Stage 8: Fastener Technology Presentation

The World’s Best Locking FastenerSince 1985

S8 Tech Ref

Agenda: “Importance of Proper Torque”

• Causes of Fastener Failures• Terminology• Class of Fit & Coarse vs. Fine• Types of Locking Fasteners• Elongation / Strain Charts• Types & Tolerances of Tightening Methods• Joint Diagrams• Torque Distribution• Fastener Tightening Methods & Accuracy

Fastener Failures

Main Causes of Fastener Failure:•Tensile Overload•Torsion / Impact Shear•Fastener Fatigue (85%)•Hydrogen Embrittlement•Corrosion (Chemical or Galvanic)•Insufficient Preload•Substandard Fasteners

Fastener Terminology

• Clamping Force: The compressive force which a fastener exerts on the joint. (aka Clamp Load)

• Breakloose Torque: The torque required to effect reverse rotation when a pre-stressed threaded assembly is loosened.

• Breakaway Torque: The torque that is necessary to start relative rotation between a locking fastener and its mating thread with no axial load on the screw.

• Proof Load (Bolt/Screw): The specified load the product must withstand without permanent set.

• Proof Load (Nut): The axially applied load the nut must withstand without thread stripping or rupture.

• Tensile Strength: The maximum tension applied load that a fastener can support prior to, or coincidental with, its fracture.

Naturally there are many more terms, but these definitions are particularly relevant to the Stage 8 locking fastener systems

Thread Cross Section

UNC (Unified National Coarse) UNF (Unified National Fine)

Basic Pitch Diameter

The Basic Pitch Diameter is halfway between the projected points of the thread crest and root. This value determines the tensile stress area of the fastener as well as the thread fit tolerance.

Class of Fit

Coatings can enlarge the pitch diameter by four times the coating thickness. Reduced distance between internal and external threads can increase the thread friction torque.

Coarse vs. Fine Threads

• Easier, Faster Assembly• Less Chance Of Cross Threading• Miner Thread Damage Less

Likely To Affect Assembly• Less Prone To Stripping In

Lower Strength Materials• Thicker Platings Possible, Less

Likely To Seize In Corrosion Applications

• Less Concentration Of Stress At Thread Root Radius, Better Fatigue Resistance

• Higher Strengths In Tension Due To Larger Tensile Stress Area

• Shorter Thread Depth Allows Threading In Thin Wall Applications

• Fine Threads Allow More Precise Adjustment Because Of Smaller Helix Angle

• Greater Strength In Limited Length Of Engagement Applications

• Fine Threads Easier To Tap In Harder Materials

COARSE THREADS FINE THREADS

Fastener Locking Types

LOCKNUTS• Prevailing Torque (Metal / Insert)• Serrated Flange• Hex Nut w/ Lock Washer• Jam Nuts

CHEMICAL ADHESIVES• Pre-Applied Micro Encapsulation• Anaerobic Sealants **

THREAD LOCKERS• Anaerobic Sealants **• O-Rings (Incl. Embedded) **• Nylon Sealing Rings **

LOCK WASHERS• Cam Faces & Radial Teeth

“Vibration, shock, temperature variations, chemical surroundings and other factors which occur during the life of a joint, cause loss of clamping load, which in turn, causes the fastener to loosen and eventually results in the fastened joint failure, owing to fatigue.“ Fastener Black Book

OTHER LOCKING TYPESCastle Nuts, Split Lock Washers, Split/Cotter Pins, Tooth & Serrated, Conical Lock Washers, Tab Washers, Wires & Wire Tie, STAGE 8 LOCKING SYSTEMS

3 Types: Free Spinning, Friction Locking, Chemical Locking

** Thread Sealers

Load-Elongation Behavior

Load-Elongation Behavior of Bolts - Hooke’s Law and the Modulus of Elasticity

Fastener Strain Curve

The Objective Is To Maintain The Fastener

Tensile Load In A Region Of

Effective Operation.

The Stage 8 locking systems are the best option for maintaining desired clamping force to keep the fastener in the design region of effective elastic tensile operation.

Difficulties In Bolted Connections

Internal Thread AxisNot Perpendicular

To Face Of Nut

Surface Not Flat And Perpendicular To Bolt Axis

Misaligned Holes

Head Bearing Surface Not Perpendicular

To Bolt Axis

Load Not DistributedOver All Threads

Hole NotPerpendicular

To Surface

Manner Of ApplyingExternal LoadMay Result In

Bending The Bolt

Because of these factors, it is rare that a load on a bolt is purely tensile.

Joint Diagram

A joint diagram is a means of displaying the load deflection characteristics of the bolt and the material that it clamps. Joint diagrams can be used to assist in visualizing how a bolted joint

sustains an external force and why the bolt does not sustain the whole of this force.

Design Preload

In order to determine the appropriate preload, the relationship with the joint AND the service load environment must be considered.

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Joint Diagram – Applied Force

When an external tensile force is applied to the joint it has the effect of reducing some of the clamp force caused by the bolt's preload and applying an additional force to the bolt itself.

Joint Diagram – Hard Joint

Importance of relative stiffness: in a ‘hard joint’, the bolt will only sustain a small proportion of the applied force due to the steep stiffness slope of the joint.

Joint Diagram – Soft Joint

In a 'soft' joint, because the stiffness slope of the bolt is greater than that of the joint, the bolt would sustain the majority of the same applied force.

Joint Influence on Fastener Design

Hard Joint

Soft Joint

For the same applied torque, the fastener preload can be different depending on the joint stiffness. In many cases, the shank of the fastener must be sized to match joint conditions.

Joint Diagram – Decompression Point

Decompression Point: zero pressure on the joint interface as a result of forces applied to the joint. Increased loading can lead to fretting at the interface, further reducing

bolt tension in a dynamic environment.

INSUFFICIENT TORQUE

Joint Diagram – Gap Formation

If the applied force is increased beyond the decompression point, a gap will form at the interface. This usually leads to bolt failure in fatigue.

INSUFFICIENT TORQUE

Joint Diagram – Yield Point of Bolt

When the external force acting on the bolt combined with the bolt's preload results in the yield of the bolt material being exceeded, imminent bolt failure is likely. Even if failure does not

immediately occur when the external force is removed, the preload will be reduced.

EXCESSIVE TORQUE

Yield Point ExceededReplaceFastener

Whether the yield strength of the fastener was exceeded in a static or dynamic environment, joint integrity has been compromised , affecting equipment reliability and availability.

Joint Diagram – Dynamic Loading

In a dynamic load environment, the variable applied forces to the joint results in fastener stress oscillation that requires fatigue limit design criteria.

Torque Distribution

This Value Can Increase With Flanged Bolt Heads / Nuts &

Friction Locks This Value Decreases With Application Of Lubricants, And Increases With Use Of Chemical Thread Locks And Prevailing Torque Locking Systems

Because the majority of the torque is used to overcome friction (usually between 85% and 95% of the applied torque), slight variations in the frictional conditions

can lead to large changes in the bolt preload.

Only This Portion Of Applied Torque

Results In Fastener Preload

Fastener Tightening Methods

Achieving the design preload must take the accuracy of the tightening method into account.

Method Accuracy Relative CostsFeel (Operator Judgment) ± 35% 1

Torque Wrench ± 25% 1½

Turn-Of-Nut ± 20% 5

Pre-Load Indicating Washers (Pre-Lubricated) ± 10% 7

Hydraulic Tensioners ± 10% 15

Tension-Controlled Bolts ± 8% 15

Bolt Elongation ± 4% 15

Strain Gages / Ultrasonic Measurements ± 1% 20Source: Fastener Black Book 1st Ed.

Torque = Correction Factor * Preload Force * Nominal Fastener DiameterT = KFd

Torque Wrench (±25%)

120,000 PSI

66,300 PSI

76,500 PSI (-25%)

127,500 PSI (+25%)

102,300 PSI

With torque wrench tolerances, An applied torque attempting to achieve an 85% preload can result in a tensile force that exceeds yield, or is close to minimum tension.

Proper Torque - Recommendations

For Critical Fastener Applications:• Design Preload Around Joint Stiffness

And Type Of Load Environment• Minimize Thread And Nutface Friction• Select Appropriate Tightening Method• Utilize Free Spinning Locking Systems