Bolt Quality question.

[47convert]

I'm looking for an opinion here on where and when a fastener should be grade 5 or grade 8. We have a discussion going on around here on that subject because a duddy of mine had the pivot bolt for the lower contriol arm on his Fatman front end break on the way to Nats South. This is a 38 Ford with a small block and a Fatman front end with Fatman's tubular lower control arm. The bolt is a 1/2" grade 5 and I understand their kits now use a 5/8" bolt. This bolt broke braking from about 65 MPH when someone cut in front of him and it came right out of the crossmember, causing a pretty scary ride as he manouvered over to the side of the road. I know the 1/2" bolt is too small and it seems ludicrous to me that they would offer the front suspension kit this way, but would a grade 8 be stronger or would it have broken easier under these conditions? I noticed since that a lot of 4 bar bolts are grade 5 too.

[enjenjo]

That bolt should not have broken, unless it was loose to begin with. It would take about 12,000 lbs pressure to break that bolt, if it was grade 5, and tight.  A grade 8 bolt would be good for about 16,000 lbs.  This is another good example of why bolts on suspension parts should be in double shear. Double shear, it woulf be about 24,000 lbs for grade 5, and 36,000 for grade 8

[Mr Cool]

Care to explain the double shear thing?

[soldermonkey]

I will try to explain the differences as I understand them;
Imagine this; One place you have double shear on a car is a spring shackle, the bolt runs through one shackle then the spring eye, then through the second shackle. In this example the bolt is in shear on both sides of the spring. This is double shear because the bolt has two interfaces where the is a side load or shear.  Now think about how it would be if instead of a long bolt that went through both shackles and the spring you had only a short bolt that went through one shackle and into the spring eye. This example would be single shear because there is only one interface appling a side load or shear to the bolt. That is why in a lot critical places, and suspension certainly is one of them, it is better to have double shear as there is a sharing of the load and also a redundant  load carring ability should either side fail even at one half the load of the single shear load.

I hope this helps, it is all I could think of.

dave

[enjenjo]

Sure. Look at a typical front control arm mount on an OEM installation. you have a bracket on the frame at both the front and rear side of the control arm. The bolt goes through one frame bracket, through the control arm, and into a second bracket. This way, the only forces on the bolt, when properly torqued, are in tension, no bending forces to speak of. This installation is double shear, the bolt would have to shear twice to fail.

On a typical street rod, there is only one frame bracket, and one control arm element. the bolt goes through the bracket, and into the control arm, that's it. The bolt not only has a tension element in the forces applied, now it has a bending force applied too. The two forces combined are more than the bolt can stand, and it breaks. The bolt has to shear only once to fail.

Adding a second bracket is generally simple, the bracket does not have to be big and ugly to be effective. And it greatly reduces bending forces, while doubling the shear load available.

[PeterR]

Care to explain the double shear thing?

As enjenjo has explained the most important contribution to the increased strength of a bolt in double shear is the reduction in the bending moment applied to the fastener.

If you imagine for a moment a bolt with the head cut off and held in a vice with about 2 inches of the shank sticking out.    Using a large hammer, take a swipe at the side of the bolt about an inch out from the vice.

The bolt will bend and after a couple of swipes it will break off flush with the vice jaws.

Now take a piece of 1" square bar, drill a hole trough it a neat size for the bolt.  Open the vice jaws 1" so the bar fits between the jaws with the bolt shank sitting across the top of each jaw.   Now pound the bar with the hammer in an attempt to shear off the bolt.   You will more likely break the vice than the bolt, because the bolt is subjected to almost pure shear.

An important consideration with high strength fasters is correct tightening.  Regardless of the application, high strength bolts subjected to cyclic loading should always be tightened up to the recommended tension to minimise the effect of fatigue.    

Most people assume head bolts are only tensioned to provide sealing between the block and head, but there is a more important reason.  Every time a cylinder fires the pressure attempts to blow the head off, and this causes cyclic loading of the head bolts.  By tightening the bolts so they have a static tension before the engine even starts, the difference between highest and lowest load in the bolt is proportionally smaller and the bolt is less likely to fatigue.    

A particularly bad situation is to have a bolt through a hollow section like a RHS frame rail without a crush tube.   You can tighten the bolt, but after a short time the walls of the RHS will bow in just enough to lose bolt tension and under cyclic loads the bolt will fail due to fatigue at a much lower load than if it had been correctly tensioned.

[32coupe]

Oh I see, it all make sense now.
It is after all, very simple, but why don't hotrod fabricators follow this simple theory :?
I can see why most people simply use a bigger bolt, but in most occasion that wont fix the problem.
Thanks for the "simple" explanation of double shear theory.
I will have to remember this one.

[C9]

That bolt should not have broken, unless it was loose to begin with. It would take about 12,000 lbs pressure to break that bolt, if it was grade 5, and tight.  A grade 8 bolt would be good for about 16,000 lbs.  This is another good example of why bolts on suspension parts should be in double shear. Double shear, it woulf be about 24,000 lbs for grade 5, and 36,000 for grade 8

How would the load pressures be in the same situation with a step up in bolt size?

To 9/16" and then to 5/8".

[enjenjo]

I want to apologize first, the figure I gave are somewhat low, I was looking at the grade chart wrong.

What I did was figure the area of a 1/2" bolt in cross section, .193 inch, rounded that to .2 inch, and divided that into 55,000 psi, which is the wrong figure for a grade 5 bolt, it should be 88,000 psi. And for a grade 8 bolt, I used 88,000 psi, rather than the correct figure of 120,000 psi. I also did a lot of rounding off, since most of it was done in my head. If the threaded part of the bolt is in shear, you have to use the minor diameter of the threads to figure area.

So to get the yield strength of a bolt, you divide the area in cross section, into the yeld strength for one square inch. It can be expressed as ; Yield equals area divided in to yield strength pre square inch for the material.

All you have to do is figure the area of any particular size bolt, and you can figure it's yield. For instance, the correct figure for a 1/2" grade 5 bolt is 17,000 lbs, and a grade 8 bolt would be 23,000 lbs. If you go to a 9/16" bolt, the figures are 27,000 lbs for a grade 5, and nearly 37,000 lbs for a grade 8

Now it should be understood that these are yield figures, not tensile strength, and should not be used for a working load figure. Yield is the point that the bolt permanently deforms, and will no longer support a load.

A properly installed bolt is designed to hold the parts without any relative movement, when properly torqued. In practice this is nearly impossible, so there are still failures, but properly designed parts will minimise this.

There have been books written about this subject, get one, it will help you understand how and why bolts fail, and easy ways to avoid these failures.

[C9]

Stepping up one size made a big difference.

Many thanks.

[Leon]

I have a Fatman front and was also concerned about this so I added a plate to put the rear in double shear.   Since the plate is 3/16 thick, I need to get a slightly longer bolt since there wasn't a lot of thread sticking through the nut before I made the addition.  I feel a little better about the front end now.

[47convert]

Looks good Leon. My friend's front end is a little different from your's but would be much improved with the additional braketry you have installed. His lower control arm is just wide enough to clear the heavy wall tube crossmember and doesn't have the extra gussetted tube that you have. I would think his being shorter would have less lever effect and would then be less prone to breakage. Interesting to see the differences in bolt strength depending on diameter and grade.

[model a vette]

I hope this isn't a hijack. It applies to some of the above:
I'm in the middle of helping a friend put disc brakes on his car and we had to replace the bolts that hold on the steering arms (and now also part of the caliper bracket). We bought grade 8 bolts but could only find course thread ones. I checked some "recommended" torque specs and notices that course threaded ones use a slightly lower spec. Any thoughts about what spec should be used on 3/8 16 bolts?

[enjenjo]

Bolt Torque is designed to stretch the bolt the optimum amount to make the strongest connection. Because of the difference in thread pitch, fine thread bolts need more torque to stretch the bolt the same amount.