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Help Needed For Engine Crane Setup

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So I’m trying to extend the arm of this engine crane, so I can lift this 65kg roof tent.

It says the maximum load when fully extended is 500kg.

How would I calculate how long I can extend the arm and what material I’ll need ect?

Will making the arm longer and therefore more of it being within the square tube where the arm goes into make me able to extend it longer too?


Engine Crane Setup.jpeg

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The maths is very simple...it is the moment around the top tube (with a catch).

Measure the top tube to the pivot in metres...then multiply the length by the safe working load at that point. If it is 1.5m, it would be 1.5 x 500 which would give you a moment* of 750 kg m. You then divide that by the 65 kg and that will give you the longest length that the tube can extend to safely (don't forget the catch though!!!)

750 / 65 = 11.5 m

Now the catch:

You will notice that the length of the boom NEVER sits proud of the legs...this is to ensure that the centre of mass is always between the three ground points for stability...if it comes out of the footprint, it could topple! There are two options here (my choice would be the second):

1. Extend both legs so the mass never exceeds the footprint.

2. Strap a large water container (far-greater than 65 litres) to the back of the unit on the upright and fill with water. This will give a balancing moment of the length of the legs (possibly about 1.6m...?) x the volume of the empty container (remember to ensure when you use it it is filled!!!). By using a water container, you can manoeuvre the the unit with the container empty with minimum inconvenience, fill the container when you need to use it, then empty again for convenience.

For the material, the bending moment at 65kg will not be an issue (the unit is designed for supporting over a ton!), so just go to your local steel suppliers and get a bit of hollow section that will slide over the boom...that will be fine!

The "*" is for the Engineers reading this...The normal SI units for moment are N m...to convert from kg m, simply multiply the kg (or kg m) by 9.81 (or for the pedants 9.80665) which converts the kg to N (F = ma).

Good luck (and great idea btw!)


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