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wbedwards88
08-31-2011, 11:33 AM
Dear Subscribers,

Our lab is equipped with an 858 Mini Bionix II materials testing device (MTS Systems, Eden
Prairie, MN). This device has one linear actuator (tension-compression) and we would like to expand its capabilities to perform torsional tests of human long-bones. We are currently developing plans with our machine shop experts to do this and we our envisioning a mechanical device that transforms the vertical displacement of the linear actuator into rotation movement of the bone using a rack and toothed wheel type gearing (i.e., the rack would be connected to the linear actuator which would rotate the gear wheel). The long bone would be mounted orthogonal to the linear actuator and potted in two cups. The distal cup will be fixed in rotation and the proximal cup will be mounted to the gear wheel.

We are curious if any of the subscribers have had experience developing such a device. If so, we were hoping you could share with us any design plans, suggestions, potential pitfalls and problems, etc. Please note that we are operating under budget constraints.

Thanks in advance,

Brent

W. Brent Edwards, Ph.D.
Postdoctoral Research Fellow
Department of Kinesiology and Nutrition
University of Illinois at Chicago

dlindsey87
09-02-2011, 11:35 AM
Brent,

We did similar thing where we used the MTS to provide a third axis of motion. We did have the 858 Bionix with the spine fixtures so we were able to run 3 simultaneous channels (axial compression/tension, axial torsion, and then a 3rd torsional axis) which we used for a combined 3-point bending/torsional experiment.

If you don't you plan to switch the machine between configurations often then you could use the servohydraulics of your 858 mini to drive a torsional actuator to do the testing directly. You'd have to purchase a torsional actuator to do that. If you are inclined to do that let me know and I can look up the details for what we used. If you have any experts in hydraulics you might even be able to valve them to be on the same system, but only run one at a time (I would likely prevent students from using that machine). I'm not sure what the MTS warranty people would think of this idea.

If you do plan to switch the setup often I would probably use the setup you have described, or possibly just a moment arm as the failure angles shouldn't be too high.

Regards,

Derek

Derek Lindsey
Research Biomedical Engineer
VA Palo Alto HCS
Palo Alto, CA

anew53
09-05-2011, 04:54 AM
Hi Brent,

Have you considered a lever-actuated system? Guiding the rack is possibly going to be complex (although I imagine you could purchase a guided rack, a "rack-on-rails" if you will). I assume you will only need a few degrees of angular motion, so a lever attached to the linear actuator might be simpler and cheaper. The end of the lever will move in a circular path (although this will approach a straight line as the lever length increases) so you will need to decouple the motion perpendicular to the line of action of the actuator, but this is easy enough to do with a slot and a rolling bearing.

Hope this helps. Let me know if you need a sketch to clarify!

Cheers

Andrew


Dear Subscribers,

Our lab is equipped with an 858 Mini Bionix II materials testing device (MTS Systems, Eden
Prairie, MN). This device has one linear actuator (tension-compression) and we would like to expand its capabilities to perform torsional tests of human long-bones. We are currently developing plans with our machine shop experts to do this and we our envisioning a mechanical device that transforms the vertical displacement of the linear actuator into rotation movement of the bone using a rack and toothed wheel type gearing (i.e., the rack would be connected to the linear actuator which would rotate the gear wheel). The long bone would be mounted orthogonal to the linear actuator and potted in two cups. The distal cup will be fixed in rotation and the proximal cup will be mounted to the gear wheel.

We are curious if any of the subscribers have had experience developing such a device. If so, we were hoping you could share with us any design plans, suggestions, potential pitfalls and problems, etc. Please note that we are operating under budget constraints.

Thanks in advance,

Brent

W. Brent Edwards, Ph.D.
Postdoctoral Research Fellow
Department of Kinesiology and Nutrition
University of Illinois at Chicago

wbedwards88
09-06-2011, 10:40 AM
Dear Derek and Andrew,

Thanks for your input. We are currently discussing the potential of a lever-based system as you both have suggested. I'll update this post with our final decision in case anyone is interested.

Thanks again,

Brent

sferguson65
09-12-2011, 03:35 AM
Are there any problems with side / moment loading on the linear actuator when you use an offset lever actuated system?

shunter67
09-13-2011, 12:55 PM
Brent,

Years ago when I was teaching a biomech lab, I needed a simple fixture for a similar situation. The solution developed by TestResources for their small tabletop testing system can be found here: http://www.testresources.net/capstan-roller-strap-sheet-grips/torsion-test-fixtures/Torsion-Test-Fixture-g493-300-nm/. It was fairly simplistic - steel cable was connected to the actuator and drove a flywheel that rotated one specimen grip relative to another - but it was adequate for our purposes. I'm not sure if the lathe-type clamps can accommodate a human femur though, we were testing small animal models.

Best of luck,
Shawn Hunter, PhD
Director, Research and Development
Community Tissue Services