Trying to model forearm bones (HELP!!!)

[saqademus]

Hi everyone, first time poster here. I'm in my final year of Mech Eng at Imperial University and using Pro E for my dissertation. I have to model the twist of the two bones that make up the forearm (radius and ulna) and am having some serious difficulties doing so.

I'm keeping it simple and just using two cylinders to represent the bones, but its the twist motion that I'm not sure how to model.

The radius at the elbow just simply spins, but at the wrist end it orbits around the ulna.

Is this possible?

I've attached a diagram to help explain what I mean.

Any help would be greatly appreciated guys!

 

[saqademus]

anyone have any tips?

 

[muadib3d]

are you asking about mechanism connections?

 

[kenppy]

I think you are talking about mechanism connections with limits, yes that's possible. can you describe how many parts you have, how they are connected and how much they move, the solution is fairly straightforward.

 

[saqademus]

well its the mechanism of the motion thats most important. so 3 pieces. 2 cylinders representing each the radius and the ulna, and a sphere for the elbow.

 

[kenppy]

saqademus,

It's often about how the problem is described, help us to help you. Sketch the parts AND show the limits to the movement.

 

[saqademus]

is there are part of the diagram i've provided that needs clarification?

 

[kenppy]

I did suggest that you show the limits to the movement, for each part.

 

[saqademus]

sorry I'm not sure how to show the limits?

The motion is 180 degrees (ie palms outwards to palms inwards) so I guess both radius and ulna will be able to rotate 180 degrees both ways. The wrist also (makeshift wrist). But the elbow will not move.

the radius part will spin on its central axis at the elbow joint (as it does in ones arm) but the whole wrist will translate 180 degrees.

I hope that helps

 

[kenppy]

do the ulna and radius stay parallel to each other at all times?

 

[saqademus]

no because they cross, just like your actual forearm bones

 

[kenppy]

I need to go back and ask again, can you define the members, the type of joint, and the limit of movement. This is purely a description of the machanism; your dissertation must start with this. I said at the start the solution is easy enough but you need to pose the problem in the correct terms.

This is in the spirit of being helpful.

 

[saqademus]

It's the mechanism that i'm struggling with, so my descriptions probably wont be that helpful.

I need to model the 180 degrees twist of the forearm. There need to be four members; two forearm bones, an elbow and the wrist.

I have researched into how a REAL forearm twists and need to model it as similar as possible.

hold your right arm out, palm facing upwards. Then turn your hand anti clockwise 180 degrees so its facing downwards. That is the motion of the forearm I need to capture.

From my research into the motion of the bones, I've found that the Radius at the ELBOW just spins in its socket, as I've tried to depict in my diagram.

Thats what I know, but arranging a mechanism that shows this is where I'm stuck

 

[kenppy]

Have a look at this, I've not applied any limits yet but it might give you someideas.

2011-11-22_115143_asm0001.asm.zip

 

[saqademus]

Just received this message as i've been in hospital for the last 4 days, I'll get a chance to look at this tomorrow so thanks for your help thus far, its appreciated

 

[saqademus]

Kenppy

Sorry for the late reply; I've just returned to pro E modelling after the xmas break and February research.

Thank you so much for the time and help you've given, its really appreciated.

The file you sent was really helpful but I have to ask how you did it? Could you tell me how you included the twist motion and the joining of the pieces (or was it a simple mate). I want to try this myself!

Then I can make the parts more detailed, add constraints (which I hope isnt too difficult) then voila, I should nearly be there. Thanks again!!

 

[kenppy]

You should be able to redefine the component and see the constraints.

 

[saqademus]

I am really struggling with getting an accurate replica of this motion.


I was in the lab a couple of weeks ago and recorded the EXACT motion I need to replicate.


http://www.youtube.com/watch?v=pogIgxdMuFQ


Notice how the radius twists in the socket but the ulna remains fairly rigid (at the elbow anyway)


as close to that video i can get the better...


is this possible? If so - can you give me a rough step by step as to how I could go about it?


Thanks