Thursday, October 13, 2011

Spring Tensioning Design




We've been discussing spring tensioning for porcelain BJDs over at the Enchanted forum. The basic idea behind spring tensioning is that instead of elastic, springs, s-hooks, swivels, and pins are used to put the ball joints under tension. Usually, a hole is drilled in a ball joint, and a pin is epoxied in the hole. An s-hook attaches to the pin, and goes through a slot cut in the ball joint. The other end of the s-hook attaches to a spring or a swivel. In the following diagram I have color-coded the various parts used in spring-tensioning a porcelain BJD. Click on any image to enlarge it.






As I learn more about springs, and spring-tensioning a porcelain BJD, I make new designs for a possible scheme to tension a porcelain BJD using springs. In the following diagram, #1 shows the head assembly with a pin, s-hook, swivel, s-hook, and spring. The spring from the head assembly is attached to an s-hook which is attached to a spring in the torso of the body #2, which is attached to two s-hooks that hold the legs onto the torso. Those s-hooks are attached to the pins in the hip joints. Then, in #4 and #5, the pin, s-hook, spring, swivel, spring, s-hook, pin scheme is used. The arms, #3, are attached from wrist ball joint to wrist ball joint, with pin, s-hook, spring, s-hook, pin. The diagram shows single joints, so if double joints are used in the knees and elbows, then two pins are used.



I have no way of knowing for sure if my design will work until I try it. There are many variables which factor into using springs for tensioning. I am still learning about springs. The springs used in a porcelain BJD are called tension springs, or extension springs. A force applied to the spring pulls the ends away from each other, and the spring wants to return to its original state. It is that tension, along with friction, that allows the BJD to pose. Friction is usually supplied by sueding with a thin leather called pliver, but that is the subject of another post.




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3 comments:

  1. This is very interesting. I am making a few BJD's for my 3rd year final project and am using Yoshida Ryo's book for reference which shows a doll being strung with elastic from, for example, the ankle straight through to the head.

    After I made my maquette, I was confused as to how a doll strung this way would be poseable if the joints weren't under tension.

    I came up with a solution similar to yours except still using elastic. Where you have your springs and swivels is where i had my elastic. I do like this method of engineering though, i'm glad i stumbled upon it, could I reference you and this idea in my research please?

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