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Crane Boom Analysis Part 2.

Picture 1: All of the loads acting on the boom.

This is a continuation of the last post and our attempt to understand the forces acting on a crane boom.  The summary of where we left it was:

  • All of the forces along the boom will compress the boom.
  • The boom weight and the load weight will try to make the boom rotate counter-clockwise and the cable controlling the boom angle is the only thing opposing that.

Tonight we will modify a few things and see what effect those modifications will have.  It will be easy to test the conclusions by doing the following test.  Hang a weight off the end of a broom stick and tie a second string to act as the “boom height” cable.   Rest the broom stick on the ground but use your foot to keep it from moving.   The pressure of the broom stick on your foot is the compressive force on the boom.

We will only deal with two of the forces in the rest of this discussion, the load force and the boom lift cable force.  We know how to reduce the force on the hoist lift cable inside the boom and we don’t have many options on the boom weight except to try to make it as light as possible.

Effect of Raising and Lowering the Boom on the Boom Cable Angle.

Lowering the boom will have two effects.  Primarily,  it will increase the force perpendicular to the boom created by the load.  It will also affect the angle between the boom cable and the boom, but this will probably be small.   This means lowering the boom will increase the tension in the boom cable and increase the compression in the boom.   Raising the boom will have the opposite effect.

The height of the point I call the “Cable Pivot Point” will have a large effect on the boom cable tension.  In this case the effect is easy to understand and summarise.  The higher the better.   Increasing the hight of this will increase the boom to boom cable angle and this reduce the tension needed in the cable to counteract the load weight.

Increasing the weight of the load will increase the boom cable tension and the compressive forces on the boom.   Things are starting to get more simple.

Since the crane boom is primarily under compression it is very much like a tower that is also under tension.  The biggest difference is a tower can have guy wires all the way around it.  These prevent the tower from toppling due to a sideways force.   The crane has the equivalent to one guy wire in the form of the boom cable.   However there is nothing to help the crane boom if there is a sideways force.  I have noticed the crane booms are more wide than tall.   This helps strengthen the crane’s resistance to sideways forces.  I have read a few crane accident reports where the crane was attempting to pull something from the side and the crane failed due to the boom buckling.  That will also be our way of dealing with it.  We just won’t do that.

The next post about the crane will talk about the structural members under compression.  We will start to put dimensions on things and come up with some tests to test our theories.   Unfortunately we are a long long way from building the crane. There will be lots of fabrication details to work out.

Gary

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