US 3563023 A
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Description (OCR text may contain errors)
Feb. 16; 1971 D. s. BREED 1 3,563,023
LIQUID ANNULAR ORIFICE DASHPOT TIMER Filed Dec. 9, 1968 Q'Sheets-Sheef 1 IINVENTOR p14 V/P JIVEED BY ga 9M, m MW ATTORNEYS D. S. BREED LIQUID ANNULAR ORIFICE DASHPOT TIMER NEY INVENTOR Feb. 16, 1971 Filed Dec.
I I a A I I 3,563,023 LIQUID ANNULAR ORIFICE DASHPOT TIMER David S. Breed, Yacht Club Drive, Spring Brook Terrace, Lakeforrest, Jefferson Township, NJ. 07005 Continuation-impart of abandoned application Ser. No.
770,205, Oct. 24, 1968. This application Dec. 9, 1968,
Ser. No. 816,132
Int. Cl. G04f 1/00 US. Cl. 58-2 21 Claims ABSTRACT OF THE DISCLOSURE The dashpot timer of this invention includes a substantially cylindrical glass tube in which travels a piston having a diameter slightly less than that of the interior of the tube. A liquid or gum defines the medium in the cylinder in which the piston is adapted to travel. The flow ingenerated in the cylinder is predominantly a pressure flow with the shear flow being relatively insignificant. The device of the present invention operates in the lubrication region whereby relatively small timers are capable of providing delays up to several months or more.
This application is a continuation-in-part of my now abandoned application Ser. No. 770,205, filed Oct. 24, 1968, entitled Liquid Annular Orifice Dashpot Timer.
The subject invention relates to a liquid annular orifice dashpot which utilizes the clearance between a ball or axisymmetric piston and an interior cylindrical wall as an orifice through which the liquid is metered.
Dashpots utilizing air as the metering fluid are known in the art and are described in Pat. No. 3,171,245. Such air dashpots are finding wide acceptance for certain time delays or where available space is no problem, applications requiring relatively long delays, ranging up to several days, months or longer times, or where space allocations are of an absolute minimum, or both, timers heretofore unavailable and unrealized are necessary. For such long period delays, the liquid dashpot timer of this invention has been found to be eminently satisfactory and acceptable and capable of yielding time delays ranging from a few seconds to several months and more.
The usual dashpot timers of the above patent also tend to be somewhat larger than the available space can accommodate. As the desired time delay increases, the size of such timers increase and the available space becomes increasingly inadequate. The dashpot timer of this invention is capable of being reduced to minute sizes while providing the aforenoted long delays. A typical timer contemplated is less than one-half inch long and three-sixteenths inch in diameter.
The liquid dashpot is also desirable in that it is relatively insensitive to changes in ambient pressure.
An understanding of the particular nature of the flow passed a ball or piston as it descends along the wall of a cylinder is important in determining the predictability and accuracy of the rate of descent. The type of liquid flow that the present invention utilizes is generally termed creeping fiow which involves very slow piston motion and, consequently, very slow liquid motion or, in other words, very small piston velocities and, consequently, liquid velocities and which are motions at very loW Reynolds numbers and in which viscous forces are predominant over inertial forces. The different types of creeping motions are distinguished essentially by the nature of the force which restricts the motion of the piston. Reference is made to the textual treatment of creeping motions in Boundary Layer Theory, McGraw-Hill Series of Mechanical Engineering by Dr. Hermann Schlichting, fourth edition, published by McGraw-Hill Book Company, Inc., New York,
United States Patent N.Y., and particularly chapters IV ind VI thereof, all of which is incorporated herein by reference.
One type of creeping motion utilizes a large clearance such that the major resistance to the motion of the piston comes from the shear flow of the fluid. It is a two-dimensional flow and the exact positioning of the piston in the cylinder is unimportant except when it gets extremely close to the cylinder wall. This type of creeping flow in fluid dynamics is characterized by the fact that the cylinder walls and the clearance between the piston and the cylinder do not effect the motion of the piston.
Another type of creeping flow utilizes the clearance be tween the piston and the cylinder and is based on the hydrodynamic theory of lubrication. When the clearance is very small, the flow is essentially one dimensional; and the forces caused by the pressure differential at opposite ends of the piston dominate. Thus, the essential characteristic of flow in the lubrication region compared to the type of creeping flow mentioned earlier is that in the latter case the total of the distributed shear forces over the entire piston creates the drag force which dominates all other forces whereas in the lubrication region, the force created by the pressue differential dominates. The basic determining factor is thus the clearance between the piston and the cylinder. For creeping flow of the first type, the ratio of the clearance to the radius of the piston :would be about 0.1 or larger Whereas for the lubrication region, the ratio of the clearance to the radius would be in the range of .01 or smaller.
Creeping flow is utilized, for example, in a common ball viscometer. It is unsuitable for long delayed devices since the rate of descent is relatively rapid. Thus, it has been found advantageous to utilize the pressure flow concept in the device of the present invention, notably, creeping flow in the lubrication region.
It is, therefore, an object of the present invention to provide a liquid annular orifice dashpot which eliminates the disadvantages, limitations and drawbacks of the prior art devices and which is exceptionally reliable, susceptible to long life and relatively inexpensive to manufacture.
The liquid dashpot of the subject invention has been successively utilized in a long term multiple time delay system. In this particular application, the dashpot incorporates two timed delays in a single piston cylinder arrangement by reason of an enlarged diameter portion at one end of the cylinder. The piston travels a distance of .075 inch at the forward end of the piston in approximately five minutes to provide the first delay. It then travels an additional .125 inch through the reduced diameter portion over a period of approximately forty-five days to provide the second delay.
Although several approaches may be taken to seal the liquid in the cylinder, the present invention proposes to seal the liquid through the use of a rubber boot enclosing the cylinder and which is punctured by a rod, pointed or sufficiently small in diameter to puncture the boot at the initiation of the delay. Thus, the dashpot of the subject invention can be stored for long periods of time without the danger of liquid loss.
Other objects and advantages will become apparent from the following detailed description which is to be taken in conjunction with the accompanying drawings illustrating an exemplary preferred embodiment of the invention and in which:
FIG. 1 is a diagrammatic perspective view of a dashpot timer incorporating the teachings of this invention utilizing a spherical piston the initial and terminal postion of which for the prescribed time delay being shown in dotted lines;
FIG. 2 is an enlarged fragmentary longitudinal sectional view of this dashpot showing the internally disposed spherical piston traveling through the selected liquid to an applied force;
FIG. 3 is a similar view showing a cylindrical piston;
FIG. 4 is a longitudinal sectional view illustrating the dashpot employed in an exemplary embodiment employing a puncturing needle prior to the initiation of the time delay sequence; and
FIG. 5 is a view similar to FIG. 4 but showing the device after the delay sequence has been initiated.
Referring now to FIGS. 1 to 3, a dashpot timer of this invention will include an outer cylinder 2 having a contained liquid 4 therein through which a piston is adapted to travel under an applied force F. This piston may assume the form of a sphere 6 or cylinder 8. The piston also defines with the inner surface of the cylinders relatively small annular orifice through which the liquid is adapted to pass.
Referring to the embodiment of FIGS. 4 and 5, a dashpot timer incorporating the features of the present invention is illustrated generally at 10. The timer 10 in cludes a cylinder 12 in which a cylindrical piston 14 is slidably disposed. The dimensional tolerances of the interior wall 16 of the cylinder 12 and the exterior wall 18 of the piston 14 provide for an annular orifice 20 through which a fluid, in liquid form, is adapted to flow.
The cylinder 12 is closed at its ends by the plates 22 and 22. The upper plate 22 has an aperture 24 positioned in the center thereof for purposes hereinafter described. A spring 26 is positioned within the cylinder and is designed to retain the piston 14 at the upper end of the cylinder in the manner shown in FIG. 4. A liquid 28 having prescribed properties fills the remaining portion of the cylinder.
In order to seal the liquid 28 in the cylinder, a rubber boot 30 is designed to overlie the upper plate 22'. If desired, the boot could completely incapsulate the cylinder, or substantially all of it, as shown in the drawings.
A plunger 32 is mounted externally of the cylinder in alignment with the aperture 24 of the plate 22'. The plunger is designed to travel along its axis, and in the illustrated embodiment is powered by a spring 34.
Upon initiation of the timing sequence, a suitable mechanism (not illustrated) releases the plunger whereby it penctrates the boot 30 and enters the cylinder through the aperture 24. The plunger 32 then strikes the piston 14 which is retained adjacent the aperture 24 by the spring 26. By design, the force exerted by the spring 34 is somewhat larger than that exerted by the spring 26 such that the piston descends along the cylinder at a controlled rate. Thus, the distance which the plunger 32 has traveled along its axis provides a convenient measure of elapsed time. Through the use of conventional mechanisms, the plunger can be utilized to trigger various devices, such as an arming mechanism or the self-destruction mechanism of a mine.
The interior wall 16 of the cylinder may, if desired, comprise two or more portions of differing diameters. Thus, in the illustrated embodiment, the interior wall 16 has an upper portion 36 of relatively large diameter and a lower portion 38 of a relatively reduced diameter. By reason of this structure, the piston will descend through the upper portion of the cylinder more rapidly than through the lower portion. In a mine, for example, this feature can be utilized to trigger the arming mechanism after a few minutes, and then trigger a self-destruction mechanism days or even months later.
While the piston 14 in the drawings is illustrated as being in the shape of a cylinder, it is also feasible to use a spherical or other axi-symmetrical piston. Such a spherical piston (or ball) has several advantages over the cylindrical piston. First, the cylindrical piston may tend to catch on the step between the diameter portions 36 and 38. With a ball as the piston, this would be eliminated. Finally, a spherical piston cannot cock, should this be a source of inconsistency. In expensive spherical metal balls 4 are available with very precise dimensions which would also contribute to the manufacturing ease of the dashpot. The motion of the piston in the cylinder depends on the geometrical characteristics of the particular device expressed through the pressure-flow relationship applicable. This pressure flow relation is:
k dP f (a) E E3 where: f(0) =fiow per unit circumferential length h=local clearance (a function of X and 6) ,u=viscosity X=coordinate along cylinder axis 6=angular coordinate along cylinder circumference.
Simultaneously, it must be that which states that the total flow rate is that which is displaced by piston moving at a velocity dX/dT. The pres sure at the high pressure side of the piston is:
F P GT2 3 where:
Fzaxial force on the piston R =radius of the piston.
The soultion of Equations 1 and 2 with the boundary conditions (3) and letting P=0 on the low pressure side of the piston, yields the relationship between the applied force (F) and the piston velocity dX/dT.
The particular geometry of a given device enters through the clearance function lz(X,B). For the particular case of a spherical piston, this clearance function h(X,0) can be approximated by:
c=mean radial clearance e=distance between the cylinder axis and the sphere center.
For the spherical case, the relation between F and dX/dT therefore is:
Which for the case where the ball is centered in the cylinder is:
|2 E)s/2 dT 97r pR R Similarly, for the case of a centered cylindrical piston, the above equations reduce to:
E Fh d T 61r,uR L
where L is the length of the piston.
Further analysis of the above equations will shown that for travel along the side of the cylinder wall, a ball will travel 1.92 times faster than for centered travel and a cylindrical piston will travel 2.5 times faster than for centered travel.
In the case of a cocked cylindrical piston, the above equations do not apply; but, nevertheless, the flow remains creeping motion in the lubrication region.
For most devices contemplated, the radius would be less than about /2 inch. The pressure could vary from about 5 psi. to several thousand psi. The clearance it would probably never exceed .001". The viscosity a could vary from 1 centipoise to over 100 million centioses. However, for most of the devices, the lower limit would probably be about 1000 centipoises. For conversion purposes, centistrokesxdensity=centipoises and 1,000,000 centiposes=.14 lb. sec./in. The usable rubber gums range in viscosity from 100 million centistokes to 1000 whereas the usable silicone fluids range in viscosity from .65 centistoke to 2.5 million centistokes. Gums and fluids of these or equivalent types are available commercially.
For most applications, however, some degree of temperature compensation would be necessary. In order to achieve temperature compensation, the ratio of the radius of the ball R to the mean radial clearance It would have to exceed 100/ 1. The viscosities of the optimum fluids change by a factor of 35 to 1 over a temperature range of -65 F. to 160 F. This fact above all others has prevented the use of liquid dashpots per so as timing elements in military fuses in the past.
The cylinder 12 is preferably made from glass or other ceramic material, since precision glass tubes are readily available. However, other materials could also be utilized. Similarly, the end plates 22 and 22' could be made from glass or a metal such as aluminum. The plates may be integrally formed with the cylinder or may be bonded to the ends of the cylinder, or the boot 30 could be utilized to retain the positioning of these members without bond- 1ng.
In summary, the present invention accomplishes and contributes the following advantages to the dishpot timer art:
1) Position of ball travel: The ball will travel approximately twice as fast if it travels near the side of the tube as opposed to being centered. The contemplated method of pushing the ball assures that the ball will be pushed over to the side of the cylinder. It would, however, be extremely diflicult to push the ball in a manner where it did not travel near the side of the cylinder.
(2) Temperature compensation: The viscosity of the best fluids change by a factor of 12 to 1 over the temperature range 65 F. to 160 F. Even over a very limited temperature range, the viscosity changes about 1%/ F. to 1.5%/ F. Consequently, if accuracy is to be achieved even over limited temperature ranges, some degree of temperature compensation is necessary.
(3) Piston rates: For most of the devices of the present invention, the rate of travel of the piston will be on the order of 1 micro-inch per minute to one-tenth of an inch per second. The most important range effectively satisfied by this invention is the one to micro-inch per minute area.
(4) Physical size: With the present invention, time delays are achieved which are significantly longer with a significantly smaller dashpot than has ever been done before. The clearance range utilized and envisioned is approximately one order magnitude smaller than has ever been used before.
(5) Sealing: The second major reason why liquid dashpots have failed in military applications has been the inability to seal silicone fluids from leakage. With the use of a rubber boot punctured by a point at the initiation of the delay, there is no sealing problem. Obviously, there are applications where sealing is not a problem.
(6) Military applications: Based on present knowledge, no liquid dashpot has been successfully applied to a military fuse in the past. Other applications and other than one-time use dashpot timer applications may be found in the above identified patent.
(7) Stepped dashpots: Based on present knowledge, no liquid dashpot has ever been devised wherein more than one rate of piston travel is achieved through the use of different diameters in the same dashpot.
(8) Precision glass tube: Based on present knowledge, no ball-cylinder dashpots have been made wherein the cylinders is a precision glass tube. Where glass tubes have been utilized in the past, they have been exercising creeping flow not in the lubrication region where the cylinder ID. is relatively unimportant.
Although a preferred embodiment of this invention has been described and illustrated herein, it should be understood that this invention is in no sense limited thereby but its scope is to be determined by that of the appended claims.
1. A liquid, annular orifice dashpot timer comprising:
a cylinder having a substantially cylindrical interior wall;
a piston disposed in said cylinder and having an outer diameter slightly less than the diameter of said interior wall whereby a substantially annular orifice is defined between the piston and cylinder, the means radial clearance between the piston and cylinder being less than .001 inch;
a liquid of relatively large viscosity in the cylinder through which the piston is adapted to move with the relative flow of the liquid being creeping flow in the lubrication region which involves very slow piston motion, very slow liquid motion, very small piston velocities and liquid velocities and which are motions at low Reynolds numbers and in which viscous forces are predominent over inertial forces;
and the piston rate of travel through the liquid in the cylinder being very small and the motion of the liquid as a result of piston travel being relatively very small.
2. The invention in accordance with claim 1 wherein the piston is substantially spherical and the piston rate of travel through the liquid being defined by the followwhen the piston is centered relative to the cylinder during its travel therein and approximately 1.92 times faster when the ball travels along the side of the cylinder where R is the radius of the piston, F is the axial force applied to the piston, ,u is the viscosity of the liquid, h is the mean radial clearance between the piston and cylinder, and dX/dT is the velocity of the piston.
3. The invention in accordance with claim 2 wherein R is less than about /2 inch.
4. The invention in accordance with claim 2 wherein is of a value of about 5 p.s.i. to several thousand p.s.i.
5. The invention in accordance with claim 2 wherein a is of a value from .65 centistoke to million centistokes.
6. The invention in accordance with claim 5 wherein the liquidis selected from the group consisting of rubber gums ranging inviscosity from 1000 to over 100 million centistokes and silicon fluids ranging in viscosity from .65 centistoke to 2.5 million centistokes.
7. The invention in accordance with claim 6 wherein means are included for temperature compensation to offset the effect of change of viscosity of the liquid upon the rate of travel of the piston resulting from changes in temperature, such means embracing a ratio in excess of 100 to 1 of the radius of the piston to the mean radial clearance 11.
8. The invention in accordance with claim 7 wherein the viscosity of the liquid changes by a factor of 35 to 1 over the temperature range of 65 F to F.
9. The invention in accordance with claim 1 wherein the piston is substantially cylindrical and the piston rate of travel through the liquid being defined by the following equation:
when the piston is centered relative to the cylinder during its travel therein and approximately 2.5 times faster when the ball travels along the side of the cylinder where L is the length of the cylinder, R is the radius of the piston, F is the axial force applied to the piston, ,u. is the viscosity of the liquid, h is the mean radial clearance between the piston and cylinder, anddX/dT is the velocity of the piston.
10. The invention in accordance with claim 9 wherein R is less than about /2 inch.
11. The invention in accordance with claim 9 wherein 'zrR is of a value of about 5 p.s.i. to several thousand p.s.i.
12. The invention in accordance with claim 9 wherein is of a value from .65 centistoke to 100 million centistokes.
13. The invention in accordance with claim12 wherein the liquid is selected from the group consisting of rubber gums ranging in viscosity from 1000 to over 100 million centistokes and silicon fluids ranging in viscosity from .65 centistoke to 2.5 million centistokes.
14. The invention in accordance with claim 13 wherein means are included for temperature compensation to offset the eifect of change of viscosity of the liquid upon the rate of travel of the piston resulting from changes in temperature, such means embracing a ratio in excess of 100 to 1 of the radius of the piston to the mean radial clearance h.
15. The invention in accordance with claim 14 wherein the viscosity of the liquid changes by a factor of 35 to 1 over the temperature range of 65 F. to 160 F.
16. The invention in accordance with claim 1 wherein the piston is cylindrical and is cocked relative to the cylinder during its travel therein.
. g 8 17. A dashpot timer for use in an ordance fuse wherein means are provided for including as a timing element the liquid annular orifice dashpot timer defined in claim 1.
18. -A liquid, annular orifice dashpot timer in accordance with claim 1 wherein said cylinder has sealing means closing at least one end thereof; and a plunger positioned externally of said cylinder and being adapted to puncture said sealing means at one end of said cylinder to exert a force on said piston.
19. The timer as set forth in claim 18 wherein a spring is positioned in said cylinder to urge said piston toward the end of said cylinder adjacent said plunger.
20. The timer as set forth in claim 19 wherein saidv sealing means at the end of said cylinder adjacent said plunger comprises a rubber boot.
21. A timer as set forth in claim 20 wherein said interior wall has a reduced diameter portion at one end thereof to decrease the clearance between said piston and said interior wall.
References Cited UNITED STATES PATENTS 2,714,927 8/1955 Stern et al. 58144 3,166,839 l/l965 Dock et al 58-144 3,171,245 3/1965 Breed 58-144 3,179,396 4/1965 Bracken 2671 3,458,992 8/1969 Breed 581 RICHARD B. WILKINSON, Primary Examiner E. C. SIMMONS, Assistant Examiner US. Cl. X.R.