US3302391A - Thermal actuators - Google Patents

Description

Feb. 7, 1967 F. P. MIHM 3,302,391

THERMAL ACTUATORS Filed Jan. 25, 1965 INVENTQR FERD P. MlHM hp Magma@ ATR N EYS United States Patent O 3,302,391 THEAL A TUA'IDRS Ferd I. Mihm, Needham, Mass., assignor to Tempstat Corporation, Hinsdale, NJH., a corporation of Dela- Ware Filed Jan. 25, 1965, Ser. No. 427,664 I() Claims. (Cl. 60-23) The present invention relates to improvements in thermal actuators, and, in one particular aspect, to novel and improved piston-cylinder units of economical construction which respond accurately to temperatureinduced dimensional variations in lill material and which uniquely maintain fluid-tight sealing between relatively movable parts.

Various forms of thermal actuators have been evolved in the past to lconvert thermal energy into mechanical forces and movements which supply motive power for operation of valves, switches, and the like. One conventional arrangement of this general type includes a rigid heat-conducting container within which is disposed a till of material, such as wax, which undergoes a marked change in rate of expansion and contraction as the result of transitions between .solid and liquid phases within a predetermined narrow range of temperatures. In translating the thermally-induced dimensional variations of the till into related mechanical strokes of a movable output member, it has been a common practice to utilize elastomeric sleeve-like diaphragms which may be tightly sealed about their outer peripheries to insure that none of the critical ll volume can be disturbed by even minute leakages. Where substantial Voutput strokes are required, such diaphragms are highly susceptible to distortion and damage by excessive stretching, rictional abrasion, and pinching or bunching effects. Moreover, in some designs the resiliencies of such seals can cause the output responses to be sluggish and can introduce hysteresis effects which prevent the output member from moving exactly to the intended positions as it is repeatedly cycled in its movements. Although piston-cylinder arrays are well suited to the type of mechanical translations which are of interest, the sealing problem has not heretofore been resolved satisfactorily, and, moreover, the expected need for precision machining of bores, pistons, rings, and the like would introduce high costs in items which should be of inexpensive construction.

In accordance with the present teachings, the aforesaid diculties may be avoided through use of unique piston-cylinder type thermal actuator units wherein a special form of dynamically-operated movable elastomeric piston and seal unit cooperates with a tubular cylinder unit in which is disposed an expansible lill.

It is one of the objects of the present invention, therefore, to provide novel and improved high-precision thermal actuators lending themselves to low-cost manufacture and to reliable non-leaking operation with improved hysteresis characteristics.

Another object is to provide unique thermal actuators of the piston-cylinder type wherein movable piston units involving resilient sealing provisions are dynamically expanded to preserve uid-tight relationships with cooperating tubular cylinders under severe operating conditions and despite wear.

A further object is to provide expansible piston units, particularly for use with thermal actuators, which maintain improved sealing in relation to relatively imprecise cylinder bore surfaces while undergoing reciprocating movements in dilerent directions.

Still further, it is an object to provide improved piston units which may be conveniently and precisely oriented and sealed within filled thermal acuator tubes to regulate accurately the responses of the thermal actuators.

3,302,391 Patented Feb. 7, 1967 ICC It is also an object to provide a novel and improved method for assembling and adjusting the responses of thermal actuators having a fill Which is at least in part liquid.

By way of a summary account of practice of this invention in one of its aspects, a thermal actuator, including an elongated tubular member closed at one end and filled at least in part with a thermally-responsive material which expands markedly upon being melted, is provided with a unique movable piston serving to en close and seal the lill near the open end of the tube. The piston comprises an assembly of a hollow cylindrical elastomeric member, which is normally of external diameter not appreciably greater than the internal diameter of the cooperating tube, and a pair of stiff washer-like elements disposed one at each axial end of the elastomeric member, and a central fastener pin extending through the washers and elastomeric member, making a tight seal with the latter, and a spring supported between one end of the pin and one of the washers exerting an axial compressive force which causes the elastomeric member to bulge circumferentially into tight sealing relationship with the bore of the tube. An actuator stem, one end of which separably engages the piston assembly, extends outwardly beyond the open end of the tube to a spring-biased movable valve member, or the like, and is caused to follow the movements of the piston assembly.

Although the aspects of this invention which are believed to be novel are set forth in the appended claims, additional details as to preferred practices and as to the further advantages, objects and features of the invention may be most readily comprehended through reference to the following discription taken in connection with the accompanying drawings wherein:

FIGURE l is a partly pictorial and partly cross-sectioned side View of a relief valve arrangement wherein teachings of this invention are practiced;

FIGURE 2 provides an enlarged illustration lof an expansible piston-seal unit `such as that used in the relief valve assembly shown in FIGURE l;

FIGURE 3 depicts the piston-seal structure `of FIG- URE 2 in a relaxe-d uncompressed state prior to lockin-g of the parts together;

FIGURE 4 illustrates an improved piston-seal unit being inserted into a lled actuator tube;

FIGURE 5 provides an enlarged detail of cooperating portions ot a valve stem, piston-seal unit, and thermal actuator tube, with certain parts 'being cut away to disclose constructional details; and

FIGURE 6 is a partial cross-section of an alternative embodiment of piston-seal assembly.

A typical application for the improved thermal actuator assembly is characterized 'by the automatic relief valve apparatus '7 shown in FIGURE 1, wherein a normallyclosed valve unit 8 is responsive to actuating vforces developed by a thermal lsensing and actuating unit 9. Such apparatus may, for example, be installed atop a hot water tank, -via the threaded coupling 10, with the elongated sensing unit immersed in the enclosed hot water. Movable valve ymember lll, which is loaded into tight seating and sealing engagement with a valve seat 12 within the valve vbody 13 by a compressed spring 14, normally prevents escape of the tank contents through inlet coupling It? and outlet passageway I5. The seating force developed by spring 14, between t-he valve imember 11 and an adjustable valve body cap I6, is set at the factory to insure that the valve is opened by tank pressure only when it falls within a predetermined narrow range of pressures. This same valving member is also .separately actuatable manually, `by an external camming handle 17 which is pinned to a slidable shaft or stem 13 sealed in relation to cap 16 and xed with member 11. The aforesaid automatic and manual pressure relief provisions are in `addition to those for `automatic valiving in response to stem movements developed by the thermal actuator unit 9.

For the latter purposes, an elongated tubular member 19 has its open end 20 fixed with a radially-spoked valve body bracket 21, through which fluid may flow, and its closed end 22 extends into the tank liquid for optimum response to its temperatures. Tube 19 is formed with a substantially even inner surface 23 of substantially one diameter, and is of material exhibiting good thermal conductivity characteristics and resistance to corrosion. The portion of valve stem 18 which projects downwardly from valving member 11 extends into the open end of tube 19 and engages a special piston-seal unit 24 mated within that tube. Preferably, the stem and piston-seal funit are separable from one another, axially, although they are maintained in engagement by the combined actions of load spring 14 and of a fill 25 of thermally-responsive material which occupies the space between piston-seal unit 24 and the closed end 22 of the tube. According to one common practice, the fill may consist essentially of a wax which has a melting point at about the critical temperature at which the valving member 11 should be lifted from its seat t-o `open the pr-otective relief valve (such as a temperature just below the boiling point of the water in the tank). Petroleum-type waxes having widely different melting temperatures are readily obtainable commercially, for example, and may be blended to achieve desired melting characteristics. As is well understood in the art, certain waxes and like substances exhibit a very substantial increase in volume when they make the transition from a solid to liquid state within a predetermined narrow temperature range, and this increase will cause the piston-seal unit 24 to move, raise the stem 18, and lift the valvin-g member 11 4from its seat, with a desirably swift and :positive action for relief valving purposes.

In the past, the tendencies for the pressurized liquid (melted) wax or like material to leak past movable seals has necessitated use of flexible diaphragms and sleeves, generally of elastomeric material, which could be clamped or otherwise bonded about their peripheries to insure that none of the liquid could escape and thereby seriously alter the fill volume and resulting operating characteristics of the actuator. Such deformable elastomeric seals tend to involve undesirably high costs, are particularly susceptible to damage, and are difficult to orient in relation to a particular volume of fill which will reliably provide the precise response sought. By way of distinction, the piston-seal unit 24 may be reciprocated without significant loss of fill, so effectively in yfact that the tube may conveniently and advantageously include a fill not only of normally solidified wax but also of a supplemental `forcetransmitting material, such as water, which is normally in the liquid state and would be expected to pose very serious leakage problems. As is shown in detail in FIG- URES 2 and 3, the piston-seal `unit is a separate integrated structure including as its principal components a tubular elastomeric resilient member 26, a pair of rigid washers 27 `and 28 one :at each axial end of member 2d, a helical loa-ding spring 29, and a central headed fastening pin member 3f). The length 31, external diameter 32, and internal diameter, of resilient member 26 in the relaxed or uncompressed state (FIGURE 3) are selected such that the member will, when compressed axially (FIGURE 2) to a shorter axial length 33, tend to bulge outwardly to a materially greater external diameter 34, while at the same time remaining in tight surrounding relationship with the central fastener 30. For the latter purposes, the axial forces exerted on member 26, when contained -by the internal diameter of the tube, produce radial pressure-sealing `forces both on the walls of the d internal diameter of the tube and on the outer diameter of the pin 3f). In addition, the inner diameter of the resilient member 26 may be made initially somewhat less than the outer diameter 35 of pin 35i, with the pointed end 3fm of the lfastener facilitating the initial assembly of the differently-sized parts. The expanded diameter 34 is selected to be materially in excess of the inner dia-meter of the cooperating actuator tube 19, such that resilient member 2d will constantly maintain Huid-tight sealing with the tube under all expected operating conditions. Before being inserted into the actuator tube, the relaxed loose assembly (FIGURE 3) is compressed axially by the desired `amount which will tend to maintain the aforesaid tight sealing, and that compressed condition is preserved by a crimping 30]) below the level of washer 27. The washers 27 and 28 lprevent damage to the resilient member 26 by distributing the axial compression forces uniformly, and these are of just slightly smaller diameter than that of the actuator tube interior. Spring 29 and the enlarged head 30C of fastener 3f) are preferably enough smaller in diameter than the tube interior to accommodate the lower end of hollow stem 18 in a surroundin-g loosely-nested relationship therewith. As appears in FIGURE 1, the expanded resilient piston-seal member 24 engages the inner side walls of tube 19 over a major portion of its length, rather than merely at the crest of the bulge shown in FIGURE 2, the dimensional change being accommodated by increased longitudinal compression of load spring 29.

An important aspect of the self-loaded piston-seal unit is recognized by considering the potentially troublesome effects which could be present, absenting spring 29, when the fill in an actuator tube contracts in volume but the spring loading by the main valve spring 14 does not act concomitantly to drive the stem 1S against the washer 28 forcefully enough to compress resilient member 26 into the needed ti-ght sealing engagement with the interior of tube 19. This condition could develop when either the internal tank press-ure -or the manually-operated lever 17 causes the valve member 11 to be raised slightly `from its seat. The resulting negative pressure effects on resilient member 26 would then tend to draw compressible air or vapor from the tank into the volume below the member 26, and, thereafter, the effective ll volume would be disturbed, as would its effective incompressibility also, and the actuator would thus fail to operate with the intended characteristics. However, in the improved dynamically-operating unit 24, including the self-loading spring 29, the latter spring is effective to maintain at least such axial compression of member 2e as Will suppress such reverse leakage even when the lseparable stem 18 is widely separated `from the washer 28. This self-loading spring also causes the unit to be self-compensating `for effects of frictional wear. Forces required to return the piston-seal unit to a predetermined position within the tube, after forced displacement therefrom and after cooling of the fill, are relatively low and predictable, such that the hysteresis characteristics of the actuator are highly desirable.

Further advantages afforded by the selfloaded unit 24 are realized while that unit is being inserted into the tube 19 (FIGURE 4). -It is important that the exterior surfaces -of resilient member 26 be smoothly applied against the cooperating inner .surfaces of the tube, and that severe pinching or other -distortions and damage be avoided. The -oversize member 26 would be somewhat vulnerable to these disturbances if the mating forces were applied directly to washer 28. However, it is found that the mating operation is improved by applying forces in the direction of 4arrow 36 to the fastener head 30C by way of a tool 37, instead. Under these conditions, any excessive forces on the resilient member 26 will result in relieving longitudinal expansion against the restraint of the load spring 29, and the resilient member will slip into position more readily and smoothly. These improvements are of particular signiiicance when the tube 19 is filled (FIG- URE 4) in part by a solidified wax 25a, or the like, and in part by a force-transmitting incompressible liquid h, such as water, which occupies the remaining volume within the tube. Some of the liquid must be permitted to escape during the insertion step, to permit the piston-unit t-o reach a predetermined mated position, and the axial resilient relief afforded by spring 29 advantageously accommodates the needed by-pass leakage 3S -at the open end of the tube. This practice involving the supplemental liquid 4lill permits a highly precise setting of the actuator to be made, and at room temperatures, whereas, if the Same technique is sought to be used with a wax ll alone, the wax must first be melted, the temperatures must be high, the clean-up released wax is troublesome, and it is extremely diflicult to control the exact critical volume of wax till which will be present in the completed actuator. The liquid used must be substantially noncompressible, should be immiscible in the wax or other lill material, should exist in the liquid state at room temperatures and at the higher temperatures at which the wax or other lill material melts, and, preferably, should have about the same thermal expansion characteristics as the solidified till material. These requirements can be adequately satisfied by substantially pure water used with a solidified wax Vtill material; one such wax fill undergoes about a ten percent change in volume upon melting, whereas the water undergoes only a relatively small change in volume within the same narrow range of temperatures. The inexpensive incompressible liquid supplemental fill material serves to transmit the forces of wax lill expansions to the piston-seal unit, and thereby permits only a relatively small amount of the more costly wax to be located at a remote sensing site for purposes of driving the piston-seal unit located elsewhere; flexible tubing may intercouple the two sites, and, in any event, the output stem -or other member driven by the pistonseal until need fbe only relatively short and costly metal may thus be conserved. Furthermore, even where only a relatively small amount of the liquid is used, it serves to overcome the problem associated with voids appea-ring between the solidified wax fill and the piston-seal unit. In this connection, FIGURE 4 discloses the type of solidication vortex 39 which commonly results from wax soliditication, and it is evident that this depression will tend -to establsh a void, between the wax Vlill and piston-seal unit, which would tend to adversely aiiect the operating characteristics of the thermal actuator. Liquid 25b eliminates this adverse iniiuence. Another unobvious advantage accrues when as little wax as possible is Iused, only to develop the minimum output movement required for any application, with a relatively large amount of liquid lill being used at the same time. By way of explanation, it has been found that wax fill material alone tends to exhibit an undesirable springiness or compressibility, which renders the operating characteristics of the actuator less predictable. When a relatively large quantity of a more incompressible liquid, such as water, is added in the actuator cavity, the total springiness or compressibility is significantly and advantageously reduced.

The detail appearing in FIGURE 5 discloses the cooperation between the piston-seal unit 24 and a hollowed separable output member or tubular stem 18a, which is generally like stern 18 in FIGURE l except that its outer diameter is large enough to permit a sliding fit within tube 19 which will guide the stem more accurately and better avoid any likelihood that the piston-seal unit will be cocked to one side by the stem. The length over which the stem and upper portion of the piston-seal unit are slidably mated is adequate to insure that they will not become entirely separated if the piston-seal unit does not follow the stem movements at all times, such as times when excess pressure or manual operation of the valve cause the valve stem to be lifted independently.

Preferably, the compressed axial length of the resilient member (substantially length 33 of resilient member 26, in FIGURE 2) is sufficient to prevent the piston-seal unit from tipping out of its intended concentric alignment with the surrounding tubular actuator member. However, in an alternative construction such as that illustrated in FIGURE 6, the resilient member, 26', may be of a relatively short length, with one of the end washers being replaced by an elongated hollow cylindrical element, 28', which is of outer diameter just slightly smaller than the inner diameter of the actuator tube 19. Load spring 29 and the central fastener 30 function substantially in lthe manner earlier described in connection with the unit of FIGURE 2, to preserve good sealing at all times, while the element 28 guides the unit 24 to prevent misalignment of member 26 in tube T19.

The resilient piston-seal members are formed of silicone rubber, or synthetic materials or rubber which is not aiected by hydrocarbons (such as those of a wax fill) and does not contain sulphur or other materials which would be likely to cause undesirable adherence to the actuator tube (such as a brass tube) at high temperatures. Where a high degree of lubricity is desired between the actuator tube and resilient piston-seal, a very thin and flexible sleeve of a relatively slippery tetraiiuoroethylene material may be disposed around the periphery of the resilient member, or the friction may be reduced by tumbling the cooperating parts in mica or by subjecting the resilient member to a surface treatment such as the so-called LCR treatment furnished by J. Royal Company, Barrington, Rhode Island, or by applying to the cooperating parts a uorocarbon such as that sold by Du Pont Corporation under the trade name Vydaxar. Although a helical load spring has been shown in association with the piston-seal units, other forms of resilient loading elements may be substituted, and, in some instances, the end of the piston-seal unit which carries the load spring may be reversed in position (i.e. may project into the fill, rather than extend toward the stem). Actuator units expressing these teachings may of course be used for purposes other than the specific valve actuations discussed herein.

Accordingly, although preferred. practices and embodiments of this invention have been shown and described, it should be understood that various modifications, additions and substitutions may be effected by those skilled in the art without departure from these teachings, and it is aimed in the Aappended claims to embrace all such variations as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

ll. A thermal actuator comprising a closed container having inner surfaces defining a rigid open-ended cylinder accommodating reciprocations of a piston unit therein, thermally-responsive till material within said container exhibiting significant temperature-induced dimensional variations within a predetermined temperature range, and an elongated piston-seal unit disposed within said cylinder and movable along the longitudinal axis thereof, said piston-seal unit including a piston member of resilient material having a cross-section normally not in excess of the cross-section of said cylinder, elongated fastening means extending through said member in the direction of said axis, and resilient loading means mounted on and movable as a whole with said fastening means in a forcetransmitting relation to said piston member, said fastening means including axially-spaced holding means movable therewith axially compressing said piston member and resilient loading means together between said holding means, whereby the cross-section of said axially-compressed piston member tends to be greater than that of said cylinder and iiuid leakage from and into said container is suppressed by said piston-seal unit.

2. A thermal actuator as set forth in claim ll, wherein said cylinder has a substantially circular and uniform cross-section, wherein said piston member is a normally substantially cylindrical hollow member of elastomeric material, wherein said resilient loading means comprises a spring, wherein said fastening means comprises an elongated member extending axially through said piston member and tightly sealed therewith, and wherein said holding means comprises enlargements at both ends of said elongated member preventing separation thereof from said piston member and compressing said spring between one of said enlargements and one side of said piston member.

3. A thermal actuator as set forth in claim 2 wherein said movable piston-seal unit further includes means distributing compression forces from said spring substantially uniformly over the ends of said piston member, at least one of said distributing means comprising a substantially rigid washer of 4diameter slightly less than that of said cylinder and mated with said elongated member, and wherein said spring comprises a helical spring disposed coaxially around said elongated member between said one of said enlargements and said washer.

4. A thermal actuator as set forth in claim 1 further including a movable output member disposed at least in part outside of said container and responsive to movements of said piston-seal unit within said cylinder, and means connecting one end of said piston member in forcetransmitting relation to said output member independently of said resilient loading means.

5.A thermal actuator comprising an elongated rigid heat-conducting tube closed at one end and having inner surfaces near the opposite end thereof defining a -cylinder of substantially circular and uniform cross-section, thermally-responsive fill material within said tube exhibiting significant temperature-induced changes in volume within a predetermined temperature range, an elongated piston-seal unit disposed within said cylinder, said pistonseal unit including a normally substantially cylindrical hollow resilient member of elastomeric material having a cross-section normally not in excess of that of said cylinder, a helical loading spring, an elongated fastening member extending axially through said resilient member and tightly sealed therewith, said fastening member having enlargements at both ends preventing separation thereof from said resilient member, and means distributing compression forces from said loading spring substantially uniformly over the ends of said resilient member, said helical spring being disposed coaxially around said elongated fastening member between one end of said fastening member and said force-distributing means, whereby said fastening member fastens said spring and resilient member together for reciprocating movements Itogether within said cylinder and with said spring compressing said resilient member and tending to increase the cross-section thereof to a cross-section greater than that of said cylinder, and whereby fluid leakage from and into said container is suppressed by said movable pistonseal unit, and means outside of said tube responsive to movements of said piston-seal unit within said cylinder and including a movable output member having a hollow end in surrounding relation to said loading spring and said one end of said fastening member and separably engaging said force-distributing means, said output member extending outside of said tube, and resilient means outside of said tube urging said end of said output member into engagement with said force-distributing means.

6. A thermal actuator as set forth in claim 5 wherein said thermally-responsive fill material includes normallysolidified Wax within said tube near the closed end thereof and a supplement fill olf liquid in which the wax is immiscible occupying the space between the wax and said piston-seal unit.

7. A thermal actuator comprising an elongated heatconductive cylindrical tube closed at one end and open at the other end, an elongated movable piston-seal unit disposed within said tube near said open end, said movable piston-seal unit including an axially-compressible cylindrical resilient member coaxially surrounding an elongated central fastener extending therethrough and axially compressing said member together with a load spring sufiiciently to expand said member into iiuid-tight sealing relationship with said tube, a normally-solidified wax within said tube near the closed end thereof, water filling the space between said Wax and piston-seal unit within said tube, and a movable output member in force-transmitting relationship to one end of said resilient member independently of said load spring.

8. A thermal actuator as set forth in claim 7 wherein said load spring comprises a helical spring coaxial with said fastener on the side of said resilient member nearer said open end of said tube, and wherein at least one end of said output member is hollow and in surrounding relationship to said spring and is axially movable in relation to said resilient member for a distance not in excess of the length of said spring.

9. The method of assembling a thermal actuator having a container which is to be closed by a movable pistonseal unit having an axially-compressible resilient member radially expansible into fluid-tight sealing engagement with the container by a load spring movable therewith, which comprises partly filling the container with a predetermined amount of a normally-solidified thermallyresponsive material, filling the remainder of the container with a force-transmitting material which is liquid at room temperatures and at the melting temperature of said normally-solidified material, at room temperature applying mating forces to one end of the resilient member in direction to mate the resilient member with the container while permitting the resilient member to elongate axially in direction opposite to that of compressixe forces exerted by the load spring and thereby permit by-pass leakage of the liquid until the piston-seal unit is in a predetermined mated position within the container, and thereafter directly applying forces from the one end of the resilient member to an output member While maintaining restraint of the load spring on the resilient member to expand it radially into duid-tight sealing engagement with the container.

10. The method of assembling a thermal actuator as set forth in claim 9 wherein the steps of filling the container comprise partly filling the container with a predetermined amount of heated wax in liquid form, permitting the heated wax to solidify, and filling the remainder of the container with water, and thereafter applying the mating forces to mate the piston-seal unit with the container with attendant by-pass leakage of excess water.

References Cited by the Examiner UNITED STATES PATENTS 2,938,384 5/1960 Soreng et al. 73-368-3 X 3,007,029 10/1961 Levine 73-368.3 X 3,180,150 4/1965 Horne 73-368-1 3,194,009 7/1965 Baker 60--23 EDGAR W. GEOGHEGAN, Primary Examiner.

Claims (1)

1. A THERMAL ACTUATOR COMPRISING A CLOSED CONTAINER HAVING INNER SURFACES DEFINING A RIGID OPEN-ENDED CYLINDER ACCOMMODATING RECIPROCATIONS OF A PISTON UNIT THEREIN, THERMALLY-RESPONSIVE FILL MATERIAL WITHIN SAID CONTAINER EXHIBITING SIGNIFICANT TEMPERATURE-INDUCED DIMENSIONAL VARIATIONS WITHIN A PREDETERMINED TEMPERATURE RANGE, AND AN ELONGATED PISTON-SEAL UNIT DISPOSED WITHIN SAID CYLINDER AND MOVABLE ALONG THE LONGITUDINAL AXIS THEREOF, SAID PISTON-SEAL UNIT INCLUDING A PISTON MEMBER OF RESILIENT MATERIAL HAVING A CROSS-SECTION NORMALLY NOT IN EXCESS OF THE CROSS-SECTION OF SAID CYLINDER, ELONGATED FASTENING MEANS EXTENDING THROUGH SAID MEMBER IN THE DIRECTION OF SAID AXIS, AND RESILIENT LOADING MEANS MOUNTED ON AND MOVABLE AS A WHOLE WITH SAID FASTENING MEANS IN A FORCETRANSMITTING RELATION TO SAID PISTON MEMBER, SAID FASTENING MEANS INCLUDING AXIALLY-SPACED HOLDING MEANS MOVABLE THEREWITH AXIALLY COMPRESSING SAID PISTON MEMBER AND RESILIENT LOADING MEANS TOGETHER BETWEEN SAID HOLDING MEANS, WHEREBY THE CROSS-SECTION OF SAID AXIALLY-COMPRESSED PISTON MEMBER TENDS TO BE GREATER THAN THAT OF SAID CYLINDER AND FLUID LEAKAGE FROM AND INTO SAID CONTAINER IS SUPPRESSED BY SAID PISTON-SEAL UNIT.

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