Questions regarding reuse of leaf spring suspension U-bolts
Questions regarding reuse of leaf spring suspension U-bolts
I would like to ask some questions and start a discussion related to U-bolts used to clamp leaf spring stacks on the suspensions of vehicles such as small to full size pickups, jeeps, and larger work trucks.
Generally, most after-market U-bolt manufacturers recommend to never reuse U-bolts. I think it is good practice to use new fasteners whenever possible, but I wonder what the position of the auto manufacturers is on the issue and whether it's necessary to replace a U-bolt simply because it has been properly torqued to specs once. At the following link, you can download a small PDF product information sheet authored by Dayton-Parts who produces aftermarket U-bolts...
In the product info sheet it states to not re-use U-bolts. It also states "A Previously torqued U-bolt will suffer from distorted threads from engagement of the deep nut. Deep nuts should be tightened and re-torqued, never loosened and re-tightened." Dayton-parts also recommends in the product info sheet to lubricate the washers and U-bolts with oil or anti-seize compound to reduce friction when torquing the U-bolts nuts.
LH rods who supplies threaded U-bolt rod to Dayton-parts and others, comments on the issue of U-bolt reuse at the following link...
At the link above, part of what LH rod says is that "Suspension U-Bolts are manufactured with a smooth rolled thread, while the mating Hi-Nuts are manufactured with sharp cut threads. When a U-Bolt is tightened to its recommended torque level, the U-Bolt threads stretch as they mate with the Hi-Nuts. Although not always visible to the naked eye, this damages the threads. Removing the Hi-Nuts from the U-Bolt will cause a cross - threading that will not allow the U-bolt to be adequately retorqued."
The specs for the threaded U-bolt rod used by LH rods are shown at the link directly below...
Basically, the U-bolt rod material is made from a cold drawn, stress relieved, micro-alloyed, 1541 modified steel that meets SAE-J429. The proof load is 120,000 PSI, the minimum yield strength is 130,000 PSI, and the minimum tensile strength is 150,000 PSI.
Is there any significance to the issue of the U-bolt threads being rolled and the high-nut threads being cut? Does ASME or any standards address the issue of compatibility between rolled threads and cut threads ? I thought they were compatible and aren't most threads rolled anyway? The issue of a fastener not producing the same clamping force upon a second use would seem to apply to any threaded fastener regardless of whether the treads of the bolt are rolled and the threads of the nut are cut. Fastenal comments on the issue of fastener reuse at the following link under "reuse of fasteners"...
Fastenal states that "On a demonstration with a 1/2-13 zinc plated SAE J429 Grade 5 hex cap screw and zinc plated SAE J995 Grade 5 hex nut with an installation torque of 70 ft-lbs to obtain a clamp load of 9000 lbs (without any added lubrication). On the second installation, this torque had increased to 95 ft-lbs to obtain 9000 lbs. By the fourth installation, we required 145 ft-lbs to reach a clamp load of 9000 lbs."
I have seen several threads online regarding the re-use of U-bolts, most are at forums devoted to trucks and the installation of lift kits and such. One such thread is at the link directly below.
In my search online I found one thread where a person claimed to have contacted several auto dealers and was told that U-bolts are serviceable re-usable items. This brings the question to mind, is there any significant difference between OEM U-bolts and aftermarket U-bolts meeting the specs mentioned above? I had always thought factory OEM U-bolts were hardened, however, I was recently told that factory U-bolts have not been hardened for about 30 years now. At another thread, one person who claimed to work for a spring shop said that they reuse U-bolts as a common practice.
It also seems there may be some misconceptions regarding U-bolts on the net. I have seen it posted that U-bolts are actually "torque to yield" fasteners. However, a typical mid to full size pickup truck will use around a 9/16" OD U-bolt and a typical nut torque of 81 foot pounds. It seems to me that 81 foot pounds on a 9/16" OD bolt is not going to produce a stress that will be anywhere near a proof stress of 120,000 PSI so are U-bolts really considered "torque to yield" fasteners ? I'm not crazy about the idea of a torque to yield fastener in any event.
Perhaps some confusion comes from statements like a reused nut will not "hold torque" or "cannot be adequately retorqued". Perhaps it would be better to say that more torque is required upon each subsequent reuse in order to create the same clamp down force as achieved on the first use. Statements such as not being able to "hold torque" also seem to perhaps create a misconception that the nuts are likely to loosen or back off on reuse. How likely is this ? If the threads are indeed slightly distorted, wouldn't this actually help prevent the nut from loosening once it is tightened down ?
It seems the main reason for not re-using a U-bolt, provided it has never been heated, you can still run the nut down on the threads easily, and the parts have not been overstressed, is that the spring clamping force will be reduced upon reuse of the U-bolts. A typical 9/16" OD bolt torqued to 81 foot pounds will have approximately 8,640 pounds of clamp down force. Each U-bolt will have two bolt shanks pulling down, so you have a force of 17,280 pounds pulling down per U-bolt, on each side of the axle-housing spring perch or platform. A force of 34,560 pounds per axle side or for each dual set of U-bolts seems huge to me unless I calculated something wrong. Of course it will vary by vehicle, but say for a full size pickup, in general, how much clamp down force is really needed to prevent leaf spring breakage, rear end shift, or spring stack center bolt breakage ?
I had read once that brand new identical bolts torqued to the same specs can have tensions that vary by as much as 50% due to friction differences http://www.surebolt.com/. When you factor in that torque wrenches themselves are not 100% accurate and consider all of the variables that can effect the clamping pressure of a bolted joint, if clamp down force is indeed critical, it seems you cannot rely on nut torque or a torque wrench anyway. You really need some way to measure axial clamping force, or you have to design the system in such a way that you will get adequate clamping force regardless of the variables, and regardless of a few reuses. This seems like the only sensible course of action, but is this how U-bolts for leaf springs are designed ?
Then there is the issue of the common recommendation that new U-bolts be re-torqued after a certain amount of driving. Most recommendations are to drive the vehicle for 100 miles and then re-torque the U-bolt nuts. However, it seems factory OEM U-bolts are never re-torqued, certainly not after any significant amount of driving. Eaton springs recommends to "re-torque the newly installed u-bolts after 50 miles of driving. Then recheck after another 50 miles. Then again after 500 miles. Finally, recheck the u-bolt torque every time you are under the vehicle". That seems a bit excessive to me.
However, isn't re-torquing the nuts at least somewhat similar to reuse? If the threads have been slightly deformed after the first initial torquing of the U-bolt nuts, then when you go to check the torque, you are not really checking the torque (or more importantly the clamp down pressure) because it will take more torque to create the proper clamp down pressure than it did upon the first tightening of the nut. If you were to loosen the nut a little before each re-torque, what would that really change or hurt as compared to just re-tightening the nuts each time ?
I read one thread where a person replaced the leaf spring U-bolts and then had to loosen the brand new U-bolts to make an adjustment on the rear end or something. Provided that the new U-bolts were torqued to specs with a hand torque wrench, the bolts were never heated or overstressed, the proper tightening sequence was followed, the vehicle was not moved or driven, and the nuts could be easily run back down on the U-bolts threads until contacting the lower anchor plates the second time, is this type of one time reuse most likely OK ?
I read at another forum where a person had some new U-bolts installed on his vehicle for one year and wanted to know if he could re-use them after doing some spring work. Aside from the fact that the U-bolts will have been installed on an in use vehicle for 1 year, & as long as the same bolt and nut conditions exists as in scenario 1 above and there is no significant rust present, is this type of one-time re-use most likely OK ?
I had a spring shop tell me once that the main reason they recommend to not re-use U-bolts is because people heat up the rusted bolts/nuts to take them off and then reuse the bolts after they have been heated. That would not be a good thing to do since it weakens the parts.
Of course the simple answer is to just replace the U-bolts, but I like to learn the details and underlying issues. Plus, there are at least a few scenarios as mentioned above where it would be desirable to reuse the U-bolts. If there are indeed spring shops out there who are commonly reusing U-bolts as indicated by a forum thread I found on-line, and with people doing rear end, spring work, or lift modifications while reusing U-bolts, do we have unsafe vehicles on the road because of this or is the clampdown pressure of the U-bolts designed to be so great that its not as much of an issue as all of the warnings instructing to not reuse U-bolts would seem to imply ?
I tend to think at least part of the reason for the recommendations is because most shops would use impact wrenches on the U-bolt nuts which really could over-stress them. Or, they may use heat on the nuts in order to remove them and reuse the bolts.
Any feedback would be appreciated.