WLL Safe Working Load Limit versus breaking strength

Hi Boc,

Yes, true. But lifting ratings and pulling ratings are completely different. Don't ask me why. The sales engineer at Beaver tried explaining it but didn't do a very good job.

As best I understand the difference is in the margin for safety and using a 4500kg rated hook on a 9000lb winch is both completely unnecessary as it is putting a hook that would operate safely PULLING at something like 20 tonne on a winch incapable of pulling more than 5 tonne.

When winching even up a steep slope the winch isn't lifting the entire weight of the vehicle. There's some sort of vector relationship and it's a very long time since I did physics but the mass of the vehicle is supported by the ground at all times, the winch is just providing the force required to get that mass to move against friction/resistance up an incline.

Even on a steep recovery the winch isn't holding the full mass of the load at any time.

A 3.2 tonne hook is rated for lifting with 10mm chain. No one uses 10mm chain in recovery, nor am I aware of any accredited 4wd course curriculum that recommends it.

A 5.4 tonne hook is rated for 13mm chain but that's just getting ridiculous.

As a Paramedic I'm acutely aware of work cover statistics. I've been to a few. It's also why I'm doing my research.

Dave

Have not done physics for a long time either , 30+yrs in fact , but it seems to be basic that a straight vertical lift of say a 3 tonne vehicle is just that , 3 tonne , a winching exercise of a bogged / stuck 3 tonne vehicle would require considerably more force than required for a straight vertical lift ,ergo a higher safety margin required , tell me I'm wrong ! Gravity is a constant , 3 tonne is 3 tonne , a stuck 3 tonne is not just 3 tonne.

Just for the people interested - ANY chain that is to be used for lifiting must have a graded stamp on it - the stuff from bunnings is not to be used for anything other than as security lock up.

formula to work out chain lift capacity - SWL(KGS)=d(squared)(mm) x G x 0.4

so - we know the chain is 10 mm - the grade will be 80 grade ( or stamped T )

so its 10 x 10 x 80 x 0.4 = 3200 KG or 3.2 tonnes

There are other grades of steel chain but 80 grade is the most common in the lifting game ( t- 80 p - 40 l-30 ( mild grade)

NOW to the one that will maybe effect winch owners if looking to change FSWR ( flexible steel wire rope )

SWL of FSWR Formula:SWR(kgs)=dsquared x 8

12 mm FSWR will be 12 x 12 x 8 = 1152 or 1.15 tones

how to work out if your rope is ready for the bin ( any kinks and it should be disguarded without this test)

6 x 24 rope ( 6 strands of 24 wires ) with the same size rop as above ( 12 mm )

length of rope to inspect is 12 mm x 8 = 96 mm
Number of wires= 6 x 24 = 144
10 % of 144 = 14.4 wires

therefore 14 broken wires over a 96 mm length means rope is no good

this covers the basics --

if you need more formula then let me know !!

Boc

Interesting reading the bit on recovery on SA_Patrol's link.

It seems the advice I had was wrong (above). It's very hard to get a consistent response to this stuff but the bit I found interesting on the Pirate 4x4 site was the bit I've pasted below.

I might even print the pics, laminate them and stick them in the glovebox for future reference. I might also be more inclined to use the snatch block.

Dave

From www.pirate4x4.com

Calculating the force of a recovery operation:

This is a critical step in both the conduct of a winch recovery (because you need to be absolutely sure that EVERY single piece of equipment used in the task is strong enough and will not fail and thereby endanger life and limb), and in the selection/purchase and decision to use a piece of recovery equipment. It is a calculation where, quite frankly, the manufacturer's recommendations are woefully inadequate. Why? Simply because if they let you in on how large the forces really are, it would leave you realizing that they are unable to economically produce a winch of sufficient capacity in anything resembling a small, light, or economical enough package. They get away with it, because, as I said, there are virtually no regulations or standards governing the industry. I'm not saying all 4x4 winches are inadequate, dangerous, or useless. But I am saying that the forces involved are often much greater than the manufacturer's would have you believe, and you will be far more capable and SAFER if you approach your 4x4 recovery KNOWING THIS, and knowing the real numbers. Realize, that for reasons of practicality and economics, your 4x4 recovery equipment is almost certainly undersized.....you can still do the job, using the correct techniques, but you will be much SAFER if you keep this in mind. Enough of the pre-amble.

Most, if not all, winch manufacturers will tell you to select a winch based on 1.5 times the gross vehicle weight. This often leads to less than satisfactory results for 2 reasons:

1) Most people are terrible at actually estimating the gross weight of their rig as it sits on the trail, full of gas, tools, equipment, food, camping gear, people, the dog...everything. Heck, in some cases the real figure can actually exceed the GVWR of the vehicle. Simple advice here - either err WAY on the heavy side, or get your rig weighed in trail trim.

2) More importantly, the "effective weight" of a "stuck" 4x4 is very often FAR more than 1.5 times the GVW. The following data on how to more accurately estimate the "effective weight", is taken from the world of professional heavy recovery - the guys that recover Tractor-trailers that have flipped on their side for instance, as well as U.S., Canadian, and UK Military recovery manuals.

Once you have accurately estimated or measured the trucks loaded weight (LW) you can calculate the resistance to be overcome in any recovery situation (this is commonly known as the ROLLING resistance). There are 4 types of resistance that must be accounted for to accurately assess the resistance that must be overcome. These are surface resistance, damage resistance, mire (stuck) resistance and grade (slope) resistance. Calculate them all as follows:
Surface resistance

A pull of 1/10 LW will cause a free wheeling truck to move on a hard, level surface.

A pull of 1/3 LW will cause a free wheeling truck to move on a softer surface, such as grass or gravel,
Damage resistance:

A pull of 2/3 LW will be required to move if the wheels cannot rotate (as if the brakes were fully applied), the pull required to overcome the resistance (drag) the truck id 2/3 or 67% of the LW. Damage resistance includes surface resistance (i.e. you only use one or the other)
Stuck (mire) resistance:

A pull of 100% of LW will be required if the truck is stuck to a depth of the sidewall on the tires.

A pull of 200% of LW will be required if the truck is stuck to the hubs.

A pull of 300% of LW will be required if the truck is stuck to the frame..

Mire resistance includes damage resistance (i.e. you only use one or the other)
Grade (slope) resistance:

Upgrade (vehicle has to be recovered up a slope or grade)

15 degrees - add 25% of LW

30 degrees - add 50% of LW

45 degrees - add 75% of LW

Vehicle recovery on level ground - no correction

Downgrade (vehicle has to be recovered down a slope or grade)

15 degrees - subtract 25% of LW

30 degrees - subtract 50% of LW

45 degrees - subtract 75% of LW
Final figure:

Add surface or damage or mire resistance and grade resistance, and this is your final figure or rolling resistance. This is the amount of pull the winch must apply in order to recover the stuck vehicle.
Example:

My trail rig fully kitted out weighs in at 5000 lbs. I get stuck down a rock ravine that's about 45 degrees steep, and there are big rocks up to the frame hanging it up. Rolling resistance is 5000lbs x 3 + (5000 x 0.75) = 18,750 lbs. As you can see, this is significantly more than the 5000lbs x 1.5 - 7500lbs the manufacturers would have you believe. You may be wondering how one could ever possibly recover the vehicle in this example, given that the largest commercially available 4x4 recovery winch is 15000 lbs and that most are in the 8-9000lb range. The answer is by using multi-line rigging, which we shall explore in a moment.

But where does any manufacturer quote the Guaranteed Breaking Strain?

If Gunnebo quote a safety margin of 4:1 and your rigging book assumes 5:1 that doesn't stack up either.

This is all getting very confusing.

Anyone know the the WLL for 8mm plasma rope?

Dave

FSWR = flexible steel wire rope

Thanks for that.

Interestingly not one of his listed suppliers in the article on hooks stock a hook with an SWL of greater than 3200kg.

Yet he insists on using a hook at least as capable as the cable.

An 8mm rope from OK Offroad is rated at 13,700 lb. I'm not sure on the conversion of US lb to kg but it's gotta be at least 6,000kg.

You could put a 6 tonne hook on it, but the winch will stall before either breaks, so your winch becomes the weakest part of the system.

Interestingly we commonly use 3,200kg bow shackles. Most I know carry a 3.2T & 4T bow shackle.

4wd Monthly did a breaking load test on those a while back but I can't recall which issue. Might go and see if I can find it.

Dave

Thanks Anthony.

I presume when you are talking 2WWL hooks you mean the 2 tonne rated hooks.

You don't happen to have the part numbers do you?

Dave

For the yellow Gunnebo hook you posted the pic, the name is Eye Type Safety Latchlok Hook Type BK, the Bullivants Order Code is 122027 (to suit 7 & 8mm chain & 2WWL or 2 tonne rate) and the order code 122028 is to suit 10mm chain and is 3.2WWL or rated 3.2 tonne.

You can check out the Bullivants info at www.bullivants.com. The Phone number in Melb is 03 95851288. I don’t have any link with them except that I spend some of my money there buying their stuff.

The interesting thing is Bullivants imports Gunnebo direct and appear to sell the Gunnebo cheaper in Aust than Gunnebo. I have not checked this claim myself. I bought a couple of the Sling Hooks with latch type EKN for 8mm chain (about $29 each). This hook needs a Hammer lock to attach to the chain. I also bought one of the safety hook type EGKN/GKN for 8mm (also about $30) for 8mm chain. The good thing about this hook is you don’t need a hammer lock to attach it to the chain. It has a pin instead of a ring, so you don’t need to buy a hammer lock.

If you want to get a hook, like the yellow one pic above, but instead of the ring (which needs the hammer lock) you get the type the can attach direct to the chain. The Bullivants catalogue lists it as a Clevis Type Safety Latchlok Hook – SHC. The order code for 8mm chain is 121066 and the code for 10mm chain is 121067. The 8mm is 2 tonne and 10mm is 3.2 tonne.

Cheers Anthony.

PS They gave me a catalogue on my list visit.

And who has rated the Chinese brand, It all depends on the quality of steel they use in production. I always buy my gear from Nobles. I know they have done lots of tests before they release a product.

That's what I figured too. The price difference is huge.

I was surprised at the price of the ARB hook, but after reading that tech page on recovery you linked before I'm not so surprised.

Dave

Money is no object. Just looking for knowledge on which to base common sense.

There's a huge gap between a 6 tonne hook and a 3.2 tonne hook in physical size.

If money was an issue I wouldn't replace my tyres at 50% tread depth for safety off & on road...

Dave

Gday Dave.
Not a dig mate, just a bit of advice from someone who has seen stuff not do its job properly.
I've seen a 2 tonne rated ratchet strap snap, and all it was holding was a 400kg piece of pipe. The force that snapped the stap was the pipe moving as a truck went around a corner. Thats why i said gravity is a bitch.
The 6 ton hook will more than likely be overkill for winching...well maybe just a tad:):).
It becomes a real pain for us, when it is left up to us to decipher what all the SWL and WLL stuff means. It would be so much easier if there was a clear and concise law with regard all of the recovery equipment we use. But, at the least, go with the brand you trust.

mewo.

rick.

No worries Rick,

No offence taken. I've seen what happens when the tie down fails and the pipe goes through the car that got hit....

Dave