Fatigue life enhancement of ratchet pawl charging mechanism through dynamic analysis

Transcription

1 nd Intnational and 17 th National Confnc on Machins and Mchanisms inacomm Fatigu lif nhancmnt of atcht pawl chaging mchanism though dynamic analysis Vishal Bagad Abstact Ratcht pawl closing sping chaging mchanism is an impotant building block of cicuit bak mchanism, sldom studid in past. It is utilizd fo tansfing ngy fom moto and stoing in th closing sping. Th ky challngs in dsigning atcht pawl mchanism a avoiding failus such as buning of diving moto, waing of pawl tips and atcht tth, waing of pawl stopps, baking of ccntic shaft which divs th pawls. In od to combat ths failus, this pap mainly consids minimization of diving toqu quid fom moto by contolling fiction, ccnticity and pssu angl. Th atcht pawl mchanism is dsignd fo fou impotant conditions: two load-handing ov positions and two xtm conditions of pawls and mathmatical lations a stablishd fo th sam. This pap focuss on th aspcts: a ffct of fiction btwn pawl and ccntic shaft on diving toqu, b ffct of fiction btwn pawl tip and atcht whl, c ffct of ccnticity, backlash, and pssu angl on diving toqu. Multibody dynamic analysis showd that th diving toqu has xponntial lationship with cofficint of fiction btwn pawl and ccntic shaft. A simila analysis showd that th unqual backlash in pawls sults in high impact load by atcht whl on pawls than th qual backlash. In th thid cas study, th undsiabl ubbing of pawl tips with atcht whl non tth potion whil closing opation is avoidd by an innovativ dsign of disngaging th pawls aft chaging opation. Th nhancd dsigns showd % impovmnt in fatigu lif in xpimntal validation. Kywods: Cicuit bak mchanism, Ratcht pawl, Closing sping, backlash, ccnticity, cofficint of fiction 1 Intoduction Cicuit baks a cucial dvics in an lctical substation. Thy a ssntially switchs that bak (i.., opn an lctical cicuit manually o automatically to potct against faulty conditions. A cicuit bak should also b abl to connct th cicuit back (i.., clos aft nomal conditions a stod. This is achivd in cicuit bak mchanisms with th hlp of potntial ngy stod in opning and closing spings. This waants quick closing and opning of th cicuit bak contacts. Th opation of a cicuit bak mchanism is dividd into th stags: chaging th closing sping, closing th contact, and opning th contact. Th focus of this pap is on th study of atcht pawl chaging mchanism. It is dsid to disngag th chaging pawls fom atcht whl whil closing opation. Pvious sachs [1-4] achivd it by using additional lmnts i.. xta sping loadd catch, pawl and cam to hold th chaging pawls. Kinmatic analysis on dul atcht pawl mchanism is sldom considd in past. In this pap, w popos a novl mthodology of analysing atcht pawl mchanism to dsign a cicuit bak sping chaging mchanism. Futh, th disngagmnt of pawls whil closing opation is achivd by a novl tchniqu. No additional Vishal Bagad Switchga R&D, Compton Gavs Ltd, Nashik, Mahaashta, India, vishal.bagad@cgglobal.com. 1

2 15 th National Confnc on Machins and Mchanisms NaCoMM lmnt is quid in th poposd tchniqu. Dsign impovmnts to duc input toqu and impov fatigu lif a discussd. High duction atio can b asily achivd in a singl stag by a atchtpawl mchanism. H two pawls a povidd to div th atcht whl. Thy a fd to as big pawl and small pawl considing th hight diffnc. On pawl divs th atcht whl and oth tails i.. tavls in vs diction along th path of atcht whl tth. Big pawl and small pawl a mountd on 180 o opposit lobs of an ccntic shaft. Moto divs ccntic shaft. Rotation of ccntic shaft is convtd into ocking motion of pawls which in tun divs th atcht whl. Each pawl advancs and tails onc duing on complt otation of th ccntic shaft. Conncting od, sping sho a assmbld with atcht whl though cank bolt to fom a slid cank mchanism. Ratcht whl itslf acts as cank and sping sho as slid. Closing sping is assmbld btwn housing (fixd and sping sho as dpictd in Fig.1. Figu 1: Ratcht pawl sping chaging mchanism Extm positions of pawls Fou conditions a considd fo th study of atcht-pawl mchanism (s Fig. : (i big pawl maximum up and small pawl maximum down, (ii small pawl maximum up and big pawl maximum down, (iii load handing ov fom big pawl to small pawl and (iv load handing ov fom small pawl to big pawl. Whn th big pawl achs th maximum hight, taking complt advantag of ccnticity, th small pawl attains minimum hight position. Th distanc btwn th cnt of th small pawl tip and th cosponding tooth of th atcht whl is calld th backlash (Fig.3.Aft attaining maximum up position, th big pawl stats its tavl in th downwad diction and th small pawl in th upwad diction. Du to backlash th small pawl cannot push th atcht whl. Hnc, th atcht whl otats in th vs diction guidd by th big pawl till th small pawl touchs th oot of th tooth of th atcht whl. Th small pawl taks ov load fom th big pawl. Th big pawl stats tailing whil th small pawl stats advancing. Whn th small pawl achs th maximum hight, th big pawl attains th minimum hight. Backlash is thn obsvd, causing vs otation of th atcht whl. Backlash is ncssay to nsu th nty of pawl in th nxt tooth of th atcht whl. It can b minimizd to duc th vs otation of atcht whl. 3 Analysis of diving pawl Kinmatic diagam containing diving pawl advancing atcht whl is as dpictd in Fig. 4. Eccntic shaft OC is pivotd at cnt O with ccnticity. Diving pawl CD is assmbld with ccntic shaft at C and its lngth is sp o bp if small pawl o big pawl is diving atcht whl spctivly. OCDO 1 foms a fou ba linkag.

3 15 th National Confnc on Machins and Mchanisms NaCoMM Angula displacmnt lationship, toqu tansmission atio btwn ccntic shaft and atcht whl can b calculatd by using fomulations of fou ba mchanism. Big pawl max up Small pawl max down Small pawl max up Big pawl max down Load handing ov Big to Small pawl Load handing ov Small to Big pawl Figu : Impotant conditions of atcht pawl mchanism Small pawl max up Big pawl max up Big pawl max down Small pawl max down Figu 3: Backlash Small pawl max up, big pawl max up Pawl tip adius is. Ratcht whl cnt O 1 is locatd at distanc z fom ccntic shaft cnt O, with x and y as hoizontal and vtical componnts spctivly. Angl θz is inclination of z with spct to X-axis. Angula position of th ccntic shaft, atcht whl, big pawl and small pawl a θ, θ, θbp, θ sp. O 1 F psnts th lin joining th atcht whl cnt and tooth con point, masuing. Lin FI at angl θ s fom O 1 H foms th tooth of atcht whl. Th diving pawl tip i.. th cicl at cnt D, foms tangnts at lin O 1 H and FI at G and E spctivly, as can b sn in Fig. 4. Link lngths of OCDO 1 fou ba mchanism, OC, CD, O 1 O a xpssd as, sp, z. Link lngth O 1 D is not known. W assum that small pawl is diving th atcht whl, fo diving xpssions. Futh sp can b placd by bp to analyz th cas of big pawl as div. Lngth and ointation of link O 1 D a xpssd as O D = + + G (1 1 ( ( F 1 θo tan ( 1D = θ Wh, FG = ( + FG θs tan( Loop closu quation fo O 1 DCO fou ba mchanism is xpssd as OC + CD + DO OO = ( By xpssing quation (3 in tms of X and Y componnt quations, aanging, squaing and adding componnt quations w gt A + A + A cos( θ + A sin( θ = 0 (4 1 3 O1 D 4 O1 D 3

4 15 th National Confnc on Machins and Mchanisms NaCoMM Wh, A = + x + y + O D sp 1 1 A = xcos( θ y sin( θ A = O D( cos( θ + x 3 1 A = O D( sin( θ + y 4 1 AB Ratcht whl tooth (5 Ratcht whl AA Diving pawl Eccntic shaft Figu 4: Kinmatic diagam of diving pawl and atcht whl Ointation of link O 1 D is obtaind by solving quation (4 as follows. A4 4A 1 4 4( A1 + A A3 A 3 θo tan ( 1D = ( A1 + A A3 Ointation of th small pawl can b calculatd as, sin( θ 1 O1 Dsin( θo 1D + y θsp = tan ( cos( θ O Dcos( θ + x Ointation of Ratcht whl quation ( θ = θ + O1 D 1 tan ( + FG 1 O1D θ can b obtaind by putting valu of angl θo 1D Toqu tansmission atio btwn th ccntic shaft and th atcht whl whn th small pawl is diving th atcht whl is givn by T T R sin( θsp θs = sin( θ θ sp Wh, TR and T a toqu tansmittd on atcht whl and toqu input fom θ whn th small pawl is diving th ccntic shaft spctivly. θ s is th valu of atcht whl. Simila quation can b wittn fo th big pawl and atcht whl by placing θ sp by θbp and θ s by θ b. Toqu on th atcht whl shaft at O 1 du to compssion of closing sping, can b computd by T R k( l0 ( xi l f sin( θ A + δ = cos( δ (6 (7 in (8 (9 (10 4

5 15 th National Confnc on Machins and Mchanisms NaCoMM Combining quations (9 and (10, input toqu quid at ccntic shaft, calculatd. 4 Analysis of tailing pawl T can b Whn on pawl divs th atcht whl, th oth movs in th opposit diction following th atcht whl tth sufac. Position of tailing pawl is ncssay to comput th backlash. Kinmatic diagam of tailing pawl is as dpictd in Fig. 5(a. H, small pawl is diving th atcht whl and big pawl is tailing in th sam divn tooth of atcht whl. Lt d1 b th ppndicula distanc of slantd potion of th atcht whl tth fom O 1. Lt PO b th ppndicula distanc of slantd potion of th atcht whl tth fom O. And lt ˆλ b th unit vcto along slantd potion of atcht whl tth. Lt ˆn b th unit vcto ppndicula to slantd potion of atcht whl tth passing though O 1. Lt γ b angl btwn O1O and OD. OJ + JK + KL + LF + FO OO = 0 ( By xpssing quation (11 in tms of X and Y componnt quations and aanging thm and dividing Y componnt quation by X componnt quation w gt ointation of big pawl θ bpt duing tailing as, 1 ( sin( sin( sin( tan ( θ + π + π θs θ y θ ( cos( cos( cos( bpt = (1 θ + π + π θs θ x Whn big pawl is diving th tailing position of small pawl can b calculatd by placing valu of θ by ( θ p wh, θ is position of atcht whl whn big pawl is diving, p is pitch of atcht whl. Applying cosin ul to tiangl O 1 FO (Fig. 5(b, lngth FO can b givn as FO = z + + z cos( θ + θ (13 z Applying sin ul to tiangl O 1 FO (Fig. 5(b, angl btwn O 1 F and FO can b calculatd as, z θo sin( sin( 1FO = a θ + θz (14 FO Angl btwn OF and FP (Fig. 5(b can b givn as, θ = ( π θ θ (15 OFP s O1 FO Hnc, backlash, lngth EL in Fig. 5(a, with big pawl whn small pawl is diving can b computd as, B = FP FE bpcos( θ ( π θ (16 s bp s Wh, FP = cos( θofp ( z + + z cos( θ + θz and FE = θs tan( Simila xpssion can b divd fo backlash whn big pawl is diving by placing θbp by θ sp, bp by sp and calculating valu of θ fo big pawl as diving pawl. Th otation of atcht whl with spct to otation of ccntic shaft is as dpictd in Fig. 6. Th paks in gaph indicat th maximum up conditions of big/small pawl. H, pawls tak maximum advantag of th ccnticity and advanc th atcht whl to th fullst xtnt. Th vallys in th gaph indicat 5

6 15 th National Confnc on Machins and Mchanisms NaCoMM vs otation of th atcht whl aft aching its pak. Th vs otation is a function of th diffnc btwn pawl hights, i.. (bp sp, ccnticity of th ccntic shaft and atcht whl pitch. Figu 5: a Kinmatic diagam of tailing pawl, b OPFO 1 fom a. Figu 6: Ratcht whl otation with spct to ccntic shaft otation Toqu quid at atcht whl cnt, atcht whl pitch, ccnticity, fiction btwn moving componnts of atcht pawl mchanism, tansmission angl btwn pawl and atcht whl a th factos affcting input toqu quid fom moto connctd at ccntic shaft. Th cas studis a psntd as follows. 4.1 Cas study 1: Effct of ccnticity, tansmission angl and backlash Modl 1 was takn as bas fo analytical study and xpimntation. Th input paamts fo atcht pawl mchanism modl 1 a as mntiond in tabl 1. Eccntic shaft was bokn aft 6500 opations duing xpimntation. Th minimum valu of is 10./4 =.55 mm. Th additional 0.95 mm ccnticity sultd in unqual backlash 0.9 mm and.43 mm fo big pawl maximum up and small pawl maximum down conditions spctivly. Th tansmission angls fo ths conditions a and Th unqual backlash ld to incasd impact foc on big and small pawls. Altnativs to duc impact foc on pawls a 1 duc tansmission angl, minimiz and qualiz backlash. nds to b ducd to minimiz th backlash and pawl hights nd to b adjustd to mak th backlashs qual. Modl is dsignd to tak ca of ths quimnts. Th 6

7 15 th National Confnc on Machins and Mchanisms NaCoMM vtical distanc btwn atcht whl and ccntic shaft is incasd to 78 mm fom 60 mm. is ducd to 3 mm fom 3.5 mm. Pawl hights a changd to 79.6 mm and 75 mm fo big and small pawls spctivly. Th dimnsional changs sultd in duction in tansmission angls to and Also, th backlashs ducd to 1.34 mm fo both. Th maximum input toqu quid is ducd to.1 Nm fom 5.3 Nm. Multibody dynamic analysis study is conductd on both modls to study th action focs on pawls and input toqu. Th maximum action foc on big pawl is ducd to N fom N. Also, th maximum action foc on small pawl is ducd to 1850 N fom 145 N. Modl xhibitd succssful opations without any failu and xpimntation was stoppd as quimnt of cycls was satisfid. Inputs Outputs Tabl 1 Ratcht pawl chaging mchanism paamts fo cas study 1 Paamt Unit Modl 1 Modl Closing sping stiffnss k N/mm ccnticity mm atcht whl cicula pitch mm big pawl hight bp mm small pawl hight sp mm c-c distanc x mm c-c distanc y mm dg dg Tansmission angl btwn atcht tooth & big pawl-whn big pawl is at max up Tansmission angl btwn atcht tooth & small pawlwhn small pawl is at max up backlash - big pawl max up mm backlash - small pawl max up mm Maximum input toqu quid at ccntic shaft Expimntation sults 4. Cas study - Effct of fiction Nm Eccntic shaft bokn aft 6500 opations. Pfomd opations. Inn diamt pawl foms volut joint with ccntic shaft lob. Th fiction in this volut pai sults in quimnt of additional input pow. Effct of vaiation of cofficint of fiction on pak input toqu is as dpictd in Fig.7. Input toqu slightly incass fom 16.5 Nm to 4 Nm fo incas in cofficint of fiction fom 0 to 0.1. Futh, input toqu incass stply till 159 Nm fo cofficint of fiction as 1. It is dsid to kp cofficint of fiction as minimum as possibl so that th input toqu quid will b th last. Th cofficint of fiction will incas du to ubbing btwn inn sufac of pawl and out sufac of ccntic shaft lob, aft sval cycls of opation. This will incas th input toqu quid fom moto. Thfo, th cofficint of fiction should main almost constant fo th dsid numb of cycls (i opations. Multibody dynamic analysis showd that th diving toqu has paabolic lationship with cofficint of fiction btwn 7

8 15 th National Confnc on Machins and Mchanisms NaCoMM pawl and ccntic shaft. Validation of th sam is don by two vaiants i.. us of dy mtallic bush and ndl oll baing. Th dy mtallic bush was aft 1000 opations and th diving toqu incass and th moto gts bund out. Th ndl oll baing lasts opations without buning of moto. Figu 7: Effct of fiction on input toqu at ccntic shaft Stopp scws Ratcht whl (a (b Figu 8: Bfo impovmnt - (a Ratcht pawl duing closing opation, (b Failus aft 1300 opations Pawls Stopp olls Ratcht whl (a (b Figu 9: Aft impovmnt - (a Ratcht pawl duing closing opation, (b Condition of componnts aft 1000 opations 8 Pawls 4.3 Cas study 3: Effct of atcht whl and pawl tip wa Pawls fly off duing otation of th atcht whl and poduc suddn shock on th stopp scws duing closing opation as dpictd in Fig. 8(a. Impact of pawls on

9 15 th National Confnc on Machins and Mchanisms NaCoMM th stopp scws sults in havy wa of scw hads. Tosion spings bing pawls back to maintain contact btwn atcht whl and pawl tip. Plain diamt of atcht whl is in contact with th pawl tip duing closing opation. It cats impssion on pawl tip. Th won out stopp scws, pawl tips and atcht tth aft 1300 opations a as shown in Fig. 8(b. This futh damags th pawl tip aa which is bnficial fo chaging. Damagd pawl tip aa sults in a foc tansf though high points on th pawls which futh sults in dtioation of th atcht whl tth. Wa of atcht whl tth and pawl tips sults in no chaging. Stopp olls a povidd instad of th stopp scws. Thy a pivotd on th ba-pin and hnc a f to otat. Thfo, fo vy closing opation, pawl will gt a diffnt sufac of th oll fo impact. This will caus unifom wa of oll. Pvntion of pawl tips fom ubbing with atcht whl plain sufac / non tth zon potcts th pawl tips fom waing duing closing opation. Th non-tth zon diamt of atcht whl is ducd by mm. Pojction on pawl is povidd so that it will touch th stopp olls and avoid contact btwn pawl tips and atcht whl plain diamt. Th pojctions on pawls a dsignd such that th locus of highst point on tip should not touch th plain diamt (Fig. 9(a. Th nhancmnts a validatd though xpimntation. Th condition of pawl tips, olls, and atcht whl tth aft 1000 opations is as shown in Fig. 9(b. Th fatigu lif of atcht whl, pawls and stopp olls is incasd by 900%. 5. Conclusions Woking of dul atcht pawl is studid fo fou xtm conditions: big pawl max up, small pawl max up and two load-handing ov positions. Mathmatical lations a stablishd fo ths conditions. Multibody dynamic analysis showd that th diving toqu has paabolic lationship with cofficint of fiction btwn pawl and ccntic shaft. Rduction in th fiction at pawl pivot joint sultd in 10 tims fatigu lif nhancmnt. Multibody analysis showd that th unqual backlash in pawls sults in high impact load by th atcht whl on pawls than th qual backlash. Expimntal validation showd that th duction in ccnticity fom 3.5 mm to 3 mm, th impovmnt in pssu angl fom 14 o to 116 o and th qual backlashs ducd fom 3. mm to 1.3 mm fo both pawls sultd in 100% impovmnt in fatigu lif. Th undsiabl ubbing of pawl tips with atcht whl non-tth potion whil closing opation is avoidd by an innovativ dsign of disngagmnt of th pawls aft th chaging opation. This sultd in nin fold nhancmnt in th fatigu lif of atcht pawl mchanism in xpimntation. 6. Rfncs [1] Alan G., Cicuit bak including atcht and pawl sping chaging mans and atcht tth damag pvnting mans, US368971, [] Patl J., Cicuit bak including sping closing mans with mans fo moving a chaging pawl out of ngagmnt with a atcht whl whn th sping mans a chagd, US368970, [3] Kiyoshi Y., Akiyoshi O., Sping opating mchanism fo a cicuit intupt, US , [4] Rog N., Jams L., And L., Dan A., Ratchting mchanism fo industialatd cicuit bak, US ,