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Note: For accurate assessment all factors must be verified by test and entered as 'User Defined' values.  
 
 
 
 
 
 
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Bolt Arrangement
Arrangement
Thread
Bolt Material
Yield Stress (MPa)
Ultimate Tensile Stress (MPa)
Youngs Modulus (MPa)
Size
ISO 898 Bolt diameter (mm)
Thread Pitch (mm)
Bolt hole diameter (mm)
Root diameter (mm)
Area thread (mm²)
Ptich centre diameter (mm)
Minimum thread engagement - same grade (mm)
Width across flats (mm)
Washer OD (mm)
Washer thickness (mm)
Bolt head thichness (mm)
Nut thickness (mm)
User Defined Set "Size" to "User Defined" for bespoke bolts:
User defined bolt diameter (mm)
User defined thread Pitch (mm)
User defined bolt hole diameter (mm)
One of the major problems associated with traditional bolt tightening methods is as the diameter of the bolt increases, the amount of torque required to tighten it increases in the third power of the diameter. Because of this, the largest size bolt a person can typically tighten by hand about 24mm. Larger diameter bolts require torque multipliers, hydraulic tools or multi-jackbolt tensioners.  
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As bolt become larger wrenches become longer to apply more torque to overcome friction. Bolts larger than M24 or so may require a toque multiplier or hydraulic torque wrenches. When bolt size increases beyond M36 a multi-jackbolt tensioner may be used or hydraulic tensioners.
Torque Multiplier
Hydraulic Torque Wrenches
Multi-jackbolt Tensioner
Hydraulic Tensioners
When bolted joint is pulled apart part of the load increases the bolt tension and part reduces the joint clamp load. The stiffer the clamped material compared to the bolt the more joint tension goes into reducing the clamp load (and less into increasing the tension in the bolt). Increasing the angle of diffusion from the bolt head increases the clamped material stiffness. The spread of the compression zone may be limited by the diameter of the clamped material. The larger the amount of bolt shank in the clamped length the stiffer the bolt is. The stiffness factor is the bolt stiffness divided by the clamped material stiffness plus the bolt stiffness. Ideally these factors should be experimentally determined.
  Material (up to 10 layers below) Thickness (mm) Bearing (MPa) Approx. Limit (MPa) D:C Limiting Diameter (mm)
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2
3
4
5
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9
10
  Total clamped length (mm)    
  Threaded clamp length (mm)        
  Shank clamp length (mm)        
  Angle of diffusion (°) typically 25-35°    
    Program Calculated    
    User defined (test)    
 
   
You can define additional clamped materials below:
  User Defined Materials Young's Modulus (GPa) Bearing Limit (MPa) Thermal Exp'n (x10⁻⁶/°C)    
1    
2    
3    
4    
5    
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Note: A low nut factor produces a high bolt preload per unit of tightening torque. It depends upon the total joint friction and thread geometry.  Ideally these factors should be experimentally determined.
Program Calculated Friction on Rotating Parts
Head Friction
Rotating nut/head
Bearing on
Head Lubrication
Estimated rotating friction
User defined rotating friction
Program Calculated Friction on Threaded Parts
Thread Friction
Thread Finish
Tapping into
Thread Lubrication
Estimated thread friction
User defined thread friction
 
     
Typical nut factor values
0.30 - Stainless steel bolts
0.26 - Zinc plated
0.21 - Machine oil
0.20 - Untreated steel
0.19 - Phosphate and oil
0.16 - Black oxide
0.16 - Cadmium plated bolts
0.16 - Molybdenum-disulphide paste
0.14 - Molybdenum-disulphide grease
0.12 - Carnaba wax (5% emulsion)
0.12 - Grease, oil or wax
0.12 - PTFE lubricated bolts
Program calculated nut factor
Preload = Torque/(nut factor x bolt diameter)
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Experiment to show how friction effects bolt preload. Notice that both the face of the rotating nut (or head) and the thread are lubricated. Notice that identical bolts under the same tightening torque give a spread in bolt pretension values.
Despite conventional wisdom, torquing on regular nuts and bolts does not deliver consistent clamp. And when you don't have consistent clamp, you risk bolt failure. The tightest bolt can end up doing the work of 4 or 5 bolts, and as a result, finally fail. Structural integrity is compromised.
See how bolt coatings produce more consistant friction values and reduce the spread in bolt pretension results.
 
Due to variations in thread friction and accuracy of the tightening method the preload in a bolt lies between the high and low extremes. This is accounted for by the tightening factor.
User defined tightening accuracy (test)
The videos below show different tightening methods and how they impact on the tightening accuracy.
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Pretension accuracy and tightening methods
Torque, preload indicating washers, torque angle
Torque angle wrench.
Torque Angle wrench adapters.
     
Note: At the micro structure level, surface high spots bear against each other yielding overtime to give embedding losses. This is an estimate of embedding losses. Retightening the joint after 24 hours can mitigate against embedding losses.  If the material is painted it is prudent to account for the full paint thickness to be lost as an embedding loss. Ideally these factors should be experimentally determined.  
Predominant loading
Surface Finish
Thread embedding loss (μm) Typical value
Under head embedding loss (μm)
Under nut embedding loss (μm)
Loss at each interface (μm)
No. of clamped interfaces From 'Clamped' Tab
Total paint thickness of all clamped parts (μm)
User adjustment (μm)
Total embedding loss (μm)
Calculate Embedding Loss
Embedding loss (N)  
 
Compare with bolt stretch (μm)
PLUS Clamped plate compression (μm)
Embedding Loss  
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If the bolt material and the clamped material have the same coefficient of expansion and they have the same increase in temperature then there is no change in bolt tension. However if the joint contains mixed metals or if there is a temperature differential between the bolt and clamped parts then keep a watchful eye on thermal effects.  
Bolt temperature rise  (°C)  
Clamped material temperature rise  (°C)  
Increase in length of bolt (μm)  
Increase in clamped material length due to temperature (μm)  
Net change in bolt pretension (N)  
 
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The separation factor sets the tightening toque for the joint. It is the maximum joint tension required to separate the clamped parts compared to the bolt yield. 
It is good bolting practice to design a joint to separate before the bolt yields (leak before break). 
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Torque (Nmm) % Yield
Bolt preload (N)
Angle (°)
Bolt Extension (μm)
Joint Separation Load (N)
Program calculated separation factor
 
Applied Torque (Nmm)
Note: Embedding losses are included in the Lo results.
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How bolted joints work Part 1.
How bolted joints work Part 2.
As joint tension increases clamp load reduces. The remaining clamp load multiplied by  the coefficient of friction is the joint slip load. The lowest clamp force and lowest shear capacity is always the lowest pretension bolt across the pretension spread.
Joint slip is the mechanism for loosening (which will be very rapid in high vibrational environments). The ultimate strength in shear may be very much greater than the slip load but if you want to prevent loosening don't ever let the joint slip.
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Joint Tension Value %Axial yield    
Bolt axial yield (N) Joint Tension
Bolt preload (N)
User defined joint tension (N)
Joint Tension (% Bolt Yield)
 
Joint Applied Tension (N)
Maximum Clamp Force (N)
Minimum Clamp Force (N)
Maximum Bolt Tension (N)
Bolt Fatigue Life (Cycles)
Joint Shear   %Shear yield    
Bolt shear yield (N) Shear Planes
User defined joint shear (N)
Joint shear (% Bolt shear yield)
 
Joint Applied Shear (N)
User defined joint friction
Friction
No. of shear planes %Axial yield
Required Clamping (N)
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Joint lateral vibration loosens bolts and preload is lost.
Joint lateral vibration loosens bolts and preload is lost.
Axial stress in the bolt is due to the bolt tension and the tightening torque gives rise to torsional shear stresses. Bolts fail on tightening due to a combination of the two stresses generally at the shank/thread interface.
σ (Mpa) % Yield
Stress in bolt thread (bolt preload plus applied load)
Thermal stress in bolt thread
Stress from parallelism of bolting surfaces; angle °
Total Stress (MPa)  
Max Principle Stress (MPa)  
Broken Bolt Parallelism
Bolt failure due to torque and tension
Parallelism
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This check is not required for same grade nuts and bolts. Generally the thread stripping capacity of a nut is twice the tensile area of a bolt so a bolt always fails in tension before the nut thread strips. This same safety factor of 2 on thread stripping is used to calculate the minimum length of engagement in a tapped hole (see below).
Tensile strength of tapped material (MPa)
Safe length of engagement (mm)
Thread Stripping
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Thread repair - Keysert comes in four styles and a variety of materials, and can be installed with standard drills and taps.
Bolted joints may incorporate secondary security devices to protect the joint against loosening and embedding. A bolted joint that satisfies this BoltExpert assessment does not require any such locking features but it is accepted that they can provide additional security when the loading or other joint parameter is uncertain. It is likely that these devices will impact on the BoltExpert assessment e.g.  torque values should be increased to account for prevailing torque or additional friction on serrated surfaces (please follow the manufacturer’s recommendations). Remember that the devices themselves may need to be included as a ‘clamped part’. The videos below provide a guide to these devices.  
Locking feature:
 
Torque Adjustment:
*Maintains bolt tension in Junker vibration test:
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Nord-Lock X series compensating for embedding (slackening) and security against vibration.
 
Other wedge lock washer products available.
HEICO-LOCK WEDGE LOCK WASHERS - alternative to Nord-Lock  washer  
WEDGE-LOCKING HEADER BOLTS - alternative to Nord-Lock washers  
Hard-Lock Nut
 
The Perfect Lock Bolt (PLB) is a self-loosening resistant, dual thread, mechanically locked bolt and two nut fastener.
 
There are many useful properties of the joint on this page whatever tightening method you choose to use (torque alone, angle/torque and angle, bolt extension, hydraulic methods). For an angle/torque method (turn of the nut) typically 20% of the full toque value is applied to bring the contact surfaces together then 80% of the tightening angle should be applied.
UTS (N)
Yield (N)
Shear yield (N)
Torque to stretch bolt (Nmm)
Thread friction torque (Nmm)
Collar friction torque (Nmm)
Total tightening torque (Nmm)
Tightening angle (°)
Bolt extension (μm)
Clamp compression (μm)
Maximum preload (N)
After tightening variation (N)
Embedding loss (N)
Thermal Loss (N)
Minimum Preload (N)
Max joint separation load (N)
 Min joint separation load (N)
Loosening torque or off-torque (Nmm)
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