Mar 10, 1989 - vented in the thirties is, except rubber, the most commonly used polymer material in industrial castors today. Cast thermoset polyurethane is the ...
Proceedlngsd)f the SKF / Chalmers symposlum on :,olytne! technology, coteborg,sverj.ge. F10 rnarch 'l989
t
PoJ.vrners
ln fildustrlal
Wfiee1s & Castore
Paul Lindhout i. SKF Transportrrielen B.V. veenenilaal, The NetheElanC.s
Àbst!act
Inèustrlal. castors are still naínfy constructeil flon sheet steel. anal cast iron for the slrlvel- anè rigiA forks. In the uh€e1s next to the tlaaltional poLytrlêls such as rubber. pofyamiae, pbênolics anA pol.yurethanes netl pofyhers anè new nanufactu!lng nethods are introduceil. The êtate of the art of wheel constluctions in poLyrTre! naterials cen be descri.beè by the elastoners suitable for tyres. À diagran of haldless velsus rebound elesticity for typicaL netelial groups is useal to indicate the inportant research anil developtnent areas. :
KEYWORDS
wheels, polyhets, tyres, injectlon moulaling
castols.
Tntrodu ct ioo
fndust!iaf q'heeIs & castoïs ale beconing Iess and. less the sole domaln of rubber, cast iron anA plesseal sheetsteel. Strong economic factors such as ove!capacity on the steeL market, \rith Iow steef p!ices and. relatively weII establlshed production techniques, have long suppreËseA the intloCuction of polyners in this applicetj.on area, UnIike in thè furnituÍe- anA stationary products, alreedy earl.ier tloninateA by polyners ana non-ferro netals for styling anA conrrnerciaf reasons, most of the opportunities sti1l. 1ie ahead. Polyners are introduceal in \"'heeIs anil tyres, but halaly in the swivel forks and rígid folks thêy are nounteél into. Starting with so1ld lubber tyles anil later pneunatic tyres, the polyner appLicatÍon in wheeLs vas originally confined to coaches, cars anil carriages. tate! spin-offs are steel \rire !einforceal plesseil-on lubbêr tyres and bcnaêa-on tyres on cast irorl wheel bodtes. Nor.rèAays pneuhatic tyres are the nost advancefl rubber products existing.They ale stilL subject of research and developnent anal a continuous source of nen patents. Íhe chemlcal industly has devel.oped, nany hore thernoset natê!1a1s aftêr their first discovêry in the seconal haff of tbe 19th
century. The phenolLc folmafdêhyd, invented èrounal 1910 still find.s its ray into tÍ1ary wheels. The thernoplastic naterial polyanidê inventeal in tbe thlrties is, except rubber, the nost connonly useal polyner natelial in lndustrial castors today. Cast thernoset polyurethane is the tnost ilulabfe tyre materi.al since the Late s
ixties
.
Designs baseal on new tyle elastoners are being Lntroduced so that the tladitionaL rubber anai polyurethanes cortent in castors stabiflzes anil in sohe arêas even decl ines
.
to nanagêr Engineerj.ng anal Quality Àssurance at SKF ?lansportnielen B.v in 1985.
.'. was appoiDteil
naterial cotnbinations êpecifically developea fo! processing in state-of-theart ilual injection moufding equiptnent have been introduceal. Chenical bonding between wheel eud. tyre without use of an êAbesive ha6 been achieveal for several polyner natef,Íals. New 'blended,' poLynèr materials anal !!ew ploductlon nethods (e.g. R.I.M.) ale êxpectêa to leail to products with rmproved p:opertiês and to new casiot co:rstruct ions . Seve!aL
PoLvners and, annuat consurnption
B.v, consurnption of various poll'ne! products in 1988, eAds up to sone 180 tonnes of natellal \tith a total noulded conponents value of soFe 3 nifllon dff. ê L€ading pol.yne! natelial is rubber NR/SBR, \rith sone 95 tonnes, fo! ty4es anil sealing conponênts. the 6econA group of polyners lE PÀ-6 PofyanÍdê, wlth sotne 70 toDnes, for wheefs, rollers anal erheelbodies fo! use wlth a ty!e. The thirê big group is polyulethanes, of cast thermoset anA noufaeil thermoplastic type together, with sone 10. tonnes. srnal.le! quantj.ties ale usea for special products onfy, Door loflers of PoM, high tehperature eheels of reinfolceil phenofic fornafdehyal aDd fillet of pofycalbonate ale sone examples of regufarfy produced alticLes witb snaLler consunptions. The SKP Transportr.rieLen
3. The castor product Rigid. ènd s iveL castols are us€tl i!! a wíile variety of industrial applications. Typical. indoo! êpplications for short Alstance transpoit equipnent o! nlaed. lnaloo!/outd.oo! use in d.istlibution oÍganlzatlons leaê to rellabj.lity arCI low rolling r€sistance requlrenents, More têchnlcal denanding applications, lrhele castors are exposeê to conbinations of nechanical, thernal anil cheÍ1ical environnents, lead to Íequilenents on natella1. anil constructLon, dulabiLity and corlosion prevention. Àlso proAuctlon techniques, ltfetine, naintenance, availabiLity and costs cont lnuous ly j.rnpose nev deslgí constraints anii open up new perspectives.
Castors càn be consi.dereè as natule products situatea in an industrial envlronnent where lnalket grovth vlrtually coincid€s \íith the gro}rth rate of the econonic activity.
aa
fodays wheels antl castor6 types produceal antl sola by SKF Transpoltwielên È.V. caÍt be grouped ès follo!'s (ftg. t):
to baal floors anal rough handLing of intelnaI transport equipment, irípact stlength anA toughness are ê1so inpoitant, The efongation percentage until. breaking shoulal be relatively high, alLo!? for bending ilue to off-centre plane loads oo the wheel oute! c ircunfeleDce. The tyre dêfolmation zone B consists of three baslc legion€. Ílhed the tyre touchês the grounil, at first ladiaI expansion together l'ith c ilcun fe rent j.al compression takês placê, Then at the legion above the tyre footprint, the opposite occurs. Àfter the footplint a second circunferentiaf conplession takes place togelher irtth ladial. expansion. The r./heel conducts the loads from thê bealj.ng seets Ín the hub part into the ltheeL threail aoa shoula wi.thstand inl]]àcts ês r.relI as forcês pressing in baII b'earings after nouldj.ng at region C. Stiffness anal cleep propertj.es ale of itnportancê not only for thê loadtating of the conpLete whee1, but also at local fastenlng points fo! sealing at1d. regteasing coÍnpone:lts. High elastlcity for the requireA haiëlness is requileal to keep the energy dissipatêd insíde the tyre naterial down to a nininun. For the 1o\,f thernal cond.uctivity values of pofyhels this is the only feasible \ray to avoial failurês due to he;t buifd-up aÈ the centre of the tyre cross sectÍon. Especielly soft tyres need a high tear strengib at the footplint region D in additioll to the elasticl-ty since thê fa!lgue effects play a alonlnant rofe. Due
- castor prod.ucts (standara and nêdiun d.uty ranges, classlc anal heavy duty .arr9e5, - loLlers - spêcial products (ai! cargo, hospitals, bakeries, etc, ) - conponenès (rims,hubs, lope shêaves, castor plvots, raifwheels ) . These groups have in connon that polyners are ahnost exclusj.vely usetl 1n r"rheêls. Wjth exception of the sealing arlangênent the swivef forks anil rigid forks arè still plod.uced fron sheetsteel or cast iron ilue to the perfolnance velsus cost benefits with rêspect to engioeêling plastics. For conpalab]e perfornancês, price ilifferences of sone 150 to 2OOg between the two, nlakê a targe scale intloduction of poLyners lnpossible at this :nonent. 4. critícaI naterial properties Bes ides 9eflêraf requirelnents (prlnarj.ly p!ice, ilurability, chenical conpat ibil tty ana - fess - afso teriperature stabil"j.ty) celtain mininum tnatelial property vafues ale Aeriveal flon specific perfornance considerations. (fig. 2) High conprêsstve Étrength and hj.gh efasticity are inrportant for foail rating anal heat build-up in the wheef body Aefornation zone À: the thin section betweên thleaa anA hub palt. CÀSTOR WHEELS CODE
CONSTRUCTION
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SPECIÀL I{HEELS ÀI]D
ROI-LERS
c!êy !!bb.r !ond.d-ón
COMPONENTS
CONSTRUCTION
EÀ
BODY
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TXRE
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th.iollcr,
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Culrent wheel designs 1n thè
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SKF
shêè:!:..I
sttlcon
Transpoltwielen product range.
clrt t!ên
càst rÍoÀ c.!r iru-
increasingLy high el.astLcity anËl alulabiLity. Àt the sane tine the thelrnosets show a rernatkablê ileveLoprnent towarËls lebound values folllle11y onLy possible to achleve \rith rubbêrs. Ín tabLe 1 a sur,lmaly of p!êseotly useà rnatêria1s aoa thel!- ploperties ls given.
st!eigih, toughness b- tytè dêao.ftáÈion zonê r é1àstici:y,hàrênêss.cónplesslve st!êngèh, the4a1 concuctlvlty r tnpàct ai:èi9th,Et1t!!êsE,c!ee9 ben.vlou!, c- Eêa.ins seèts !hê!hàl coe:ftciënÈ of èxFà.slon i hà!drêss.u€a:.tear sEref,c:1,ê1àsLtc1Èv C- Tvre foo::rL.c àréà
Fi9. 2 Crltical nheel Íratêrial. propert!es. 5. "State of thê art" À convenient diagram for solia tyle na-
têliaLs is the oíê shown in fig. 3. The tyle hardness is put on the vertical axis, the rebounil resilience along the horizontáI axis, These t\ro naterial propelties deternine the nain \.rheeI pêrformalrcê factor: tbê ro11lng !ês lstancê. Fo! wheefs with lhe same construction but lrith differenÈ outef, diaríeters the rolling resist,ance of any of these rrheels can be apploxinateal by a constant factor nultiplied w j.th the wheêfload. The factor is usualfy expressed as a percentage of the wbeeL load. Tbê figule shows lines of constant percentages. Fo! Iower barAnesses the factor i,s alnost plopoltional to the rebounil resifience. For higher haidnesses the tyre flefornation becones fesê Ínportant. Àt the top of the glaph the non-efastoner mate!iafs ale indicateè ( for reÍerence onLy). Major treods arê i.dentifieA by arrons. The rubber tyre situation is characteriseCl by two clearly separated main denrand area's. First the seni-e1astic, traditional rubbers group Ëhere (far-east) p!ice conpetition and caÈitel intensive nulticavity injection noulding equipnent sêt thê ênvirónrnent. secondLy, thê hÍgh]y elastic rubber tyres, proaluced with the compressi.on noulding technique, where attacks cone flotn two z si.des. One being the replacement by cast e.l.astomers, vith similar perfolnancê to cosÈ ratios anal the othêr the changê to injectLon nouldil1g proèuction techniques offering conpetltiveness at the expense of a snral-ler a;14 smaffer perforlxance d
6. Research & Developr]lent activlti.ês with the ,'stète-of-the-art,, as a background it is clear that Jnost of the toplcs concern tyles. Àt present an activity on injectioÀ noulding of tyres is ongoing at seve!al. supplLêrs. Rebounal values of 65 to 70? havê been obtaineA for acceptable têalstrength values. BotlAing of thernoplastic polyurethaoe tyles to polyanide-6 r./heêlbodies has been attenpted the lnechanicat Íray for a fong tirne with aêceptebfe results. Chenical bonding is available for tbelnoset types wj.th slightly beitêr results, but ihlrovernent is neeAed. High tenperature silicone rubbers nêed Lnplovenent in tenpelature stability and r,lechanical proÈerti,es . thernoplastic eLastotners have reacheai sone 60 to 65? rebounal values but neea to be d.eveloped further to becone aitractj.ve fo! tyre applications. and :olfers coulal be produced of 'iheelbodies nylon block copolyner naterials lrith the R.I.M. technique. h that case equal or bêtter perfohence is obtainecl than for cas! polyurethanes. Àn ad.hesivê systen to bonal these to steêf is not ava.ilablê. In genexal attractive design to cost ad.vantages can bê achieveA by conbined thelnal and. lnechanlcal finite eLenent analys is on Hheelbody designs. À cLassic problen is the availebility of electricalLy conductive !íheelboalies and. tyles. À ploblen still not sofved at the lnonent is antistatic grey efastic xubber to comnercial plices. Prod.uction of lrheeLs but also of the - nuch smaffer - sealing arranger,rent could be rnade rnore efficient by introduction of dual. injection noulding in nore products
is advantage.
Àl-so a thila, nuch less signLficant, denand area can bê alef-ined, thê high ten-
perature tyle. In this appLication sone silicon rubbers ê!e colnnerciafly available but the area neeéls inprovement on both perfolrnance anal costs,
The polyurethane sl.tuatioa is chalactelisea by tI-Io types of natelial. Thê traAitional cast therrnoset poLyurethanes face a threat
fron therFoplastic elastoners uith
RTBOUND RESITIENCE
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Fig.3 State of the art fo! solLd tyre matelials.
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