Gas spring and corresponding production method

Abstract

A gas spring (1) comprising: an outer casing (2) internally provided with a closed cavity (3); a piston-rod (4) which is fitted in the outer casing (2) in axially slidable manner and is structured so as to form / delimit, inside said closed cavity (3), a variable-volume closed chamber adapted to contain a pressurized gas; and one or more annular gaskets (7, 10, 11) that are interposed between the piston-rod (4) and the outer casing (2) and / or between separate pieces (5, 6) of the outer casing (2) and at least one of which is made of a material transparent oorr semi-transparent to visible light.

Description

[0001] "GAS SPRING AND CORRESPONDING PRODUCTION METHOD"

[0002] Cross-Reference to Related Applications

[0003] This Patent Application claims priority from Italian Patent Application No . 102023000014262 filed on July 7 , 2023 , the entire disclosure of which is incorporated herein by reference .

[0004] Technical Field

[0005] The present invention relates to a gas spring and to the corresponding production method .

[0006] More in detail , the present invention relates to a gas spring adapted to be used in the field of sheet metal forming . Use to which the following disclosure will make explicit reference without thereby losing generality .

[0007] Background of the Invention

[0008] As is known, the gas springs which are used in the moulds for sheet metal moulding generally comprise : a cup-shaped body substantially cylindrical in shape ; a bushing substantially cylindrical in shape , separate and distinct from the cup-shaped body, which f luid-tightly closes the mouth of the cup-shaped body via the interposition of at least one annular sealing gasket ; and a substantially cylindrical-shaped, piston-rod which is fitted in pass-through and axially slidable manner in the hole at centre of the bushing, so as to be able to move axially with respect to the cup-shaped body between a retracted position and an extracted or extended position .

[0009] The piston-rod is moreover fitted f luid-tightly into the bushing with the interposition of at least one annular sealing gasket , so as to form / delimit , together with the cup-shaped body and the bushing, a variable-volume closed chamber that is filled up with a high-pressure gas .

[0010] The pressuri zed gas thus keeps the piston-rod in the completely extended or extracted position, i . e . with the inner end of the piston-rod in abutment on the body of the bushing, until the piston-rod undergoes an axial force capable of overcoming the thrust of the gas .

[0011] In the gas springs most widespread on the market , the nominal gas pressure inside the variable-volume closed chamber ( i . e . the gas pressure with the piston-rod in the completely extended or extracted position) usually ranges between 20 and 200 bar .

[0012] As a result , during the movement of the piston-rod, the pressure of the gas inside the variable-volume closed chamber can rise up to easily reach values above 500- 600 bar, with the sealing problems that this entails . Problems that unfortunately directly af fect the reliability of the device .

[0013] The Applicant , in fact , has carefully examined a statistically signi ficant sample of the gas springs that highlighted mal functions during the first year of li fe , and has found that most of these mal functions are caused by an abnormal under-pressure deformation and / or early rupture of one of the annular sealing gaskets of the gas spring .

[0014] By analysing the defective composite , the Applicant has furthermore found that the early rupture and the abnormal under-pressure deformation are both due , in almost all cases , to the presence of small bubbles of air or other gas not necessarily globoidal in shape , that are hidden inside the body of the annular sealing gasket .

[0015] The annular sealing gaskets used in the gas springs , in fact , are commercial type annular gaskets , and are made of polymeric material filled with carbon black and / or other additives , via inj ection moulding . The bubbles of air or other gas occasionally form inside the body of the annular sealing gasket during inj ection moulding of the piece .

[0016] Unfortunately, to date there are no economic and reliable quality control systems that are capable to detect , in a non-destructive manner, when there are bubbles of air or other gases hidden inside the annular sealing gaskets . The low production cost of the annular sealing gaskets made of polymeric material , in fact , makes this type of structural checks economically non-af fordable .

[0017] For the same reasons it is economically unthinkable to use ultrasonic equipment , X-ray equipment or similar machinery to detect bubbles or other structural discontinuities within the opaque body of the annular sealing gaskets that are used on assembly of the gas springs . This type of equipment , in fact , has economically unsustainable costs .

[0018] Summary of the Invention

[0019] Aim of the present invention is to overcome the problems connected with the defectivity of the annular sealing gaskets without excessively increasing the production costs of the gas springs .

[0020] In accordance with these aims , according to the present invention there i s provided a gas spring as defined in Claim 1 and preferably, though not neces sarily, in any one of the claims depending on it .

[0021] Moreover, according to the present invention there is provided a production method of a gas spring as defined in Claim 9 and preferably, though not necessarily, in any one of the claims depending on it .

[0022] Brief Description of the Drawings

[0023] The present invention will now be described with reference to the attached drawings , which show a non-limiting embodiment thereof , wherein :

[0024] - Figure 1 is a perspective view a gas spring reali zed according to the teachings of the present invention;

[0025] - Figure 2 is a side view of the gas spring shown in Figure 1 , sectioned along the midplane and with parts removed for clarity' s sake ;

[0026] - Figure 3 is a perspective and schematic view of a step of the production of the gas spring shown in Figures 1 and 2 ;

[0027] - Figure 4 is a side view of a second embodiment of the gas spring shown in Figures 1 and 2 , sectioned along the midplane and with parts removed for clarity' s sake ; whereas

[0028] - Figure 5 is a side view of a third embodiment of the gas spring shown in Figures 1 and 2 , sectioned along the midplane and with parts removed for clarity' s sake .

[0029] Detailed Description of the Embodiments

[0030] With reference to Figures 1 and 2 , number 1 denotes as a whole a gas spring that can be advantageously used in the presses or dies for the cold forming of metal sheets and the like .

[0031] The gas spring 1 comprises : an outer casing 2 with a rigid and sel f-supporting structure , which is internally provided with a large closed cavity 3 advantageously complementary in shape , which is insulated from the outside ; and an oblong-shaped piston-rod 4 , which is fitted in the outer casing 2 in axially slidable and fluid-tight manner, so as to protrude cantilevered outside of the casing 2 , and is shaped / structured so as to form / delimit , inside and together with the inner cavity 3 of casing 2 , a variablevolume closed chamber that contains / is adapted to contain nitrogen or other pressuri zed gas .

[0032] Clearly, the pressuri zed gas present inside the closed cavity 3 , or rather inside the variable-volume closed chamber, has a pressure higher than the external one so as to maximi ze the volume of the closed chamber .

[0033] More in detail , the piston-rod 4 is fitted in axially slidable manner in a substantially complementary-shaped straight hole , which is made in the outer casing 2 so as to connect the cavity 3 directly with the outside .

[0034] In addition, the piston-rod 4 is f luid-tightly coupled to the outer casing 2 via the interposition of at least one annular sealing gasket , which is structured so as to prevent the leakage / passage of pressuri zed gas in the interstitial space between the piston-rod 4 and the outer cas ing 2 , and optionally also with the interposition of at least one protective annular gasket , which is structured so as to prevent oils , powders and / or other solid or liquid contaminates from reaching the inner cavity 3 of casing 2 .

[0035] Preferably, the gas spring additionally includes one or more guide and / or centring rings , which are placed inside the straight hole of the outer casing 2 and are adapted to be engaged in axially slidable manner by the piston-rod 4 .

[0036] Preferably the piston-rod 4 is moreover structured / shaped so as to protrude cantilevered also inside the closed cavity 3 . In addition, the part of piston-rod 4 that remains stably inside the casing 2 , or rather protrudes cantilevered inside the closed cavity 3 , preferably has an anti-extraction structure that is adapted to prevent the complete coming out of the piston-rod 4 from the casing 2 due to the thrust exerted by the pressuri zed gas .

[0037] The pressuri zed gas present inside the closed cavity 3 , or rather inside the variable-volume closed chamber, is therefore adapted to bring and keep the piston-rod 4 in a completely extended or extracted position ( see Figures 1 and 2 ) , wherein the anti-extraction structure of the piston-rod 4 abuts on the inner surface of the cavity 3 .

[0038] In the example shown, in particular, the nominal pressure ( i . e . the pressure of the gas with the piston-rod 4 in the extracted or extended position) of the gas contained in the cavity 3 , or rather in the variable-volume closed chamber, preferably ranges between 20 and 200 bar .

[0039] Preferably, the outer casing 2 is furthermore divided into at least two complementary-shaped pieces , which are f luid-tightly coupled to each other via the interposition of one or more annular sealing gaskets , which is / are structured so as to prevent the leakage / passage of the pressuri zed gas in the interstitial space between the same pieces of the outer casing 2 . At least one of the pieces forming the outer casing 2 , moreover, is preferably provided with the straight hole that is engaged in axially slidable manner by the piston-rod 4 .

[0040] In addition, at least one among the annular gasket or gaskets that are interposed between the piston-rod 4 and the outer casing 2 , and / or at least one of the annular gasket or gaskets that are interposed between the various pieces of the outer casing 2 , is made of polymeric material and further has a structure transparent or semi-transparent to visible light .

[0041] In other words , said annular gasket or gaskets with a transparent or semi-transparent structure is / are made of a polymeric material that is transparent or semi-transparent to visible light , advantageously via inj ection moulding .

[0042] More in detail , said annular gasket or gaskets with transparent or semi-transparent structure is / are preferably made substantially entirely of a transparent or semitransparent polymeric material , advantageously via inj ection moulding .

[0043] In addition, said transparent or semi-transparent polymeric material is preferably an elastomeric polymer advantageously of thermoplastic type and / or preferably has a surface hardnes s ( ISO 868 or DIN 53505 ) preferably higher than or equal to 90 ShoreA.

[0044] More in detail , said transparent or semi-transparent polymeric material is preferably a transparent or semitransparent polyurethane ( PU) .

[0045] In other words , at least one among the annular sealing gasket ( s ) and / or protective gasket ( s ) that are interposed between the piston-rod 4 and the outer casing 2 , and / or among the annular sealing gasket or gaskets that are interposed between the pieces of the outer casing 2 , is preferably made ( entirely) of transparent or semi-transparent polyurethane , advantageously via inj ection moulding .

[0046] As an alternative , said polymeric material may also be a fluorinated elastomer ( FKM or FFKM) , traditionally called Viton, transparent or semi-transparent to visible light .

[0047] With reference to Figures 1 and 2 , in particular, the outer casing 2 is preferably substantially cylindrical in shape , and its inner cavity 3 is preferably substantially cylindrical in shape and preferably extends inside the casing 2 substantially coaxial to the longitudinal / central axis A of the same casing 2 .

[0048] The piston-rod 4 , in turn, is preferably substantially cylindrical in shape and preferably extends substantially coaxial to the longitudinal / central axis A of casing 2 .

[0049] In addition, the outer casing 2 is preferably divided into a plurality of substantially complementary-shaped pieces , advantageously made of metallic material . The piston-rod 4 , on the other hand, preferably has a monolithic structure and is advantageously made of metallic material .

[0050] More in detail , the outer casing 2 preferably comprises : a cup-shaped body 5 advantageously substantially cylindrical in shape , which preferably has a monolithic structure and is preferably also made of metallic material ; and a plug body 6 , separate and di stinct from the cup-shaped body 5 , which has a shape substantially complementary to that of the mouth of the cup-shaped body 5 , and i s at least partially fitted into the mouth of cup-shaped body 5 so as to form / delimit , inside the cup-shaped body 5 , the closed cavity 3 . Furthermore , similarly to the cup-shaped body 5 , the plug body 6 preferably also has a monolithic structure and is preferably made of metallic material .

[0051] In addition, the plug body 6 is f luid-tightly coupled to the cup-shaped body 5 via the interposition of at least one annular sealing gasket 7 , which is adapted to prevent the leakage of pressuri zed gas in the interstitial space between the cup-shaped body 5 and the plug body 6 , and is preferably made of transparent or semi-transparent polyurethane , advantageously via inj ection moulding .

[0052] Preferably, the annular sealing gasket 7 moreover has a surface hardness ( ISO 868 or DIN 53505 ) ranging between 90 and 95 ShoreA.

[0053] In a di f ferent embodiment , however, the annular sealing gasket 7 may be made of a di f ferent transparent or semitransparent elastomeric polymer .

[0054] Preferably, the outer casing 2 is furthermore provided with a locking member 8 , which is placed at the mouth of the cup-shaped body 5 and is adapted to lock / retain the plug body 6 stably in place inside the cup-shaped body 5 .

[0055] With reference to Figures 1 and 2 , in addition, the piston-rod 4 is preferably fitted in pass-through and axially slidable manner in the plug body 6 .

[0056] In other words , the plug body 6 is preferably provided with an advantageously circular-shaped, straight through hole that preferably has a section substantially complementary to that of the piston-rod 4 , and is engaged in pass-through and axially slidable manner by the piston-rod 4 .

[0057] Preferably, the piston-rod 4 is thus f luid-t ightly coupled to the plug body 6 via the interposition of at least one annular sealing gasket 10 , which is placed in the through hole of plug body 6 , is structured so as to prevent the leakage / passage of the pressuri zed gas in the interstitial space between the piston-rod 4 and the plug body 6 , and is preferably made of transparent or semi-transparent polyurethane , advantageously via inj ection moulding .

[0058] Preferably, the annular sealing gasket 10 moreover has a surface hardness ( ISO 868 or DIN 53505 ) higher than or equal to 94 ShoreA.

[0059] In a di f ferent embodiment , however, the annular sealing gasket 10 may be made of a di f ferent transparent or semitransparent elastomeric polymer .

[0060] Preferably, the gas spring 1 additionally comprises at least one protective annular gasket 11 , which is placed at the outer mouth of the straight through hole of plug body 6 , and is structured so as to prevent oils , powders and / or other solid or liquid contaminates from penetrating inside the plug body 6 .

[0061] Furthermore , similarly to the annular sealing gasket 10 , the protective annular gasket 11 is also preferably made transparent or semi-transparent polyurethane , advantageously via inj ection moulding .

[0062] Preferably, the protective annular gasket 11 moreover has a surface hardness ( ISO 868 or DIN 53505 ) higher than or equal to 92 ShoreA.

[0063] In a di f ferent embodiment , however, the annular protective gasket 11 may be made of a di f ferent transparent or semi-transparent elastomeric polymer .

[0064] Preferably, at least one guide and / or centring ring 12 advantageously made of a hard polymeric material , i . e . with surface hardness ( ISO 868 or DIN 53505 ) higher than or equal to 98 ShoreA, is moreover placed inside the straight through hole of the plug body 6 .

[0065] Optionally, the polymeric material forming the guide and / or centring ring ( s ) 12 may be furthermore fi lled with graphene and / or graphite and / or carbon fibre .

[0066] More in detail , the gas spring 1 is preferably provided with two guide and / or centring rings 12 , which are placed inside the straight through hole o f plug body 6 on opposite sides of the annular sealing gasket 10 .

[0067] With reference to Figure 2 , in the example shown, in particular, the plug body 6 preferably substantially consists of a bushing roughly cylindrical in shape , which preferably has a monolithic structure , is preferably made of metallic material , and has a shape substantially complementary to the mouth of cup-shaped body 5 . The bushing is moreover provided with a substantially circular-shaped, central through hole having a diameter roughly complementary to that of piston-rod 4 .

[0068] Similarly, the annular gasket 10 is preferably a lip gasket and is preferably accommodated inside an annular groove made inside the central through hole of the bushing .

[0069] The guide and / or centring rings 12 , on the other hand, are preferably accommodated inside respective annular grooves made inside the central through hole of the bushing, respectively above and below the annular groove accommodating the annular sealing gasket 10 .

[0070] The annular sealing gasket 7 , in turn, is preferably a double-lip gasket , and is preferably accommodated inside an annular groove made on the outer periphery of the bushing .

[0071] The locking member 8 , on the other hand, preferably includes a Seeger elastic ring or the like , advantageously made of metallic material , which is preferably adapted to removably engage an annular groove made in the cavity of cup-shaped body 5 , close to the mouth of the latter .

[0072] With reference to Figures 1 and 2 , in turn, the piston- rod 4 preferably consists of a cup-shaped body substantially cylindrical in shape , which is made of metallic material , and is fitted in axially slidable manner into the plug body 6 , or rather into the bushing, with the concavity facing the inside of cup-shaped body 5 .

[0073] In addition, said cup-shaped body advantageously has , at the mouth, also a protruding annular flange , which is adapted to prevent the complete extraction of the cup-shaped body from the plug body 6 , or rather from the bushing .

[0074] With reference to Figures 1 and 2 , finally the gas spring 1 is preferably provided with a one-way valve 13 and / or with a safety plug 14 , both advantageously located on the bottom of the cup-shaped body 5 .

[0075] Through the one-way valve 13 it is possible to introduce the pressuri zed gas into the inner cavity 3 of casing 2 , or rather into the variable-volume closed chamber .

[0076] The safety plug 14 , on the other hand, is structured so as to automatical ly discharge , outside of gas spring 1 , the pressuri zed gas present in the inner cavity 3 of casing 2 , or rather in the variable-volume closed chamber, when the pressure of the gas present in the same inner cavity 3 exceeds a predetermined maximum limit , and / or when the piston-rod 4 performs an "overtravel" , i . e . when the piston- rod 4 re-enters inside the casing 2 exceeding the maximum stroke allowed by the dimensions of the gas spring 1 .

[0077] More in detail , the bottom wall of casing 2 , or rather of cup-shaped body 5 , which contributes to delimiting the variable-volume closed chamber, is preferably provided with two through ducts that connect the variable-volume closed chamber with the outside .

[0078] The first through duct accommodates the one-way valve 13 , while the second through duct is f luid-tightly closed by the safety plug 14 .

[0079] Preferably, the mouth of the first through duct is also f luid-tightly closed by an additional closure plug 15 , preferably with a mushroom structure .

[0080] More in detail , the first through duct is preferably L- shaped, and preferably ends inside the cavity 3 on the top of a protuberance 16 that rises from the bottom of the same cavity .

[0081] Preferably, the protuberance 16 and / or the final section of the first through duct is / are substantially coaxial to the longitudinal / central axis A of the same casing 2 .

[0082] The protuberance 16 allows the lubricating oil that should reach the inner cavity 3 to accumulate at the bottom of the cavity without reaching and / or damaging the one-way valve 13 .

[0083] The second through duct , on the other hand, is preferably substantially straight and advantageously also parallel to and spaced from the longitudinal / central axis A of the casing .

[0084] The safety plug 14 , moreover, is preferably structured so as to protrude cantilevered ins ide the inner cavity 3 of casing 2 , so as to be hit by the pi ston-rod 4 when it performs an "overtravel" , i . e . when the piston-rod 4 re-enters inside the cavity 3 exceeding the maximum stroke allowed by the dimensions of the gas spring 1 .

[0085] The one-way valve 13 and the safety plug 14 are already known components easily available on the market , so they won' t be further described .

[0086] Operation of gas spring 1 is similar to that of the gas springs currently present on the market , and therefore does not need any further clari fications .

[0087] The production of gas spring 1 , on the other hand, basically comprises the steps of :

[0088] — reali zing at least one of the annular gasket or gaskets 7 and / or 10 and / or 11 in a polymeric material that is transparent or semi-transparent to visible light , advantageously via inj ection moulding;

[0089] — checking, via optical and / or visual inspection, the structure of said at least one annular gasket 7 and / or

[0090] 10 and / or 11 made of transparent or semi-transparent polymeric material to detect the presence of any bubbles inside said annular gasket ; and

[0091] — using said at least one annular gasket 7 and / or 10 and / or 11 made of transparent or semi-transparent polymeric material in the assembly of said gas spring 1 , only i f the same annular gasket 7 and / or 10 and / or

[0092] 11 made of transparent or semi-transparent polymeric material is without bubbles of air or other gas ins ide itsel f or has , inside itsel f , bubbles of air or other gas with a nominal volume ( i . e . the volume of each individual bubble of air or other gas ) below a predefined first maximum threshold and / or with an overall volume ( i . e . the sum of the nominal volumes of all bubbles of air or other gas present inside the body of the annular gasket ) below a predefined second maximum threshold .

[0093] More in detail , the production of gas spring 1 preferably entails reali zing the annular gasket or gaskets 7 and / or 10 and / or 11 substantially entirely in said transparent or semitransparent polymeric material , via inj ection moulding .

[0094] Clearly the annular gasket or gaskets 7 and / or 10 and / or 11 in transparent or semi-transparent polymeric material may be made by a natural or legal person other than the one assembling the gas spring 1 .

[0095] Preferably, the first maximum threshold is furthermore equal to 0 , 01 % o f the overall volume of the annular gasket and / or is equal to about 0 , 3 mm3( cubic millimetres ) . Preferably, the second maximum threshold is instead equal to 0 , 02 % of the overall volume of the annular gasket and / or is equal to about 0 , 6 mm3( cubic millimetres ) .

[0096] Preferably, said transparent or semi-transparent polymeric material is moreover an elastomeric material advantageously of thermoplastic type .

[0097] More in detail , said polymeric material transparent or semi-transparent to visible light is preferably a transparent or semi-transparent polyurethane .

[0098] With reference to Figure 3 , preferably the production of the gas spring 1 additionally provides for checking the structure of the annular gasket via an optical inspection electronic device 100 , which is capable to detect the presence of bubbles of air or other gas inside the body of an annular gasket made of transparent or semi-transparent polymeric material , and advantageously also to determine the volume of the individual bubble or bubbles of air or other gas possibly present inside the same annular gasket .

[0099] More in detail , the optical inspection device 100 preferably comprises : at least one , advantageously high resolution, video-camera or camera 101 which is adapted to acquire one or more images of the annular gasket 7 and / or 10 and / or 11 made of transparent or semi-transparent polymeric material to be checked; and a data processing unit 102 , which is programmed / conf igured so as to process the images acquired from the video-camera / s or camera / s 101 , to detect the presence of any bubbles of air or other gas inside the annular gasket 7 and / or 10 and / or 11 to be checked .

[0100] Preferably, the data processing unit 102 is moreover programmed / conf igured so as to process the images acquired from the video-camera / s or camera / s 101 , to determine the volume of the individual bubble or bubbles of air or other gas possibly present inside the annular gasket 7 and / or 10 and / or 11 made of transparent or semi-transparent polymeric material to be checked .

[0101] In other words , the checking of the structure of the annular gasket preferably provides for placing the annular gasket 7 and / or 10 and / or 11 made of transparent or semitransparent polymeric material to be checked in front of the lens of the video-camera / s or camera / s 101 of the optical inspection electronic device 100 , for a suf ficient time to acquire one or more images of the annular gasket .

[0102] Preferably, the optical inspection device 100 is furthermore provided with a lighting device (not shown) which is selectively adapted to illuminate the annular gasket 7 and / or 10 and / or 11 made of transparent or semi-transparent polymeric material to be checked .

[0103] Clearly, the checking via optical and / or visual inspection may also be carried out manually by an operator .

[0104] With reference to Figure 2 , in the example shown, in particular, the assembly of gas spring 1 preferably comprises the steps of :

[0105] — reali zing, preferably by mechanical machining with removal of material , the cup-shaped body 5 and the plug body 6 of the outer casing 2 ;

[0106] — reali zing, preferably by mechanical machining with removal of material , the piston-rod 4 ;

[0107] — placing the annular sealing gasket or gaskets 10 and, advantageously, also the annular protective gasket or gaskets 11 in the plug body 6 ; — inserting the piston-rod 4 into the plug body 6 , engaging the annular sealing gasket or gaskets 10 and advantageously also the annular protective gasket or gaskets 11 ;

[0108] — fitting the annular sealing gasket or gaskets 7 on the periphery of the plug body 6 ; and finally

[0109] — inserting the plug body 6 into the mouth of the cupshaped body 5 , so as to close the same mouth in fluid- tight manner .

[0110] In addition, before inserting the piston-rod 4 into the plug body 6 , the assembly of gas spring 1 preferably also comprises the step of placing the guide and / or centring ring or rings 12 in the plug body 6 , so that they are subsequently engaged by the piston-rod 4 .

[0111] Preferably, the assembly of gas spring 1 additionally comprises the step of placing the one-way valve 13 and / or the safety plug 14 in the cup-shaped body 5 .

[0112] Furthermore , after inserting the plug body 6 into the mouth of the cup-shaped body 5 , the assembly of gas spring

[0113] I preferably also comprises the step of introducing nitrogen or other pressuri zed gas into the cup-shaped body 5 , so as to bring to a predefined pressure the closed space / volume delimited by the cup-shaped body 5 , the plug body 6 and the piston-rod 4 .

[0114] Finally, before introducing the pressuri zed gas inside the cup-shaped body 5 , the assembly of gas spring 1 preferably also comprises the step of locking the plug body 6 rigidly to the cup-shaped body 5 , preferably by means of the locking member 8 .

[0115] The advantages connected with the particular structure of the annular gaskets 7 and / or 10 and / or 11 are remarkable .

[0116] The realisation of the annular gas kets 7 and / or 10 and / or

[0117] I I in a polymeric material transparent or semi-transparent to visible light allows to greatly reduce the quality control costs to check the presence of bubbles or other structural discontinuities inside the gasket .

[0118] Thanks to this drastic reduction, quality control costs now cause only a slight increase in the overall production costs of the gas springs . Increase that the market can j usti fy and accept in consideration of the greater reliability given to the gas springs .

[0119] It is finally clear that modi fications and variations may be made to the gas spring 1 and the above-mentioned production method without , however, departing from the scope of the present invention .

[0120] For example , with reference to Figure 4 , in a first embodiment variation the piston-rod 4 of gas spring 1 engages in axially slidable manner the plug body 6 , but it is fluid- tightly coupled directly to the cup-shaped body 5 .

[0121] In other words , instead of being placed inside the through hole of plug body 6 , the annular sealing gasket 10 is preferably fitted directly on the piston-rod 4 , or rather on the protruding perimeter flange of piston-rod 4 , so as to slide directly on the lateral surface of the inner cavity 3 , or rather on the inner surface of the cup-shaped body 5 , and is adapted to prevent the leakage / passage of the pressuri zed gas in the interstitial space between the piston-rod 4 and the cup-shaped body 5 .

[0122] More in detail , the annular sealing gasket 10 is preferably a lip gasket , and is preferably accommodated inside an annular groove made on the periphery of the protruding annular flange of the cup-shaped body forming the piston-rod 4 .

[0123] Clearly also in this embodiment the annular sealing gasket 10 is preferably made of transparent or semitransparent polymeric material or, more in detail , of transparent or semi-transparent polyurethane .

[0124] In this embodiment , moreover, the gas spring 1 is preferably provided with a single guide and / or centring ring 12 , clearly located inside the through hole of the plug body 6 , or rather inside the central through hole of the bushing .

[0125] Since the annular sealing gasket 10 is interposed between the piston-rod 4 and the cup-shaped body 5 beneath the plug body 6 , the gas spring 1 may optionally also lack the annular sealing gasket 7 .

[0126] In this embodiment variation, therefore , the assembly of gas spring 1 preferably comprises the steps of :

[0127] — reali zing, preferably by mechanical machining with removal of material , the cup-shaped body 5 and the plug body 6 of the outer casing 2 ;

[0128] — reali zing, preferably by mechanical machining with removal of material , the piston-rod 4 ;

[0129] — placing the annular sealing gasket or gaskets 10 on the piston-rod 4 and optionally also placing the annular protective gasket or gaskets 11 and / or the annular sealing gasket or gaskets 7 on the plug body 6 ;

[0130] — inserting the piston-rod 4 into the plug body 6 , engaging, i f present , the annular protective gasket or gaskets 11 ; and finally

[0131] — inserting the plug body 6 into the mouth of the cupshaped body 5 , so as to close the same mouth in fluid- tight manner .

[0132] With reference to Figure 5 , in a second embodiment variation, moreover, instead of engaging in axially slidable manner the plug body 6 , the piston-rod 4 is inserted in pass- through and axially slidable manner in the bottom of cupshaped body 5 , or rather in the bottom wall of the cup-shaped body 5 , and is f luid-tightly coupled to the cup-shaped body 5 via the interposition of at least one annular sealing gasket 10 , which is adapted to prevent the leakage of pressuri zed gas in the interstitial space between the piston- rod 4 and the cup-shaped body 5 , and is preferably made of transparent or semi-transparent polymeric material or, more in detail , of transparent or semi-transparent polyurethane .

[0133] In other words , in this second embodiment variation, the annular sealing gasket or gaskets 10 and, i f provided, the annular protective gasket or gaskets 11 are placed at the straight through hole present on the bottom of the cup-shaped body 5 .

[0134] Similarly, i f present , the guide and / or centring ring or rings 12 are located inside the straight through hole present on the bottom of the cup-shaped body 5 .

[0135] Clearly, the plug body 6 is still f luid-tightly coupled to the cup-shaped body 5 via the interposition of at least one annular sealing gasket 7 , which is adapted to prevent the leakage of pressuri zed gas in the interstitial space between the cup- shaped body 5 and the plug body 6 , and is preferably made of transparent or semi-transparent polymeric material or, more in detail , of transparent or semitransparent polyurethane .

[0136] Also in this second embodiment variation, moreover, the plug body 6 is preferably locked in place at the mouth of the cup-shaped body 5 by means of the locking member 8 .

[0137] In this second embodiment variation, in addition, the one-way valve 13 and / or the safety plug 14 is / are preferably located in the plug body 6 .

[0138] More in detail , the plug body 6 is preferably provided with two straight through ducts that extend parallel to the longitudinal / central axis A of cas ing 2 so as to connect the variable-volume closed chamber with the outside .

[0139] The first through duct accommodates the one-way valve 13 , and is preferably f luid-tightly closed by the additional closure plug 15 .

[0140] The second through duct , on the other hand, is fluid- tightly closed by the safety plug 14 .

[0141] In this second embodiment variation, therefore , the assembly of gas spring 1 preferably comprises the steps of : — reali zing, preferably by mechanical machining with removal of material , the cup-shaped body 5 and the plug body 6 of the outer casing 2 ; — reali zing, preferably by mechanical machining with removal of material , the piston-rod 4 ;

[0142] — placing the annular sealing gasket or gaskets 10 and, advantageously, also the annular protective gasket or gaskets 11 in the cup-shaped body 5 ;

[0143] — inserting the piston-rod 4 into the cup-shaped body 5 , engaging the annular sealing gasket or gaskets 10 and advantageously also the annular protective gasket or gaskets 11 ;

[0144] — fitting the annular sealing gasket or gaskets 7 on the periphery of the plug body 6 ; and finally

[0145] — inserting the plug body 6 into the mouth of the cupshaped body 5 , so as to close the same mouth in fluid- tight manner .

[0146] Finally, in a less sophisticated embodiment , only some of the annular gaskets 7 and / or 10 and / or 11 may be made of a transparent or semi-transparent polymeric material or, more in detail , of transparent or semi-transparent polyurethane . The remaining gaskets , on the other hand, are made of an opaque polymeric material or, more in detail , of opaque polyurethane .

Claims

CLAIMS1. A gas spring (1) comprising: an outer casing (2) having internally a closed cavity (3) ; a piston-rod (4) , which is inserted in the outer casing (2) in axially slidable manner, and is structured so as to form / delimit , inside said closed cavity (3) , a variable-volume closed chamber adapted to contain a pressurized gas; and one or more annular gaskets (7, 10, 11) that are interposed between the piston-rod (4) and the outer casing (2) and / or between distinct pieces (5, 6) of the outer casing (2) ; said gas spring (1) being characterized in that at least one of said annular gaskets (7, 10, 11) is an annular gasket made of transparent or semi-transparent polymeric material.

2. The gas spring according to Claim 1, wherein said transparent or semi-transparent polymeric material is an elastomer .

3. The gas spring according to Claim 1 or 2, wherein said transparent or semi-transparent polymeric material is polyurethane .

4. The gas spring according to any one of the preceding claims, wherein said annular gasket made of transparent or semi-transparent polymeric material has a surface hardness greater than or equal to 90 ShoreA.

5. The gas spring according to any one of the preceding claims, wherein said annular gasket made of transparent or semi-transparent polymeric material is an annular sealing gasket or an annular protective gasket.

6. The gas spring according to any one of the preceding claims, wherein said annular gasket made of transparent or semi-transparent polymeric material is a lip or double lip gasket .

7. The gas spring according to any one of the preceding claims, wherein the outer casing (2) comprises a cup-shaped body (5) and a plug body (6) , separate and distinct from the cup-shaped body (5) , which is at least partially fitted intothe cup-shaped body (5) so as to delimit said closed cavity (3) , and is coupled in a fluid-tight manner to said cupshaped body (5) via the interposition of at least a first annular sealing gasket (7) ; said first annular sealing gasket (7) being made of transparent or semi-transparent polymeric material .

8. The gas spring according to any one of the preceding claims, wherein the piston-rod (4) is coupled in a fluid- tight manner to the outer casing (2) via the interposition of at least a second annular sealing gasket (10) ; said second annular sealing gasket (10) being made of transparent or semi-transparent polymeric material.

9. A production method of a gas spring (1) comprising: an outer casing (2) having internally a closed cavity (3) ; a piston-rod (4) which is inserted in the outer casing (2) in axially slidable manner, and is structured so as to form / delimit, inside said closed cavity (3) , a variable-volume closed chamber adapted to contain a pressurized gas; and one or more annular gaskets (7, 10, 11) that are interposed between the piston-rod (4) and the outer casing (2) and / or between distinct pieces (5, 6) of the outer casing (2) ; said production method being characterised by comprising the steps of:- realizing at least one of said annular gaskets (7, 10,11) in a transparent or semi-transparent polymeric material ;- checking, via optical and / or visual inspection, the structure of said annular gasket made of transparent or semi-transparent polymeric material to detect the presence of any bubbles inside said annular gasket; and- using said annular gasket made of transparent or semitransparent polymeric material in the assembly of said gas spring (1) , if said annular gasket made of transparent or semi-transparent polymeric material is without bubbles or has, inside itself, bubbles with nominal volume below afirst predefined maximum threshold and / or with overall volume below a second predefined maximum threshold.

10. The production method of a gas spring according to Claim 9, wherein said at least one annular gasket (7, 10, 11) made of transparent or semi-transparent polymeric material is produced by injection moulding.

11. The production method of a gas spring according to Claim 9 or 10, wherein said at least one annular gasket (7, 10, 11) made of transparent or semi-transparent polymeric material is made of transparent or semi-transparent polyurethane .

12. The production method of a gas spring according to any one of Claims 9-11, wherein said first predefined maximum threshold is equal to 0,01% of the overall volume of the annular gasket and / or is equal to approximately 0,3 mm3.

13. The production method of a gas spring according to any one of Claims 9-12, wherein said second predefined maximum threshold is equal to 0,02% of the overall volume of the annular gasket and / or is equal to approximately 0, 6 mm3.

14. The production method of a gas spring according to any one of Claims 9-13, wherein the checking of the structure of said annular gasket made of transparent or semi-transparent polymeric material is carried out by an optical inspection electronic device 100 that comprises: at least one videocamera or camera (101) adapted to acquire one or more images of said at least one annular gasket (7, 10, 11) made of transparent or semi-transparent polymeric material; and a data processing unit (102) which is programmed / conf igured so as to process the images acquired from said video-camera / s or camera / s (101) , to detect the presence of any bubbles inside said annular gasket.

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