Rod Bolt Stress

[adrag]

I have a situation where I need to calculate the stress on a rod bolt. I have a 3/8-24 rod bolt which is torqued to achieve a bolt elongation of .006" as measured with a bolt stretch gauge. I have calculated the inertial load at the max rpm and I want to add that to the clamp load to get total load and then stress on the bolt. I’m using the equation:
dl = (F*L)/(E*A)
rearranged to
F=(dl*E*A)/L

My question is what number should I use for L? Should it be distance from under the bolt head to the first thread? To the middle of the threads? To the parting line of the rod cap? It makes a significant difference in the final stress.

For A, I’m using the smallest cross section of the bolt which occurs in the shank. I attached a jpeg with a cross section of the bolted joint. This should be a fairly simply calculation but this one detail is tripping me up. Thanks in advance.

 

[Tmoose]

"I have calculated the inertial load at the max rpm and I want to add that to the clamp load to get total load and then stress on the bolt......."

If you are planning simply straight addition of clamping force plus inertial load to "calculate" bolt load I think you will severely ( and incorrectly) over-estimate the resulting bolt load.

The slightly over simplifed situation is, the loading experienced by the bolt will not increase until the preload is overcome and the rod/cap parting faces separate.
Page 52 here -
" Suppose a joint has been tightened to a preload Pi and additional load, Pe, tending to separate the members is applied. In general in rigid
assemblies, as long as the external load is less then Pt it primarily decompresses the joint and has little effect on the tension in the screw.
Thus even if such a load is repeatedly applied, the fastener will not fail in fatigue. However, if a repeated external load greater than Pi must be
applied, it should be kept to a minimum, since it produces cyclic tensioning in the screw and may lead to fatigue failure.

http://www.holo-krome.com/pdf/Consolidated_Tech_Manual.pdf.

Then there is the "prying" action the bolt will be subjected to as the con rod big end deforms at high rpm exhaust TDC.

Along the lines of this -

http://img.photobucket.com/albums/v14/SaabTuner/ViggenData/TensileConRodDistortion.jpg

http://garage.grumpysperformance.com/index.php?threads/connecting-rod-strength-h-vs-i-beam.1168/

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As far as effective "L" I'm thinking it is a little less than the distance from bolt head face to approximately the first thread. The variation in diameter really limit the use of that formula, unless I solve it as a bunch of pieces

 

[CoryPad]

The length between measurement locations (conical regions in screw head and screw end) is the correct way to measure average strain (dL/L) in the screw, although the majority of the strain occurs in the waisted shank of the screw. Without a calibration technique, you will have unknown error in your calculation. You need to measure force vs.change in length of the clamped length of the joint, and change in length of the screw (between conical regions), to generate the correlation.

 

[desertfox]

Hi

Take a look at this link, on page six it gives a diagram of effective bolt length to use for the purposes of bolt load versus stretch calculation.

http://www.hexagon.de/rs/engineering fundamentals.pdf

“Do not worry about your problems with mathematics, I assure you mine are far greater.” Albert Einstein

 

[tbuelna]

Threaded holes with long engagement lengths, like that shown, are often thread milled with a progressive pitch. When a bolt with a constant thread pitch is installed in this threaded hole, it produces a tensile preload in the external bolt threads and a compressive preload in the mating internal threads. This helps to produce a more even stress distribution among the engaged threads.