US2609752A - Delay electric blasting cap - Google Patents

Description

INVENTOR.

John E. Jeffery J. E. JEFFERY DELAY ELECTRIC BLASTING CAP Flled July 12, 1946 Sept. 9, 1952 Patented Sept. 9, 1952 UNITED STATES DELAY ELECTRIC BLASTING CAP John E. Jeffery, Shelton, Wash., assignor to Atlas Powder Company, Wilmington, Del., a corporation of Delaware Application July 12, 1946, Serial No. 682,995

6v Claims.

This application relates to delay electric blasting caps.

Antobject of the invention is the provision of a delayelectric blasting cap having a very short delay time.

Another object of the invention is the provision of a delay electric blasting cap having a short delay period and a low standard deviation.

A further object of the invention is the provision of delay electric blasting caps having standard deviations within relatively closely de-y terial located between the electric ignition element and the detonating charges in the cap. The train of deflagrating material, by the time it takes to burn, produces the desired delay pe-- riod between the firing of the ignition element and the detonating charges in the initiator.

There have been two principal types of delay electric blasting caps utilized to date.

Since black powder produces large quantities of gas on-burning, conventional delay electric blasting caps provided with black powder delay trains additionally include vents in the shells to provide an exit for these combustion gases. To prevent premature firing of the explosive charges to be initiated by hot gases from the delay train, vented delay electric blasting caps are usually further provided with flame damping shields of various sorts. l

Another type of delay electric blasting cap is the gasless type. Such initiators make use of delay trains which burn without the production of. large quantities of gas and, consequently, require no vents.

Neither of these types of delay electric blasting caps has been successfully produced to give delay periods of very short duration. Most commonly they have been made in delay periods of one second or more, though some shorter fractional second delays have been made in the vented type.

When it is attempted to make vented very short delay electric blasting caps with gas pr ducin Of these, one type employs a black powder delay train;

delay trains, it is found that the trains required are so short that the flame from the electric ignition element often flashes through the delay train practically instantaneously or the delay train is blown out'of position, and, in any event, no consistent delay times are obtained.

The problem is quite similar when it is attempted to make very short delay electric blasting caps of the ventless, gasless type. Such small amounts of delay powder are required that the flame flashes through the train causing instantaneous or highly ragged firing. Furthermore, delay electric blasting caps having gasless type delay trains do not ordinarily provide as consistent timing as do the vented delays having gas producing delay trains, and when it is attempted to produce short delay electric blasting caps this raggedness becomes proportionately much too high. By means of the present invention it has been found that delay electric blasting caps can be prepared, having average delay times of from about 10 to about 250 milliseconds, and standard deviations from their average times between about 6 milliseconds and about milliseconds.

Delay electric blasting caps of the present invention utilize a train of propellent powder consolidated under high pressure and located inside a ventless shell. Preferably the delay train is contained in the bore of a tubular delay element located between a deflagrating' ignition element and a detonating explosive.

The invention will be further described i connection with the drawings in which:

Figure 1 is a cross sectional view ofa specific delay electric blasting cap which has an average delay period of about 40 milliseconds.

Figure 2 is an elevation, partly in section, of a specific delay electric blasting cap which has an average delay period of about 115 milliseconds. j

Referring to Figure 1, into a drawn gilding metal shell 0.260 inch in inside diameter and approximately 0.007 inch thick represented by H], a base charge [2 of tetryl, a primary detonator charge I 4 of nitromannite, and a flash charge l6 composed of diazodinitrophenol and 25% nitromannite are pressed in conventional manner under an inner capsule 18. A brass delay tube 20, 5% inch high, is pressed in tight fitting engagement into shell I0 above inner capsule l8. Delay tube 20 is provided through its center with a inch diameter bore 22 into which is pressed a charge 24 of 0.04 gram of granular 3096 mesh black powder of 1.84

" of attachment to electric match 39.

to 1.37 grain density, under 450 pounds pin pressure(3l,000 pounds per square inch) to a height of 0.068 inch. A conventional electric match ignition assembly is'located over delay tube This assembly is composed of insulating tube 26- into which sulphur 28 is cast around electric 'match 38 bearing copper acetylide type match head 3! and to which: are attached leg'wires 32 and, 34 which are insulated above their point Over this I :iiagrating delay powdersgburn more rapidly under of operation, it maybe pointed outthatthe delay electric blasting caps of this invention appear to function by a combination of two actions both of which depend upon ventless construction; Under the closed conditions, the heat and gas production from the burning'elements de-j velop high gas pressuresin, the caps.'- Since dehigh pressure conditions, longer'delay trains may ignition assembly are two poured sealing plugs,

35 of asphaltic material and 38, of sulphur.

be employed than would otherwise produce the desired short delay periods.

"trains, flashing through the delay trains ispre- Shell It is provided with conventional bead 4t which assists in anchoring the sealing plugs.

The space shown around match head 3| and in bore 22 above delay train 24, amounts to about In the operation of the cap shown in Figure 1 a current is passed through leg wires 32 and 34 i I to electric matchtll, firing match head 3!, the

flame from which ignites black powder train 24 which in turn initiates flash charge 16 to cause detonation of the cap. These delay caps are found to have a delay period averaging about 40 milliseconds and a standard deviation of about 16 milliseconds. 1

Standard deviation is a statistical term which is a measure of the dispersion of values about an average. Specifically it is the root mean square deviation, or in mathematical terms ,where d is the deviation of each specimen of a sample from the average and n? is the number of specimens in the sample. v,

The standard deviation of the caps just described was determined by firing 419 of the initiators, averagingthe delay times obtained, andcalculating the root mean square deviation of the delay times from the average delay time.

g Their standard deviation isa particularlyvalw able characteristic of the caps of this'invention.

' ing cap detonation; Thus it may be seen that in" a the present delay caps the delay period appears,

vented and more, consistent timing maybe obnamed. The high gas pressures developed also tend to force the'burning delay trains out of the bores of delay elements, when they are employed, and into the detonating explosives. Such blowing out of delay trains is of course undesirable until the desired delay times have beenobtained. In the present caps the high consolidation of the delay powders in the bores 'of the;

delayv elements causes the trains to resist blowing out for a'time suificient to provide the desired delay; However, after the delay trains have burned away a suihcient amount of delay powder,

and after pressures have built up sufiiciently high,

the remaining portions'o'f the delay, trains are blown down into the detonating explosives insurat least in part,.to be caused by asort of valve action wherein the'delay train acts as aplug holding back the initiating force from the detonating explosive and then after a period of timethe detonating explosives. i a

To provid'ethe desired delay action the volume releasing so that the burning powder spews into of. the space above "the" delay train should 'be I sufiicientlysmall to permit development of the It can be shown by statistical methods that the difierence in firing time between any two of the present caps picked, at random will be expected in about 90% of the cases to be greater than 0.18 I

multiplied by-the standard deviation and in about 90% of the cases to'be less than 2.3 multiplied by the standard deviation. Co-pending application'Serial Number6'70,193, filed May 16, 1946, by David M. McFarland, for Process of Blastingdiscloses that peculiarly desirable efiects .are obtained when adjacent explosive charges areset oif withinfrom about 2 to about 150 milliseconds of one another. Consequently, in accordance with the above principles, these desirable elfects may be expected to be substantially obtained when delay electric blasting caps having standard deviations between about 11 milliseconds I and about milliseconds are employedlin. a row of bore holes. Caps of the present invention provide standard deviations within this range. A discussion of the statistical principles involved will be found in S'imons Engineer's Manual'of Statistical Methods, John Wiley & Sons, Inc., New York, 1941.

Without limiting the invention to any theory necessary pressures, but;v on the other hand, it should be large enough to prevent bursting of the shell and seal and consequent premature release of pressure.

The cap shown in Figure 2 is injge'neral similar tothat shown in' Figure lexcep't that it is de signed'for a somewhat longer delay time. Similar parts are numbered similarly in the two drawings.

. Delay train i thowe'ver, :is made up'of, 0.08 gram of the black powder and reaches to a height of 0.133 inch in the delay tube. Since this larger amount of powder produces considerably more gas than is. produced by the delay train in the foradditional resistance to plug blow-outs.

This initiator of Figure 2'has been found to,

give an average delay time of about milliseconds and a standard deviation of about 15 milliseconds.

Delay electric blasting initiators having other delay periods than those obtained according to the specific embodiments just described maybe made by varying the delay train length and by varying the'amo'unt of free space in communication with the delay train and with the ignition element. The invention will be found applicable to the production of caps having average delay With longer delay periods as low as about '10 milliseconds; and as high as about 250.- millisecondsi l It is of. assistance. to thetproper functioning of thecaps of thetype' just described that there. be a tight joint between: the delay tube and thegshell walls Gtlierwise, it maysometimes be found that the flame from. the. ignition. element bypasses: the delay train and. directly ignites. the explosivesiin the base of the" cap. Usually. it is preferred. to obtain a tight 'seal,"as'in.the= specific should be..;sufliciently. strong that the pressures built up inside theinitiators are resisted until after." the delay action has taken place. It. is readily apparent that many different types of sealsimaybe used,;and'those shown in the drawings ar'e only"illustrative.. For. instance, rubber plugscrimped into thexinitiator shellsmake excellent seals.

The invention may be practiced with different types of propellent powders. As examples'of propellent powders, in additionto the blackpowder' employed. in. the specific embodiments: described above, may be mentioned other granulation'siand densitiesof black powder, or smokelessnpowder of. the. single base or double base variety in. either grain or stick form. More specifically a delay initiator having an average delay time of about 40. milliseconds may be prepared: by pressing 0.05 gram of 18 mesh single base' smokeless powder under 400 pounds pin pressure..;(28 ,000 pounds per square inch) 'into pressures as high as about 40,000 pounds perplosives; This consolidation of the delaytrain is not only important tdth e' obtention of sufficient delay times but also to the obt'enti'on of consistent delay times; Some'variation in delay times may be obtained by variation in the consolidation' pressures employed, the higherconsolidation pressures .producing' longer" delay times.

The cross sectional area of the delay train and its height generally contribute to the obtention of consistent delay times. If the cross sectional area of the delay train is greater than about 0.05 square l inch; the" timing may tend to be erratic, and to obtain consistent timing and sufficient retention of the delay powder in the tube, it has been found that heights of delay powder of at least about inch are often necessary.

The delay tube is preferably made of metal but other rigid materials may be used, it being desirable that the delay tube be sufiiciently rigid so that the delay powder will remain adequately consolidated in the bore. Often, and particularly with the wider bore diameters, it is found desirable to constrict the lower end of the bore slightly as by providing the bore with a slight taper or with a small shoulder at its base. Thus bore 22 in the delay tubes shown in the drawings has a diameter of about 0.141 inch at its top below the flaring portion and about 0.138 inch at its base. A constricted bore makes for consistent delay times as it reduces variations in the time at which the delay powder will be blown from the delay element.

As has been outlined above the volume of space in the cap in communication with the gas-pro:- ducing elements is important since it determines the pressure under which the delay train burns and a high pressure is necessary to obtain blowing out of the delay train and to obtain burning 7 speeds sufficient that enough delay powder can The sealing means and the shells employed the: delay tube. of Figure12. to a height of 3% inch. .In someiinstances andparticularly when delay electric blasting" caps of the present invention are to beprepared having delay times near the. upper endof' the range,: propellent delay powders may be mixed with inertmaterials or slower burnin materials prior to consolidating them in the. delaytrains.

Conversely, accelerating materials may be incorporated when the shorter. period delay capsare to be prepared.

It will also be readily apparent that the inventionis applicable to: caps of many different sorts. For example, shells of. different materials such as aluminum or plastic compositions may be used provided they are sufiiciently strong to resist bursting before desired delay times are attained. Also many different explosive materials wither without inner capsules may be used in the initiators of the invention. For example, trihi-trotoluene or pentaerythritol tetranitrate base charges may advantageously .be. used, as'may diazodinitrophenol. or lead azide primary detonator compositions and various flash charges such as mercury fulminate may also be employed. If preferred the flash charge may be eliminated with some primary detonator compositions, or the irivention may be used with single charge initia ors.

Similarly many types of ignition compositions may be used such as lead styphnate or diazodinitrophenol. Even gasless ignition elements may be employed providing the space in the cap in communication with the combustible delay train is made sufiiciently small to compensate for any reduction in the amount of gases produced. It also is readily seen that the invention is applicable to different types of ignition elements as, for example, the bridge plug type or the bead type commonly employed in electric blasting caps.

What is claimed is:

1. A delay electric blasting cap comprising a ventless shell, a detonating explosive in said shell, a deflagrating electric ignition element in said shell, a charge of defiagrating propellent powder selected'from the group consisting of black powder and smokeless powder consolidated under a pressure of at least approximately 7,000 pounds per square inch and positioned in plug-like engagement within a passageway between said detonating explosive and said ignition element, means defining an enclosed space in communication with said deflagrating propellent powder on the side thereof away from said detonating explosive, the volume of said space and the amount of said deflagrating propellent powder providing, upo'n burning of said ignition element and said deflagrating propellent powder, detonation of said cap between about l milliseconds and about 250.

milliseconds of the time of firing said ignition element, a seal closing said shell, and. said shell and said sealhaving a strength sufiicient to prevent their burstingprior to the detonation of said cap. i

2. delay electric blasting cap according to claim'l in whichhthe ignition element is position'ed to initiate .directly deflagration of. said deflag rating propellent powder. V 1

3. A delay electric blasting cap comprising a ventless' shell, a detonating explosive in said shell,

a defiagrating electric ignition element in, said shell, a rigid tubular' element fitting tightly in said shell and'positioned between said detonating explosive and said ignition element, a deflagrating propellent. powder selected from the group consistingof' black powder and smokeless powder in plug-like engagement in said tubular element consolidated under a pressure .of at least approximately 7,000 :pounds per square inch, means defining an enclosed space in communication with said deflagrating propellent powder, the volume of said space'and the amount of saiddeflagrating propellent powder-providing, upon burning of said ignition element and said defia'grating propellent powder, detonation of said cap between about milliseconds and about'250 milliseconds of the time of firing said ignition element, a seal closing said shell, and said-shell and said seal having a strength sufficient to prevent their burstingprior to detonation of said cap. v

4.JA delay electric blasting cap according to 'claim' 3 'in which the defiagrating "propellent powder is black powder.

55A delay electric blasting capcomprising' a ventless shell, a detonating explosive in said shell,

a deflagrating electric ignition element in said shell, a rigid tubular element fitting tightly in saidshell-and positioned between said detonating explosive and said'ignition element, a charge of black-powder inplug-like engagement .in said tubular element consolidated under a pressure of atileast approximately 7,000 pounds per'square inch to a heightof at least approximately 3 2111011,

means defining an enclosed space in communication with said black powder,- the volume of said c space and the amount of black powder providing, upon burning of said ignition element and said black powder, detonation of said cap between about 10 milliseconds and about 250'milliseconds after the firing of said ignition element, a seal closing said shell and said seal and said shell being suificiently strong to prevent their bursting prior to detonation of said cap.

6. A delay electriciblastingl cap according'to" claim 5 in which the bore of said tubular element is smaller at-it"s base than at its top.

[JOHN E. JE E Y. ,REF ERENcEs CITED f 1 "The following references are of record in the file of this patent:

I I N TEnsTATEs rArENirs Number V OTHER REFERENCES I plosive Arthur Marshall, London 1915,'page

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