TechReportPlastics have, since the 1950’s, shown the ability to out perform iron or steel in manyapplications. The crane industry, for example, has seen increased wire rope life andimproved equipment operation since the introduction of nylon as a sheave material.Wire rope running over a nylon sheave can have its serviceable life extended as muchas 450% through less crown and tangential breaks. Equipment operation is improvedsince nylon is lighter and has self-lubricating properties. These qualities allow forgreater loads on booms and lower maintenance costs.

Nylon is one of a large family of thermoplastic polymers. Simply, nylon is made up oflong chains of repeating chemical units arranged into crystal-like formations (crystal-lites). Crystallites are formed by these chemical units organizing themselves in closelyordered groups. These groups are surrounded by areas of disorder (amorphous re-gions). These crystal-like regions give the nylon its abrasion resistance. The amor-phous regions give nylon its resiliency.1

Nylon, by nature, lends itself to bearing materials and other friction reducing applica-tions. Nylon has a low coefficient of friction and resists abrasion. It provides a sur-face able to handle repeated abuse while retaining its original shape. Nylon will re-tain its inherent physical properties after continuous service at temperatures approach-ing 220°F.

In order to enhance the anti-friction qualities, nylon is often blended with either liq-uid or solid lubricants. The addition of lubricants does not significantly affect thephysical properties of the nylon, while substantially adding to nylon’s wear proper-ties. One lubricant used is molybdenum disulfide (MoS2). a natural lubricant. ThePolymer Corporation of Reading, PA, has developed proprietary formulas of nylonand molybdenum disulfide called Nylatron® nylon.

Nylatron® nylon is available in a wide range of formulas in both nylon 6/6 and nylon6 bases, each specially designed for specific needs. Nylatron® GS and GSM nylonshave proven to be excellent all around choices for sheaves in the lifting industry.Nylatron® GS nylon is a resin that is cost effective for high speed injection molding.The manufacturing process of injection molding limits the maximum cross-section ofa sheave, thus restricting it’s load carrying capabilities. Nylatron® GSM nylon, how-ever, is produced from a patented process of polymerizing material at atmosphericpressure. This process allows for large and complex sections to be produced free ofvoids. Nylatron® GSM nylon castings up to 48" in diameter and 700 pounds havebeen produced by The Polymer Corporation. In 1985, H & H Specialties Inc. intro-duced the Nylatron® GS nylon loft block sheave to the theatrical industry. Currently,loft block sheaves are manufactured using injection molded Nylatron® GS nylon andhead blocks cast from Nylatron® GSM nylon.

NYLA

TRON

®HH Specialties Inc.&

manufacturer of stage equipment

Bearing Installation

Nylatron® nylon sheaves can easily accommodate manydifferent styles of bearings. As a general rule, most bear-ings and bearing materials may be pressed directly intothe sheave with satisfactory results. Consideration mustbe given to the thermal expansion coefficient of Nylatron®nylon which is several times that of cast gray iron. Thisexpansion must be considered to insure that the bearingsdo not move within the bore of the sheave. To determinethe correct bore, use the following equation:2

Bore = Bearing OD - Press Fit AllowancePress Fit Allowance (d) = .009/DD = OD of bearing

Example: Find The correct bore for a precision bearingwith an OD of 1-3/8”:d=.009/Dd = .009/1.375"d = .009•1.173"d = .011"Bore = Bearing OD - Press Fit AllowanceBore = 1.375" - .011"Bore = 1.364"

Experience with relatively thin cross-sectioned Nylatron®GS nylon loft block sheaves has shown that satisfactoryresults can be accomplished with pressing bearings di-rectly into the sheave bore. The contact area betweensheave material and bearing surface is adequate to retainthe bearing in the bore. This procedure has proven ac-ceptable at loads normally used in the theatrical industry.Additionally, the bearings are mechanically retained inthe assembly by the sheave side plates.

Installation of bearings into sheaves with wide cross-sec-tions or extreme loads requires positive retention of thebearings within the bore. Nylatron® GSM nylon is inher-ently resilient and flexible. Under high loads, this flexingmay cause the bearings to move within a thick cross-sec-tion of material, resulting in bearing misalignment andpoor performance. In the theatrical industry, head blocksare subjected to greater loads than loft blocks and thepercentage of bearing contact surface is considerably lessthan a standard loft block assembly.

The most effective solution is to press a steel sleeve intothe sheave material. This sleeve is mechanically retainedin the bore with external snap rings, then bearings arepressed into the steel sleeve per manufacturer’s recom-mendations. This is the procedure normally followed onsheaves used in the crane industry.

An additional consideration is the pressure that the bear-ing will exert on the bore into which it is pressed. Themaximum allowable bore pressure for normal operationof a Nylatron® GSM nylon sheave is 4000 psi. The fol-lowing equation may be used to calculate the bore pres-sure.3

Pb = 2(U/Fd)/Db • Wh

Pb= Max. Bore Pressure (psi)Db = Bore Diameter (in.)Wh = Width of Bearing in Contact with Bore (in.)U = Breaking Strength of Wire Rope (lb.)Fd = Design Factor for Wire Rope

Groove Pressure

As wire rope bends around a sheave, the individual wirescontacting the groove surface exert pressure on the sheavematerial. This is known as groove pressure. Excessivegroove pressure can result in premature wear and/orsheave failure. In sheave design, the type of material usedand groove pressure are two important considerations. Thetheatrical industry has nominally used these allowablegroove pressures for various materials: 480 psi for ASTMA48, Class 30-35 gray iron and 1000 psi for steel. Groovepressure can be calculated by using the following equa-tion.4

P = 2T/DdP = Groove Pressure (psi)T = Load on Rope (lbs.)D = Tread Diameter (in.)d = Rope Diameter (in.)

Example: Find the groove pressure for a sheavewith a tread diameter of 8" and a load of 500pounds on 1/4”, 7 x 19 aircraft cable.

P = 2T/DdP = 2•500#/8"•.25"P=1000#/2"P = 500 psi

The Polymer Corporation publishes the following in theirNylatron® Nylon Sheave Design Manual, “If the groovepressure does not exceed 3500 psi, a Nylatron® (GSMnylon) sheave should perform satisfactorily under nor-mal operating conditions.”5

Wire Rope Life

The Polymer Corporation’s tests have shown that the useof Nylatron® GSM nylon can extend wire rope life asmuch as 450%. One test involved running wire rope overboth steel and Nylatron® GSM nylon sheaves with a loadof 10% and 20% of the rope’s breaking strength. Duringthe test, the ropes were rigged in such a manner that onerope ran only over the steel sheave and one rope ran overonly the Nylatron® GSM nylon sheave. The steel sheavewas machined from cast steel with the groove flame hard-ened to Rockwell C32 per crane industry standards. TheNylatron® nylon sheave was machined from Nylatron®GSM nylon solid stock. Both sheaves were machined tothe same set of dimensions. At set intervals, the test wasstopped and the ropes were inspected. The criteria usedfor rope failure was ANSI Standard B30.5. This standardstates that for running lines, “six randomly distributedbroken wires in one lay or three broken wires in one strandin one lay” are reason for replacing the rope.6

The test apparatus was loaded to 10% of the wire ropebreaking strength and run for approximately 136,000cycles before the conclusion of the test. Wire rope on thesteel sheave required replacement six times during thetest, while the rope on the Nylatron® GSM nylon sheavewas replaced once. Rope on the Nylatron® nylon side ofthe test apparatus lasted 4.5 times longer than the steelside. During the second part of the test the ropes wereloaded to 20% of capacity and run for 68,000 cycles.During this test the steel side needed five wire rope re-placements. The Nylatron® nylon side required two re-placement ropes. During this phase, the Nylatron® nylonside showed an improvement of 220% over steel.

In both tests, crown breaks were cited as the leading causeof wife rope failure on the steel sheave side. Crown breaksoccur when a cable contacts a material of similar or greaterhardness than itself. During this contact, tremendous pres-sure is exerted on the individual wires; in time these wireswill break. One solution is to provide a material which issofter than wire rope and able to handle the extreme pres-

sures. Nylatron® GSM nylon with its inherent resiliencyprovides an excellent solution. As the rope bends aroundthe sheave, the sheave material deforms slightly to pro-vide increased contact area for the individual wires. Thislowers the pressure on the wires and lessens the likeli-hood of crown breaks. The relationship between steel andthe Nylatron® GSM nylon sheave material is expressedin the following Hertzian equation. This equation definesthe maximum pressure at the center of the contact sur-face q0 is a function of moduli E1 and E2 of the two con-tact materials.

q0 = f [3/4(E1 • E2/E1 + E2)2]

Substituting E1 = 30,000,000 psi for steel and E2 = 400,000for nylon or the ratio of 1 to 0.013, the numerical value inthe brackets of the above equation will become .087. Atthe steel sheave side, E1 = E2, and the value in the brack-ets becomes 1. This indicates that the maximum contactpressure at the steel sheave side is 11.5 times of the con-tact pressure at the nylon sheave side.

This Hertzian equation shows that wire rope life can begreatly improved through the use of a material with amodulus of elasticity significantly different than steel.7

Endurance

In the past, many companies have considered replacementof steel sheaves with alternative materials. Two compa-nies investigating the performance of Nylatron® GSMnylon sheaves were Clark Equipment Company andUnited States Steel Company.

Clark Equipment Company tested the ability of aNylatron® GSM nylon sheave to handle various shockloads under different temperatures. Their test involvedrunning a wire rope over a Nylatron® GSM nylon sheavewith one end attached to a load cell. The other end of thewire rope was attached to a 5,000 pound weight whichwas dropped one foot. Additionally, the fleet angle waschanged from 0° to 6° in various increments. Clark Cor-porate Laboratories stated:

“The shock load measured using a nylon sheave was 90-103% of the amount measured when using a metal sheave.No damage to either the nylon or metal sheave was ob-served.” Clark Laboratories calculated the groove pres-sure peaked at 7,500 psi.8

United States Steel Corporation was interested in whethera Nylatron® GSM nylon sheave can support loads up tothe breaking point of wire rope. A test was conducted byrunning a 1/2" IWRC wire rope over a Nylatron® GSMnylon sheave and securing both ends of the rope to thetest bed. The sheave was attached to a load cell and aload was applied to the point of rupture. UnitedStates Steel Corporation reported:

“The total load on the platform at point of rupture of thewire rope was 54,900 pounds. There was no distortion ofthe sheave and only small superficial wire rope impres-sions were observed in the groove.” The calculated groovepressure on the Nylatron® GSM nylon sheave at point ofrupture of the wire rope was over 9,000 psi.9

Cold Flow

The phenomenon commonly called “cold flow” or creep,is a subject that has been studied in depth as it applies toengineered thermoplastics such as Nylatron® GSM ny-lon. Published data allows engineers to accurately pre-dict the stresses which can cause creep, as well as defin-ing the maximum deflection (or set) that a material willexperience over an extended period of time at a givenload (stress). Below a given minimum stress, creep is neg-ligible and is not a design consideration. The PolymerCorporation’s typical recommendation for minimum stressbelow which creep will not occur is approximately 2,000psi for Nylatron® GSM nylon. Since the sheave groovepressures in the theatrical industry are typically 1,000 psior less, any deflection will be “elastic”, meaning it willbe fully recoverable once the load is removed.10

The engineering principles are the same for Nylatron®GS nylon injection molding resin, however, cross-sec-tional area and design of the sheave will determine actualelasticity under load. A study performed at the fa-cilities of H & H Specialties Inc. in 1985, indicated thatunder a static load test of approximately 1,400 poundsfor over 90 days, the injection molded Nylatron® GSnylon sheave showed no abnormal deformation. The loadimposed was almost twice that recommended by the manu-facturer under normal operating conditions.

Theatrical Applications

In industries that have looked toward different ideas forimproving their products, Nylatron® GS and GSM ny-lons have shown to be viable alternatives to cast iron,steel or other materials. The theatre and entertainmentindustry is no exception.

H & H Specialties Inc. has been the leader in the use ofNylatron® nylons in the theatre industry in NorthAmerica. Since 1985, over 35,000 Nylatron® GS nylonloft block sheaves and over 1,600 Nylatron® GSM nylonhead block sheaves have been installed in over 500 the-atres all over North America. In every instance, the over-all performance of the rigging system has been improved.The installation of the equipment is considerably easiersince Nylatron® nylon is 1/7 the weight of cast iron.During a performance, personnel notice the differences

in operational characteristics. Nylatron® nylon sheavesoperate more quietly than cast iron or steel. Also, the re-duced weight of the sheave along with the self-lubricat-ing qualities of Nylatron® nylon and lower coefficient offriction, greatly reduces the amount of effort needed tooperate even the heaviest counterweight set.

Nylatron® nylon is being used in other areas of the the-atre. For years, arbors were guided with wood or pressedfiber products. These materials provided a hard wearingsurface, but did little to reduce friction. H & H Special-ties Inc. was the first to enhance the standard shoe guidearbor with the introduction of the first roller guided arborin 1972. A roller guide utilizes a small ball bearing sheavethat rolls along the guide rails. By rolling, rather than slid-ing, friction is reduced and operation is dramatically im-proved. Originally, these guides were equipped with stan-dard nylon ball bearing sheaves. Today’s design utilizesNylatron® GS nylon ball bearing sheaves, providing aguide that not only rolls along the guide rail, but requiresno outside lubrication.

Throughout history, manufacturers have sought to improvetheir products and consumers have demanded better per-formance and longevity. The crane and materials handlingindustries have seen that engineered plastics can providebetter performing and more cost effective alternatives toiron or steel. The Nylatron® nylon family of productshas shown superior durability and performance when usedas a sheave material. H & H Specialties Inc. was the firstto take this technology and introduce it to the theatre in-dustry in 1985. Though skeptically received at first bysome, the theatrical industry now accepts and embracesthis material. From the largest rigging sheave to the small-est track component, from high schools to Broadway,Nylatron® GS and GSM nylons are providing exceptionalperformance economically.

Model# 832N25 with 3 Idlers

Typical Properties of Nylatron® GSM and GS Nylons

Property Units TestMethod

Value ValueASTM GSM Nylon GS Nylon

Mechanical/Physical PropertiesTensile Ultimate StrengthTensile Elongation at BreakTensile ModulusCompressive StrengthShear StrengthFlexural StrengthFlexural ModulusTensile ImpactHardness, RockwellSpecific Gravity

Thermal PropertiesMelting PointCoefficient Linear

Thermal ExpansionDeformation Under Load(122° F, 2000 psi)Deflection Temperature

264 psi66 psi

Continuous ServiceTemperature in Air (Max.)

Flammability

Chemical PropertiesWater Absorption

24 hoursSaturation

AcidsWeak, 73° FStrong, 73° F

AlkaliesWeak 73° FStrong, 73° F

HydrocarbonsAromatic, 73° FAliphatic, 73° F

Inorganic Salt Solutions73° F

Bearing PropertiesCoefficient of Friction

(Dry vs. Steel)Dynamic

PSI%

PSIPSIPSIPSIPSI

ft. lbs/in2

R-

°F

in/in/°F

%

°F°F

°F-

%%

--

--

--

-

-

D-638D-638D-638D-695D-732D-790D-790

D-1822D-785D-792

D-789

D-696

D-621

D-648D-648

-D-635

D-570D-570

--

--

--

-

-

11,000-14,00010-60

400,00012,00010,500

16,000-17,500400,000

1501201.16

430±10

5.0x10-5

0.5-1.0

200400-425

200Self-Extinguishing

0.6-1.25.5-6.5

AcceptableUnacceptable

AcceptableAcceptable

AcceptableAcceptable

Acceptable

.25-.28

13,50015

450,000-600,00013,00010,50016,500

460,00050-180

1191.16

482-500

3.5x10-5

0.5-2.5

200400-490

220Self-Extinguishing

0.5-1.46.0-8.0

AcceptableUnacceptable

AcceptableAcceptable

AcceptableAcceptable

Acceptable

.15-.35

Note: Property data shown are typical average values and will vary on specific production lots and by size andconfiguration of product.All values shown are based on bone dry specimens.Nylatron® is a registered trademark of the Polymer Corporation Division, DSM Engineering Plastic Products, Inc.

Footnotes1. John H. Chen and Robert A. Alberts, EngineeringPlastics: Components in Construction Equipment(Earthmoving Industry Conference, April 1982) p.2.

2. Nylatron® Nylon Sheave Design Manual (The PolymerCorporation, 1984) p.6.

3. Ibid.

4. Wire Rope Users Manual (Committee of Wire Rope Produc-ers, American Iron and Steel Institute. 1981) p. 38

5. Nylatron® Nylon Sheave Design Manual (The PolymerCorporation, 1984) p.8.

6. John H. Chen and Paul E. Gage, The Polymer Corpora-tion, Improved Wire Rope Endurance Life with NylonSheaves. (13th Annual OTC, May 1981, Offshore Technol-ogy Conference, 1981) p.444

7. John H. Chen and Paul E. Gage, The Polymer Corpora-tion, Comparison of Wire Rope Life Using Nylon and SteelSheaves--Part 2: New Concept to Improved Predictability ofWire Rope Remaining Strength After Cycling (Off-HighwayVehicle Meeting & Exposition, Sept. 1979, Society ofAutomotive Engineers, Inc., 1979.) p.2.

8. Low Temperature Shock Load Testing of Nylatron® GSMSheaves. (Construction & Material Handling IndustryTechnical Bulletin. The Polymer Corporation, 1979).

9. Static Load Testing of Nylatron® GSM Nylon Sheaves(Construction & Material Handling Industry TechnicalBulletin. The Polymer Corporation, 1980).

10. Unpublished letter from Thomas Connelly, IndustryManager, The Polymer Corporation to Reid Neslage,February 23, 1993.

Selected Project List

Issaquah High SchoolIssaquah, Washington

Fox TelevisionLos Angeles, California

El Camino CollegeTorrance, California

Ohlone CollegeFremont, California

Moorpark CollegeMoorpark, California

MGM Grand HotelHollywood TheatreLas Vegas, Nevada

MGM Grand HotelGrand TheatreLas Vegas, Nevada

All statements, technical information and recommendations contained in this publication are presented in good faith based on careful research andpractical experience. The reader is cautioned that H & H Specialties cannot guarantee the accuracy or completeness of this information. It is thereader’s responsibility to determine suitability of this information as it relates to their own individual application.

© 1993, 1995, 1998 H & H Specialties Inc., P. O. Box 9327, South El Monte, CA 91733 (626) 575-0776 • (800) 221-9995. TR-1 Rev. 11/04

James K. Polk TheatreTennessee PerformingArts CenterNashville, Tennessee

Florida Community Collegeat JacksonvilleJacksonville, Florida

Pensacola Christian CollegePensacola, Florida

Mannequin Dance TheatreDisneyworldLake Buena Vista, Florida

ABC TelevisionNew York City, New York

Minskoff TheatreNew York City, New York

Six Flags Fiesta TexasTheme ParkSan Antonio, Texas

Music Mansion TheatreDollywoodPigeon Forge, Tennessee

Mel Tillis TheatreBranson, Missouri

Western Illinois UniversityMacomb, Illinois

Krannert CenterUniversity of IllinoisUrbana, Illinois

Andrew Jackson HallTennessee PerformingArts CenterNashville, Tennessee

New York-New YorkHotel & CasinoLas Vegas, Nevada

Fine Arts ComplexArizona State UniversityTempe, Arizona

Orpheum TheatrePhoenix, Arizona

Eastern New MexicoState UniversityPortales, New Mexico

Wortham Theatre CenterHouston, Texas

WFAA-TVDallas, Texas

Majestic TheatreSan Antonio, Texas

BibliographyChen John H., and Alberts, Robert A., The Polymer Corporation. Engineering Plastics: Compo-nents in Construction Equipment. Earthmoving Industry Conference, April 1982. Society ofAutomotive Engineers, Inc., #820641, 1982.

Chen, John H., and Gage, Paul E., The Polymer Corporation. Improved Wire Rope EnduranceLife with Nylon Sheaves. 13th Annual OTC, May 1981. Offshore Technology Conference, OTC4001, 1981.

Chen, John H., The Polymer Corporation and Ursell, C. R., Southwest Research Institute. Com-parison of Wire Rope Life Using Nylon and Steel Sheaves--Part 1: Test Methodology and Com-parison of Wire Rope Endurance Life. Off-Highway Vehicle Meeting & Exposition, Sept. 1979,Society of Automotive Engineers, Inc., #790904, 1979.

Chen, John H., and Gage, Paul E., The Polymer Corporation Comparison of Wire Rope LifeUsing Nylon and Steel Sheaves--Part 2: New Concept to Improved Predictability of Wire RopeRemaining Strength After Cycling Off-Highway Vehicle Meeting & Exposition, Sept. 1979, Soci-ety of Automotive Engineers, Inc., #790905, 1979.

Connelly, Thomas W., Industry Manager for The Polymer Corporation, a letter to Reid Neslage,VP H & H Specialties Inc., dated February 23, 1993.

Low Temperature Shock Load Testing of Nylatron® GSM Sheaves. Construction & MaterialHandling Industry Technical Bulletin. The Polymer Corporation Reading PA, ST-1, 1979.

Nylatron® Nylon Sheave Design Manual. The Polymer Corporation. Reading, PA, BR-29A.1984.

Procedure for Installing Two-Row Double-Cup Tapered Roller Bearings in Nylatron® GSMSheaves. Construction & Material Handling Industry Technical Bulletin. The Polymer Corpora-tion, Reading, PA, ST-3 (Rev.) 1981.

Static Load Testing of Nylatron® GSM Nylon Sheaves. Construction & Material Handling Indus-try Technical Bulletin. The Polymer Corporation, Reading, PA, ST-2, 1980.

Wire Rope Endurance Life Comparison for Nylatron® GSM vs. Steel Sheaves. Construction &Material Handling Industry Technical Bulletin. The Polymer Corporation, Reading, PA, WR-lr,1981.

Wire Rope Users Manual, Second Edition. Committee of Wire Rope Producers, American Ironand Steel Institute, 1981.