FOR FURTHER INFORMATIONON ZA ZINC BEARING ALLOYS CONTACT

eastern alloys, inc.BOX Q. MAYBROOK. N.Y. 12543 (845) 427-2151

ZINC-ALLOYBEARINGS

CHALLENGETHE BRONZES

First used in wartime Europe in the 1940s assubstitutes for bronze bearings, zinc-aluminum

alloy bearings have maintained a rather lowprofile in the engineering universe until recently.

Now, their low cost and longer life are againattracting attention-this time in the U.S.

WILLIAM MIHAICHUKEastern Alloys IncMaybrook, N.Y.

ALTHOUGH zinc alloys havebeen used in Europe for manyyears for plain bearings, U.S.design engineers are onlyrecently beginning to recognizethese metals as possible alter-natives to traditional bronze.The key engineering advan-tages of zinc alloy bearings arelower cost, longer life, a higherload-bearing capability, andbetter emergency (dry-run-ning) characteristics. Zinc alsooffers benefits to foundries inthe form of pollution-free han-dling and significant energysavings in melting and casting

operations.The zinc alloys now being

used in the U.S. are general-purpose foundry alloys, devel-oped for gravity casting ofstructural components. But bycoincidence, their compositionsclosely approximate the alloysdeveloped in Germany specific-ally for bearings during WorldWar II.

The alloys are genericallyidentified as Zn-8, Zn-12, andZn-27. They are available fromseveral sources in the U.S. as

This array of nested, zinc-alloybearing products shows some of

the size variations manufactured byVoest-Alpine AG, Linz, Austria.

well as in Canada, Europe, andAustralia. During the past fewyears, two of the alloys, Zn-12and Zn-27, have been identifiedas good bearing materials.

The introduction of zincfoundry alloys for bearings istimely in light of recent con-cerns over bronze. One concerncenters on the high cost andprice fluctuation of copper, thebase material of bronze. Theother problem is that bronzebearing producers are report-ing increasing difficulty inmeeting OSHA air-qualitystandards for lead. (Lead is animportant alloying element inmost bronze bearing grades.)

Some bronze bearing found-ries are being forced out ofbusiness by the high cost ofcompliance with environ-mental regulations; others arerefusing to handle leadedbronze alloys. Bronze bearingbuyers are thus facing theprospect o f hav ing fewer

With all these advantages, itwould appear that the zinc al-loys would be expected to dis-place all bronze bearings in thenear future. Not so. The zinc al-loys are temperature limited,so bronze will continue to beused in applications involvingservice temperatures of 300°Fand higher. Bronze will also bethe choice where maximumcorrosion resistance is requiredand where abusive service con-

Cost of the zinc alloys, on avolume basis, is about one-third that of bronze alloys. Thecost difference is even greaterfor special, high-tin bearingbronzes. Furthermore, themelting-energy requirementsfor zinc alloys are substantiallylower than those for bronze, theworld supply of zinc is abun-dant, and the price is relativelystable.

suppliers and of paying higherprices for traditional bronzebearing products.

Block bearingin an 800-hpIngersoll-Randindustrialcompressor,sand-cast inzinc alloy Zn-12from BunkerHil l Co.,operatescontinuously at120°F and islubricated witha flood-typesystem. Theequipmentcycles at 168rpm, and the bearing is loaded to 5,100psi. A micrometer reading taken at 4,000h of service detected no measurablebearing wear. Cost of the zinc bearing,which has been operating continuouslyfor nine months, was 30% lower than theSAE 660 bronze bearing it replaced.

ditions warrant the toughnessof the aluminum-bronze alloys.

Zinc-alloy properties

Research on bearing perfor-mance of the zinc alloys is cur-rently underway at BattelleColumbus Laboratories underthe auspices of ILZRO (Inter-national Lead Zinc ResearchOrganization). ILZRO is azinc-industry-sponsored or-ganization that identifies, con-ducts, and manages researchprojects for the benefit of itsmembership.

A n e a r l y s c r e e n i n g a tBattelle comparing Zn-12 to

An eight-month test showed that bushings of Zn-12 for a hot mill runouttable had double the life of SAE 64 bronze bushings, had superior

dry-running characteristics, and cost 20% less. In comparative runswhen both types of bushings were lubricated weekly for four months, theZn-12 bushings showed no measurable wear; the bronze bearings wore0.030 in. (diameter). All the runout-table bushings at Crucible Stainlessand Alloy Div. have since been converted to Zn-12 except those whereslow-moving hot steel would heat the bearings enough to soften them.

A switch from SAE 64 bronze to zinc alloyZn-27 provided equivalent wear and cut thecost of this gear cover/wear ring almost 50%.The cover, which is used on a Ridge ToolCo. portable power drive, is subjected tohigh radial and thrust loadings from a steelface gear as the drive is used in threadingheavy-wallsteel tubing. The part has been inproduction for three years.

Properties of the bearing alloysZinc alloys

Zn-12 Zn-27 SAE 660Bronze alloys*

SAE 64 SAE 40Sand Perm mold Sand Sand† (CDA 93200) (CDA 93700) (CDA 83600)

*Properties of sand-cast components.†Heat Treated: 3 h at 610°F and slow furnace cool.80.5% offset.

SAE 660 and SAE 40 for wearresistance shows that the zincalloy outperforms the bronzesby a wide margin (up to 3 to 1)in antifriction characteristicsand wear resistance. The onlylimiting factor appears to bethat of service temperature.

The aim of an on-goingBattelle study is to providecomplete design guidelines forthe application of Zn-12 andZn-27 as bearing materials.The investigative methods em-ployed parallel those used by

the Cast Bronze Bearing Insti-tute in the development of itstechnical data.

Testing is presently beingconcentrated on the low-speed(to 90 fpm), high-load applica-tions, where the alloys are ex-pected to perform best. The testmethod employs 2-in.-ID sand-cast bushing samples, installedin a test jig in which bearingloads and shaft speeds can bev a r i e d . S h a f t m a t e r i a l(AISI1045, Rc30) is ground andpolished, and the bearings are

well lubricated with grease.Bearing clearance is nominally0.005 in.

Results of these tests showthat the Zn-27 alloy exceedsthe load capabilities of 660bronze over the entire speedrange investigated and appearsto have superior wear resis-tance. Zn-12 may also besuperior to 660 bronze undercertain low-speed conditions.The report further indicatesthat the superior load-bearingcapability of the zinc alloys is

the result of the lower coeffi-cient of friction of zinc alloyscompared to that of 660 bronze.These results confirm earlierEuropean findings and clarifythe reasons for the superiorbearing performance of theU.S. alloys in some industrialapplications.

Emergency operation

When unlubricated, drybronze bearings generate con-siderable heat, causing inter-mittent seizures and shortrecoveries prior to completebearing failure and total sei-zure . The f r i c t ional heatusually causes fusion of bronzeto the steel shaft and, some-times, deep grooves and scoremarks.

Under similar conditions,zinc-aluminum alloys usuallyperform satisfactorily. Thebearings wear, but shafts arenot damaged. When intermit-tent seizures occur, zinc-a l u m i n u m b u s h i n g s c a nrecover and return to normalrunning. However, if heat gen-eration is too great, full seizureoccurs as temperatures reachthe 700 to 800°F range.

At these temperatures, how-ever, the zinc does not fuse tothe steel. Although layers ofzinc alloy may adhere to theshaft, they can be easilyscraped off and the shaft re-used. Scoring of the shaft is un-likely and, if grooves do form,they are shallow. Another fac-tor that helps emergency run-ning characteristics is that zincalloys have a greater affinityfor lubricants, which can pro-mote longer bearing life underdry conditions.

The low coefficient of frictionand the low melting point ofzinc alloys account for their ex-cellent emergency runningcharacteristics. Ironically, thelow melting temperature ofzinc alloys is also their greatestlimitation. Maximum operat-ing temperatures for zinc bear-

ings is in the range of 250 to300°F.

Longer bearing life

Several applications haveshown that Zn-12 and Zn-27provide longer life compared tobronze alloys such as 660, 64and manganese bronze. Some-times the life is several timeslonger. The zinc alloys are notnormally considered for self-lubricating bearings, butwhere lubrication cannot betolerated or where bearings areinaccessible for lubrication, thezinc alloys wear less and standup longer than bronze. Thelower coefficient of friction andthe higher hardness of Zn-12and Zn-27 appear to account forthis performance. In addition,the wear residue of bronzes isgenerally more abrasive thanthat of zinc.

Bronze bearings are madeprimarily from continuouslycast shapes, sand castings, andcentrifugal castings. Zn-12 andZn-27 bearings can be produced

by these same methods, withthe exception of the centrifugalcasting of Zn-27. (Preliminarystudies show alloy-segregationproblems.) However, thesecastings normally require ex-tensive machining to providefinished bearing dimensions.

More precise casting meth-ods can be used with the zincalloys, and this can drasticallyreduce machining. Thesemethods include die casting,permanent-mold casting, andgraphite-mold casting (a newpermanent-mold process thatprovides tolerances similar tothose of investment casting).These manufacturing methodscan be used not only to makebushings themselves but alsoto produce cast-to-size, one-piece designs of complete as-semblies that incorporate bush-ings. The combined effect ofcast-to-size technology and lowmetal costs opens many pos-sibilities for one-piece designseliminating or reducing sec-ondary machining and assem-bly costs.

Reprinted from MACHINE DESIGN December 10, 1981

Copyright 1981 by Penton/IPC. Inc. Cleveland, Ohm 44114

Another Success Story from Eastern AlloysZA-12 ZINC BEARINGS OUTPERFORMBRONZE IN STEEL MILL

ZA-12 zinc runout tab/e bushings replaced bronze in a steel hot mill environment. Tablerollers are seated in ZA-12 bearings and support fast moving hot steel from break down millsimilar to above. Zinc bearings provided service life double that of bronze.

A tough steel plant environment for abearing is at Crucible inc., Stainless & AlloyDivision, Midland, Pennsylvania, in their hotrolling merchant mill, where problems havepersisted with SAE 64 bronze bearings intheir runout tables. A year and a half ago,Crucible tried zinc bearings and the servicelife was found to be double that of bronze.As a result, Crucible is replacing the bronzebearings with zinc bearings.

The zinc alloy which is now being used ISEastern Alloys’ new high strength ZA-12(zinc-aluminum) foundry alloy. The bronzebushings were high maintenance items hav-ing a life of from one week to three years,depending upon location and lubricationfrequency. These simple lubricated splitflanged bushings see service when hotsteel from Crucible’s 10-12-14-inch rodbreakdown mill passes over rollers, whichare seated in the bushings, The steelcrashes into the rollers at speeds up to 600ft./minute and the 2-15/16 and 3-inch IDbushings must carry intermittent impactloads in excess of 520 Ibs.

Controlled bearing tests revealed thatwhen greased once a week for four months,ZA-12 showed no measurable wear and SAE64 bronze wore 0.030 inches. The same fourmonth test using an automatic oil lubrica-tion system showed no measurable wearwith ZA-12 and 0.005 inch wear for bronze.Under dry (no lubrication) conditions bronzetended to rapidly heat up and wear out.

ZA-12 zinc under these emergency condi-tions, where excessive heat from the steelwas not a problem, performed well. The zincalloy held lubrication longer and developeda smooth glaze which wore less. The resultwas longer service life with ZA-12 bearingsand a reduction in maintenance costs.

But that’s not all! The zinc bushings arepriced 20% lower than the identical bronzeparts.

Albco Foundry and Machine, Inc., Lisbon,Ohio, are specialists in steel mill bearingsand supply Crucible with their ZA-12 sand-cast bearings. Gary Staffeld, Vice Presidentof Operations, states, “We are getting simi-lar feedback from other customers on thegood performance of zinc. However, zincshould not be considered as an across-the-board substitute for bronze bearings. But,as we found with Crucible, there are manyareas where zinc does work well and atlower cost than bronze.”

“We like zinc foundry alloys” advisedAlbco’s Mr. Staffeld. “They give us a com-petitive edge because zinc alloys are muchless expensive than bronze alloys. In addi-tion, zinc takes less than half the energy to

Typical split flanged ZA-12 steel mill tab/ebearings now used by Crucible Inc., Mid-/and, Pa. Zinc bearings provided longerservice life and superior emergency running(no lubrication) characteristics at lowercost than bronze

melt compared to bronze and provides near-ly pollution-free handling in our foundry.”

Albco worked with Crucible for eightmonths on a test program before Crucibleswitched to zinc. The only area where ZA-12zinc could not be used was where hot steelwas moving too slowly, which allowed heatto build up and soften the zinc bearings.(Zinc’s melting point is about one-half thatof bronze.) In Crucible’s case, the hot appli-cat ion accounts for about 50% of theusage. The remaining 50% of the tablebushings have been changed to zinc.

Want to learn more about zinc alloys forbearings or other engineered applications?Come to the technical experts on zincfoundry alloys. Find out how high strengthZA zinc casting alloys (ZA-12, ZA-27 andZA-8) are being applied as sand castings,permanent mold castings, graphite perma-nent mold castings and die castings. We’llwork with your engineers and designers todevelop cost-saving ZA alternatives. Justcall or write Derek Cocks. (Complete tech-nical literature and case histories on bear-ings and other applications available uponrequest.)

eastern alloys, inc.Box Q Maybrook, N.Y. 12543 (845) 427-2151