Curt 2" receiver: Grade 5 bolts???
01-03-2008, 08:40 AM
#21
Sponsoring Vendor
One of the engineers from Warn expressed it to me like this. If your pulling straight up/out on the head of the bolt, you want grade 8. If it's a sheer you use grade 5. I have personally seen three winches fail (snapped the bolts holding them to the mount) and all had been replaced with grade 8 hardware.
01-03-2008, 09:06 AM
#22
JK Enthusiast
Join Date: Nov 2007
Location: Albuquerque, NM
Posts: 123
Likes: 0
Received 0 Likes
on
0 Posts
One thing not touched on is the effect of less-than-uniform stress on the bolt head. If the head doesn't make uniform contact all the way around with it's seating surface the stresses are very uneven, leading to early failure. Harley-Davidson had an issue with the first year's twin cam engine from this, where the hardened bolts holding the cam gear onto the end of the cam were snapping because the stamped flat washer between the bolt and gear, wasn't flat. It was usually off just enough that the bolt head was twisted a bit, and they would snap in service. The final cure was precision grinding the gear, washer, and bolt flat to match.
On a JK the frame rails are made of hardened cream cheese and will deform, and there's not enough room inside for a really big washer to back up the nut; I'd expect the nut and washer to pull through the frame rail before a grade 5 bolt would fail; so brittle fracture at the bolt head isn't likely for the bolts holding the plate to the frame because the frame will deform enough to relieve uneven stresses. The winch mounts, on the other hand, and the plate itself, are beefy items and aren't going to deform, so any unevenness is going to stress the bolt at the head. That would explain the fracturing of grade 8 bolts holding the winch down; uneven loading (and possibly poorly-made hardware from Home Depot).
So Warn supplies grade 5 bolts to mount their winches? Interesting. I'd be inclined to trust their judgment on this, they have more specific experience than anybody.
On a JK the frame rails are made of hardened cream cheese and will deform, and there's not enough room inside for a really big washer to back up the nut; I'd expect the nut and washer to pull through the frame rail before a grade 5 bolt would fail; so brittle fracture at the bolt head isn't likely for the bolts holding the plate to the frame because the frame will deform enough to relieve uneven stresses. The winch mounts, on the other hand, and the plate itself, are beefy items and aren't going to deform, so any unevenness is going to stress the bolt at the head. That would explain the fracturing of grade 8 bolts holding the winch down; uneven loading (and possibly poorly-made hardware from Home Depot).
So Warn supplies grade 5 bolts to mount their winches? Interesting. I'd be inclined to trust their judgment on this, they have more specific experience than anybody.
01-03-2008, 09:20 AM
#23
JK Enthusiast
Thread Starter
Join Date: Jul 2007
Posts: 189
Likes: 0
Received 0 Likes
on
0 Posts
If you really are interested shoot me a PM and I'll go through my "build-book" from when I built the CJ7 and get the info for you. It was special order, definitely not Home Depot material.
01-03-2008, 09:23 AM
#24
JK Enthusiast
Thread Starter
Join Date: Jul 2007
Posts: 189
Likes: 0
Received 0 Likes
on
0 Posts
01-03-2008, 04:50 PM
#25
JK Enthusiast
Join Date: Feb 2007
Location: Michigan
Posts: 348
Likes: 0
Received 0 Likes
on
0 Posts
The test on grade 8 bolts that found them inadequate doesn't surprise me. The military and aircraft industry have had problems with this for years. For critical applications they have a paperwork trail all the way back to the raw materials providers. You really have no idea what you are getting at a store unless they can identify the manufacture of the bolts at a minimum.
One thing that wasn't mentioned here (but maybe in the links) is the surface to surface friction that is part of most properly designed shear systems. Loading fasteners in shear is typically bad for a number of reasons, not only because the fasteners are weaker in that plane. So why are they used so often? Because the friction of the two surfaces bolted together is substantial and carries much of the applied load. What you have in an application like a hitch mounted to the sides of a frame rail is that the two metals clamped together actually carry all of the load. The bolts are actually in tension and are simply holding the two surfaces in contact firmly. The bolts could be made of plastic, and as long as the friction between the clamped metals was sufficient they would hold.
I don't deny that this clamped interface must be properly engineered with the proper bolts rated for the load in shear. If the clamped friction interface slips and or the bolts come loose the bolt are now in shear. But, if you keep the bolts properly torqued, and the interface is properly designed, the bolts should never really be loaded in shear. Says a lot about using a torque wrench and retorwquing fasteners occasionally.
One thing that wasn't mentioned here (but maybe in the links) is the surface to surface friction that is part of most properly designed shear systems. Loading fasteners in shear is typically bad for a number of reasons, not only because the fasteners are weaker in that plane. So why are they used so often? Because the friction of the two surfaces bolted together is substantial and carries much of the applied load. What you have in an application like a hitch mounted to the sides of a frame rail is that the two metals clamped together actually carry all of the load. The bolts are actually in tension and are simply holding the two surfaces in contact firmly. The bolts could be made of plastic, and as long as the friction between the clamped metals was sufficient they would hold.
I don't deny that this clamped interface must be properly engineered with the proper bolts rated for the load in shear. If the clamped friction interface slips and or the bolts come loose the bolt are now in shear. But, if you keep the bolts properly torqued, and the interface is properly designed, the bolts should never really be loaded in shear. Says a lot about using a torque wrench and retorwquing fasteners occasionally.
01-03-2008, 07:28 PM
#26
JK Enthusiast
Thread Starter
Join Date: Jul 2007
Posts: 189
Likes: 0
Received 0 Likes
on
0 Posts
.......Automated torque guns (like the car manufacturers use) usually tighten a graded bolt until the torque levels off as measured by a strain gauge. At this point, the bolt is beginning to stretch because it has hit its yield point. Some bolts will then use an additional quarter turn. This is why your Chiltons book has you replace some bolts regardless of their condition. When a bolt is tightened to its yield point, it has stretched AND WILL NOT RETURN TO ITS ORIGINAL SIZE OR STRENGTH. If you remove the bolt an tighten it again, you are threading it in that NEW lenght then adding MORE stretch to it. That will cause failure well below a new bolts ratings.
.......
Bryan.
.......
Bryan.
Maybe I should go check it and if I've torqued to too much replace the bolts (at least it's going to be in the 60's this weekend). The only problem though, I don't know if there's enough clearance to get a torque wrench in there without dropping the muffler (which I can't figure out how to do without cutting the rubber mount and buying a new one).....
Last edited by amc78cj7; 01-03-2008 at 07:40 PM.
01-03-2008, 07:59 PM
#27
JK Junkie
Join Date: Mar 2007
Location: Mount Carmel, PA
Posts: 3,122
Likes: 0
Received 0 Likes
on
0 Posts