US 3064686 A
Description (OCR text may contain errors)
Nov- 20, 196 J. GRATZMULLER 3,064,686
HYDRO-PNEUMATIC ACCUMULATORS 2 Sheets-Sheet 1 Filed Aug. 5, 1958 Nov. 20, 1962 .GRATZMULLER 3,064,636
HYDRO-PNEUMATIC ACCUMULATORS 2 Sheet heat 2 FIG.2
United States Patent F 3,664,686 HYDRO-PNEUMAHC ACCUMULATORS Jean Louis Gratzmuller, 66 Blvd. Maurice Barres, Neuiliy Sur Seine, France Filed Aug. 5, 1958, Ser. No. 753,358 Claims priority, application France Aug. 23, 1957 2 Claims. (Cl. 13831) In the American Patent No. 2,724,412, filed on September 19, 1951, there was described a hydro-pneumatic accumulator of the free piston type, in which the said piston is provided around its periphery, with a rabbet having a cylindrical wall and a bottom substantially normal to the axis of the cylinder, a packing ring made of a plastic and elastic material being housed in the said rabbet and subjected, through an annular member bearing against the free face of the ring, to a permanent pressure exerted by a spring and higher than the difference between the pressures liable to prevail on either side of the piston.
The materials preferably used by the applicant up to now were of the neoprene or perbunan type; these materials, when subjected to a unitary pressure of about kg./cm. are capable of undergoing a suflicient plastic deformation to fill substantially the whole capacity of the rabbet while keeping that degree of elasticity required to ensure tightness.
In practice, the hydro-pneumatic accumulators of the above-mentioned type are often intended to be used under pressures of about 200 to 300 atmospheres. It will be easily understood that as long as liquid remains in the accumulator, the difference between the pressures acting "on the piston faces is either nil when, the piston is stationary, or comparatively small when it moves, since it results exclusively from friction of the ring material against the cylinder wall material.
However, when the piston reaches the end of its stroke, after having completely expelled the liquid out of the accumulator, which is the case in particular during a prolonged stocking of an inflated accumulator, the pressure to which the ring is subjected by the compressed gas may reach about 200 kg./cm. i.e., more than 10 times the pressure to which it is subjected by the compression spring.
It will be easily understood that, under these conditions, while the ring undergoes in operation but a reasonable plastic deformation under the action of the pressure exerted by the spring, it is subjected during stocking to a far higher pressure and, hence, it risks undergoing a deformation capable of causing its extrusion between the piston and the cylinder.
In order to obviate this drawback, particular arrangements had to be provided.
Such arrangements have been described, in particular, in the American Patent No. 2,804,094, filed by the applicant on September 4, 1952. V
The applicant'has observed, however, that at certain critical temperatures, the extrusion may become objectionable even With the above-mentioned particular arrangements.
An object of the invention is to ensure perfect tightness between the two faces of the piston, not only when a certain amount of liquid is contained in the accumulator, but also when the latter is stocked with all the liquid expelled out of its cylinder. Another object of the invention is to ensure tightness in operation, as well as in stockage, whatever may he the temperature, within the usual practical limits.
Still another object of the invention is to provide a hydro-pneumatic accumulator of the type described wherein the packing ring is made of a plastic and elastic material which requires (to be deformed plastically to asufiicient extent for ensuring continuous contact, along a circum- 3,064,686 Patented Nov. 20, 1962 ference, with the cylinder wall and to keep the elasticity required for ensuring operation under the conditions exposed in the first reference cited above), a pressure at least equal to the maximum pressure liable to prevail in the accumulator, the said material moreover offering the property of having a very low friction coefiicient on the cylinder metal, in order that the overall friction of the ring on the cylinder wall be not objectionable.
With a ring according to the invention, when the piston reaches the end of its stroke, after having completely expelled the liquid out of the cylinder, the ring is always subjected to a pressure equal to the sum of the spring pres. sure and that of the gas. However, this sum is at most equal to twice that pressure to which the ring is normally subjected instead of the pressure ten times higher than the spring pressure to which the ring is subjected when it is made of neoprene or perbunan. It is clear that such a reduced pressure difference does not risk to determine any unwanted extrusion between the piston and the cylinder.
It will be understood, moreover, that owing to the low friction coefficient between the ring according to the invention and the cylinder metal, the overall friction which is equal to the product of the contact area by the said coefiicient, is not in the least objectionable. As a matter .of fact, the said friction opposes no serious difficulty to the displacements of the piston and, moreover, it does not tend to cause an extrusion of the ring material between the piston and the cylinder during such displacemerits.
A further object of the invention is to choose among the materials which may be used to make a ring in accordance with the above-mentioned conditions, a polyfluorethene. In an alternative embodiment of the invention, the ring is made of polytetrafluorethylene. This specific material offers both properties required to ensure a suitable behaviour of the ring according to the invention; moreover, it also offers the inherent property of keeping its mechanical characteristics at least between and 260 C.
This additional property ensures the required tightness in operation, as well as in stockage, within a temperature range which up to now seems to correspond to all possi ble conditions of use of. the hydraulic circuit elements to be used with such an accumulator.
Moreover, the above-mentioned material requires, to be deformed under the desired conditions, a unitary pressure of about 600 -kg./cm. so that in an accumulator having an inflating pressure of 200 kg./cm. the ring is only subjected -in 'stockage to a pressure which is higher than its normal working pressure by only one third'of the latter. a
It is yet another object of the invention to provide a hydro-pneumatic accumulator of the type described, wherein thering is constituted by a simple wire the ends of which, owing to the easy sliding properties of the material, ,are strongly applied against each other.
Still a further object of the invention is to provide a number of constructive arrangements which permit of fur-therimprovin'g the results 'obtained by the use of the above-defined materials.
Another object of the invention is to provide several dispositions making the use of these materials generally easier, these dispositions being even indispensable in certain cases to make this use possible at all, namely, when the size and weight --of the accumulator are to be as reduced as possible.
Apaekingringof a material having the above-mentioned properties and, in particular, that of resisting compression stresses sufficiently to require the use of considerable pressures, offers, as compared with the :rings previously used by the applicant, aconsiderably reduced size. As-anillustration, a according to the invention .jected' to thermal deformations.
preferably has before being deformed, an axial height equal to about of the cylinder diameter so that, once compressed, this dimension becomes still smaller than This particular arrangement offers a double advantage.
.First of all, as it will be easily understood, by reducing the height of the ring, one diminishes the contact area be- .tweent the ring and the cylinder and hence, the total friction between the ring and the cylinder since the latter .friction is proportional to the friction coeflicient of the material, as well as to the contact area. The reduction of size of the ring also afiectsits width (in a plane normal to thecylinder axis), the saidwidth preferably being of the sameorder of magnitude as the ring height. With this arrangement, the surface of the ring on which the spring pressure is exerted is considerably reduced, which permits of obtaining the desired .a cylinder having an inner diameter of 50 mm., a 1 mm.
wide ring still requires to becompressed to 600 kg./cm. a spring having a strength of about 900 kg. It is clear that such a spring is normally very heavy and, moreover, hassuch an outer diameter that it cannot be lodged within a cylinder having an inner diameter of 50 mm.
Another object of the invention is, to provide, in addition to the already described arrangements, a particular disposition permitting, on the one hand, of lodginga sufficiently strong spring within the accumulator and, on the other hand, of reducing to the strict minimum the weight of the said spring. 'In the caseof airborne materials, it is precisely essential to reduce to a minimum the weight of all elements, so that an accumulator intended to be used with such materials has to be as light as possible.
According to this featureof the invention, there is provided to transmit to the ring the compression required for its suitable operation, a coil spring the winding diameter of which is greater than its wire diameter but only from'2.5 'to ,3 times, which is in absolute contradiction with all admitted standards, in which the ratio between the said diameters is limited. to 7.
The spring of the accumulatoraccording to the invention thus works under conditions which are usually considered as inadmissible and, in fact, far above the limit of elasticity of the metal. In practice, the-spring according to the invention, will work in the neighborhood of the deformation capable of breaking the metal it is made of (by torsion shearing).
The applicant has moreover observed that, owing to the special function of the spring according to the invention, the latter may be easily manufactured in spite of all usually admitted principles. As a matter of fact, to keep the ring in the required compression state the spring has only to work within an exceedingly small stroke with respect to its mean position. a 1
The importance of thisvariation will be better understood if one considers that, for example, the spring according to the invention will have a height of about 40 mm.
for a displacement with respect'to its normal compression, of about 1 mm.
Another consideration is that the spring is practically subjected to no play since its length will only tend to increase as the ring wears out, or, again, when it is sub- Since the latter are exceedingly small, due to the minute dimensions of the ring, the play of the spring may be considered asnegligi ble. In other words, it may be said that the spring is a static one.
- Still a further object of the invention is to provide a particular mounting of the spring which consists in put-- 4 ting the spring on the compressing member of the ring and then assembling the annular abutment of the piston on a special apparatus capable of exerting the desired force, this whole assembly being so designed that the spring is but very slightly deformed beyond the length it will have in operation, this supplementary deformation being, if desired, limited to that which is strictly necessary to subsequently lock 'the annular abutment on the piston.
The piston and spring and the degree of deformation of the spring, when assembled, may be chosen sufliciently great to subject the spring to such a permanent deformation that all peripheral fibres of the spring wire will work at an optimum rate.
It is yet another object of the invention to provide several mechanical features ensuring a suitable guiding of.
the annular member, transmitting to the ring the pressure of the spring, as well as a suitable guiding of the latter, the importance of these features increasing as the dimen sions of the ring decrease.
For example, with a ring having a width of 1 mm., all clearances must be reduced to the strict minimum, since it is obvious that they can only amount to a fraction of the ring width, viz., a few hundredths of 1 mm. Moreover, it is obvious that if the clearances are very small, there are more risks of jamming, which implies particular care to avoid such jammings.
Independently of the mechanical arrangements which will be described in detail hereafter, another object of the invention is to provide a characteristic arrangement permitting of avoiding another risk resulting from the reduced dimensions of the ring. I I
It will be easily understood that if the clearance be tween the piston considered as a whole and the cylinder, is of about one tenth mmgfor example, any difference of thermal expansion between the piston and the cylinder will be objectionable. a It is yet another object of the invention to provide a hydro-pneumatic accumulator, wherein the cylinder and the piston (or at least the body of the piston in which is formed the rabbet and the member transmitting the compression to the ring) are made of the same metal. This arrangement avoids any risk of jamming, or excessive clearance due to differences of expansion; 7 It is to be understood that by same metals there is meant here metals having the same coefiicient of expansion such as for example, ferrous metals which permits, if desired, to use steels of .difierent grades for the various elements. Y 7 In a preferred embodiment of the invention,-to reduce the clearances which might result from expansions affect.- ing diflzerently the piston and the cylinder, both are made of steel which, among the metals of sufliciently low price andhaving satisfactory mechanical features, is the one offering the lowest coefficient of expansion. r To avoid any deterioration of the cylinder surface, due to the steel-to-steel friction which could cause subsequently a deterioration of the packing ring, it is still a further object of the invention to chromatize the steel of the cylinder internal walls, while making the piston and the compression member of ordinary'steel.
This arrangement permits, in particular, of using without any risk of scratching, a steel of the same grade for the cylinder as well as for the piston (which permits of making'the latter lighter). I V p Hereabove, the invention has been described in its application to'the free piston of hydro-pneumatic accumu lators, since in this application its advantages are particu- In the first prior specification referred to in the beginning of this specification, it has been explained that owing,
to the general arrangement of the packing device described in the said patent, the tightness may be considered an ab-'" M Il 5 solute if the pressure (with which the packing ring is applied along a continuous circumference against the annular member, such as the cylinder with which it cooperates) is higher, even but slightly, than the difference between the pressures prevailing on either side of the packing device.
Under these conditions, it is clear that a packing device designed as described above may be incorporated, within the scope of the invention, to a hydraulic cylinder piston, if the pressure difference on either side of the said piston can never be higher than the pressure with which the ring is applied against the cylinder surface.
As described above, said pressure is normally of about 600 kg./cm. As a result, the invention may be applied in any hydraulic cylinder in which the pressure difference between the two faces of the piston is lower than the said pressure.
Therefore, the invention covers, in addition to hydropneumatic accumulators comprising a free piston such as defined above, any hydraulic cylinder the piston of which is equipped in the same manner.
Other objects and advantages of the invention will be hereinafter described with reference to the accompanying drawings, given merely by way of example.
In these drawings:
FIG. 1 shows a hydropneumatic accumulator according to the invention.
FIG. 2 shows, on a larger scale, the piston of the said accumulator, and
FIG. 3 is a part view on a still larger scale, of an element of the said piston.
Referring to the drawings, there is shown at 1 the cylinder of an accumulator which is divided into two compartments by a free piston 2.
In FIG. 1, the space located above piston 2 is intended to be filled with compressed gas while the space located under the said piston is intended to be filled with pressure liquid, the said liquid being sometimes expelled out of the accumulator, in particular when the latter is stocked. In this state, the piston 2 is applied by the gas pressure against the lower head 3 of the accumulator.
The free piston 2 is designed in the same manner as described in the above first cited prior specification. However, according to the invention, the ring 4 is made of a plastic and elastic material ofien'ng the above-mentioned features. This ring thus requires, to be deformed plastically, a very high pressure which is supplied by a strong spring 5 acting through an annular member 6, as shown in the drawings. Moreover, the dimensions of the ring 4 are considerably reduced, as compared with those of the ring described and shown in the above-cited specification. As a matter of fact, in the example shown, the ring 4, when free (i.e., before being deformed by the spring 5) has a height of about & of the diameter of cylinder 1. After the deformation of the ring, the said height is still smaller. That surface of the ring 4 which is in contact with the cylinder wall is thus reduced to the strictest minimum. Since, moreover, the coefficient of friction of the ring material on the cylinder metal is very low, the overall friction between the ring and the cylinder is extremely reduced, which ensures an excellent operation of the accumulator.
In the case of a ring made of polytetrafluorethylene, the strength of the spring is such that the pressure exerted on the ring is of about 60 kg./cm. Assuming that the accumulator operates between 200 and 300 kg./cm. it will be understood that when the piston 2 is compressed against the head 3, the ring 4 is subjected to an additional pressure of 200 kg./cm. i.e., an extra A of the normal pressure to which it is permanently subjected by the spring 5. Thus, the said ring does not risk to be crushed under the action of the additional pressure, which could cause extrusion of the said ring between the piston and the cylinder.
The invention comprises, in addition to the above-mentioned general arrangements, a number of supplemental dispositions intended to improve the conditions of operation of the accumulator. In such an accumulator, it is important to avoid any risk of jamming or objectionable friction without increasing the clearances between the member 6 and the cylinder, as well as between the piston and said member 6.
According to a feature of the invention, the annular member 6 is guided by a cylindrical portion 7 formed on the rod 12 which carries the abutment 13 against which bears the spring 5, the said guiding being ensured by a rounded portion 11 of the inner edge of the member 6. Thus, the latter is positively located along a circular line of small diameter. This arrangement is very important since it is indispensable that the member 6 keeps a certain freedom of angular displacement to ensure a uniform compression of the ring 4. Since the clearance between the member 6 and that portion of the piston on which it is guided is proportional to the diameter of the said piston portion, it is necessary and sufficient to reduce the said diameter, which is obtained by means of the above-described arrangement. Moreover (see details in FIG. 3), the outer wall of the annular member 6 is so designed as to leave a small clearance between the cylinder wall and the said member 6. The preferred shape of the outer wall of the member 6 is that shown in FIG. 3, the space provided between the member 6 and the cylinder wall having the form indicated at 10. This form ensures the required angular freedom of the member 6 without increasing the necessary clearance in the neighbourhood of the ring between the said member and the cylinder.
As shown in particular in FIG. 2, the rod 12 offers, independently of the guiding cylindrical portion 7, two other cylindrical portions having a diameter greater than that of the rod 12, viz., the portions 8 and 9. These portions are intended to ensure guiding of the spring 5.
In FIG. 2, is shown another feature of the invention, viz., a particular shape of the spring 5, as well as the manner in which it is mounted and precompressed. As shown in this figure, the ratio between the mean winding diameter D of the spring and its wire diameter d is smaller than 3 which, as already mentioned, is in absolute contradiction with the ratios usually admitted in the manufacturing standards.
FIG. 2 also shows that the mounting and precompression of the spring may be ensured by a more axial displacement of the annular abutment member 13 which, once the spring has undergone the desired deformation, is locked by means of a piano wire 14. It will be noted that the distance 0 between the face 15 of the abutment 13 and the face 16 of the shoulder 9 is reduced to the strict minimum necessary to permit of mounting the piano wire 14.
This is made to take into account the fact that it is important to avoid, once the spring has been suitably precompressed, any increase of its length.
It will be easily understood that, if the abutment member 13 is locked by means of a nut or any similar means, the clearance 0 may be completely suppressed.
It is to be noted that the invention is not limited to any particular material of the ring (provided that the said material correspond to the two above-mentioned conditions), nor to any specific dimensions of the said ring with respect to the dimensions of the accumulator.
What is claimed is:
1. In a hydropneumatic accumulator comprising a cylinder and piston slidably mounted in said cylinder to divide the same into a liquid pressure chamber and a gas pressure chamber, said piston being constituted by a diskshaped element including an annular step formed by a rabbet around the periphery of said disk-shaped element and an axial extension carrying an abutment, said annular step forming an annular open chamber bounded radially by the cylinder wall and by a concentric cylinder surface on the piston, and the bottom of said annular chamber being formed by the annular surface of said step extending from said cylindrical surface toward said cylinder wall, the combination comprising an annular sealing ring located in said annular open chamber and of an elastically and plastically deformable material of a hardness requiring, for an axial compression of the ring capable of determining a radial deformation thereof sufiicient to insure substantial tightness, an axial pressure at least equalto the maximum fluid pressure for which the accumulator is calibrated, a seal-compressing ring slidably mounted in said chamber and resting against said sealing ring, said annular sealing ring having a height not more than 6 of the cylinder diameter and being of relatively small width, and a coiled wire spring located between said seal-compressing ring and said abutment and calibrated to exert said axial pressure on said sealing ring,
8 said spring having a winding diameter between 2.5 and 3 times'its wire diameter and being confined between the seal-compressing ring and said abutment so that it is prestressed beyond its limit of elasticity.
2. The structure of claim 1, and wherein said seal-compressing ring has a rounded inner annular edge surrounding and engaging said axial extension.
References Cited in the file of this patent UNITED STATES PATENTS in ass-b.2146