exactly the same way as it was mounted in a central portion of the housing 12 in the previous embodiment, and is linked to the piston rods 15 to drive the two pistons 11A and 11C to reciprocate back and forth in their respective cylinders. FIGS. 7 to 9 are simplified diagrammatic views of the reciprocating device apparatus 10 attached to two pistons for simplicity. In these Figures, each slide member and associated connecting plate is simplified to a single, unitary slide member with a mounting portion for the respective connecting rod. These drawings also illustrate the ball bearing connections or rotary bearings 57 between connecting link 42 and the pivot pin 55 of the slide member/connecting plate, and between the connecting link 42 and the pivot pin 59 at the end of the crank shaft 19. Similar connections are provided between the other connecting link 40 and the slide member and crank shaft 18.

[0040] FIGS. 7 and 8 illustrate the piston 11C at one end of its stroke, at the upper end of the glass tube 65 in which it slides. In this position, slide member 45 will be at one end position of its axial stroke on slide rail 32, specifically the upper end as viewed in FIG. 7. The piston 11A at this point is at a mid point in its stroke, traveling towards one end of the tube 65, with the slide member 44 at a mid point in its travel along slide rail 30. In FIG. 9, piston 11C has traveled to the opposite end of its stroke, at the lower end of tube 65, while piston 11A has returned to the mid-point of its stroke traveling towards the opposite end of its glass tube 65.

[0041] In each of the above embodiments, rotation of each crank shaft 18, 19 causes the associated connecting link 40,42 to rotate about pivot connection 55 due to rotation of the offset end of the crank shaft about the axis of rotation of crank shaft 18, 19, while the pivot connection 59 rotates relative to the end of the link to which it is connected. At the same time, the connecting link pulls the associated slide member up and down the linear bearing slide rail on which it is constrained to travel. This provides a relatively simple mechanism for converting rotational motion into true linear motion with little or no side loads.

[0042] Although the reciprocating drive apparatus of FIGS. 1 to 9 is used to convert rotational motion of a rotary drive into linear motion, it could alternatively be reversed with a linear drive connected to the linear bearing and the crank shafts linked to a device to be rotated. The pump assembly using the reciprocating drive may be used to pump any type of fluid, but is particularly intended for pumping gases for use in medical and other applications, for example a ventilator pump for supplying air or oxygen to a patient. The reciprocating drive apparatus may alternatively be used to drive a suction pump, an automobile or airplane pump, or in any other applications where it is necessary to convert rotational into linear motion, or vice versa.

[0043] The reciprocating drive apparatus of this invention is particularly useful in applications requiring a relatively small pump, such as medical equipment where the pump must be small enough to fit into the limited space available. Such pumps are also required to be low maintenance and cannot use lubricants. This means that there is little tolerance for side loads which often occur in existing reciprocating drives. This invention converts rotational to linear motion using a miniature linear bearing which is the key to delivering linear motion substantially free of any side loads to the piston. The drive apparatus has the capability of driving up to four pistons simultaneously, further reducing size requirements.

[0044] Although some exemplary embodiments of the invention have been described above by way of example only, it will be understood by these skilled in the field that modifications may be made to the disclosed embodiments without departing from the scope of the invention, which is defined by the appended claims.