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Unread 01-26-2010, 03:14 PM   #16
pulsepb
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Quote:
Originally Posted by ratmonkey View Post
10k pulling force is pretty conservative for a pull on a good stuck.
You want it to fail at a higher rating than your pulling strap. Calculate your forces at 30k and add in another 20% for safety.
A conservative value would be one that errors on the side of caution. 30 k would be a conservative value.

I'm writing this on my iPhone as I walk so I'm not writing it out in full but 30k + 20% would result in a shear stress of just over 27,000 psi which is way under the yield strength of mild steel.

Pretty much I'm not going to continue argue the strength of it unless you have your own calculations that disprove my own.

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Unread 01-26-2010, 06:50 PM   #17
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if you have access to ansys or a similar modeling program at school i suggest you run the whole frame rail system.

use a shock loading of 40k to start. use some off angle pulls too. like i said, you won't be pulling the plate out, you'll be bending the rails. at the stress concentration points where they were designed to bend.


i've been building industrial systems for a while. don't let the trade classification fool you, i've got the same strength of materials course work under my hat that you do. i just took a different career route after my 3rd year.

oh yeah, by conservative i meant that you were underestimating the forces involved in unstuckifying a vehicle.
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Unread 01-27-2010, 02:38 AM   #18
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. . . as far as my mounting to the frame Im confident it is sufficent as well. The 1/2 bolts are torque to a spec of 106 lb-ft and the 7/16' bolts to 69 lb-ft. this creates a sufficient pre-load where the friction of the plate on the frame is so great that the bolts will never see any shear effects as long as it it under the calculated load. . .
Don't assume that the preload is uniformly distributed over the surface area; it tends to concentrate under the head of the bolt which in turn can lead to very high surface shear stress in that region.

30K lbs is a ridiculously high design load for a winch pull; 10k is much more realistic although you can certainly generate more with a double line pull. As a point of reference, the factory tow hooks are rated for something like 5K lbs.

Doing a simple strength of materials calculation by making assumptions about the stress distribution (that its uniform) ignores stress concentrations arising from geometric discontinuities (holes). Bearing stress from the bolt shank on the hole combined with frictional shearing stress that also happens to concentrate in the same area can easily exceed the yield stress of the material at a load much smaller than 30K lbs. Another point to consider is that the load will not distribute evenly to all the bolts; the lions share will be felt by the front bolt(s).

I believe the OP's design is plenty adequate for normal recovery loads.
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Unread 01-27-2010, 05:27 AM   #19
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factory tow hooks are 10 or 15k iirc. the throat on the hooks usually opens up before you rip em out.


if someone has to give you a jerk to get you out of a stuck, it can easily surpass 30k instant force.

how much force does an 86 suburban on 42's going from 15mph to 0 in 2ft exert?
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Unread 01-27-2010, 07:16 AM   #20
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factory tow hooks are 10 or 15k iirc. the throat on the hooks usually opens up before you rip em out.


if someone has to give you a jerk to get you out of a stuck, it can easily surpass 30k instant force.

how much force does an 86 suburban on 42's going from 15mph to 0 in 2ft exert?
I was wondering about the stock hooks. Mine bent the bracket getting "jerked" on. I figured the bolts would snap long before the hooks would unravel.
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Unread 01-27-2010, 07:37 AM   #21
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I did a quick search and find that the factory tows hooks are 5.5k each; thats 11K for the pair.

Bad things can happen by jerking. Using an elastic tow strap tends to minimize the effect of impact loading. Factory tow hooks are specifically designed to fail before the load causes more severe structural damage to the vehicle. This should tell you something when you go about designing your own tow hooks or winch bumper.
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Unread 01-27-2010, 12:04 PM   #22
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Ratmonkey- I have soldiworks and cosmos works on my laptop and contemplated doing some analysis but getting accurate FEA data would be more time consuming than I'm willing to do right now

technohead- I know I very much generalized some things but I wasn't gonna get into the stress concentration around the holes and derive equations for impact when most people are unable to even understand basic loading physics and material science.
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Unread 01-27-2010, 10:39 PM   #23
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It may be worthwhile to those of us that like such information.
Which is more here than you may guess.

I am simply trying to make the case that you want the load distributed over two planes is all. Much nicer to the frame when pulling from an angle.

Shock loading with a heavy strap is sometimes the only way to get unstuck. I have seen a truck pull a 2 ton tow truck off the road it was stuck so bad. A few jerks got it out though.
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Unread 12-08-2011, 07:58 PM   #24
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I know this is a very old post and all but i was wondering if you still had the dimensions for these shakle mounts. i work at a metal fab shop and this would be pretty easy to do. Im most likely going to end up measuring it out myself but if you had the dimensions on hand itd be extremely helpful
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