56 APRIL-JUNE 2014 engine professional

Product improvements and the need toreduce costs are constant in today’scompetitive market. As engines becomelighter, faster, more powerful and moreefficient, customers demand higherperformance, durability, reliability andreduced costs.

This continuous evolution in thedevelopment of engines increases themechanical requirements of thecomponents and joints, and therefore thefasteners, following the trend to be smallerand carrying higher dynamic loads. In thisscenario, fatigue and creep remain seriousconcerns for special engine builders.

SPS Technologies, established in 1903,has companies specialized inmanufacturing of critical automotivefasteners, such as connecting rod bolts,cylinder head bolts, main bearing bolts, forboth gasoline and diesel engines, withseveral locations in the United States andabroad. SPS Technologies is a subsidiary ofPrecision Castparts Corp. (NYSE:PCP), aworldwide manufacturer of complex metalcomponents for aerospace, automotive andindustrial applications.

SPS Technologies has complete fastenerlaboratories and engineering teams thatcan develop new products to meetcustomer needs and up-grade existingprojects. Extensive research anddevelopment, along with superiormanufacturing facilities and strict qualitycontrol, has made SPS Technologies aleader in the critical engineered fastenerindustry.

When a bolted joint works under anexternal load, the fastener must be suitablydesigned and tightened, in order to assureproper clamp load during any time of theworking cycle, avoiding issues such as jointopening, leakage, etc.

However, if this external load is highand dynamic, the bolt will work underhigh alternating stress, and then the fatiguestress acting on the bolt must be lowerthan its endurance limit to avoid failure byfatigue during the life of the joint.

The endurance limit of the boltdepends on the combination of severalfactors, such as:

• Material• Heat treatment• Forming design and process – cold,warm or hot

• Thread rolling process• Superficial discontinuities• Coating• Clamp load achieved during assembly

All of these factors must be carefullydefined through proper design andaccurately controlling the manufacturingand assembly processes.

Critical engine fasteners have featuressignificantly different from non-criticalfasteners. As a consequence, it requiresfurther knowledge, improved design andtest capabilities, special tools and modifiedshapes, special manufacturing processesand quality control techniques in order toguarantee joint performance for the

expected life of the engine. Theseconditions usually require a dedicatedplant to produce such critical fasteners.

Fatigue Calculation

The basis for a proper design of boltedjoints is presented on VDI 2230.

Threaded fastener fatigue resistance iscalculated for two thread rollingconditions. These two methods (A and B,shown below) were developed by Germanresearchers Kloos and Thomala:

a) Thread Rolling Before Heat Treatment:

b) Thread Rolling After Heat Treatment:

and

where:d = Nominal size of the bolt threadK = Ratio between the assembly clampload after relaxation (FV) and the yieldlimit force (F0.2) of the bolt.

(continued)

Cylinder Head Bolts Main Bearing Bolts Connecting Rod Bolts

Critical Engine FastenersBY SPS TECHNOLOGIES

57 APRIL-JUNE 2014 engine professional

ENGINE FASTENERSBY SPS TECHNOLOGIES

Fig. 1: Standard Values of Fatigue Resistance for Quenched and Tempered Boltswith Property Classes 8.8, 10.9 and 12.9, according to Kloos and Thomala.

The fatigue resistance of bolts with threads rolled after heattreatment is higher compared to a bolt with threads rolled beforeheat treatment, because of the compression tension induced onthe surface during the thread rolling process. The figure 1 abovepresents the fatigue resistance as a function of the thread rollingtype and the bolt size, as per Kloos and Thomala.

To achieve the fatigue resistance according to Kloos andThomala formula is necessary to follow specific procedures on alldesign, manufacturing, quality control and assembly steps thatwill assure the superior performance required by highperformance equipment, and that is SPS Technologies expertise.

The ability to simulate an assembly under working conditionsand test for the fatigue life of a bolt is accomplished through theuse of a Fatigue Test Machine, like the one in figure 2, and a testbench that will simulate the tightening method, as presented infigure 3.

Fig. 2: INSTRON Fatigue Machine (250 kN)

Fig. 3 (above): Test Bench

Fig. 4 (right): Fatigue Failure

Fig. 5 (below): Strain Gauge

Concepts

What is fatigue?Fatigue is the process of progressive localized permanentstructural change occurring in a material subjected to conditionsthat produce fluctuating stresses and strains at some point orpoints and that may culminate in cracks or complete fracture aftera sufficient number of fluctuations (as per ASTM E 1150).

How does creep or elongation occur?Creep occurs when a bolt is subjected to a stress at a hightemperature, which causes plastic deformation and high loss ofpreload. Elongation is a plastic deformation, which may occurdue to excessive torque or an inadequate friction coefficient, forassemblies controlled just by torque. However, a small and acontrolled amount of elongation is expected and desired forfasteners tightened with yield or angle control, as in most of the

58 APRIL-JUNE 2014 engine professional

ENGINE FASTENERSBY SPS TECHNOLOGIES

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critical bolts of engines (head, mainbearing, con rod, flywheel, and damper).

What are the consequences of too muchelongation?Too much elongation may take the bolt tonarrowing of its shaft beyond its maximumtension. When this happens, the fastener isconsidered failed, because in thisexcessively elongated region the clampload and endurance limit are severelyreduced.

How and what do you look for in material or processing to prevent elongation?SPS uses the correct alloy steel andintensive quality control when coldforming, thread rolling, heat treating andcoating processes to ensure the best boltfor the desired application. A co-design ofthe bolt/bolted joint/assembly parameterswith the customer’s engineering team isalso important to prevent undesirableelongation because it depends on theinteraction between bolt and bolted jointsurfaces, and the combined resilience of the joint.

What should an engine company look for when selecting a vendor?Engineers with deep know-how on designof optimized critical bolts and joints; fullyequipped laboratory for full studies andtests of bolts and bolted joints; capablemanufacturing process for criticalcharacteristics, such as under head bearingand thread dimensions, surfacediscontinuities control, thread rolling, heattreating and coatings.

Summary

SPS is a leading company in design andmanufacture of critical fasteners anddifferentiates itself by having skilledengineers using special resources such as:Finite Element Analyses (FEA) software,strain gage verification for stress andstrain, fatigue test machines, bench test forassembly simulation and to developtightening parameters, torque-tensionmachines to determine friction coefficients,vibration machine to test vibrationresistance, ultrasound equipment, modern

heading and thread rolling machines,special tooling and quality controlprocedures to guarantee fasteners withoutsuperficial discontinuities, like laps oncritical areas, and 100% sorting machineswith metallurgical and dimensional checks,among other techniques.�

Fastener Division of SPS Technologies, a PCC CompanyAutomotive OperationFor more information, please call 215-572-3000 or go online: www.spstech.com.