Need a simplistic testing rig to rotate two materials against each other for friction measurements

[Naveen93]

To start with, I am not a mechanical engineer and physics was never my strong point. I feel like what I'm asking for is pretty simple, so apologies if it's not suited for this board.

I want to build or purchase a simplistic testing rig that will help me analyze some of the frictional forces in a knee. Basically, the rig should hold each end of the specimen's leg and rotate it so that it resembles a natural physiological knee movement. I am interested in analyzing the frictional forces that occur within the knee during this basic bending movement, so ideally the rig will allow for a consistent and reproducible motion (but hopefully the motion and the force can be varied if I wanted) It doesn't have to be super precise or complicated though. I am looking for something easy to put together and cheap to build/purchase.

I have some primitive ideas in my minds involving a network of springs and pulleys, but like I said, I'm out of my element here so I would have no idea where to begin with actually constructing it. I just feel like there probably already exists something that can accomplish what I want, but when I search for things like this, I keep getting directed to these very expensive tribometers and universal testing rigs.

Note: The way I'm measuring friction is irrelevant to my problem and does not have to be incorporated into the rig. Again, my only issue is to create a simple rig that can reproducibly create this motion.

 

[KENAT]

So I'm sure you found these guys

https://www.bruker.com/products/surface-analysis/tribology-and-mechanical-testing.html,

did you read any of publications at the links they provide?

Simplistically all you need to be able to do is have your 'simulated knee' set up in a way that you can:

1. Measure normal force between the mating parts.
2. Measure force applied to make the knee bend.

2. Is pretty easy with some string, some masses/weights and some pulleys and some basic understanding of mechanics/levers & geometry to equate the force you apply away from the joint to the force you see at the joint. Gradually increase the mass until the leg starts to move (if you care about kinematic friction it'll get a little trickier).

1. is a bit more tricky but you should be do able in a similar manner - perhaps orient you simulated leg/knee vertically with the 'thigh' fixed and the lower leg 'hanging' with the simulated knee joint held together by strings (attached to the lower portion) over pulleys (attached to the 'thigh') with a fixed mass on each to give constant force.

Disclaimer: I may or may not have business links to the company I mention above.​

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[KENAT]

Oh, and of course to take to extremes of simplicity do you really need to worry about the fact it's a rotational effect?

Either dragging the 2 surfaces across each other via string/pulley/mass or cover some kind of 'board' with one material place a piece of the mating material on there (with suitable mass on it) then tilt the board until it slips and measure the angle, work back (vectors/trig) to the relevant F/R=µ.

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(probably not aimed specifically at you)
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[rb1957]

recognise that a simple model of a knee may well not be (won't be ?) an accurate model. even if it's "just" two materials rubbing against one another ... the devil's in the details ...

another day in paradise, or is paradise one day closer ?

 

[3DDave]

There are no significant friction in a biological knee. It is well lubricated and self-renewing. The shear forces of tendons in sheaths is higher, but still not very high until high rates of motion are seen, such a sprinting.

If you really are intent on this, then look at measuring the temperature rise in the knee near the interface and then look at a thermal model to see what power absorption is required to generate a similar temperature rise. I don't know what to do about the lubricant if the leg is no longer attached to a living animal.

See

http://hypertextbook.com/facts/2007/ConnieQiu.shtml

http://www.researchgate.net/profile/Sarit_Sivan2

published a paper on measuring cartilage friction; I tried to get a straight link to it; not the 2000 character link Google makes, but did not succeed.

 

[romke]

i guess it will be rather difficult to make a simple "knee testing rig" that gives meaningful results. basically human joints are fully lubricated, so what you actually measure would be mainly fluid friction and not the friction between the mating surfaces. unless industrially made lubricants, the lubricant in our joints is highly effective - one of the reasons being that its viscosity increases substantially under load and thus keeping a full film lubrication regimen intact. thus when you would be able to measure the friction, you would actually measure the changes in friction due to varying "lubricant properties", with not much reference to the surfaces moving relative to each other. sports like weightlifting or powerlifting would not be possible if we were not equipped with this very specialized and not yet industrially reproducible lubricant in our joints!

 

[Strong]

Here is a link to check out for low cost force sensors for Medical-Joint analysis:

https://www.tekscan.com/application-group/medical/joint-analysis?tab=applications

Walt

 

[downranger]

The majority of the force in a joint can come from the muscle forces acting to move the limb. So it may be insufficient to load the knee with simply the mass of the body it is supporting if you want to see realistic lubrication or wear characteristics. Bone-on-Bone joint loads, as opposed to Intersegmental joint loads, must be considered when selecting the load.

Also, like KENAT pointed out, you don't necessarily need to consider the rotation of the knee. At a certain amount of flexion, the femoral condyle begins to translate across the cartilage inside the knee. So a simple translational test may still give realistic results.

I find this text to be very useful: Basic Orthopaedic Biomechanics and Mechanobiology

 

[JStephen]

In some cases, it might be of interest to let the item swing back and forth and measure the decay. The decay would be a function of friction in the joint and also the result of air on the moving items.