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• # Shaft loads static or fatigue calculation ?

Discussion in 'The main mechanical design forum' started by Yuri, Dec 11, 2013.

1. ### YuriMember

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Hi

I have a new question. I need to calculate shaft strenght.

the configuration is shaft with two self align bearings and a pulley in the midlle.
the radial load have constant direction, the torgue change direction (spining is reversed)

Now the question: is the shaft static loaded or i have to consider and calculate for fatigue failure ??
I am using Ivertor design calculator will do and FEA analysis.

The main thing is i have to choose safety factor which is different for the two types of loading

I hope it makes sense.

2.
3. ### ErichWell-Known Member

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Your shaft is subject to fatigue loading. Consider the radial load from the viewpoint of the shaft. As the shaft spins it appears that the radial load reverses direction. You have fatigue. How frequently does the shaft change directions?

4. ### YuriMember

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I also find out that is subject to fatigue now trying to understand the calculations. I think i have Alternating /Reversing Stress due to bending of the shaft compresion/tension on every 180 degrees. I think i have to find Se-modified fatigue strenght ?

I will post all the data I have and hope somebody will help.

direction of rotation is changed normaly every 30-40 seconds. Using VFD for smooth change CW to CCW

5. ### YuriMember

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Fatigue calculation

SeÂ´=0,5.Sut
Se=Ka.Kb.Kc.Kd.SeÂ´

Sut- Ultimate tensle strength
SeÂ´- Endurance limit stress
Se- Endurance limit stress of component
Ka- surface finish factor = 1,58. 0,57 = 0,89 (ground)
Kb- Size factor = 0,85
Kc- Reability factor (90% survival ) = 0,897
Kd- modifyin factor to account for stress concentration = 1/Kf
Kf- stress concentration factor

I need to find out stress concentration factor Kf ? D/d=1,28 r/d= 0,03 material steinles steel 420

6. ### YuriMember

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Update

using eAssistant http://www.eassistant.eu/en/startseite.html for the fatigue calculation according DIN 743. Keyway section in the middle fail.
Can somebody confirm if the shaft is rotating and have constant radial load in one direction is it subject to alternating bending or constant bending ?

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8. ### JonGreeneNew Member

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I agree with Erich, you certainly have alternating/fully reversing bending. I would start with a bending moment diagram, torque diagram, and angle of twist diagram; These diagrams will help you define the stress within the shaft:

Bending moment diagram will help you define the moment at any point along the shaft. If you take an element at the surface of the shaft you can use M(x) to calculate stress in the axial direction of the shaft (Axial Stress = Mc/I).

Torque diagram will help you define the shear stress at any given point in the shaft (Shear Tau = Tc/J). The shear stress is felt by the same element as above; Your element now possesses shear and axial stresses.

Be careful as your shaft is certainly in combined loading; the pulley produces both a torque and a static force etc.

Once all the stresses on an element are known, you can convert them to principle stresses and use the octahedral stress equation to obtain the Von Mises stress or single equivalent stress on the element. Compare this to your yield stress and you can understand the factor of safety built into the shaft.

The rule of thumb when it comes to twist is your shaft should not twist more than 1 degree within a length 20 times it's diameter. An angle of twist vs shaft length diagram will help you see if you are meeting this criteria with the current material (angle of twist = TL/JG)

Hope this helps.

9. ### vyasangNew Member

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Should we design a rotating shaft with steady bending loading only for fatigue failure or should we design it for static and fatigue failure both?

10. ### ErichWell-Known Member

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As with all things engineering, it depends.

It depends on your specific design requirements.

Is this shaft going to used in a piece of equipment that will turn many millions of revolutions? Yes, design for fatigue.

Is this shaft in a disposable toy being given away for free? Static is good enough.

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