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Engineering Question

Joined
19 August 2002
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Location
Miami, Florida US
How much force does it take to this to a bolt. It was holding a pulley and correctly tightened.


Armando
 

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Comparing to the size of the penny, that looks like a 3/8"-24 fine thread hex head cap screw.

The gold color tells me that it is plated, so it is probably a "garden variety" fastener from a local hardware store, perhaps a grade 6. Look at the top of the hex head. If there are six equally spaced radial lines, it is a grade 8 fastener.

I'll do some calculations.
 
It also depends how many times the bolt was tightened, bolts fatigue and stretch with reuse. Many application require bolt replacement at a certain number of uses, sometimes after only one. Usally the more important and the higher the stress dictates the design of the bolt, these would include rod bolts, crankcase bolts, brake caliper bolts, cylinder head bolts as some examples.
 
AndyVecsey said:
Comparing to the size of the penny, that looks like a 3/8"-24 fine thread hex head cap screw.

The gold color tells me that it is plated, so it is probably a "garden variety" fastener from a local hardware store, perhaps a grade 6. Look at the top of the hex head. If there are six equally spaced radial lines, it is a grade 8 fastener.

I'll do some calculations.
Andy

There are 3 lines on the left bolt and 6 on the right one. If the fix we talked about earlier dosent do it, Whats your feeling on tapping the shaft to accept a larger bolt?

Armando
 
Three lines = grade 5 bolt.

Six lines = grade 8 bolt.

Tensile strength of a grade 5 bolt is 85,000 PSI. To fail that this bolt in tension requires 6800 LB of force. To fail this bolt in torsion, as in over-tightening, requires only 47 FT-LB. Assume we torque the bolt to only 75% of this value for safety's sake to 35 FT-LB.

For a grade 8 bolt, the tensile strength is 120,000 PSI. Therefore, it will fail in tension with 9600 Lf of force or 66 FT-LB or torque.

To put into perspective, the oil pan plug is tightened to 33 FT-LB and that is a larger bolt. Another reality data point is the wheel lug nuts......slightly larger threads torqued to 80 FT-LB.

Assuming the grade 5 nor grade 8 bolt was over-torqued, the failure mode may be from fatigue, which is somewhat representative of the picture, but I'm not 100% sure because of my browser's image resolution. As Arata mentions, a fastener that is repeatedly used, will over time it fatigue fail. However, this bolt is a one-time application so the fatigue source would be more difficult to trace.

As far as using a larger bolt, the next largest is 5/16". The female hole in the end of the shaft has to be drilled out. If the remaining 3/8" threads are still present even a bit, then you have to go to the next largest bolt which is 1/2" Relative to the shaft diameter, that is huge and may invite higher shaft stress in the bolt area.
 
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Bolt failure

The failure looks like fatigue to me. I would get new grade 8 bolts, clean up the female threads and blue loctite the new bolts in place at their called out spec. If there is no spec, go with Andy's recomendation of no more than 75% 66 '-lb..

Make sure the bolt is not bottoming out in the hole, you may need to put washer under the head to get the right clearance. That is a long shot seeing you could get the stub out of the hole.
 
One other thing you need that it does not look like you have is 1.5 times the bolt dia being threaded into the female threads.

Example: 8 MM bolt needs 12MM of thread engagement.
 
thread engagement?

Do you think more engagement would prevent this type of failure? To me it looks like a failure in a tension mode. I thought 1.5 time rule had to do with bending loads and when a bolt is in tension with a standard class B fit, only 2 threads are actually transmitting load.

What was the part that these bolts broke off of? How many bolts were holding it together and how many of the bolts broke?

Nice chart, bookmarked it!
 
On the right bolt it looks shorter as the bolt broke in 3 parts, (one is not in the picture) I replaced it with a #8 and torqued it as per Andys sugestion, I also cleaned everything out and added blue thread lock. The bolt is holdng a pulley which in turn is spinning the blower.


thanks guys for your input ill let you know if it lets loose again.

Armando
 
AndyVecsey said:
As far as using a larger bolt, the next largest is 5/16".

Andy – lots of good info!

As far as English bolt diameters go, 7/16 is one size larger if your broken ones were 3/8” From the looks of the bolts in the picture, id say they are either ¼” or 5/16” but it’s easy to determine using a caliper or comparing to known sized nuts.

Getting larger, English bolt/screw diameters:
4 / 16 =1/4
5/16 = 5/16
6/16 = 3/8
7/16 = 7/16
8/16 = 1/2

As far as 10.9 grade: I believe this is the standard hardened grade for metric fasteners, I don’t know how it compares to the English hardness grade. Most English screws sold in hardware stores are grade 2 (shear bolts), but most stores have a grade 5 (3 marks) and grade 8 (5 marks and gold looking) section. If there are no marks on the top of the screw or just a triangle then assume grade 2.

DanO
 
DanO said:
Andy – lots of good info!
As far as 10.9 grade: I believe this is the standard hardened grade for metric fasteners, I don’t know how it compares to the English hardness grade. Most English screws sold in hardware stores are grade 2 (shear bolts), but most stores have a grade 5 (3 marks) and grade 8 (5 marks and gold looking) section. If there are no marks on the top of the screw or just a triangle then assume grade 2.

DanO

He mad no mention of wether this is a SAE or metric thread I assumed metric. You can't always tell by the marks on the top of the bolt, which was my point with the chart. If you look you will see more than one type of bolt has 6 lines on top.

http://metricmcc.com/catalog/Ch10/10-1021.pdf

http://metricmcc.com/catalog/Ch10/10-1020.pdf

http://metricmcc.com/catalog/Ch10/10-1027.pdf
 
If your current fix fails, you can try socket head cap screws. These are usually much stronger than grade-8 or metric 10.9 fasteners. I think the rating is somewhere around 180,000 PSI tensile.



Also, I'd be careful about undertorquing the bolts as suggested.
Many joints are dependent on the friction caused by the tightened bolt. The friction is what keeps the parts from moving, not the bolt in the hole. Undertorquing might not produce enough friction and will allow the assembly to shift under load. This movement will fatigue the bolt and lead to eventual failure. Loose wheel lugs fail in this fashion.


How clean the mating surfaces are will also have a big effect. Clean steel on steel has a cf of ~0.8 whereas oily steel is closer to ~0.2. An oily joint will support far less load than a clean one.

-- Joe
 
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