Re-Chargeable Servo Cylinder

The prevent invention relates to fluid power components generally and to closed-loop servo cylinders specifically.

In particular, the present invention pertains to a linear actuator energized either hydraulically or pneumatically by a working fluid under pressure in a self-contained rechargeable system that employs tubular housing as fluid reservoir comprising of two fluid chambers separated by a piston with appropriate seals therebetween to maintain fluid pressure therein so as to either lock piston and rod assembly in a selected axially variable position or to permit axial motion within the stroke limitations provided in order to find it's own position, consistent with closed-loop servo system operation. Such servo cylinders normally operate with integrated circuits and rarely are rechargeable self-contained units. In turn, all servo cylinders of the present state of the art are extremely complicated and costly.

The present invention provides a servo cylinder of this general type which is simple in design, and hence low in cost, and reliable in operation. One aspect of the invention makes possible the utilization of simple unidirectional check valve in the cylinder blind end provided with a port for cylinder charging, be it an all metal or an all plastic servo cylinder.

Depending on the magnitude of forces, speeds and strokes including other parameters that may influence the selection of such cylinders, the ultimate choice of materials of construction can always be based on the minimum fabricating cost for a given cylinder design. The integration of the check valve into the cylinder charging port permits the utilization of injection and pressure molding with thermosetting or thermoplastic materials or a combination thereof with equal success as making such cylinders by conventional techniques from metal. However, producing servo cylinders from plastics multiplies the advantages of the present invention considerably. It provides means for integrally molding at least one end cap with the cylinder body in one operation. It also enables production of an all-plastic one-piece piston rod assembly drastically cutting down the manufacturing cost of such piston assembly. In another embodiment, the piston rod assembly may be of a rubber-plastic composition wherein the piston is of rubber while the rod is of plastic. Or it may be that such composite piston rod construction is employed with a metal housing. Or a metallic gland serving as rod end closure may be preferred is some applications since such glands are already in use with metal cylinders as standards. In some cases it may be imperative to use metal cylinder with plastic piston rod assembly, in particular in large force and velocity applications requiring high pressures for load position change.

It is obviously desirable to provide servo cylinders of this type which are not too complicated and perform superior function in service at less cost.

Further, the cylinder re-charging provision makes it possible to use the same cylinder design for a plurality of applications. By simply charging cylinder with a working fluid of higher pressure changes the cylinder classification, a task impossible with servo cylinders of prior art, factory precharged with fixed pressure for fixed force and velocity application and not otherwise, in particular if the force must be slightly augmented for better results in the field. In turn, logistics are greatly improved with the servo cylinder of the present invention at practically no cost.

These and other objects and advantages of the invention will become more fully apparent from the following description of an embodiment of the invention, taken together with the accompanying drawings.

FIG. 1 is a cross sectional view of a typical all plastic servo cylinder identifying pertinent structural details.

FIG. 2 is a cross section of identical servo cylinder in metal with gland modified.

FIG. 3 is a piston and rod assembly in section with modified valving for high pressures.

FIG. 4 is a piston and rod assembly in section with optional valving means.

Shown in FIG. 1 is an all plastic one-piece molded cylinder housing 1, having a tublar body 2 with open receiver end 3 and closed opposite end with integrally molded end cap 4 which is formed and provided with substantially conical inner face 5. In the end cap 4 a central port 6 for supply of working fluid under pressure during cylinder charging is provided with an integral floating check valve 7. A seal 8 secured to a stem 9 provided with a larger head 10 at one end and a flattened large tee 11 at another stem end of check valve 7 is shown abutting face 5 of cap 4 when the valve 7 is closed due to internal pressure effect after charging the cylinder which when disconnected from fluid supply source becomes a self-contained energized servo unit. During cylinder charging, however, seal 8 will float away from face 5 along with stem 9 as far as the tee 11 will permit when subjected to pressure of the working fluid supplied to port 6,rendering valve open. Disconnecting port 6 from the source of supply of the working fluid renders valve automatically closed due to the directional change of fluid flow, making check valve 7 unidirectional device of simple design. In fact, stem 9 can be inserted without seal 8 through port 6 so as to have head 10 pass end opening 12 into a first fluid chamber 13 at the cylinder blind end 14 far enough to place seal 8 from open receiver end 3 over head 10 during check valve 7 assembly into the housing 1 prior to placing piston and rod assembly thereto. Due to the angular configuration of face 5, the force vector acting over seal 8 is downward, towards the inside corner of head 10 marked by arrow 15 when valve 7 is closed, legitimizing the structural integrity of such simple check valve design, because if the face 5 were not on an angle, seal 8 could be easily unseated from stem 9 under pressure due to force direction otherwise acting parallel to the axis of heat 10. It is not to say that in some high pressure and force applications head 10 may have to be enlarged beyond the size of opening 12 so as to command the use of two piece check valve construction. This could be easily accomplished by separating stem 9 with head 10 from the tee 11 initially until final assembly inside opening 12 is performed placing the seal 8 over stem 9 against large head 10 first before inserting from the receiver end 3 to meet tee 11 inserted via port 6 for final assembly. Large head two piece design would insure sealing capability of the check valve 7 irrespective of the construction and configuration of end cap 4 as well as the magnitude and the direction of the forces prevailing. Since the majority of the servo cylinder applications fall into low to moderate pressure and force ranges, extensive tests with one piece stem 9 shown in FIG. 1 and FIG. 2 proved conclusively that the design of check valve 7 is more than adequate for purpose intended, in particular that the seal 8 of O-ring configuration shown can easily be replaced with a seal of rectangular configuration cut out from an elastomer tubing of dimensions needed (not shown) when so indicated by an application subject to seal improvement. Port 6 may also be closed by a plug per FIG. 2.

An all plastic one-piece molded piston and rod assembly 16 of FIG. 1 is comprised of piston 17 including appropriate grooves 17a for piston seals 18, and a piston rod 19. With the cylinder receiver end 3 open, the piston rod assembly 16 is slidably inserted into tubular body 2 of cylinder 1. Rod end 20 provided with seals 21 and 22 is slipped over piston rod 19 for subsequent securing permanently in the receiver end 3 with seal 39 thereby completing the cylinder assembly. This cylinder alone serves as novel conventional actuator except for the check valve 7 which obviously would be omitted from the end cap 4 in conventional actuator but added as mandatory to re-chargeable servo cylinders of the present invention incorporating check valve plus flow control means so as to enable closed-loop servo system operation, typical to servo cylinders. In turn, central to piston 17, at an inner end of a a bore 23 of piston rod 19, FIG. 1 shows an integral flow control valve means 24 for control of fluid flow between piston sides 17b and 17c respectively via a central fluid passage 25 exhausting into the first fluid chamber 13 at the cylinder blind end 14. When control rod 26 is depressed externally at the end 26-a to unseat internal seal 27 secured inside a groove 28 adjacent rod end head 29 spaced inside a counterbore 30 of piston 17, working fluid can circulate between a second fluid chamber 31 comprising an annular space formed by piston rod 16 and tubular body 2 and a first fluid chamber 13 at the cylinder blind end 14 via an opening 32 leading to central fluid passage 25 thereby establishing closed-loop system that permits unit operation. A seal 33 spaced inside bore 23 against a rod shoulder 34 of control rod 26 insures that the working fluid never leaves the fluid reservoir comprised of the two fluid chambers 13 and 31 respectively after the cylinder becomes energized hydraulically or pneumatically at selected pressures determined by applicational needs, speeds and forces. However, since the seal 27 adjacent rod end head 29 is purposely made of a size smaller than the seal 33 inside bore 23, the size differential produces different end forces due to the working fluid pressure inside bore 23. Since the end force over seal 33 is larger than the end force over seal 27, the integral flow control valve means 24 automatically stays closed with seal 27 pressing against a conical piston face 35 inside counterbore 30 until the rod end 26-a in effect is depressed by a larger external force in the direction opposite to the direction the end force over seal 33 acted to establish fluid communication between the sides of the piston 17b and 17c respectively. Conversely, when the flow control valve means 24 is closed, the piston and rod is locked in a given position because of no fluid communication between piston sides is permitted. However, when the control rod 26 becomes activated by pressing on the rod end 26-a, the piston and rod can be moved inside the piston proper due to the fluid flow between piston sides via fluid passages provided therein until the external force becomes removed to automatically render flow control valve means 24 closed. Simultaneously the axial movement of piston and rod inside the servo cylinder stops at a position which is infinite between the maximum and minimum stroke limitations.

In operation, with piston rod re-tracted as shown in FIG. 1 the flow control valve means 24 is also closed, without any movement of the piston and rod assembly 16 until rod rod end 26-a is depressed opening fluid communication through the valve and starting to extend the piston rod 19 for as long as the rod end 26-a stays depressed and until piston 17 completes the stroke abutting a face 20-a of rod end 20. This travel is automatic due to an end force developed by the working fluid pressure over the piston side 17-b equivalent to pressure times cross sectional area of piston rod 19. For example, if piston rod is of 1" diameter having 0.78 in.sup.2 surface area, and if the cylinder was charged with a 100 psig working pressure, be it hydraulic or pneumatic, the end force of 78 lbs will always be present when the flow control valve means 24 become open so as to permit flow of the working fluid from chamber 31 to enter chamber 13 as a result of the piston travel caused by the end force prevailing. As stated before, the axial piston travel inside tubular body 2 can instantly be stopped when the flow control valve 24 becomes closed which takes place also automatically when the force from control rod end 26-a becomes removed prior to completing full stroke, or it may be restarted again to complete the stroke or, if the force to the rod end 19-a exceeds the driving force of 75 lbs, the direction of piston travel inside the cylinder can be reversed so as to return the piston back into original position shown in FIG. 1, and vice-versa. This type of servo cylinder design is therefore ideal for applications in chair column assemblies wherein the chair seat weighing circa 50 lbs can be lifted up to a maximum height permitted by the stroke. After adding body weight to it when seated such person can lower the seat to any other position downward he may select as a result of the total external force which is larger than the internal force of the working fluid over piston rod of a specific servo cylinder. This is done by activating the protruding end of the control rod so as to open integral flow control valve that permits closed-loop servo system operation in accordance with the preceding description. Locking of piston and rod assembly in any selected axial position inside the cylinder takes place when working fluid is prohibited from communicating between the piston sides. In turn, piston becomes locked in a fixed position by the working fluid from both sides until the communication becomes established across the piston sides urging piston motion inside cylinder in the direction where least resistance prevails as a result of dominant force-action previously described. It should be emphasized here that for non-compressible working fluid the fluid chambers 13 and 31 must be sized equally in order to complete the stroke shown in FIG. 1. For this purpose it was necessary to incorporate a long counterbore 36 into rod end 20 of FIG. 1 acting as a compensator of volumes, since volume of chamber 13 is always longer than the volume of chamber 31 that has to accomodate the piston rod 19 during the piston position change inside tubular body 2 in operation. Using compressible working fluid such as air or carbon dioxide or the like the need for compensator of volumes becomes academic, and as such rarely used with servo cylinders which often are called gas cylinders. Gas cylinders in effect are preferred due to the cushion effect they provide because of the inherent compressibility of the gases serving as working fluids. In turn, more and more gas cylinders appear on the market for use not only in seating adjustment of chair columns, but also in applications such as high-low beds, operating tables and even in automotive industry. Many untapped applications necessitated consideration of servo cylinders produced from metal, in paritcular with cylinder re-charge capability using non-compressible hydraulic fluids at high pressures for cylinder charging.

FIG. 2 identifies such servo cylinder produced from metal. As can be seen from FIG. 2 in cross section, except for a few details, the servo cylinder illustrated is identical in construction and operation to that of FIG. 1.

The servo cylinder of FIG. 2 is comprised of a cylinder housing 40 which may be produced in one piece by forging or casting with an end cap 41 integral or it may be fabricated by conventional methods from two separate pieces wherein a tubular body 42 is connected to cap 41 by threads or tie rods using seals therebetween (not shown). FIG. 2 shows an integral floating check valve 43 inside central fluid supply port 44 which is closed by a plug 45 after cylinder is charged for protection against tampering with check valve 43 to insure long life of the cylinder in service due to this redundance in double sealing from leaks. A piston and rod assembly 46 with seals 46-a and a flow control valve means 47 terminating with external control rod end 48 is also shown in FIG. 2. Opposite to rod end 48 an internal rod head 49 with a seal 50 controls fluid communication between piston sides exposed to fluid chambers 51 and 52 respectively. A seal 53 at an internal rod shoulder 54 of actuating rod 55 of a size larger than the size of seal 50 insures that the flow control means are automatically closed due to the force differential induced by the working fluid in accordance with the preceding description while discussing details of FIG. 1. End closure 56 which is nothing more than a conventional rod gland modified to incorporate a volumetric compensator 57 provided with seals of which an O-ring seal 56-a protects the tubular body 42 from leaking while seal 56-b may be an O-ring or a U-cup to protect a hollow piston rod 58 from leaking along with additional seal 56-c which also serves as a rod wiper to maintain the protruding piston rod 58 clean during the cylinder operation completes the assembly of this actuator. In operation, the conditions are identical to those already discussed when describing FIG. 1 operation, except for the requirement to properly proportion volumes of chambers 51 and 52 so as to benefit from total provision of stroke which in FIG. 2 starts with an end 56-d of gland 56 and continues along the axial length of tubular body 42 up to and until reaching a piston 59, adjacent which is opening 60 for fluid communication. Provisions for load attachment means to the servo cylinder of the present invention in applications include many options. Application of thrust bearings to either end of the servo cylinder of FIG. 1 provided with threads 37 and 38 respectively can be employed if desired or a load carrying platform may be attached to an external taper 19-b of piston rod 19 of FIG. 1. Likewise in FIG. 2 an external self-locking taper 61 formed over protruding section of piston rod 58 may servo as a first cylinder anchor point for a platform of a chair or the like (not shown) provided with a mating taper while the end cap 41 provided with threads 62 may serve as a second anchor point for attaching the servo cylinder of the present invention directly or through a thrust bearing into a system requiring height adjustment without the use of external circuitry for supply of the working fluid.

FIG. 3 represents a modified flow control valve means adaptable to FIG. 1 and FIG. 2 as well in that it shows in a greater detail the valving means in general and means for metering flow across the piston to facilitate speed control means for axial piston travel inside the cylinder in particular, including a modified disposition of the valving means that may be preferred in high pressure applications. In FIG. 3 a piston head 70 having a single peripheral seal 70-a is provided with a hollow piston rod 71 which is sectioned together with a concentric control rod 72 terminating at a shoulder 72-a with a tapered valving stem 73 of the valving means also shown in FIG. 1 and FIG. 2. Piston head 70 includes an inwardly extending small diameter central fluid passage 74 entering a larger diameter central bore 75 of a hollow piston rod 71 for communication of the working fluid with a perpendicular side hole 76 adjacent head 70 passing a wall 71-a of piston rod 71. Stem 73 is provided with a stem head 73-a of a size larger than the size of passage 74. A peripheral seal 77 adjacent head 73-a enters passage 74 together with stem 73 to render valve closed automatically due to an end force exerted by the working fluid over larger seal 78 spaced adjacent shoulder 72-a, in accordance with the preceding description. In FIG. 3 the head 73-a will be seated inside a chamber 74-a of fluid passage 74 to structurally and physically sustain the heavier load high pressure applications of valving means 79 experience in contrast with FIG. 1 and FIG. 2 light duty service wherein seals 27 and 47 respectively carry the end load alone. However, the tapered stem 73 of FIG. 3 when used in FIG. 1 and FIG. 2 will insure the often needed speed control of the piston inside cylinder by virtue of metering capability of the flow through passage 74 allowing more flow to pass along the taper when control rod 72 pushes stem 73 therethrough to extend more while protruding into the blind cylinder end during the actuation thereof.

FIG. 4 identifies an optional valving means with piston head 80 including a single peripheral seal 81 and an internal seal 82 for control of fluid flow between piston sides by a tapered axially movable stem 83 of a control rod 84 inside piston rod 85.

It should be noted here that the piston and rod assembly 86 of FIG. 4, like that of FIG. 3, can be employed with linear actuators that convert pressure energy of the working fluid into a linear motion of the present invention described and shown in FIG. 1 and FIG. 2 with equal success.

The choice of valving means is optional and can be selected from those identified in FIG. 1, FIG. 2, FIG. 3 or FIG. 4, depending on applicational requirements that control basic selection parameters such as speed, force and position, coupled with associated characteristics such as corrosion resistance, lubrication, including materials of construction in order to result in a least costly operational unit.

It can be stated without hesitation that the servo cylinder of the present invention offers many advantages, of which the provision for cylinder re-charge in the field predominates. In fact, there is no cylinder on the market that can be field maintained. The capability of converting a given unit with fixed stroke and force range into another selected stroke by placing inserts into the tubular body, if need be, or by repressurizing the servo cylinder with either gas such as compressed air or carbon dioxide or liquids such as hydraulic oils or a combination of gas-liquid as a working fluid under pressure of a magnitude the construction permits is contributory to the state-of the art in servo cylinders. Force, stroke and speed control with servo cylinder shown in FIGS. 1-4 and described in the foregoing specification meets the objectives set forth by this invention.

Although a preferred embodiment of this invention is disclosed, it is to be understood that various modifications and rearrangements of parts may be made without departing from the scope and the spirit of the invention disclosed and claimed herein.