oldbloke wrote:

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Bit surprised the tow pin gave up so easily.

When you look at a ball it looks very strong, but just not designed to take that amount of force/stress. Also the metallurgical strength of the material would be a factor including fatigue.

I wonder if you removed the ball and used the hole on the tongue if it would be ok with a decent shackle??

We need a mechanical engineer on this forum.

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Like I said, remove the ball and attach a shackle in its place. Should do the trick assuming a bit of common sense is used. There is a bit of leverage there, and don't know about the metallurgy. But it's solid.

 

But I'm sure someone will find fault with the idea.

 

The way I see it, people use those snatch straps with way too much force. Better to do a bit more digging. 

 

 

 

Brenda and Alan wrote:

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Consider this if you are ever tempted to use one of these. The retaining pin is 5/8 in Dia (16mm).  Cross-sectional area is 200 sq mm. A total area of 400 sq mm.

A towball shank root dia is 20.5 mm. Cross-sectional area 330 sq mm.The un-threaded section at the top is 22 mm dia. Cross-section area 380 sq mm   

The steel used for both is chosen for sheer strength not tensile strength, so would be similar.

But in the case of the pin if the loading is not in a straight line but at an angle one side of the pin bears more of the load than the other in the instant of the snatch.

Of the two possible projectiles which would you prefer not to see heading your way. A 1 kg tow ball or a 2.5 kg lug and rated D shackle. Alan

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Hi Good figures. BUT IMHO the steel in most generic towballs would not be selected for any particular strength quality but as there is a lot of machining to be achieved for the final shape and thread I might suspect a free machining type of steel would be used. That is not a good quality steel for cyclic or impact use. Chrome plated critical components were banned for racing unless heat treated to remove likely embrittlement. Consider those things when buying one.

A rated shackle should be formed from some impact resistant steel and even partly forged to shape. Then proof tested also. But who knows what is actually used or done in some cheap O/S  factory. 

Use care in all selection of equipment. It is hard to judge these days where the quality is found. 

jaahn

What could go wrong? A young girl was killed about a year ago on a Charters Towers' property.

Using a dampener on the strap can take out some of the whiplash.

And tow balls often break under normal use (cheap Chinese junk?)

Brenda and Alan wrote:

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>50 mm tow balls are supposed to comply with AS 4177.2 which stipulates a testing

>regime which includes a lateral loading on the ball of 85 kilo newtons (8.67 tonnes

>force) but I have never seen this compliance stamped on one yet, only the usual 3.5 tonne.

 

Can anyone recommend a tow ball of known quality which complies with Au standards?

 

 

>As you may gather from my post on relative strengths of pins and balls I am not a

>fan of snatch straps, preferring a simple tow or winch for recovery.

 

A large elastic band stretched to an unknown and uncontrolled tension to which is applied an unknown force in an attempt to move an unknown mass and to do the whole thing in an instant with no opportunity for safety assessment. Not what I would call a sensible solution for anything and certainly not a solvable engineering equation.

Are these breaking from heavy jerking on strap ? I have seen some bad accidents on boat ramps also from breaking cable or straps !

Aus-Kiwi wrote:

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Are these breaking from heavy jerking on strap ? I have seen some bad accidents on boat ramps also from breaking cable or straps !

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 In 1965 I worked my passage to London on a Swedish cargo ship. We were in Fremantle and were instructed to move the ship two berths sternward. Two hawsers were used, one at the bow and another at the stern to drag the ship along the wharf. They were hemp hawsers. A steel cable cable spring was used to keep the ship up against the wharf. The spring ran from the stern and towards the bow. As the stern hawser dragged the ship sternward the bow line was slacked off and so was the spring.

Something went wrong and for whatever reason the spring was not fed out. The stern line was being taken in and the ship was moving. This over tensioned the steel cable and it started jerking and making a snapping noise. Eventually it jumped off the wharf bollard and slammed into the side of the ship. The railing stopped the cable from landing on the deck but the part from the winch was jumping around and seamen scattered and waited for the cable to stop jumping around. 

Of course the ship swung out away from the wharf; the captain was furious, he kicked the third officer off the rear deck and took over himself.

A friend in Norway, working on a tugboat. went onto the deck, which is a big no-no when towing and copped a broken hawser in the face. When I next saw him I only recognised him by his voice.

All tethers under tension are dangerous. Think back to school and how the teachers leather strap hurt. Now imagine something 20 or more times the mass moving 10 or more times as fast hitting you in the gob. 

Always stand clear of tow ropes.