US3603072A

US3603072A - Liquid annular orifice dashpot timer with modified liquid

Info

Publication number: US3603072A
Application number: US1340A
Authority: US
Inventors: David S Breed
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-01-08
Filing date: 1970-01-08
Publication date: 1971-09-07
Anticipated expiration: 1988-09-07

Abstract

The dashpot 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 modified 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.

Description

United States Patent Inventor David S. Breed Box 270, RD. 2, Boonton, NJ. 07005 Appl. No. 1,340 Filed Jan. 8, 1970 Patented Sept. 7, 1971 Continuation-impart of application Ser. No. 816,132, Dec. 9, 1968, now Patent No. 3,563,025, which is a continuation-in-part of application Ser. No. 770,215, Oct. 24, 1968, now abandoned.

LIQUID ANN ULAR ORIFICE DASHPO'I TIMER WITH MODIFIED LIQUID 28 Claims, 5 Drawing Figs.

US. Cl 58/1, 267/1 Int. Cl G04f 1/00 Field of Search 188/139,

[56] References Cited UNITED STATES PATENTS 2,604,163 7/1952 Exline 58/1 2,714,927 8/1955 Stern etal 58/144 Primary Examiner-Richard B. Wilkinson Assistant Examiner-Edith C. Simmons Attorney-Kane, Dalsimer, Kane, Sullivan and Kurucz ABSTRACT: The dashpot 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 modified 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.

PATENTED SEP 7197i 3503072 SHEUIUFZ INVENTOR .DlV/D S. BREfD ATTORNEYS PATENIED SEP 7 1911 saw 2 OF 2 FIG. 4

INVENTOR DAV/0 S. BREED ATTORNEYS LIQUID ANNULAR ORIFICE DASI-IPOT TIMER WITH MODIFIED LIQUID This application is a continuation-in-part of application Ser. No. 816,132, filed Dec. 9, 1968 which, now Pat. No. 3,563,023 in turn, was a continuation-in-part of application Ser. No. 770,215, filed Oct. 24, 1968, now abandoned. The subject invention relates to a modified 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 modified liquid is metered.

Dashpots utilizing air as the metering fluid are known in the art and are described in U.S. 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, are described in my pending Pat. application Ser. No. 816,132. For such long period delays, the liquid dashpot timer of that application 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. It has been found that substantial improvements in the operation and simplicity of the liquid annular orifice dashpot can be achieved if the liquid is modified by the addition of solid particles, a soap and/or a gelling agent.

For cases where the liquid annular orifice dashpot must operate over a wide temperature range and where the variation in time delay over this temperature range must be minimized, the clearance between the piston and cylinder is chosen so as to yield the minimum variation in the time delay with temperature. Over the temperature range from 65 F. to +160 F. and using the clearance for optimum temperature compensation the time delay will still vary by a factor of 3 to l for the silicone fluids and up to 2 to l for the silicone rubber gums. It has been found that through the addition of the various soaps or solid particles in the form of very fine powder to the liquid that the variation in time over this temperature range can be almost completely eliminated yielding a dashpot timer whose time delay remains substantially constant at any temperature between -65 F and +160 F.

For most liquid annular orifice dashpot timers some means must be provided to seal the timer to prevent the leakage of the fluid during storage. By the addition, however, of a gelling agent, a soap or of solid particles to the liquid, the liquid can be converted into a grease or compound which will not flow until subjected to a minimum threshold pressure. This then entirely eliminates the need of sealing the dashpot since the so modified liquid will remain indefinitely in the cylinder until the piston is subjected to a force sufficient to overcome this flow threshold.

In addition, it is necessary in most liquid annular orifice dashpots to provide an internal support biasing spring to maintain the piston at the top of the cylinder until the device is activated. This is necessary to prevent the possibility of the piston from traveling in the cylinder under the influence of gravity as would happen during long term storage. By the use of a modified liquid the need for this spring is completely eliminated.

It has been found that in addition to yielding a far more constant time delay over wide temperature ranges through the use of a modified liquid, the clearance between the piston and the cylinder can be made substantially larger than was heretofore possible while still maintaining a constant time delay over the temperature range. Whereas for the standard liquid annular orifice dashpot the upper limits for the clearance for most applications was approximately 0.001, inch through the use of a modified liquid good results over the extended temperature range can be achieved using clearances of 0.020 inch or larger. This permits the use of this dashpot for very short time delays as well as the very long time delays. In fact, the range of operation extends from a few milliseconds up to several years in length. The short time delays were not practical with the liquid annular orifice dashpot because of the necessity of using very thin fluids which require scaling to prevent leakage of these fluids out of the cylinder.

The possibility of using significantly larger clearances greatly reduces the need to hold very precise tolerances on the piston and cylinder. Out-of-roundness and taper in the cylinder, for example, which heretofore typically had to be controlled to 0.000025 inch or better now can be relaxed significantly greatly improving the producibility of the cylinder and consequently reducing its cost. Further, the range of materials which is now available for the piston and cylinder is greatly increased. For example, successful dashpots have been constructed using plastic balls and metal cylinders. The plastic ball having the advantage of being considerably less expensive and having a substantially higher thermal coefficient of expansion which permits even larger clearances to be used while still maintaining good results in the time delay over the extended temperature range.

An understanding of the particular nature of the flow passed a ball or axisymmetric 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 flow 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 textile treatment of creeping motions in BOUNDARY LAYER THEORY-McGraw-Hill Series of Mechanical Engineering by Dr. Hermann Schlichting-Fourth Editiom-Published by McGraw-l-lill Book Company, Inc., New York, NY, and particularly Chapters 1V and V1 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 charac terized 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 between 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 pressure 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 radial 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 0.01 or smaller for a liquid. For a modified liquid, however, it has been found that the range of clearances over which flow in the lubrication region is believed to exist is greatly extended. For example, for a ratio of the radial clearance to the radius of the piston of 0.2, there still remains a strong dependence of the time delay on the radial clearance.

It is, therefore, an object of the present invention to modify the liquid used in the liquid annular orifice dashpot to eliminate 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 modified liquid dashpot of the subject invention has been successfully utilized in a short term time delay system. In this particular application, the piston travels a distance of 0.20 inch in approximately 8 seconds to provide the delay. Although several methods may be used to exert a force on the piston, it has been found eminently satisfactory when using a ball piston to press on the ball with a rod having a slight angle so as to tend to force the ball against the side of the cylinder. It has been found that a great variation in the rate of piston travel occurs when the piston travels down the center of the cylinder as opposed when it travels against the side. For this reason it is important that the relative orientation of the piston in the cylinder remains constant in order to achieve constant time delays. This could be achieved with a cylindrical piston by cocking the piston in the manner described in the pending application, Ser. No. 878,703, filed Nov. 21, 1969, now Pat. No. 3,553,954 entitled Dashpot Timer Having a Cocked Piston.

Any one of a variety of the standard soaps which are used to form greases from silicone or petroleum fluids could be used to modify the fluids. Similarly, since it is the physical properties of the powders which seem to be the most important, any one of a variety of powders could be used to achieve specific effects. Teflon powder, for example, has been shown to improve the low temperature properties of the base fluid. Zinc oxide powder has performed well over the entire temperature range whereas silica powder when added to a silicone fluid has a more drastic modifying effect due to the affinity of silica and the silicone fluids. The amount of the powders added and the base viscosity of the fluid to which they are added is dependent upon the given application and, accordingly, can be readily determined. For example, a commercially available compound, Dow Corning l25U, a low-viscosity silicone rubber gum having powder additives added by the manufacturer, has proven eminently satisfactory when used with a /8- inch I.D. glass cylinder and a 0.005 inch smaller in diameter steel ball, to achieve a time delay of 4 seconds for a travel of 0.120 inch with a 10 angle on the pushing rod.

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 position 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 modified 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 an angled pushing rod 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 modified 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 annular orifice through which the modified 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 includes a cylinder 12 in which a piston 14 is slidably disposed. The dimensional tolerances of the interior wall 16 of the cylinder 12 and the exterior wall of the piston 14 provide for an annular orifice 20 through which a modified liquid is adapted to flow.

The cylinder 12 is closed at one end by the plate 22. A modified liquid 28 having prescribed properties fills the remaining portion of the cylinder.

A plunger 32 having an angled face is mounted externally of the cylinder. 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 enters the cylinder and strikes the piston 14. By design, the force exerted by the spring 34 is somewhat larger than needed to overcome the threshold shear stress of the modified liquid causing it to begin flowing 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 40 or the self-destruction mechanism of a mine or hand grenade.

For most devices contemplated, the radius would be less than about 1 inch. The pressure could vary from about 5 p.s.i. to several thousand p.s.i. The radial clearance would probably never exceed 0.2 time radius of the piston. The viscosity of the base fluid could vary from 1 centipoise to over million centipoises or 0.65 centistokes to 100 million centistokes.

For many applications, 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 especially since for some applications the tolerances could be greatly relaxed. Similarly, the end plate 22 could be made from glass or metal such as aluminum. The plate may be integrally formed with the cylinder or may be bonded to the end of the cylinder.

In summary, the present invention accomplishes and contributes the following advantages to the dashpot timer art:

1. Position of ball travel: The ball will travel faster 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.

2. Temperature compensation: The viscosity of the best fluids change by a factor of about 35 to I over the temperature range 65 F. to F. Even over a very limited temperature range, the viscosity changes about l%/ F. to l.5%/ F. Consequently, if accuracy is to be achieved even over limited temperature ranges, some degree of temperature compensation is necessary. It has been determined that through the addition of modifying agents such as soaps or finely divided powders, that temperature compensation can be much more effectively achieved and at a wide variety of clearances as opposed to the single optimum clearance for the liquid annular orifice dashpot timer of the parent application.

. Piston rates: For most of the devices of the present invention, the rate of travel of the piston will be on the order of l microinch per minute to 1 inch per second.

4. Physical size: With the present invention, time delays are achieved which are over a significantly wider range than has ever been practical before.

5. Sealing: With the use of modifying agents, the need for seals is completely eliminated.

6. Military applications: Based on present knowledge, no modified 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. Clearances: Based on present knowledge, no temperature compensated liquid or modified liquid dashpot has been devised using clearance greater than 0.001 inch thus permitting a general relaxation in the precision required in the piston and cylinder.

8. Materials: Based upon present knowledge, no liquid or modified liquid annular orifice dashpot has been devised wherein the piston material is plastic.

9. Modified liquid: Based upon present knowledge, no liquid annular orifice dashpot timer has ever been devised wherein the liquid has been modified by the addition of a gelling agent, soaps or finely divided powders.

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 modified liquid annular orifice dashpot timer comprismg:

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;

a modified liquid comprising a base liquid 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 predominant over inertial forces;

and the piston rate of travel through the modified liquid in the cylinder being very small and the motion of the modified liquid as a result of piston travel being relatively very small, with the modification of the base liquid being accomplished by the introduction of an additive, the additive being selected from the group consisting of powders, soaps, gelling agents and thickening agents.

2. The invention in accordance with claim 1 wherein the piston is substantially spherical.

3. The invention in accordance with claim 2 wherein the radius of the piston is less than about 1 inch.

4. The invention in accordance with claim 2 wherein the radial clearance between the piston and cylinder is less than 0.2 times the radius of the piston.

5. The invention in accordance with claim 2 wherein the viscosity of the base fluid is of a value from 0.65 centistokes to 100 million centistokes.

6. The invention in accordance with claim 5 wherein the base liquid is selected from the group consisting of rubber gums ranging in viscosity from 1,000 to over 100 million centistokes and silicone fluids ranging in viscosity from 0.65 centistokes 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 modified liquid upon the rate of travel of the piston resulting from changes in temperature, such means embracing a ratio in excess of 5 to l of the radius of the piston to the mean radial clearance between the piston and cylinder.

8. The invention in accordance with claim 7 wherein the viscosity of the base liquid changes by a factor of 35 to 1 over the temperature range of 65 F. to +160 F.

9. The invention in accordance with claim 1 within the piston is substantially cylindrical 10. The invention in accordance with claim 9 wherein the radius of the piston is less than about 1 inch.

11. The invention in accordance with claim 9 wherein the mean radial clearance between the piston and cylinder is less than 0.2 times the radius of the piston.

12. The invention in accordance with claim 9 wherein the viscosity of the base fluid is of a value from 0.65 centistokes to I 100 million centistokes.

13. The invention in accordance with claim 12 wherein the base liquid is selected from the group consisting of rubber gums ranging in viscosity from 1,000 to over 100 million centistokes and silicone fluids ranging in viscosity from 0.65 centistokes to 2.5 million centistokes.

14. The invention in accordance with claim 13 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 5 to l of the radius of the piston to the mean radial clearance between the piston and cylinder.

15. The invention in accordance with claim 14, wherein the viscosity of the base liquid changes by a factor of 35 to I over the temperature range of 65 F. to +1 60 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.

17. The invention in accordance with claim 2 wherein a rod is used to transmit the force to the ball piston with an angle on the face of the rod so as to urge the ball piston against the side of the cylinder.

18. The invention in accordance with claim 17 wherein the angle on the rod is less than 30.

19. A modified liquid annular orifice dashpot timer comprising:

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;

a modified liquid 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 predominant over inertial forces;

and the piston rate of travel through the modified liquid in the cylinder being very small and the motion of the modified liquid as a result of piston travel being relatively very small, with the modification of the liquid being accomplished by the introduction of an additive, the additive is a powder chosen from a group having a maximum particle size of less than 0.000040 inch.

20. The invention in accordance with claim 19 wherein the powder is Teflon.

21. The invention in accordance with claim 19 wherein the powder is silica.

22. The invention in accordance with claim 19 wherein the powder is a metallic oxide.

23. A dashpot timer for use in an ordnance fuse wherein means are provided for including as a timing element the liquid annular orifice dashpot timer defined in claim 1.

24. The invention in accordance with claim 1 wherein the piston is free from any spring-biasing pressure being exerted thereon.

25. The invention in accordance with claim 1 wherein the modified liquid possesses a flow threshold below which it and consequently the piston remains stationary in the cylinder.

26. The invention in accordance with claim 1 wherein the clearance between piston and cylinder is greater than 0.001 inch.

27. The invention in accordance with claim 26 wherein the clearance is at least 0.02 inch and larger.

28. The invention in accordance with claim 1 wherein the dashpot timer is free of any seals for the liquid.

Claims

1. A modified 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; a modified liquid comprising a base liquid 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 predominant over inertial forces; and the piston rate of travel through the modified liquid in the cylinder being very small and the motion of the modified liquid as a result of piston travel being relatively very small, with the modification of the base liquid being accomplished by the introduction of an additive, the additive being selected from the group consisting of powders, soaps, gelling agents and thickening agents.

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 modified liquid upon the rate of travel of the piston resulting from changes in temperature, such means embracing a ratio in excess of 5 to 1 of the radius of the piston to the mean radial clearance between the piston and cylinder.

8. The invention in accordance with claim 7 wherein the viscosity of the base liquid changes by a factor of 35 to 1 over the temperature range of -65* F. to +160* F.

9. The invention in accordance with claim 1 within the piston is substantially cylindrical

10. The invention in accordance with claim 9 wherein the radius of the piston is less than about 1 inch.

12. The invention in accordance with claim 9 wherein the viscosity of the base fluid is of a value from 0.65 centistokes to 100 million centistokes.

14. The invention in accordance with claim 13 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 5 to 1 of the radius of the piston to the mean radial clearance between the piston and cylinder.

15. The invention in accordance with claim 14, wherein the viscosity of the base liquid changes by a factor of 35 to 1 over the temperature range of -65* F. to +160* F.

18. The invention in accordance with claim 17 wherein the angle on the rod is less than 30*.

19. A modified liquid annular orifice dashpot timer comprising: 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; a modified liquid 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 predominant over inertial forces; and the piston ratE of travel through the modified liquid in the cylinder being very small and the motion of the modified liquid as a result of piston travel being relatively very small, with the modification of the liquid being accomplished by the introduction of an additive, the additive is a powder chosen from a group having a maximum particle size of less than 0.000040 inch.

20. The invention in accordance with claim 19 wherein the powder is Teflon.

21. The invention in accordance with claim 19 wherein the powder is silica.