_‘_&¢.:W.‘A‘WmJ¥-Zgfi Q ban.-- M" h «‘25 ‘- N... 2“ ‘i‘. ‘1» 41““ ~ W 53“" ‘5 I'm" 7r”?- SOME ENGINEERING FEATURES OF A MODERN BREECH :\'\ECHA:\IS:\1 Thesis for Degree of M. E. Henry Jacob Schneider 19.51 \IHIIHHIWlllllllllHHlUlHHHlllHllIl\llilllulllHlHl 31293 01577 9535 PLACE IN RETURN BOX to remove this checkout from your record. TO AVOID FINES return on or before date due. DATE DUE DATE DUE DATE DUE MSU Is An Affirmative Action/Equal Opportunity Institution gunman-9.1 SOME ENGINEERING FEATURES OF A MODERN BBEECH MECHANISM Thesis for Degree of I. E. Henry Jacob gohneider 1931 THESIS ‘ SOME ENGINEERING FEATURES OF A MODERN BREECH MECHANISM The term breech mechanism may be defined as an apparatus by means of which the breech end of a gun is opened in order to load the amunition and also closed and locked when the gun is to be fired. The gun is essentially a hollow cylinder and -the breech mechanism is used for Opening and closing the rear end 01' the gun into which the projectile and powder are inserted. In the older forms of cannon the rear end was closed. These. therefore. required no breech mechanisms. This type of cannon was loaded from the muzzle end. This was a laborious and slow method. The need for greater speed in firing brought about the 0pm ended gun with its breech mechan- ism. The earlier forms of breech mechanisms were very simple. inefficient and slew of cpera- tion. In great cmtrast to these. the heavy power cperated breech mechanisms of today appear. The progress and develOpment in this branch or ordnance is the result or years or study. re- search and experimentation. The modern breech 10353:) mechanism has reached a high state of perfection and has kept pace with the great progress which has been made in other lines Of’mechanical engi- neering. In small fire arms a great variety of breech mechanisms are used and many automatic features are employed which are not suitable in larger guns. For the latter class dependability. sim- plicity. speed. and ease of Operation are essen- tial features. The various parts. although de- signed tO withstand severe stresses and rough usage. must. nevertheless. be well balanced and carry no excess weight. No undue time should be required for the Operation of Opening and closing the mechanism. Parts requiring fre- quent adjustment and attention can not be util- ized. The mechanism must have a positive action. be easily Operated and dependable. It must be fool proof and safe in the hands of the Opera- tors. It must be easy to disassemble so that thejparts may be readily inspected. To give a detailed description of a.modern breech mechanism is beyond the scope of this article. It is the intention of the writer to present a few Of the engineering problems which are encountered in the design Of a modern breech mechanism of a major caliber gun. \ There are two principal types Of mechanisms in use today viz: the sliding block type and the interrupted screw type. In the former the breech block which closes the rear of the bore against the force of the charge. slides trans- versely to the axis of the bore on suitable guides. The second type employs a breech block. called a plug. having interrupted threads around its circumference which engage similar threads in the rear end of the bore of the gun. The plug moves in the prolongation of the axis. The latter type only will be treated in this ar- ticle. The screw threads are an important feature in this type of. mechanism. They will be treated in detail later. A plug with a continuous thread engaging the threaded bore in the rear of the gun is not adaptable to a speedily Operated breech mechanism because of the great number of revolu- tions necessary to engage or disengage the plug from the gun. The time required for Opening and closing the mechanism would be prohibitive. The - Operating time must be reduced to the minimum. This is accomplished by employing what is known as interrupted threads. Threads are not cut around the whole periphery Of the plug and gun. but the circumference is made up of several threaded bands and blank bands arranged alter- nately and extaiding parallel to the axis. For example. the circular arcs Of quadrants l and 3 will be threaded while those Of quadrants 2 and 4 will have the threads removed. This is true Of both the plug and the gun. By means of this arrangement only ninety degrees Of rotation Of the plug are required for engagement or disen- gagement of the threads. Thus instead of a great number of revolutions of the plug being required to release it from the gun only one- fourth of one revolution is necessary. Figures 1 an! 2 illustrate this principle. A Pig. 1 shows the plug A with threads re- moved in quadrants l and 3. rig. 2 shows the gun. B. with threads rmoved in quadrants 2 and 4. It will be seen that the plug. A. may be inserted in the gun. B. if the horizontal axes . x-x are coincident and the Y-Y axes of figures l and 2 coincide. When the plug is thus in- serted in the gun its entire length am thus rotated. the threads of the plug will engage those Of the gun. By continuing this rotation through ninety degrees the 1-]: axis Of the plug will coincide with the 1-! axis of the gun. all the threads will be hearing throughout their en- tire length and the mechanism will be locked. The design produces a mechanism which can be quickly Opened and closed. but half of the bearing threads have been rancved to provide clearance. Thus fifty percent of the original bearing surface has been sacrificed. In order to regain this the length of the plug must be doubled. This will require a very heavy plug which will be cumbersome to Operate. This de- sign is used for small mechanisms but is not practical for large sizes on account Of its inefficiency and the great weight necessary to produce a plug having mfficient thread area tO stand the firing loads. A mechanism of greater efficiency is pro- duced by reducing the length Of the blank arcs. thus utilizing more of the periphery of the a: maul plug for threads. This is accomplished by the use Of stepped threads. The plug and the bore at the rear of the gun are slotted to form num- erous sectims. let us say. for example. twelve sectors. These twelve sectors will consist of four groups. each containing one blank sector and two threaded sectors one Of which is Of a little greater radius than the other. The blank sector is made a little wider than the threaded sectors to permit clearance for the withdrawal of the plug. Virtually each quadrant is divided into threaded sectors A and B am a blank sector 0 as shown in Pig. 3 which represents the plug. rig. 4 represents the gun. Threaded sector A has a radius 2 Ba. threaded sector B has a red-- ius Rb and blank sector 0 a radius Ra. Let the thread depth be represented by h_. Row B], is rude less than Re by an amount a little greater than 2. the depth Of the thread. an} B.3 is smaller than Rb by the same mount. The bore at the rear end of the gun is ma- chined in the same manner except the blank sec- tor. D. is cut to a radius a little greater than Ba. By this arrangement it is necessary to re- tate the plug only 30° tO engage or disergage the threads. The plug A can be inserted in the gun B if the 1-1: axes rmain coincident an! the Y-Y axis of the plug coincides with the Y-Y axis of the gun. Thus sector A of the plug enters into the blank sector D of the gun. the sector B of the plug enters into sector A of the gun and sector 0 of the p113 enters into sector B Of the gun. When the plug is thus inserted in the gun its entire length and rotated thirty degrees all Of the threads will be bearing throughout their entire length and the mechanisms will be locked. The improvement Of this design over the pre- vious design is obvious. Only 30° of rotation are necessary instead of 90°. The total width of the blank sector has beat reduced from 2 x 90° 8 180° to 4 x 30° - 120°. In other words. 360° - 120° 3 240°. approximately. or 2/3 or the cir- cumference of the plug is threaded and provides bearing area to carry the firing load. The efficiency Of this plug. disregarding all clearance cute. is approximately 350° - 120° = 240° . .66 2/3 "3565' 36'6" while that Of the first design was approximtely 360° ~ 180° . 180° 3 .50 335° 36'6" This type Of mechanism may be further im- proved by carrying this same idea of reducing the width Of the blank sector still farther and by increasing the number of stepped sectors. For example. let the face of the plug and gun be divided into three groups. each containing (me blank sector and three stepped threaded sec- tors Of different radii. The blank should be a little wider than the threaded steps to permit clearance for withdrawing the plug from the gun. This arrangement is shown in Figs. 5 and 6. Fig. 6 representing the plug and Pig. 6 the gun. Each group or sector Of 120° consists Of three stepped threaded sectors A. B an! 0 am one blank sector D. The radius R], of the threaded sector B is again made less than R3. the radius Of the threaded sector A. by an amount a little greater than the depth Of the thread h. The radius Rc of sector 0 is made less than R}, by the same amount. Rd. the radius Of the blank sector D is similarly less than R0. The bore - at the rear and of the gun is machined infidefll same manner except that the blank sector‘\E is slotted out to a radius a little greater than R. to provide clearance for the plug. The plug A cm new be inserted into the gun B as in Figs. 3 and 4 above if the 1-1 axis and the Y-Y axis of the plug coincide respectively with. the X-I axis and the 1-! axis of the gun. Thus again the threaded sector A Of the plug enters into the blank sector . B. Of the gun; the threaded sector. B. of the plug enters into the threaded sector. A. Of the gun and the threaded sector. 0. Of the plug enters into the threaded sector. B. of the gun and finally blank sector. D. Of the plug alters into the threaded sector. 0. of gun. When the plug is thus inserted its entire length and rotated in a clockwise direction the threads Of the various steps of the plug will en- gage the corresponding stepped threads in the gun. When the plug has been rotated about 30°. all of the threads will be bearing throughout their en- tire length and the mechanism will be locked. Let us compare this design.with the preced- ing one. Here we have the face Of the plug di- vided into three parts of the 120° each. Each one of these parts is divided into four sectors of approximately 300 each. The total width of the blank is 3 x 30° : 90° approximately 360° - 90° 2 270° approximately or about t of the circumference Of the plug is threaded while i-is 'blank. The efficiency is 360° - 90° 3 .75 which is an improvement of 26% over the preceding one. By increasing theznumber Of stepped sectors. the lengths of the threaded ones become shorter. Thus by continuing this process the blank arcs will become very short resulting in a plug of maximum threaded circumference and a consequent reduction in the length of the plug. This means that a lighter. more efficient and more easily operated mechanism can be produced. Practical considerations. however. will determine the max- imum number of threaded steps which canfibe used in any design of a breech mechanism. The machine Operations necessary for cutting the threads on these stepped arcs are laborious and costly. It is clear that in order to insure -10- an even distribution of load the workmanship on the threads Of both plug and gun must be very accurate and in order to maintain this distri- bution the material must be Of uniform quality and hardness throughout. The cost of fabricat- ing a stepped thread mounts rapidly as the number of steps is increased. There is a practical limit beyond which the refinement Of increasing the number of threaded steps should not be carried. Clearance Cuts. Thus far we have considered only the engage- ment of the plug threads and the amount Of rota- tion of the plug which is necessary to free it from the gun. In order to Open the end of the gun so that ammunition may be inserted. the plug must be moved out of the way after it has been disengaged from the gun. In this type Of mech- anism this is accomplished by supporting the plug on.a sort Of hinge which is commonly called a car- rier. This carrier which pivots about a bracket bolted to the outside of the gun is provided with a spindle upon which the plug is mounted so that the plug can be rotated and its threads be disen- gaged from the threads of the gun. When thus freed the plug can be swung out Of'the gun by means of the carrier. leaving the end of the gun Open for inserting the ammnition. The path Of the plug when swinging out of the gun necessitates the removal of some Of the metal from both the plug and the gun in order tO provide clearance. In order to sacrifice as little metal as possible. these swing clear- ances are calculated very accurately. .Iuch study and careful panning are required to es- tablish the most satisfactory locations Of the stepped threads and blanks in relation to the center of the hinge in order to keep the clear- ance cuts down to the minimum. When metal must be sacrificed for'clearance. investiga- tions are made to insure that only the least de- sirable metal is rennved. For example. clear- ance cuts. if possible. should be made on the blanks rather than on the threads. The location of the hinge center is import- ant. It has a decided bearing on the nature of the clearance cuts. 11k --- —— -——J FK377 Pig. 7 shows a sectional view through the breech end Of a gun. plug anl carrier. A rep- resents the plug which is mounted axially on the spindle. S. Of the carrier. 0. The carrier is hinged to the gun at D. I is the distance from the center of, the hinge to the center line -13- Of the gun. .1 is the distance from the center Of the hinge to the rear face of the gun. I rep- resents the length Of the plug. By making the distance Y large. the radius to the extreme point of the plug. such as n. will be long. producing a somewhat flat arc and a shallow depth Of clear- ance cut at h. The distance I”. however. must not be made too long lest a long and weak carrier. which is easily deflected will be the result. Furthermore. the plug will swing on a long arm requiring much space at the end Of the gun when the mechanism is in the Open position. ’The dimension.X is also important. If X approaches the length. 2* then the depth of the cut at h_approaches zero. At the same time. however. the clearance cut in the gun which is required by the point.‘t. becomes deeper. Here again the importance of keeping the length of the plug at a minimum becomes evident. The shorter the plug is made. the smaller the clearance cuts become. Hush thought and study should be devoted tO the determination of the location Of the hinge center because of its influence on the clearance out a. Determination Of Swing Clearances. Having established the location of the hinge center. the next problem is to determine the clear- ance cuts which are necessary in order that the plug may swing clear of the gun. In determining the amount and location of the clearance cuts. the chief aim is to reduce to a minimum the cutting away of any threaded section. Studies and layouts are nude to determine the least Objectimable loca- tion Of these cuts. whether the metal be removed from the plug or the gun. Sometimes a compromise is made and metal is removed from each member. Here again considerations Of the nature and diffi- culty of the machine Operations involved are of ut- most importance. lest. in the effort tO gain effi- ciency an impossible or impracticable condition will be produced for the machinist. Often the advantage gained in maintaining a certain dimen- sion is not Justified by the complications and added co at which are involved. Let us now consider the plug A and its motion in swinging out of the gun. B. Fig. 8 shows the end view of the plug and gun and a vertical sec- w .. .. r... s . Pt ,- _.. ..~ ...._..H a... . . “she. .“Usw lud..lis e i. . .. Vs; Vie .. A .\ l a . 4 .. free- 0 Fa... .. .. .3 d The... A _.. . u. a. .5... i“ . .I. \l. u .s. '..‘.I u ,s V r e i with, ........_.e§ .. . ....,... .. “v... ,... - _Q——- . n . . . . aw w. .I I .l-.l.. [:9]? .ep .. .-”.sb ... . Z. a 3.1% eifiuxu. e a, aflLH "2...... fir...‘ .t. me. -—i ot ‘ I .is- a erbo‘t‘i'l' '- - 7‘ u.” “4". ....‘f‘.-.... .. a us. 44 n ”7W . esteem; Illll -I I III) | I l I 'Eii'fi s ' telll. I I Ir- eds" Illl‘III-elil I ’ e.. u. .0. 3.x r - .— ._ ( I ' I s ., . t I . J x e , I g . .e .t .\|\ . I .. “ll alt Ii. 1! ii Ii il. it It '- tion through the gun. 1-1! is the hinge axis about which the plug and carrier swing and Y-Y the axis upon which the plug rotates to engage and disengage the threads of the gun. The plug is shown in the unlocked position i.e. it has been rotated sO that the threads are disengaged and it _is now ready to be swung out Of the gun. The paths described by the points a. b. g. 2. etc. . on the plug will be considered as the plug is swung about the axis 11-1 in the act Of Open- ing the mechanism. The plug must have an unob- structed movement. Therefore. the path of every critical point of the plug must be investigated so that the interference. if any. may be removed. The radius or point a 3 Ba 'W where Xa is the horizontal distance Of point a from the axis 1—1 and Y‘. the vertical distance from this axis. The radius of point b : Rb : ‘\/X7B+Yb7- ; 1b. Yb being the horizontal and vertical distances Of point ‘3 from the axis. The radius Of point c = BO : \/ Xé-i—Ycz' and so on for all other critical points of the plug. Thus by making the vertical distances of the points a_' y g' in the gun from the axis 1-! a -15- little greater in length than the radii Ra. Rb, Bc. the depth of the clearame cut in the gun for the free passage of arc a b g of the plug is defined. In like manner the clearance neces- sary for the arc d g g and 3 h i. etc. . are de- termined. It should be pointed out here. that due consideration mat be given to the determi- nation of the difference in the radii of the successive threaded steps. The radius Of the» arcd_e_ f should be made shorter than that of the p 1‘0 I9 b _c_ by an amount which will enable the arc 2 lo IH Of the plug to swing unobstructed in the are d' 3' y of the gun when the p113 re- volves about axis 1-1. If this is done none Of the threads of these arcs will be sacrificed for clearance. If. on the contrary. the differ- ence in the successive steps is not made suffi- ciently large. a condition may arise in which the plug will be free in the gun when the threads are disengaged but when swung about the axis X-X the length of the radii Rd. Re. Rf. etc.. of the plug may be sufficient to cause interference with the gun. In this event either some of the threads must be cut away near the forward face Of the plug so as to reduce the radii Rd. Re. Rf or some of the threads will be sacrificed in the arc _d_ _e_ f of the gun. In this manner the paths of the limiting points of the plug must be anal- yzed and sufficient clearance provided. Whether these clearance cuts will be made on the plug or in the gun or on both will depend upon which ar- rangement will cause the least loss in thread area. Referring again to Pig. 8 note that the clearance cuts are shown in red. The area g'b_g. d g f_ and g h_ Of the plug clear the gun as shown. The difference in height of the successive stepped sectors was made sufficiently large so that no clearance cuts are needed at these points. The forward edge of the radial surface of the plug 5 _1_ interferes with the gun at k" 1' when the plug is swung on.the axis x-x. 4A out is made in both the gun and the plug in order to leave the thread area intact. The cut in the gun is shown at I" and in the plug at 57. In swinging the point. 9, of the plug necessitates the out in the gun which is shown at m'. The are p g . requires a cut in the gun at p' 1' and the point.j_:_. -13- a cut at 3'. These cuts are shown at 3f and if in the sectional view. Clearance cuts are also re- quired at the points 2 and y_. lletal is removed at the forward end Of the plug as shown at u? and I? in the sectional view. - When all Of the clearance cuts have been deter- mined. the total length Of thread which has been lost is computed and this amount is deducted from the total length of thread on the plug before clearance cuts were made. Thus the effective thread area is determined and calculations for strength are made. Screw Threads The form Of screw thread best adapted for this type of mechanism is the buttress thread in.which the pressure or bearing side of the thread makes the more acute angle with the normal to the pitch line. A normal section of the thread is shown in Pig. 9 below. p_represonts the pitch of the thread in inches; 2, the effective height; 5 the radius at tap and bottom of the threads: d, the thickness at the bottom of the thread. If we let L denote the length of the plug then L-+»p_will equal the number of threads in the length of the plug. Compute the length of one thread around the per- iphery of the plug by taking the sum of the lengths Of a single thread on each step. measured at the pitch line Of the thread. If this length of thread be designated by't, then the total length of active thread will be t_mnltiplied by the number of threads or 13(15): L75: . new if P 3 the total pressure in pounds which is exerted on the plug then P -% the length Of thread : 73‘3" L75: = 1E1: will represent the load exerted on each inch Of thread length. The bearing pressure per square inch Of thread will then be 1 ' ' P i, =22 - 138' C Ltc where c - the effective height Of the thread Stress in the Threads Considering a section of thread one inch in length let us analyze the stresses in section Id. The bending due to a counter-clockwise moment. M, will be e 1’ P S: __ 7’ M gab 0ft€(z+*)~§%b , now d,the thickness of the tooth.=h and b=i‘.I Pp(c+2 r) = :93.” therefore ‘2.L.t 6 or the stress per sq.in. S= aPpcc+zr) i_rd1’ The bending due to a clockwise moment will be = Sbh’i Sd". Fish" noh‘ M ‘s‘ “ a - L3 ' the stress per sq. in., 8,: éplig'éi‘flalf Compression will be PNfanfia) Ltd’ The resultant compression at x will then be 3ijc+2r)_ 6Pp(tana)1’- P tana Ltd'z- Ltd? + _EEEJ The resultant tension at y will be 3Pp(c+zr) __ 6Fp(tanajf_ P tana td‘ Lt LtdL L The shear will be LEE-E- :EE i.t Ci irtd Stress in Threads - Section v-w -21- Bending due to counter-clockwise moment will be M,= _h_h -_— EEC 6 ZLt the stress per sq. in., S): GPPC = 39ch ,. ZLtgL Ltgl Whue b= I and h= 5 Bending due to clockwise moment will be M: 5.1937; PJD(tana)e LI the stress per sq. in., S: 6PP(W’IQ)€ Ltgl Compress ion will be PP (tan a) Ltg The resultant compression at v will be SPpc, __ 6P£(tana)e + Pp(t'an a) Ltgz Ltqz- Ltg ' The resultant tension at w will be SPEC- 6PP(tan a)e_Pp(tanaj Ltq 2- Ltg L Uta The shear will be l:E.+.g'= I19 Lt‘g Stresses in Threads - Assuming the Load Equally Distributed. Tension. c'clockwise Compression. c'clockwise Tension clockwise Compression clockwise Compression radial Besul tant compression at X : Resul tant tens ion at Y a Section X-Y Section V~W At- x At Y At v At W 3? c.+zr 3PPC —E%1—L. ) Ltgz BPP C+Zr BPPQ 'L—Fé'L—J Ltd?- 6P tanaf We t Ltd]?— 6Pp§tana)f @PEitanaZe. Lt Li-gl F (tuna Fpfl'ana) Elegant!) PEfitanai ‘Pi—ta—J L. Ltq Ltg 3 Pp(c.+zr)_ EFL Hm + P tana Lid LT 3431:3(ct2r) __ 6(tan aw... tan za] 3 Ep(c+zr)_ ePpH-anafi... Ltd‘ Pyiana): 1: 31cm r1... efianafit. tan aJ Resultant compression at v 3 3}:ch _ 6Pp£§coctque¥thaqna§ a t {item «w Result ant tension - 3F c _6Fp(tana)e_P t a = at W .. -T.—tECTL Ltqu _Rét‘fiLl 1’ 3c,__ ektana e tang] DE, [7? a + Shear at x and Y : EB. Ltd Shear at V and W : Bearing Pressure {4&1 t? If these values shoe that the metal is stressed beyond the safe limit of the material then the shape of the thread must be changed to provide greater strength. It, for exasxple. the bearing stress is found excessive then the depth or the thread may be increased. This will add to the bearing area and reduce the unit bearing stress. 1!. the shape of the thread can not be changed to provide greater strength then the plug must be redesigned and more thread area added. This may be accomplished by changing either the length or the diameter of the plug. In this manner several sets of calculations and studies must be made. as a rule, before a satisfactory breech mechanism is designed. -35-