towhooks & grade 5 or 8?

TOW HOOK Discussion on FSJ-List...

--) --) From: Dennis F --) --) Date: Monday, July 28, 1997 7:43 AM --) --) --) --) Spend an extra $5.00 and get 20,000 lb. tow hooks. The 10,000 pound ones --) --) are NOT strong enough to take a good yank. Also, make darn sure you use --) --) GRADE 8 BOLTS to attach it, nothing less, and DO NOT WELD the hooks on. --) --) The life you save may be your own. I also recommend spending an extra --) --) couple of bucks for safety clips. That's kind of a misnomer, they really --) --) don't have much of an effect on the safety of a hook-up, but they sure do --) --) help to keep a strap from falling off a hook. --) --) JenHen wrote: --) Good point about the 20,000 lb hooks, Doc. In the 7/97 issue of 4x4 Power --) someone wrote in recommending Grade 8 bolts. The mag wrote back --) recommending Grade 5 bolts for the tow hook application since, as they put it, --) --) "Grade 8 bolts have a very high-tensile strength. This means they are very --) hard and have great resistance to stretching. They are also easier to break --) than Grade 5 bolts when used in shear. Grade 5 bolts are slightly softer --) and will give a little rather than snap in half. Since the hook attaching --) points are held in single-shear, Grade 5 bolts are recommended." --) --) Any comments from the MEs out there on the Grade 5 versus Grade 8 tow hook --) bolt subject? --) --) ======================================= Tom Anhalt Date: Tue, 29 Jul 1997 09:36:07 -0700

Subject: Re: Grade 5 versus Grade 8

What's more important than whether the bolts are grade 5 or grade 8 is to make SURE that they are torqued to the correct values. In bolted joints, the idea is to create enough preload in the fastener to take up to 80-90% of it's yield strength. You might be saying "Gosh, if I tighten it that much, it won't take much loading to cause it to yield and fail!!" Au contraire, when you tighten a threaded fastener, not only do you put the fastener in tension, but you also put the parts you're clamping together into compression. Think of these two things as a pair of springs in parallel. Typically, the fastener is a lot softer of a spring than the parts being clamped. What this means, is that with any external loading, the majority of the load will go into relieving the compression in the clamped parts (the stiffer spring) and only a small portion of the load will go into increasing the tension in the fastener. This all holds only if the proper preload is maintained. Once you relieve all of the compression loading on the clamped parts, the fastener then has to support ALL of the loading and typically fails in a quick manner. Now, in a tow hook application, the conservative way to look at the loading is to consider that the bolts are loaded purely in shear. In actuality, this is not the case since the preload in the fasteners will cause a large friction "load" between the 2 parts being clamped together. In order to load the bolts in shear, this frictional force has to be overcome first. Also, since the load point of the hook isn't directly in line with the clamped interface, there will be a bending load (moment) applied to the interface as well. As you can see, there will be a multi-stress state and the shear stress in the bolts can be very high even though the direct shear loading doesn't look that high. I have never seen any numbers that show that grade 8 bolts have a lower shear strength than grade 5. I do know that the tensile yield stress in grade 8s and 5s are typically 120ksi and 85 ksi respectively. In wrought steels, the shear yield stress is typically approx. 58% of the tensile yield stress, so a grade 8 should have a higher shear yield stress than a grade 5 (70 ksi vs. 50 ksi). But, like I said, I have never seen the shear strength numbers for the two. As far as whether one or the other is more likely to yield (more ductile) than the other, I'm not so sure. One measure of the ductility of a material is the difference between the yield stress and the ultimate stress of the material. This difference is approximately 30 ksi for grade 8 and 35 ksi for grade 5, so you can see, the two grades aren't that much different. Personally, I think that grade 5s are more than adequate for any automotive/light truck applications IF they are properly torqued and thats what I typically use. In my experience, the most common cause of fastener failure is lack of proper preload due to improper torqueing or loosening under vibration. I'd be more worried about whether the bolts I buy are ACTUALLY the grade they say they are (not counterfeit) than if they are grade 5 or 8. Just MHO. -- -Tom Anhalt Santa Barbara, CA

wagoneers.com

Select pages from WAGONEERS.COM:

the Full Size Jeep decal! © 1998-2000 John Meister

Buy this FSJ decal... ...use PayPal to pay for it!

a viable option to bloated and unstable code

AMSOIL synthetic lubricants and filters
(please use customer #283461)

Independent Lifetime Dealer John Meister - customer #283461 - since 1983

Shop AMAZON.com from wagoneers.com!

Search:
Keywords:

Join the fight against spam (UCE)! Eliminate spam!

Washington State has a $500 fine for "spam"
(unsolicited commercial email)!

read the Bible