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Check your U-bolts!!

That being said, the u-bolts do need to stretch during proper install. There is a risk of false readings if they are not torqued evenly. Pull one side too far and it could partially rotate around the bottom of the axle tube. After driving, the tension will want to equalize. This could lead to things moving and/or loosening.
 
I think that's the biggest key when installing them, each side needs to be torqued in small, equal increments. Not torquing one down, then the other...

Run each down in small increments to let the u-bolt settle in it's fitted place.

Still recheck them after a few hundred or shorter miles.
 
campfire said:
Martin - It's certainly possible that both my trucks had been tampered with by previous owners. But it does seem odd that both trucks would end up matching... :dunno:
Click to expand...

I cut up a LOT of trucks. I have never seen a 9/16" u-bolt on a 1973-1987(1991) Gm truck unless it was an aftermarket u-bolt, and they stand out. I have also never had to ream out a u-bolt plate to install a 5/8" u-bolt....

Martin
 
82355 said:
campfire said:
Martin - It's certainly possible that both my trucks had been tampered with by previous owners. But it does seem odd that both trucks would end up matching... :dunno:
Click to expand...

I cut up a LOT of trucks. I have never seen a 9/16" u-bolt on a 1973-1987(1991) Gm truck unless it was an aftermarket u-bolt, and they stand out. I have also never had to ream out a u-bolt plate to install a 5/8" u-bolt....

Martin
Click to expand...

I hear ya. And I'm not doubting ya (2 trucks is not a large sample by any measure). I just thought it was odd. My plate holes were certainly large enough to accommodate 5/8" u-bolts. How (for the less-experienced) can one tell an after-market bolt from a stock one?

And why the use of coarse threads on a part that receives shock-loading? I thought that was a no-no... :dunno:

(That last one may be a Kenny question...)
 
The stock 5/8" u-bolts are fine thread. I run a die down them before I take them apart to clean the rust out of the threads.

Martin
 
This article offers a good explanation of why bolts come loose.

On an axle bolted to a spring pack, there is ample opportunity for bolt side sliding. This is amplified when things like shims are installed.

In short, to prevent bolt loosening, you must completely eliminate any sliding of the nut relative to the rest of the assembly (i.e.: u-bolt plate). Tough to do with an aftermarket spring pack and/or shims...but achievable with good geometry and sufficiently high clamping force. The clamping force requirement will be different amongst configurations - a GM spec for torque is for a stock configuration and can be waaaaaay off for the same bolt with an aftermarket setup.

As a safety measure, one could either drill the U-bolts and use castellated nuts and cotter pins or even put a tack weld on the nut after you're confident it doesn't need to be tightened any more (or disassembled).
 
I find it interesting that conventional split washers actually help the fasteners loosen up.

I would expect the torque spec needed will increase as the length of ubolt and angle of shim are increased.
 
kennyw said:
I would expect the torque spec needed will increase as the length of ubolt and angle of shim are increased.
Click to expand...
Correct.

Bolt strain is measured in elongation per unit length:
Strain = ΔL/L0

Bolt stress (F/A) is clamping force over bolt cross-sectional area:
Stress = F/A

Modulus (E) of the steel is a constant.

Stress = F/A = Strain x E = [ΔL/L0] x E

Rearrange to isolate L0:
L0 = [A x ΔL x E]/F

So, for a given bolt diameter, alloy (modulus), and stretch, the clamping force would have to decrease to keep the stress/strain relationship the same as bolt length increases. If the ratio is not kept constant you run the risk of moving up beyond the elastic portion of the stress/strain curve (i.e.: yielding the bolt). Keeping the ratio constant, however, is also in direct opposition to keeping clamping force sufficiently high.

What really needs to happen is for the bolt diameter to increase as U-bolts get longer (increase A in proportion to L0).

Regarding shim angle, as angle added in either direction from perfactly perpendicular (bolt shank to spring plate 90 degrees), the nut will load up assymetrically. As this happens, the load will shift to a smaller surface area. With a decrease in nut-to-plate contact area, clamping force must go up in order to maintain the same friction force which prevents side slipping.
 
longbedder said:
Regarding shim angle, as angle added in either direction from perfactly perpendicular (bolt shank to spring plate 90 degrees), the nut will load up assymetrically. As this happens, the load will shift to a smaller surface area. With a decrease in nut-to-plate contact area, clamping force must go up in order to maintain the same friction force which prevents side slipping.
Click to expand...

I don't think the U-bolt/nuts can tell there's a shim or not. The whole thing just rotates (U-bolts and spring plate) (well, actually the U-bolts and spring plate stay still and the axle rotates within it). The only weird place is between the shim and perch. The clamping will be trying to squeeze the shim out, and I guess only the center pin is really stopping it unless there's enough friction. If the shim were wider than the U-bolts, that would give it something to rest against.

On my Scout, the shims were wide and I shaved away material to make space for the U-bolts. Seems more secure that way...
 
Mastiff said:
I don't think the U-bolt/nuts can tell there's a shim or not. The whole thing just rotates (U-bolts and spring plate) (well, actually the U-bolts and spring plate stay still and the axle rotates within it). The only weird place is between the shim and perch. The clamping will be trying to squeeze the shim out, and I guess only the center pin is really stopping it unless there's enough friction. If the shim were wider than the U-bolts, that would give it something to rest against.

On my Scout, the shims were wide and I shaved away material to make space for the U-bolts. Seems more secure that way...
Click to expand...

Or better yet, if the shim had about a 1/2" lip on the thin end that rested against the spring perch. This would probably mean that the shim would need to increase in overall thickness a little to accommodate this.
 
Mastiff said:
I don't think the U-bolt/nuts can tell there's a shim or not. The whole thing just rotates (U-bolts and spring plate) (well, actually the U-bolts and spring plate stay still and the axle rotates within it).
Click to expand...

That depends on how the shim is designed. As long as it keeps the axle centerline in the same spot, it would have no effect on the u-bolts. However, the cheap shims probably don't account for the spacing between the axle tube centerline and the face of the spring pad to correct for the axle rotation properly. The axle will move forward as the shim angle increases because it is being rotated about an axis on the face of the spring pad rather than the axle tube centerline. This assumes your top plate is restrained from moving forward by the spring pin.
 
I think many of us (including me) have top plates with big grooves in them to allow some forgiveness for different kinds of installations. The top plate is only held in place by the U-bolts.

The more I think about it, I'm kind of uneasy with the way the shim seems to only be held in place by the center pin. Since there is also the machined "pocket" to keep the center pin mounting points parallel, there's that much less of the center pin head to reach into the perch too.

Here's a super exaggerated pic just to help think this out:

attachment.php


I wonder about welding some "wings" onto the shim to keep it captured between the U-bolts. Or am I overthinking this? I guess even without a shim, sideways (vehicle front/back) impacts to the axle would only be resisted by the center pin without significant friction at the spring/perch interface. As long as the U-bolts are plenty tight, I guess all is well in either case...

shim.gif
 
Center pin should only play a roll for installing springs to axle. The center pin shouldn't see much stress if U bolts are tight....
 
kennyw said:
That depends on how the shim is designed. As long as it keeps the axle centerline in the same spot, it would have no effect on the u-bolts. However, the cheap shims probably don't account for the spacing between the axle tube centerline and the face of the spring pad to correct for the axle rotation properly. The axle will move forward as the shim angle increases because it is being rotated about an axis on the face of the spring pad rather than the axle tube centerline. This assumes your top plate is restrained from moving forward by the spring pin.
Click to expand...

I think we have a winner! The point that it would rotate is at the center pin where the spring mates to the shim. The angle that it rotates would of course be the same angle as the shim. Toss in a little geometry and we can figure out how far the center line of the axle shifts forward.
The distance from the point of rotation would be the radius of the axle + perch thickness + shim thickness(at the spring center bolt).
This would be 1.6875" (for a 14BFF) + .75" + .36"(for an 8* shim, determined through calculation). That is approximately 2.8" from the bottom of the spring to axle center line. If we rotate it 8* ... (toss the #'s into here http://www.calculator.net/triangle-...2.6&vy=2.6&va=&vz=&vb=&angleunits=d&x=73&y=13 ) and you see that it moves forward .36" ... almost 3/8".
I doubt the relief for the center pin on the u-bolt spring plate has enough play in it to move forward that much. And I don't think there is a way to make corrections in the shim to compensate for this.
 
I don't think I understand what it is you are trying to compute. The offset of the center pin relative to the U-bolts? The top plates all have slots to account for stuff like this if so.
 
If the u-bolt spring plate can't move forward the same distance then when you torque the u-bolts you're applying force against the head of the center pin, which could cause it shear.
I'm not any where near my truck so I don't remember how much play the spring plate has for the center pin.
 
rampage said:
If the u-bolt spring plate can't move forward the same distance then when you torque the u-bolts you're applying force against the head of the center pin, which could cause it shear.
I'm not any where near my truck so I don't remember how much play the spring plate has for the center pin.
Click to expand...

Yeah, that's what I've been saying, the plates have slots to account for stuff like this:

U-bolt%20reversal.bmp


UD%20spring%20plates-400x400.jpg
 
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