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How to Analyze a Devil's Hook

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Ron247

Structural
Jan 18, 2019
1,052
Just curious if anyone has ever analyzed a Devil's Hook to show mathematically how it works. I had one of these in the 60s but lost it. Never knew what it was called. Recently found out it was called a Devil's Hook and found them for sale in Magic Shops and some other novelty internet sites. Attached are some photos and a sketch of the dimensions. I know it is all or partly related to loading through the shear center. These cost about $2 and both kids and adults are amazed by them.

Devils_Hook_2_tzqtcx.jpg


Devils_Hook_3_xtqpts.jpg


Devils_Hook_4_yalvt0.jpg


[URL unfurl="true"]https://res.cloudinary.com/engineering-com/image/upload/v1570801609/tips/Devils_Hook_Dimensions_lnsvc0.pdf[/url]
 
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Agent666 said:
The centre of the gravity force of the hanger thingee and the belt must align horizontally with the pin support. Equilibrium tends to do that.

Don't you mean vertically?

BA
 
Same horizontal location, vertically below the support point to clarify.
 
Yes Agent666, the resultant force of the Devil's Hook plus belt must act through the support point which I take to be the end of shelf.

BA
 
Ron, I do not have any references for the balancing cylinders trick. I worked out the math when working on developing very low friction, but high load, pivot mechanisms for a machine I was developing. When the math revealed that only a smaller bar will be stable when balanced on another bar, I then vaguely remembered seeing the trick before. The principle can be demonstrated with many common objects that have a smooth, cylindrical section, such as wine bottles and pens.
 
No mystery, and fairly simple maths. The combined centre of mass coincides with the support point (as does any plumb bob).

There’s an associated bending moment in the hook (and belt), as there is in any hook.
 
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