Dan,
I always wonder the same thing. I would love it know.. Why get a 8ton Strap is the shackles are only rated to 4 ton...
Quote:
Originally Posted by wovenrovings
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Dan,
I always wonder the same thing. I would love it know.. Why get a 8ton Strap is the shackles are only rated to 4 ton...
Quote:
Originally Posted by wovenrovings
Straps are rated for their minimum breaking strain (when new), so 8,000 kg strap should break at a load just above 8,000 kgQuote:
Originally Posted by disco_2008
Rated shackels have a safety factor of 5X their rated load so 3,700 kg shackel should fail at 18,500 kg.
What your tow hook fails at, or more importantly the other vehicles tow hook is another story!
The testing is independant. Actually, I tend to think that the ARB strap winning is really the result of a previous comparo where the ARB strap did poorly. I think that ARB pride themselves on the products that they make and sell and were surprised when the strap came apart at the stiching. Aparently they revised what they were doing to produce the current crop of straps.Quote:
Originally Posted by Xavie
I've heard comments from them regarding other products that they sell that have been along the same lines, notably trying to work out why the Warn winches with the same power and same gearing pull less than some of the other cheaper winches..... and it came down to drag in the winch brakes. Again likewise with the revision to the ARB compressors which are apparently under rated by them in their performance
Absolutely. These are also dry breaking strains.Quote:
Originally Posted by jimbo110
I think that there are a few recovery points out there that will fail before the strap does.
So if you dont have a 11 ton recovery point on your car ther is no point for a 11 ton snatch strap? ..
Thats correct in terms of outright force, but the 11 ton strap will last longer than the 8 ton strap, so you will have increased life.Quote:
Originally Posted by disco_2008
Its not quite as simple as ratings. As Jimbo said their are 5x fold ratings for shackles, same applies for most rigging and lifting gear as safety margins are put in place. Rope rescue is the same with an 8:1 safety margin observed, so infact these items are way under rated. Of course that does not mean you should exceed the SWL.
The difference is most 4x4 recovery gear such as snatch straps are rated to their maximum breaking strain and have no real safety margin. It is worth enqiring about safety margins when you buy a product and I doubt most 4x4 retailers would be able to tell you. Also important why you buy rated shackles not just generic hardware store ones.
There are also some subtle differences eg the force transfered to the recovery point may be no where near the force actually generated into the strap. Generally the force generated in the strap will concentrate in the centre, but the eyelets are the weak point and if they snap it is usually the stitching in the eyelets that goes.
Recovery points are rather subjective in their capacity, but should always be chassis mounted with high tensile bolts.
Some testing on this would be interesting. I think more emphasis on recovery point ratings is needed.
The hook type recovery points should actually bend before the bolts break, hence a safety factor. The strap will come off if they do and can cause damage or injury but not to the same degree as a shackle. Using D type shackles is another matter as they will deform but usually this will not be seen until they break by the untrained or not so observant and then become lethal. However it is unlikely a 4.8 tonne shackle will fail before a 11,000kg strap. A point to remember is if you go to 11, 15 or 19,000kg straps you should also go up in the shackle SWL to match. While as said a 4 tonne shackle has a safety of 20 tonnes built in if used with a 19,000kg snatch strap is right on its safety limits if loaded up and way outside the safety margin.
The major factor here is to inspect all your equipment prior to and after use for any damage and if possible observe any deformaties when being used (without putting yourself in danger). Any distortion in shackles or recovery points should be assessed and retired immediately (put a grinder through them so that they can not be used again). The recovery straps and winch ropes are a bit different and they are allowed to accommodate a certain amount of damage and still be serviceable within certain tolerances. If you are unsure or untrained get a rigger or someone with inspection qualifications to inspect for you. Consider whatever damage is visible to your eye, there is probablly many more times that damage within the item being checked.
Most 4x4 shops can supply you with 7.2 tonne SWL D shackles, they are the most common along with 4.8 tonne but there are other variants out there depending on manufacturer. Higher rated shackles are also available but may be special order for most 4x4 retail shops.Quote:
Originally Posted by disco_2008
If you are concerned go to a specialised rigging supplier and their reps are usually qualified riggers and inspectors of such equipment and can allay an fears eg Bullivants.
As I have said in another post in here the 4 tonne rating on a shackle has a safety of at least 5:1 so is in effect a 20 tonne breaking strain, a snatch strap is rated at its breaking strain and generally has no safety margin. Confusing and stupid to have 2 sets of rules.
I have had custom made 15tonne and 19 tonne snatch straps made and there are suppliers that will get these for you, but they become significantly dearer. We get them to recover trucks. Also you will need to uprate your shackles to match.
The shackle has a WLL with a huge safety factor for lifting applications etc. I don't recall the test figures, but they fail at something like 23000kg and undergo significant plastic deformation (permanent bend) before they fail. It's more likely that you'll tear the point or the chassis out than break a 4T shackle.Quote:
Originally Posted by disco_2008
In my past I have made and tested webbing equipment used in caving and climbing. The risk here of personal injury is much higher then in the 4 x 4 area if things go badly wrong., so standards have to be extremely high. Once had a well used, if not abused caving harness returned about two years old as the guy had replaced it with a new one that I had made. Would I be interested in testing it. Lets say that this harness was so badly abraded that I would not even contemplate using it. Surprising enough the harness passed the UIAA (climbing association ) official tests.
This was due to no significant damage to stitch blocks or nicks/cuts in from edge of webbing
So my two cents worth on webbing strops used in 4 x 4 area, assuming no manufacturing fault in webbing or stitching and what weakens them and when to bin them. Not in any particular order
Stitch blocks.
Are there any signs of stitching unravelling? Are there any broken stitches? The odd broken stitch is not too bad get multiple broken stitches then it could be a sign that the strop has been extremely close to its breaking limit.
It might be prudent then to replace it
A good stitch block should in my experience be stronger then the webbing.
Lateral Damage
Nicks and cuts from the side. As a general rule a cut going into the side of a strop say 10% of its width and you will lose normally just over 10% of its strength. So a 8,000 kg strap is now say a 7,000 kg strap. If two or more 10% cuts on same side to same depth in general terms you will not lose any more strength. If you have 2 10% cuts on both edges of strap and your 8,000 kg strap is now about 6,000 kg in strength
Surface Abrasion
This is more difficult to quantify and describe. If it is light and general 'hariness' spread around the strap then you may lose say 5-10% of strength.
If it is concentrated in one area and is the full width of the strap then it is possible to lose significant amount of strength. The percentage loss in this case could easily reach 25+% depending on how deep the abrasion has gone
Internal Abrasion
Filaments inside the strap can be damaged by grit being forced into the strap and the sharp edges of minute particles of sand /grit etc cutting the small diameter filaments which make up a strap. Difficult to quantify the percentage lost in strength in this case. Ideally you would not use a recovery strap in a muddy/sandy situation and avoid this problem BUT you normally get stucky in muddy/sandy/boggy areas so have to live with it. :p
Washing excess mud off can limit surface mud being later on forced into the strap. Do NOT use pressure washers to clean straps as that will force grit into strap. If you use the wives washing machine, do not use chemicals and do it when she is not at home for your own safety. :D
UV Degradation
Normally takes reasonable amount of time say 1-2+ years before becoming significant. Store them clean and dry in a bag or box and no problem. I personally would be more worried about cuts and abrasion
Chemical attack
Nylons are considered alkaline resistant and polyester are acid resistant. Note the word resistant they are not proof against these chemicals. Acids will attack nylons and alkalines will attack polyester. What material is the stitching mad from????
Damage by chemical attack is difficult to quantify and is best avoided. Avoid straps coming into contact with any acid, alkaline, oil, solvent etc. If you suspect any contact with chemicals bin them.
Storage
Keep straps in a clean, dry, light proof environment and they should be fine
In Use
Keep them as clean as possible (yeah, yeah I know :p) Avoid straps rubbing against trees, rocks ground especially when under load. Keep the webbing as flat as possible. Twisted straps are not as strong due to uneven loading on filaments
Remember a shackle will lose very little of its strength over a long time period if used/stored correctly. A brand new strap can be severely weaken by being badly used once!
Inspect your straps regularly, bin them if you have any doubts. A new strap is a lot cheaper then a windscreen or a visit to the hospital
Regards
Brendan
PS Where is the soap box store here? Time I put mine away :p