By Ken Sibly

A recent incident, reportedly involving a synthetic rope and a snatch block, prompted me to do some research and gather some information from experts on the subject of the proper use of snatch blocks; all in the interest of safety.

The incident apparently started when a truck got stuck attempting to drive through a bog hole. The story goes that a snatch tow to extract him was attempted but failed. While the driver of one of the preceding vehicles offered to winch the stuck vehicle out his kind offer was refused in favour of another snatch tow attempt.

Two 4WDs were linked together and then hooked up to the stuck 4WD with a long, 20mm diameter, polypropylene rope and both gave a good snatch tow. The tow vehicles were around a bend in the track so the result was that they only succeeded in pulling the front of the stuck 4WD sideways.

So, the next step was to put a snatch block part way along the long rope, so they could pull the stuck vehicle out straight, with the next go.

However, there was a catastrophic failure of both the snatch block and the long rope, on this attempt, and the stuck truck was finally winched out of the hole. In hindsight, perhaps thats what should have been done in the first place.

Equipment Used

The equipment used in the incident, in addition to the 20mm polypropylene rope, was a Titan snatch block. I have actually seen the broken snatch block and from my personal observation, one side, along one third of the circumference, appeared to have broken off.

The snatch block was rated as having a capacity of 8000kg and was designed for 11mm diameter wire rope. It had a cast iron sheave (pulley) with three 20mm holes. It was made in China.

Its important to be aware of some facts about ropes and sheaves. Ropes are manufactured in a great variety of constructions to meet widely varying demands. A cheap and common rope used for 4WD recoveries is the very 20mm polypropylene used in this example.

The rope is manufactured using what is called a laid rope construction. There are various brands available but they usually have a breaking load of about 6000kg. The safe working load of this rope is one sixth of the breaking load, or 1000kg.

Care should be taken with all man-made fibre ropes, like this is, as they give little or no warning when they are about to break. Obviously, we exceed the safe working load of this rope, when it is used for snatch towing, so it is extremely important to use the rope with the utmost care and we shouldnt use it for purposes, or with equipment, that it was not intended for.

Snatch blocks are used for turning a rope around a corner and it is vital to use a rope that is suited to the sheave, otherwise failure of the rope, or the sheave, or both is possible. The effect of using a too small sheave, in comparison with the rope used, is the accel-erated wear and damage of the rope and also the sheave groove.

The pres-sure of the rope on a sheave should ideally be spread as evenly as possible for a given load and the strongest part of the sheave is the bottom of the groove. However, as you can see from the photograph that if the rope is too large, it is just contacting the top edge of the sheave groove and there is a large gap under the rope.

With a load applied to the rope it is forced down into the groove but not all the way and is, therefore, applying the most pressure to the sides of the groove. So, it is quite possible for the rope to burst the side out of the sheave, as apparently happened on this occasion. The resulting sharp edges would also likely sever the rope immediately.

In addition to the rope applying pressure to the sides of the groove the edge of the top of the groove will likely cut into the rope as it is forced down into the groove thus also causing rope failure.

If you are using a pulley or sheave, make sure the sheave is of adequate size to hold the rope your are using. The groove of the sheave should support between 135 and 150 degrees of the ropes circumference and the width of the groove should be 10 percent wider than the ropes diameter to give the longest service life.

The sides of the groove should be tapered out, at approximately 30-degrees, to permit the rope to enter and leave the groove smoothly. Too small sheaves can cause the rope to build up turn (due to the spiraling of the strands), which may cause kinks in the rope, with a further resulting strength loss. Ropes can also become jammed in a sheave that is too small.

Snatch or power tow?

The term snatch block is a bit of a mis-nomer because while it can be used in a power tow situation, if theres not room to do a straight-line pull, it shouldnt be used to do a snatch tow, under any circumstances.

When you are setting up a snatch block, whether it be directly attached to a vehicle or connected to a rope around a tree, it will hang down or be laying on the ground. In a power tow the rope is tensioned slowly and it will slowly raise the snatch block, so that sheave is parallel to the rope, and allow the rope to run through the sheave correctly.

If you attempt to snatch tow a lot of nasty things happen all at once: The slack is still being taken up in the rope; The rope is being stretched; The snatch block is being lifted and; The rope is running through the snatch block.

Its not necessarily happening in that order but is trying to happen all at the same time and, with hugely increased loads involved, its a recipe for disaster. As the rope is being pulled through the snatch block and the block is being lifted the rope will already be running against the relatively square edge of the side plate, not against the sheave, causing chafing and potentially jamming.

This problem will be accentuated if the rope is running through a narrow block, like the ones designed for 11mm wire rope. The side plates on many snatch blocks overlap the sheave so rope chafing is inevitable if the wrong size is used but even if the correct size block is being used the rope will still chafe on the side plates causing failure if you use it to snatch tow..

Available snatch blocks

A survey of 4WD outlets showed that the only snatch blocks commonly available were designed for the use with 11mm wire rope. Of these, Goldford appeared to be the most common brand, with a cast 125mm diameter sheave.

Ridge Ryder was stocked by a well known generic parts supplier; it had only a 105mm sheave, which is a bit on the small side for an 11mm wire rope. Black Rat was similar to the Goldford block, with slightly different side plates.

ARB stood out, appearing the best-made snatch block available and it was the only one that gave its breaking strength, which happened to be 14,500kg.

There were no 4WD blocks readily available for use with 20mm Polypropylene rope. So, if you want a snatch block to use with 20mm rope you will have to get one made. The block, shown in the accompanying photograph, has been made specifically for use with 20mm polypropylene rope and has a sheave machined from a piece of stock 100mm diameter, 35mm wide aluminum. There is a 22mm diameter stainless steel pin through the sheave and side plates of the block.

Apparently the owner of the 20mm rope that was broken, in the earlier incident, did have a suitable block available but, for some reason, it was not used.

There are some suppliers and 4WDrivers who have said that it is OK to use 20mm rope with snatch blocks designed for 11mm wire, but I simply cannot agree at all; for the very reasons outlined, failure is more than likely to occur.

In addition to using the right piece of equipment for the job, the quality of the equipment should be inspected. The manager of one well-known recovery equipment supplier assured me that the sheave in the snatch block they supplied was machined from a billet of steel. This was simply not the case, as I witnessed the testing technician of the same company take one of these blocks apart and it was found to be sand cast.

So there is obviously not only a question of quality but also supplier product knowledge. Anyone can get it wrong, check it out for yourself.

For several reasons, the result of snatch towing, using 20mm polypropylene rope through a snatch block, that was designed for 11mm wire rope, was catastrophic failure of the sheave and a broken rope. A failure like that could have easily resulted in injury or death, let alone damage to property.

(While not party to the actual event recounted in this story, regarding which we therefore have no opinion, we support Kens personal concerns regarding safety and the proper and thoroughly planned use of equipment and techniques. DK).: