Portlandjib.com
cranes
 

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I've come up with over a dozen crane models. Here's one of the earlier versions.

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The white string holding up the load (the cardboard box) is called the LOAD LINE.

String and rope are referred to as line.

  

     You can see the load line go up and then through an eye-bolt at the end of the boom.  Then the line runs down the length of the boom and finally under the body of the crane. But what's holding up the boom? 
The BOOM LINE which attaches to the boom  a little ways down from the tip of the boom ---

boomline.JPG

and then goes over the white pulley wheel (sheave) on top of the crane body and ends up at the windlass.

barrel.JPG

A windlass is a wheel attached to an axle.

windlass.JPG

You can see the 6" diameter blue wheel and the axle is the 3/8" thick aluminum rod that attached to the center of the wheel.  When you turn the wheel, you're also turning the axle and the barrel (the thick wooden rod that fits over the axle).  That's how you wind up the line to raise or lower the boom - (or the load)

windlassJPG.JPG

 
 
The later model cranes have wheels and a lazy susan.

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The brick is the counter-weight- without it the crane would tip over. 

see Overturning Moment in the Engineering page

With many of the cranes, the angle of the boom can change.

 With this crane, the boom is fixed, first at an angle of 60 degrees. 

60boom.jpg

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The weight of the crane body, -and the 2 bricks (which weigh 9lbs. together ) are sufficient to keep the crane level.

To change the boom angle to 45 degress-

boom45.jpg

  I repositioned where the boom attaches to the horizontal beam, and I replaced the #2 beam that supports the boom with a #3 beam.

Now to change to a boom angle of 30 degrees

2xboom30.jpg

  I simply went back to the #2 beam to support the boom, and attached it to the #3 hole

( instead of the #4 hole) in the boom. 

 

                           Why is the crane tipping over?

for the formula go to Overturning Moment in the Engineering Page 


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This crane is lifting a 4lb. nail gun. The load line takes the full 4lb. load.

I added a small pulley.

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 now the load line goes from the white plastic pulley wheel (sheave) in the end of the boom,

down through the pulley ,and then is tied off to an eye-bolt attached to the boom.

That means the 4lb. load is being supported by 2 parts of line.

The part that is tied to the eye-bolt is holding up 2 lb. and the other part of the line ( which holds up the other 2lbs.), goes over the sheave and then down to the windlass. 

Imagine that you're pulling on the end of the load line

-forget about the windlass for now. The weight on the line is only 2lb.- the eye-bolt is holding up the other 2lb. That means by using the pulley, we only need about 2 lbs. of muscle to lift the 4 lb. load.


70boom.JPG

 Here's a split boom- 2 separate beams connected to make a stronger boom. The boom angle is fixed at around 65 degrees.

splitcu.JPG

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It was easy to change the angle of the boom- simply move the position of the #3 beam that holds the boom.


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         Not the usual looking boom. The load is hanging so far away from the pivot point,
that a brick counter-weight was necessary.

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Here's what happens if you forget the counter-weight.

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