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Using CAD in initial design – More work on the Popsicle Crane

The Base Drawing of the Crane.

The Base Drawing of the Crane.

People normally think of CAD drawings being used to produce finished plans. While that is probably the main use for drawings, CAD is very useful in all stages of design including conception and analysis.  I have decided that I will do more work on the Popsicle Crane and now I need to do more analysis before proceeding.

This “little project” has become in many ways a case of reverse engineering and the only thing little about it is the size of the crane.  As I drive around if I pass a construction site that has a lattice boom crane, I usually stop and take a look at the crane. Most modern cranes use a very thick cable doing the function of lifting the boom.  This cable is fixed length and ties into a hinged frame behind the cab.  That is the next step in design of this crane.  Although this new frame could be made with a lattice work similar to the boom,  this is an opportunity to learn some new skills and I will probably create it out of some more hefty material than Popsicle sticks.  Details of construction will be later, but for now this post is dedicated to using CAD for conception and analysis.

In the first and base picture, I drew the arc of the boom and another arc showing the point where the boom height control cables connect to the boom. The solid line at 45 degrees represents the centerline of the boom at 45 degrees. The lines fanning out from that are 10 degree increments from 5 degrees to 85 degrees above horizontal.  Lines are being used to represent structures, because that is all that is really needed for this purpose and also to keep from making the drawing too messy.  It will be messy enough very quickly.

pix2_8h6b setup

Determining the Length of the fixed cables.

In the second picture, a vertical line was drawn six inches behind the main boom hinge and another line 8 3/8 inches above the hinge.  These numbers were arrived at by trial and error.  In other words, there was more than one attempt to arrive at a satisfactory answer. The small circle represents the swing radius of the new frame. A line was drawn from the cable connection point of the 45 degree boom to the intersection of the vertical line and the small circle.  The new frame will be vertical when the boom is at 45 degrees.  A large circle was drawn centered on the boom-cable connection point and with a radius equal to the cable length.

max and min positions of hinge frame

The Maximum and Minimum angles of the Hinged Frame.

The large circle was copied by moving the center from the 45 boom angle connection point to the same point on the 85 degree and the 5 degree boom positions. At this point the cable can simply be drawn by drawing a 2 point line from the intersection of the two circles to the connection point on the boom. By copying the circle this cable will be exactly the same length as before.

The new hinged frame max and min swing angles.

The new hinged frame max and min swing angles.

The next picture shows how I zoomed in to draw the maximum and minimum swing angles of the new frame. This was the point where I decided the height and setback of the new frame was acceptable.



Determining all of the positions of the new frame.

Determining all of the positions of the new frame.

Now that I had decided to go with this height and setback combination, the drawing continued by simply copying the large circle to all of the boom positions. The intersection points were connected to draw the cable and the new frame lines for each boom positions.

pix6 lines added circles removed

Lines added, circles removed




After all of the lines were drawn the layer of the circles were changed to a layer that is no longer visible to remove some of the confusion in the drawing. I chose to move these circles to a non-visible layer instead of deleting them in case I made a mistake and needed to see them again.


Measuring the boom to cable angle

Measuring the boom to cable angle


From the other force analysis work we have done, we know angle measurements are the key to understanding the forces on the cables and other structural members. We are now at the point where we can acquire the angle measurements.  The first set is the boom to cable angles.


The angle from the cables to the new frame.

The angle from the cables to the new frame.


The next set of angles to measure is the cable to new frame angles. These angle measurements are functional, but not pretty.  This is a drawing for our analysis use and not finished plans to be constructed.  These angles were drawn on a new layer and the layer with the previous angles was made not visible to make the drawing “less busy”.


A new cable to pull the new frame.

A new cable to pull the new frame.

Now that we have laid-out the new frame, we need some way to control it. This will be done with another cable. I picked an attachment point for this new cable and drew it in each of the positions of the new frame. Next I measured the angle for each of these positions.

Now that we have all of these angles, the next step will be to enter all of this information into a spreadsheet and use trigonometry to calculate all the forces on the beams and cables,.

That will be for a future post.


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