fti jg pp presentation torque tension 090419

8/3/2019 FTI JG PP Presentation Torque Tension 090419

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Torque Tension


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Copyright Fastener Training Institute Page 2

Torque Tension

1. What does torque-tension mean?

2. What is more important for insuring a tight joint,

torque or tension?

3. What happens to joint tension if you apply thesame torque as in the past, but this time you adda lubricant to the bolt and/or nut?

4. If you have two identical nuts except one hasserrations on the bearing surface, what happensto the tension in the joint?

5. What is the most commonly used torquecalculating formula and what are the variables in

the formula?

Preview Questions


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Torque Tension

95% of all fastener failures are the result of

poor selection or improper installation!


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Torque Tension

Why do fasteners fail?

1. Poor fastener selection.

Fastener strength less than operatingloads occurring in the application

2. Loss of tension during operation:

a. Joint relaxation

b. Joint creep

c. Thermal expansion and contraction of

joint3. Fastener not properly loaded (stretched)

during tightening.


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Torque Tension

Think of a rubber band as a

fastener similar to a bolt and nut.


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Torque Tension

This rubber band looks tightas does the bolt and nut.


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Torque Tension

But when forces are exerted on the assemblyit becomes obvious that the fastener is

insufficiently tensioned!


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Torque Tension

When the fastener is put under greater tension

it withstands the forces exerted on the assembly!

Properly tensioned bolts stay tight!


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Torque Tension

Bolts act like heavy coil springs

They must be stretched to be effective!


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Torque Tension

Bolts must be stretched and staystretched to avoid failure!

Hooks Law:

Steel bolts stretch .001 inches per inchbetween bearing surfaces for every30,000 PSI of tension.


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Torque Tension

Hooks Law Example:

Fastener: 5/16 18 X 3 3/4 Grade 5 HHCS

Distance between bearing surface: 3 inches

Yield Strength (YS): 92,000 PSI

75% of Yield Strength: 69,000 PSI

Grade 5 Stretch: = (69000/30000) X 3

Stretch length to achieve 75% YS: .0069 inches


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Torque Tension

For joints to maintain tightness, the boltstension must exceed the greatest possible

service load.


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Torque Tension

Fastener Tightening Analysis


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Torque Tension

Bolt tension-elongation curve


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Torque Tension

Torque-Tension Analysis Equipment


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Torque Tension

Loading the tension analyzer


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Torque Tension

Torque-Tension Relationship:-20 Grade 5 bolt and nut


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Torque Tension

Pre-set click wrench to 10 ft.lb. for-20 Grade 5 bolt (75% yield strength).


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Torque Tension

-20 Grade 5 torque-tensionusing click wrench set at 10 ft.lb.s:


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Torque Tension

Bolt tension increases when lubricity isimproved and torque remains the same.


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Torque Tension

Serrated nuts and bolts provideseveral assembly benefits..

..but, installation torque values should be

adjusted to achieve proper bolt tension.


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Torque Tension

Bolt tension decreases when bearing surfaceis serrated and torque remains the same.


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Torque Tension

36 foot pounds was required toachieve 2400 pounds of tension.


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Torque Tension

Serrations change thetorque-tension relationship

A. Serrations act like plow, dramaticallyincreasing friction.

B. For a given amount of torque, tensiondrops dramatically.

C. Installation torque should be raised.


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Torque Tension

What happens when a bolt is over-tightened?

It stretches beyond its elastic limit and

elongates between the threads.

Nut goes

on before

tightening.

Nut will not

go on after

tightening.


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Torque Tension

Calculating Tightening Torque Values

Formula: T = D P K

T = Torque (inch pounds)

D = Bolt Diameter (decimal)

P = Pounds of Clamp Force

- 75% of Yield Strength (PSI) X

Tensile Stress Area (TSA)

K = Friction Factor (estimate)


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Torque Tension

Calculation Example:

Example:

-20 X 2 SAE Grade 5, HHCS, Zinc

D = .250P = .75 (92,000) X .0318 = 2194 pounds

K = .22

T = .250 X 2194 X .22 = 120 inch pounds

T = 120/12 = 10 foot pounds


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Torque Tension

Example: Same as previousexcept Dacromet L finish on bolts

Seating torque for

1/4-20 Grade 5 bolts with wax finish

T = .250 X 2194 X .15

T = 77/12 = 7 foot pounds

Bolt tension induced if

torque is not adjusted:

10 ft.lb.s = .250 X P X .15

P = 10 ft.lb. / (.250 X .15)

P = 3200 pounds force

(DANGER! 3800 lb. Is minimum UTS)

K Factors:Zinc: .22

Dacromet L: .15


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Torque Tension

Bolt tension varies by +/-25% whentightened with an indicating torque wrench.


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Torque Tension

Determining Torque TighteningValues By Experimentation

Assembly variables:

Joint StiffnessSurface Hardness

Surface Roughness

Surface Flatness

Bolt and Nut Grades

Bolt and Nut Finish


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Torque Tension

Determine seating torque by testing.

A. Torque 10 or moreparts to yield(rotationwithout torque increase) or failurein the actualapplication.

B. Averagethe yield or failure values.

C. Use a seating torque equal to 65% - 75%ofobtained in B above.


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Torque Tension

What helps keep fasteners tight andwhy?

Properly selected and stretched bolts

seldom come loose!


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Torque Tension

Match Bolt and Nut Grades By Strengths

Rule: The proper nutfor a given bolt isthe one having a proof load strengthequal to or greater than the matingbolts ultimate tensile strength.


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Torque Tension

SAE Grade Hardness Strength 3/4 10

Bolt Gd 2Rb 80/100 74 KSI UTS 24,700 lbs

Nut Gd 2 Rb 68/Rc 32 90 KSI PS 30,100

Bolt Gd 5 Rc 25/34 120KSI UTS 40,100

Nut Gd 5 Rb 68/Rc 32 120 KSI PS 40,100

Bolt Gd 8 Rc 33/39 150KSI UTS 50,100

Nut Gd 8 Rc 24/38 150KSI PS 50,100

Match Bolt and Nut Grades By Strengths


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Torque Tension

When things fail, harder

is not always better!

Harder bolts are more brittle than softerbolts.

Softer bolts are more ductile.

Move to Grade 9 and Property Class 12.9very cautiously. Grade 8 and PropertyClass 10.9 properly used are generally

superior.

Beware of hydrogen embrittlement andstress corrosion cracking.


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Torque Tension

Use drivers with torque control

ora combination of driver

and torque wrench in critical joints.


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Torque Tension

Control impact wrench torque

Torque sticks based on torsion bar principle.

30 ft.lb. to 475 ft.lb.


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Torque Tension

How much torque was applied?

Part is seated to an

unknown torque

Mark head

Mark surface

Mark socket


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Torque Tension

This is approximately the amount of torqueoriginally applied to the fastener!

How much torque was applied?

Break part loose

Re-tighten bolt to original positionAnd observe torque


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Torque Tension

Tension Distribution in Bolts and Nuts

Long lengths of thread engagement

do not increase joint strength!

Bolt

Nut


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Torque Tension

When short bolts vibrate loose,increase length to allow more total stretch.

Stretch for Grade 5 bolt tensioned to 69 KSI (75% YS):

.00421.500

.0014.500

StretchGrip length


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Torque Tension

Conclusion

When tightening is critical, select torqueby experimentation.

When anything changes re-evaluateseating torque values.

Properly tightened fasteners seldomcome loose.


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Torque Tension

Calculating minimum thread depth

for assembly components

See section A of the IFI 7th Edition Inchand 3rd Edition Metric Books


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Torque Tension

Calculating minimum thread depthfor assembly components


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fti jg pp presentation torque tension 090419

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