Must Have Basic 4x4 Recovery Kit

[tkn]

This post is long, but I think all of it has to be said.

The way this thread has been progressing opens a window on why people get killed and injured recovering vehicles.

Mechanical Systems

Any mechanical system comprises of a composite of different components and assemblies that together can be assembled to form a complete system suitable for a particular operation.

A “recovery kit” is just a specific name to describe a means of assembling a system for doing work i.e. safely recovering a vehicle.
Just as a car’s braking system is intended to work by bringing the vehicle safely to a halt.
Where there is a mismatch of components or assemblies – including the work capacity of each, whether that is called “Working Load Limit (WLL)” or some other specification – the system is inherently unstable and potentially dangerous i.e. it might kill someone.

Vehicle recovery is always a potentially dangerous operation, no matter who is doing or supervising it.
A “recovery kit” should be a readily usable system for doing the work and also a control system, such that the work may be completed SAFELY!

SAFETY.

You will find screeds written about it, but there is a disjunct when putting it into practice.

For example, a pallet of goods has a known mass, thus, the force that must be applied to lift that load is known.
In vehicle recovery no one can accurately quantify the force required in any situation,
MudrunnerDT has posted some usable figures and in the absence of calculations that show MudrunnerDT’s figures are wrong or highly inaccurate they stand for the time being.
As I understand it, according to MudrunnerDT’s figures, a “bellied” vehicle will be about 300% of its “loaded mass” i.e. (its Tare plus load) x 3 – for a fully loaded GQ Patrol tray back that would be about 9450Kg.
A recent article I read indicated that a bellied vehicle would need force of about 5300Kg to initiate a recovery. The vehicle mass was said to be 3.2Tonnes and from the accompanying photograph it appears that the vehicle was bellied on a flat stretch.
If their vehicle had of been on an incline like the vehicle in MudrunnerDT’s photo then at least 30% to 75% might be added to the 5.3Tonnes i.e. 6.89Tonnes to 9.275Tonnes

Working Load Limit (WLL)

A WLL is not a rating based on a failure point i.e. the point in time or load figure at which a piece of equipment (e.g. snatch strap, chain, “D” shackle, etc.) becomes inoperable.

A WLL includes a safety factor to enable the equipment to be safely matched to and operated with other equipment.
It has been inferred by some that a WLL is excessive when considering vehicle recovery.
In the article I referred to earlier, words to that effect may have been used. The same article included a photograph of a bow or “D” shackle with pulley block attached – the load was shown as applied to the “legs” of the shackle and not to the crown of the shackle and to the shackle pin.
It is an example of one of the reasons why the WLL of a shackle includes a safety margin of about 500% i.e. failure of a 4.7Tonne WLL shackle must be more than 28Tonnes.
Another reason is that a constant straight line test cannot replicate the shock and stress of a shifting load e.g. when there is a “bounce” during the lifting operation.

Safety Factors

The article I read also said something to the effect that the WLLs for load restraint chains are less than for shackles, which is good news because they are over engineered for recovery purposes.

As I read it, the gist of the article, which is reflected in some comments posted in this forum, is that WLLs are for some unexplained reasons irrelevant to recovery equipment.

Problem is that the result is a recovery system that includes some equipment that have safety factors of 500%, some that have safety factors of 200% and some that may have safety factors of about 10%.
The outcome is mismatched equipment i.e. WLL equipment and equipment with small safety margins.
A number of commercially available recovery kits include mismatched equipment.
THE SAFETY FACTOR OF THE WHOLE KIT MAY BE REDUCED TO 10% OR 30%

In my view, a safety margin of 10% OR 30% is ridiculous in any mechanical system!

Motive Forces

Winch capacity is a limiting factor in a winch driven recovery system.
If the winch (electric, PTO, hand or any other kind) can pull say 4000Kg then that by default sets the limit of the system and all parts of the system must at least be able to safely withstand application of that pull.
The same applies to a system that includes a winch that can pull 8000Kg – all parts of the system must at least be able to safely withstand application of that pull.
For the sake of clarity, a chain rated for lifting 10,000Kg with a “D” shackle at each end rated for lifting 4500Kg could be used as a bridle assembly and may be considered as capable of withstanding 8000Kg with a safety margin in addition to its WLL of about 12%.

A problem with a snatch strap is the only thing known about it is that it might fail some way short of say 8000Kg or a little above 8000Kg, for example.
The inertia of the vehicle applying force to the snatch strap may easily exceed 8000Kg – there is no inherent limiting factor, such as the capacity of an electric winch or shear pin in a hand winch.
The snatch strap itself does not limit the force that may be applied to a recovery system except by failing, which I think we can all agree is an undesirable outcome.
The solution IS NOT to include a snatch strap that will probably fail with application of a load in excess of other part(s) of the system – such an arrangement is dangerous and may be lethal.

THE FAILURE TONNAGE OF A SNATCH STRAP IS NOT A DE FACTO WLL.
There is no safety factor – a test to destruction means an inoperable product.
A snatch strap cannot have a safety factor – see above.


A few points may be summarised as follows –

1. A basic recovery kit should be suitable for common eventualities and unfortunately, drivers burying a vehicle in soft ground is commonplace – whether sand, mud, river bottom, river bank, roadside drain, sink hole, beach quicksand, leaf litter, (hidden) seaweed bank, etc., etc.
2. The recovery of a “bellied” vehicle will probably involve a pull exceeding 8000Kg.
3. Any person (especially one with little technical skill) should be able to assemble and operate the system to enable a driver to safely extract a vehicle from soft ground, etc.
4. There can be no presumption that all persons are equally competent at vehicle recovery.
5. For safety there must be certainty in the recovery process.
6. Guesswork fails.
7. Making do kills.


A “recovery kit” enables the assembly of different systems of work for a variety of situations, but safety must be –

(a) Constant, consistent and stable; and
(b) Must never be compromised for the sake of convenience or to save a few dollars.

Lets try again.