CosmosWorks 101 Bolt Connectors

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The COSMOS CompanionModeling Connectors - Bolts

Volume 101

What is the COSMOS Companion?

• The COSMOS Companion is a series of short subjects to help design engineers build better products with SolidWorks Analysis

• Video presentations and accompanying exercises

• A tool for Continuous Learning on your schedule

• Pre-recorded videos are accompanied by a more detailed webcast with Q & A – Download videos and review webcast schedule at:

http://www.cosmosm.com/pages/news/COSMOS_Companion.html

• It is not an alternative to instructor-led introductory training – We highly recommend you take a course with your local reseller to build a

solid knowledge base

Bolted Joints

• Fastener loads are an important output of a Finite Element model– Simple to calculate on simple problems

F L

F

F/2F/2

L/2

Bolted Joints

• Fastener loads are an important output of a Finite Element model– Not so straightforward as model complexity increases

Bolted Joints

Primary Causes of Bolted Joint Failure• Failure to Provide Sufficient Clamping Force

– Preload must exceed external loads

• Bolt Overloaded by External Loads– Bolt loaded beyond yield strength – Weakened Joint– Bolt loaded beyond tensile strength – Joint Failure– Shear Failure also critical

• Fatigue Failure– Repeated loading requires a preload in excess of peak alternating loads to

minimize cyclic loading of bolt

• Excessive Bearing Pressure– Compressive pressure under the bolt head or nut should not exceed the

compressive yield strength of the joint material

• Thread Stripping– Shear failure of threads as a combination of preload and external loading

Bolted Joints

Bolt Connector Output• Component Forces in

Shear, Axial, and Bending• Use these forces to size

bolts• Remember that in a

redundant system, bolt loads will be a function of their stiffness– Stiffer Bolts will carry more

load

• If bolts are resized, simulation should be repeated with new sizes

• Calculate bolt acceptability using standard tables and calculations for proof strength and thread failure

Bolt Connector Basics

Bolt With Nut• Choose Head & Nut Contact

Faces• Must be on separate parts• Should be Split Lines at Bolt

Head, Washer, and/or Nut Diameters

• Bolt holes don’t necessarily need to be selected or even exist.

• Bolt and Nut Contact Faces should be concentric but COSMOSWorks will allow selection of non-concentric faces!


Bolt without Nut• Select Head Contact Face on one

part and Shank Contact face(s) on another part.– Shank Contact Face corresponds to

the threaded hole in the second part involved in the bolted joint.

– Shank Contact Faces must be cylindrical, coaxial, and on the same part.

– Icon appears to allow 2 or more parts to be selected but only one Shank Contact Face is allowed

– Head Contact Face should be a Split Line at the diameter of the bolt head or washer & concentric with Shank Contact Face

• COSMOSWorks doesn’t require a split line, concentricity or a clearance hole in Component 1


Grounded Bolt• Grounded Bolt is the only Bolt Connector

option available in a Part document. Others require an assy

• Select Head Contact Face on Component 1– Head Contact Face should be a Split Line at the

diameter of the bolt head or washer • COSMOSWorks doesn’t require a split line,

concentricity or a clearance hole in Component 1

• Select a Plane representing the contact face which the Bolt preloads Component 1 against

– This must be the plane a Virtual Wall contact condition is (or will be) defined with.

– COSMOSWorks allows you to exit the input form without selecting this plane but the solve will fail!

– Must be a reference plane, not a flat solid face– If a Virtual Contact condition is not defined, the

solver will return with an error message instructing you to define it.


Tight Fit Option• The tight fit option requires the selection

of a thru hole• It is assumed that the bolt has no radial

clearance and, essentially, ‘plugs’ the hole.

• In reality, COSMOSWorks makes the hole around the bolt rigid so that there are no local deformations.

• This results in an overly stiff bolt/hole and should be used with caution.


• General Assumptions– The Bolt Head/Washer and the Nut contact faces

always remain in contact with the Bolted Components and cannot slide due to shear loads (infinite friction).

– If the Tight Fit option is selected, the Bolt shank is rigidly tied to the thru holes. They cannot slide or deform under shear loading – No clearance or elasticity in the bolt.

– Otherwise, the Bolt Shank does not interact with the thru holes in any way.


• Define a material or choose one from the COSMOS Material Library

• If a library material is chosen, COSMOSWorks copies the Young’s Mod, Poisson’s Ratio, and CTE to the Bolt definition…it doesn’t link the Bolt to the material– Changes to a library material will not be

reflected in any existing Bolt definitions– Failure properties (Yield, Ultimate Strength,

Fatigue Strength) are not used by the Bolt


• Preload can be defined as an axial force or as a Bolt Torque

• An axial preload imposes an internal force on the Bolt that imparts a compressive load on the bolted parts– A spec of 1000 lb preload gives you a

1000 lb preload

• A Torque preload converts input torque (as might be applied to the head or nut) to axial preload.

Preloading Bolts

• The Torque to Preload conversion uses these relationships:– Bolt with a Nut: Faxial = T/(K*D) **Nut Torqued– Bolt w/o a Nut: Faxial = T/(K*D*1.2) **Head Torqued

• The 1.2 factor compensates for shank wind-up when the head is tightened.

• T = Applied Torque

• D = Nominal Bolt Diameter

• K = Torque Coefficient

Preloading Bolts

Torque Coefficient, K• The calculation of K involves thread diameters, thread lead

angle, friction coefficients, and thread angles• Since friction coefficients are very difficult to estimate in a

real-world application, it is recommended that published ‘ball park’ K values be utilized:– Non-Plated K=0.30– Zinc-Plated K=0.20 – 0.28– Lubricated K=0.18– Cadmium-Plated K=0.12 – 0.15

• When in doubt, use 0.20 for initial analyses but it is highly recommended that a more empirical correlation of torque to preload be made using the actual parts, bolts and tightening system to ensure better predictability of preload

Preloading Bolts

More thoughts on preloading:• Remember that the torque that contributes to preload includes a component of

“running torque” which is the torque required to overcome friction before load is engaged. This also varies greatly and must be subtracted from the total applied torque

• High levels of friction or the use of thread lock can cause high shear (torsional) stresses to occur in a bolt shank that must be added to axial stress when considering applied stress. It is recommended that the total stress in a single-use bolt not exceed 90% of the Yield Strength

• Up to 85% of the measured torque can be attributed to losses that don’t contribute to preload such as under-head friction and thread deformation. Again, correlation in a controlled test is ideal

• Typical error levels for different preloading methods:– Operator “Feel” +/- 35%– Torque Wrench +/- 25%– Angle (Turn of Nut) Control +/- 15%– Load Indicating Washer +/- 10%– Measure Bolt Elongation +/- 5%– Hydraulic Bolt Pretension +/- 1-10%– Strain Gage/Ultrasonics +/- 1%

Bolts in Shear

Clearance Hole0.36” Dia

100#

100#

0.25”

100#

0.25”

M=100# * 0.25”= 25 lb-in

M

M

Bolts in Shear

Bolt Connector:Solid Bolt:

Difference:

0.00039”0.00037”5%

Bolts in Shear – Tight Fit

30% DifferenceNon-Conservative if bolt is expected to carry shear load

Bolts in Shear – Tight Fit

ConnectorNote Holes retain Cylindrical Shape

Solid BoltNote Holes Deform as expected

Bolts in Tension

100#

Bolts in Tension

• Bolt Strength = Proof Strength, Sp– 90% of 0.2% Yield Strength

• Proof Load: Fp = At * Sp– At = Tensile Stress Area

• ¼ - 20 UNC Bolt Grade 5– Nominal Diameter, Dn = 0.25 in– Tensile Stress Area, At = 0.0318 in2

– Grade 5 Proof Strength, Sp = 85,000 psi– Proof Load, Fp = AtSp = 2,700 lbf

• Guideline Preload, Fi– Fi = 0.75Fp : Reused Connections– Fi = 0.90Fp : Permanent Connections

• Preload for Permanent Connection = 2,430 lbf

Source: Mechanical Engineering Design 5th Ed.; Shigley & Mischke; McGraw-Hill; 1989

Bolts in Tension

Von Mises Stress Contact Pressure

Bolts in Tension

F=2430#D = 0.000172”Kj = F/D = 14,125,000 #/in

A0.25 = 0.049 in2

E=30e6 psiL = 0.50 in.Kb = AE/L = 2,945,000 #/inPreload Extension = F/K = 0.000825 in.

Joint Stiffness Calculations

Compression Stiffness of Joint, Kj

Extension Stiffness of Bolt, Kb

D

L

Bolts in Tension

Joint Diagram

JointCompression

Preload

Bolt ExtensionPreload

3,000

0

2,000

1,000

App

lied

Load

(Lb f

)

Prel

oad

( 2,4

30 lb

f)

Sepa

ratio

n Lo

ad (

2,95

0 lb

f)

Bolt CarriesEntire Load

Extension/Compression (in^10-4)

Bolts in Tension

Separation Load

P3 Stress

Contact Pressure

P1 Stress

Presentation Summary

• In this COSMOS Companion unit, we explored the use of Bolt Connectors to represent bolted joints

• Inherent assumptions for each Connector type was reviewed

• Bolt Connector output compared favorably to theoretical results

• The concept of Joint Diagrams was introduced as a means to estimate separation loads for bolt sizing including preliminary preload calculations

Conclusion

• For more information…– Contact your local reseller for more in-depth training or

support on using Connectors and modeling bolted joints– Review the on-line help for a more detailed description

of the features discussed– Attend, or better yet, present at a local COSMOS or

SolidWorks user group. • See http://www.swugn.org/ for a user group near you

– References on bolted joints:• Mechanical Engineering Design 5th Ed.; Shigley & Mischke;

McGraw-Hill; 1989• Unbrako Engineering Guide; SPS Technologies; Cleveland, OH;

www.unbrako.com

Documents

CosmosWorks 101 Bolt Connectors