Cantilever Bridge/Crane

[lekh2003]

I am doing an engineering project and my team has been assigned to build a cantilever which should be able to hold 5kg minimum, 1 m away from the base. I will be using thin wood pieces, PVA glue, and string with the restrictions: height 60 cm, length 1.25 m, width, 20 cm.

I have an idea of building cantilevers, and I was just looking for general advice or tips on building the crane (techniques, basic structures recommended). I am also looking for past cantilever designs in real life which I can use as a guide to help me choose what to put in my final design. I am not looking for a final design, just inspirations.

If this thread needs to be in the homework forum, I can post there instead.

 

[berkeman]

It's probably okay here for now. Can you post some of the more interesting cantilever designs that you've seen? That will help us to see where we can fill in with other suggestions for things for you to look at. I have a particular design in mind which is pretty cool and practical, but I'd like to see if you have seen it or considered it yet.

Also, what are the other constraints on the project? Are there any overall size limitations? Limitations on the amount of material? Etc.?

height 60 cm, length 1.25 m, width, 20 cm.

Are those dimensions an overall box that you have to stay within? What are the shape and size of the weight?

 

[berkeman]

Also, how is the overall assembly anchored to the ground?

 

[Baluncore]

Use the wood for compression members and the string in tension. Think spaceframes.
The structure must be stable and flexible, it should not fail suddenly if it twists or bends under load.
I assume the 5kg mass will hang from a point. If not it will be a real challenge to provide a platform.
The cantilever will taper gently at first from the as yet undefined “anchor plate” to the load bearing point.
What is the section of the wood, and how much is available? Bamboo would be good.
How strong is the string, and how much is available?

 

[lekh2003]

The restrictions. The abutment will be clamped to a table and the entire cantilever must be attached to the abutment via glue. Imagine that it must fit in a box of those dimensions.

I have seen a few designs already, but they are currently at my school and its the weekend. I have however already though up a few ideas, maybe you can give your thoughts.

We are being graded on our angle of deflection, how the cantilever moved downward when the weight is applied, so I had the idea that the anchor point be at the very end of the cantilever. I thought this because I didn't want any of the weight of the wood contributing to the effect of deflection.

Now for actual designs. Most designs I have visited are long cantilever attached to a tower which is attached to an abutment. One cantilever was attached to the bottom of the tower and was suspended by tensioned string. This cantilever had a rectangular prism shape.

The next one was entirely triangular and the cantilever was built directly into the tower with triangles and further enforced with tensioned string from the top of the tower. I genuinely thought this was the best idea. I am well aware of the strength of triangles and this cantilever was quite strong. The angle of deflection would also be minimized since the cantilever can't really move very much since it's part of the building.

Other design were similar to the first.

One more thing, I'm not required of this, but how can I calculate the angle of deflection. How am I supposed to account for the weakness of the adhesive and wood in a statics problem? (My physics course conveniently decided to skip over strain and tension in objects).

 

[Baluncore]

One more thing, I'm not required of this, but how can I calculate the angle of deflection. How am I supposed to account for the weakness of the adhesive and wood in a statics problem?

You design to minimise the deflection by applying the material sensibly in the design. It is important that deflection does not result in structural failure. The deflection should transfer load to other paths through parallel members.
If glue strength is a problem you need to re-design the joints. Glue will hold things in place. Consider dividing a short piece of string into smaller threads that can be used to lash a joint, then coat the thread and joint with glue to lock it in place.

The 200mm square base plate will clamp flat to a table top. It should have a high timber 'A' frame hinged at the front edge, supported by string guys to the rear corners. The cantilever is then an 'A' that stands hinged against the front edge of the plate. By hinge I mean a joint that can flex like a pin joint, as the hinge needs to ensure that the 'A' legs remains straight when deflection occurs as they are under compression.

The 'A' side legs can be fabricated from three strips of wood each, in the form of an 'I' beam. Wooden cross bracing, like rungs of a ladder, between those keeps them straight. I would consider tapering the I-beams in height like a fusilla, thinner at the ends but separated in the middle.

The guy strings from the rear corners of the plate pass first to the 'A' frame tower peak, then to the tip of the cantilever where the 5kg load is applied.

 

[lekh2003]

You design to minimise the deflection by applying the material sensibly in the design. It is important that deflection does not result in structural failure. The deflection should transfer load to other paths through parallel members.
If glue strength is a problem you need to re-design the joints. Glue will hold things in place. Consider dividing a short piece of string into smaller threads that can be used to lash a joint, then coat the thread and joint with glue to lock it in place.

The 200mm square base plate will clamp flat to a table top. It should have a high timber 'A' frame hinged at the front edge, supported by string guys to the rear corners. The cantilever is then an 'A' that stands hinged against the front edge of the plate. By hinge I mean a joint that can flex like a pin joint, as the hinge needs to ensure that the 'A' legs remains straight when deflection occurs as they are under compression.

The 'A' side legs can be fabricated from three strips of wood each, in the form of an 'I' beam. Wooden cross bracing, like rungs of a ladder, between those keeps them straight. I would consider tapering the I-beams in height like a fusilla, thinner at the ends but separated in the middle.

The guy strings from the rear corners of the plate pass first to the 'A' frame tower peak, then to the tip of the cantilever where the 5kg load is applied.

This was very helpful, I will be sure to use the advice.