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## Detailed Bolted Joint Calculations.xls

Description:
A video accompanies this calculation.

Purpose of calculation:
Determine force distribution in a bolted joint.
Calculation Reference
Machine Design Juvinall
Scahum's Machine Design
http://em-ntserver.unl.edu/Negahban/em223/note16/note16.htm
http://en.wikipedia.org/wiki/Bolted_joint
http://www.boltscience.com/
Use 'Torque Pretension' worksheet also consult with the following references
Calculation Validation

1) Material and basic geometry geometry
Young's modulus of bolt
Poisson ratio of bolt
Young's modulus of clamped plate
Poisson ratio of clamped plate
upper clamped plate thickness
lower clamped plate thickness
total clamped plate thickness

2) Bolting details
bolt diameter
bolt clearance hole
(note: could be increased using a flanged headed bolt or a washer)
nut thickness
(note: enter 0 if threaded into flange)
bolt length assumed equal total clamped plate thickness + hnut

3) Torque Tightening and Bolt PreLoad Calculation
Method of torque tightening determines accuracy of bolt preload obtained.
It is not unusual to increase bolt size because the torque tightening method cannot guarentee a minimum preload.
The effects of the torque tightening method are shown on the joint diagram below.
Variation in preload (+ and -)
Tightening Torque (to yield bolt see Eqn. 37)
Coefficient of friction between screw and nut thread
Coefficient of friction at head/nut bearing collar
Angle between tangent to tooth profile (on the loaded side) and a radial line
Efficiency of the screw mechanism (ratio of useful work out to work in)
Torque to stretch Bolt
Torque to overcome collar friction
Joint pretension constants to compare with other threads
Typical K factors for comparison purposes
0.30 as received, stainless on mild or alloy
0.20 as received, mild or alloy on same
0.14 molybdenum-disulphide grease
0.12 PTFE lubrication
Off Torque - torque required to loosen the nut (if negative then torque needs to be applied to hold nut still)
Bolt Pretension and material properties
Bolt ultimate tensile strength
Bolt yield stress
Force on joint at which bolt yields (ignoring torsional effects)
% of bolt yield stress used by bolt preload

Tensile stiffness of a bolt shaft
Shear stiffness of a bolt head
Shear stiffness of a nut
Total stiffness of bolt

cone angle (normally 45° dispersion from bolt head/nut)
Compressive stiffness of clamped plates (in series)
Shear stiffness of clamped plates
Total stiffness of all compressive loadpath elements under bolt

6) Calculate Basic Joint Forces
Force in bolt when preload is lost (joint separates and contact force between clamped plates = 0)
Fbreak/Fleak: resulting safety factor

7) Calculate bolt loads for a particular external load applied to the joint
External load applied to the bolted joint
External shear load applied to the bolted joint
Contact force between clamped plates
Coefficient of friction between clamped plates
Shear capacity of joint

8) Graphical Representation of Results

9) Bolt Stress Analysis
Direct stress in the bolt
Shear stress in bolt (using torsion in circular shaft formula)
maximum principle stress
minimum principle stress
Von Mises Stress (note s3=0 for biaxial stress system)

10) Bearing stress under head of bolt
Note excessive bearing stresses lead to creep effects which will result in a loss of preload over time.
This relaxation is a function of:
Typically the bearing stress should not exceed the clamped plate yield stress.
Maximum bearing Stress

11) Joint deflections and nut rotation.
Bolt extension
Flange compression
Angle turned to extend the bolt

12) Losses at Clamped Surfaces
Paint compression factor
Maximum paint thickness (specification)
Number of painted surfaces
Maximum paint thickness (specification)

A simpler version of this calculation is available here.

Calculation Reference
Handbook of bolts and bolted joints

American Machinists' Handbook and Dictionary of Shop Terms

Basic Principles for Construction, Cengage Learning

Machinery's Handbook

Version History
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Submitted On:
25 May 2012
Submitted By:
File Date:
25 May 2012
File Author:
John Doyle
File Version:
1.2
File Size:
519.00 Kb
File Type:
xls
XLC:
396
Rating:
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#7 roymech1 2012-08-30 11:33
Hello quick correction to previous comment
kf = kf1.kf2/(kf1+kf2)

#6 roymech1 2012-08-30 11:29
Hello responding to comment on joint stiffness from brab2. I do agree and I suggest the following changes.
A separate calc for d1 and d2 with hf1 and hf2 in the equations and with the initial 2 deleted in each equation. A separate calc for kf1 and kf2 based on d1 and d2 ( and possible using separate E values (Ef2 and Ef2 ) although I do not think this is important.
kf = kf1+kf2 /(kf1.kf2).

I have created a modified version of the spreadsheet if John Doyle wants a copy.

#5 brab2 2012-08-29 17:06
Looking further into your spread sheet, there are further enhancements which could be made. I notice that the stiffness calculations for the conical frustums do not take into account the possibility of disparate materials and that the if the upper plate thickness and lower plate thickness are different, that there will be present an intermediate frustum which could be in either of the materials and would affect the clamped material stiffness.

#4 brab2 2012-08-29 15:26
Clearly a great deal of effort and thought put into this spread sheet but I think a couple of enhancements would make it perfect: