will_warne Posted December 4, 2005 Share Posted December 4, 2005 I need to look at front recovery points again and I've come up with a design but I really need high tensile countersunk machine screws to make it work. I'd be using 4 per recovery point, bolted through one of the faces of the bumper onto a spreader plate behind. I'd prefer not to weld as the bumper's only 5mm. So, has anyone heard of them before? TIA Quote Link to comment Share on other sites More sharing options...
LR90 Posted December 4, 2005 Share Posted December 4, 2005 I need to look at front recovery points again and I've come up with a design but I really need high tensile countersunk machine screws to make it work. I'd be using 4 per recovery point, bolted through one of the faces of the bumper onto a spreader plate behind. I'd prefer not to weld as the bumper's only 5mm. So, has anyone heard of them before?TIA Yes, HT hex socket head, countersunk. Used them when I did my 4.2 conversion so the bolt heads were flush with the adapter plate Quote Link to comment Share on other sites More sharing options...
minivin Posted December 4, 2005 Share Posted December 4, 2005 not sure on the benefits of bolts through to a spreader plate or welding without talking to a stress engineer at work, but all I can think at the moment is that you will still have the issue of a 5mm bumper trying to disperse the load. I take it you are making a rather wide and tall spreader plate to spread the load out further than what the original recovery point you are fitting will do if welded? Quote Link to comment Share on other sites More sharing options...
Les Henson Posted December 4, 2005 Share Posted December 4, 2005 (edited) ltwt's have a similar strengthening and they have a NATO hook on the front bumper.The limiting factor is always the weakest point, so if you have the bumper strengthened, but the attachment to the chassis the same as standard, then the weak point will be the bumper itself. I think having a bracket that goes from the dumb iron to the bumper would be the best set-up, and this is what a ltwt set up similar to. It might be worth mentioning that a hi-tensile bolt may not be as strong as you might like when in shear, but up to the job in tension. Les. Edited December 4, 2005 by Les Henson Quote Link to comment Share on other sites More sharing options...
will_warne Posted December 4, 2005 Author Share Posted December 4, 2005 Thanks guys. The backplate will be VERY strong and will use fins to spread the load over most of the face of the bumper. The bumper itself is more than up to taking the load as its well designed to begin with and it will get strengthened 'slightly' when the winch gets breathed on. Its just that on the flat faces it does bend as its only 5mm. Mountings won't be a problem; its got 3 bolts to each dumb iron and then it will be attached to the cage too. Quote Link to comment Share on other sites More sharing options...
simonr Posted December 4, 2005 Share Posted December 4, 2005 Countersunk socket head cap screws/bolts are easily available. Most bolts are technically High Tensile (8.8 grade), but much stronger bolts are readily available as cap bolts. On a rear tow point, it uses 2 x 8.8 grade M16 bolts. I would suggest as a minimum, you need the same area of steel which equates to 400mm^2 or: 4 x M12 6 x M10 If you use stronger bolts, then so much the better. It is also worth remembering that this assumes a straight pull where the load is spread equally over the four bolts. Pulling at an angle will load some bolts higher than others. They reckon that a LR can develop up to 30 ton of pull in a snatch recovery - so that is a reasonable loading figure to work on. Where's Bish the bolt expert when you need him! Si Quote Link to comment Share on other sites More sharing options...
Turbocharger Posted December 4, 2005 Share Posted December 4, 2005 Bear in mind that, as Si says, the geometry of the bracket can load some bolts more than others. Also, a given bolt is about half as strong in shear. [THREADJACK!] Do I need high-tensile bolts holding my winch in? It's pushed towards the bumper under load, and the bolt holes are only tapped into the (aluminium?) casing of the Milemarker. Quote Link to comment Share on other sites More sharing options...
will_warne Posted December 4, 2005 Author Share Posted December 4, 2005 Cheers again, 4 x M12s was what I was going to go for. Most of the time I just use the fronts to take the winchline when doing a double line pull so the loads aren't quite as bad. Whenever I do a snatch recovery its always off 2 points using a bridle (be it front or rear). Quote Link to comment Share on other sites More sharing options...
simonr Posted December 5, 2005 Share Posted December 5, 2005 [THREADJACK!]Do I need high-tensile bolts holding my winch in? It's pushed towards the bumper under load, and the bolt holes are only tapped into the (aluminium?) casing of the Milemarker. No - the Al will give before even weak bolts. Si Quote Link to comment Share on other sites More sharing options...
dollythelw Posted December 5, 2005 Share Posted December 5, 2005 Milemarker threads have stainless inserts - I run them "6 bolted", four flat down onto the floor and the two for the fairlead all wrapped up in a devilishly chocolatey braced milk tray of 6mm, 12500lbs pull and no worries. Will - just and idea but have you got enough space for a perpendicular brace across the back? less lard more strength? Quote Link to comment Share on other sites More sharing options...
BogMonster Posted December 5, 2005 Share Posted December 5, 2005 I've often wondered about winch bolts... We worry about bolting towing attachments on with 6" thick backplates and loads of high tensile bolts and yet a winch (a normal winch, before Nige comes along to say his has 48 bolts fixing it on ) is held on with 4 x fairly short 3/8" bolts into alloy.... and with the geometry of the thing, most of the load is going to be on 2 of those. If somebody turned up for a marshal inspection with a towing hitch held on with 4 x 3/8" bolts they'd be told to b****r off I should think! As Dolly said the MM mounting bolts are helicoiled, though IIRC the two on the front aren't, as they are only supposed to be for the fairlead mounting. Quote Link to comment Share on other sites More sharing options...
will_warne Posted December 5, 2005 Author Share Posted December 5, 2005 Milemarker threads have stainless inserts - I run them "6 bolted", four flat down onto the floor and the two for the fairlead all wrapped up in a devilishly chocolatey braced milk tray of 6mm, 12500lbs pull and no worries. Will - just and idea but have you got enough space for a perpendicular brace across the back? less lard more strength? Mmm, there might be space but it'd be tight. Onto winchbolts, I don't think they really need to be HT as your not using them in tension. Most of the load goes through the feet of the winch. Quote Link to comment Share on other sites More sharing options...
bishbosh Posted December 5, 2005 Share Posted December 5, 2005 Better late than never Si!! The bolt loads will be very dependant on the geometry - both of the bolts and of the position of the load relative to the bolt group. As there are so many variations, I couldn't comment on the strengths / weaknesses of them, but if you want to post up some sketches with details I can do a quick calc or two.... ...here're some I prepared earlier 'ickle HSFG M24's too.! 6mm plate Nige? Pah! My first time...... Nothing like a 60m cantilever to make you sweat a bit... Quote Link to comment Share on other sites More sharing options...
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