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• # Pulley Tendon Mechanism

Discussion in 'Calculations' started by Hadia, Jan 6, 2016.

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Hi.

I am trying to design a pulley- cable system for a mechanical hand's finger.
What my design will essentially do is drive the tendon cable over the three joints over pulleys attached at the each.

I am confused about the kinematics of how a pulley system is going to work in tension to flex and extend a finger's tendon when handling a weight.
Could someone point me in the right direction as to the type of pulley system I should preferably use here and the direction I should take with the design calculations.

Help needed asap.

2.
3. ### robertjefferyActive Member

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a picture is worth a thousand words

I have made a massive assumption of what you might be looking for here,

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Their are hundreds of solutions in engineering, and every one has its pros and cons.

what kind of calculations are you looking for? clamping force, stress in components, actuator size / speed.

Just before any one comments on the design, the cables ideally want to pass though the centre of the pivot points on the joints, but realistically I don't think it will make massive difference, also the d1 on the distal phalanx should be twice the distance as the d1 on the middle phalanx so that the middle phalanx travels though 90degs where the distal phalanx only travels 45degs.

I do hope this is of some use.

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That was extremely helpful. Thank you.

However, could you please explain the assumptions for the distances you took. I need to make equations for torque transfer and I am a little stick with that.

Also.
Assuming I eliminate the leaf springs and use pulleys instead, where would i need to start?
Because leaf springs would have a shock absorption problems, no?

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Also. What are those actuators for the lower tendons?

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Okay so about this design. What I'm trying for is one motor to drive all of these tendons. Could you please suggest a mechanism?

Also. How is your mechanism for flexion and extension working. And could you please explain the part about the angles and d1,d2. A little hazy on it.

Also. I'm trying to incorporate trusses instead of solid plates to reduce my weight. Could you help me with the calculations?

7. ### robertjefferyActive Member

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First you need to decide on how your mechanical finger wants to move. My first concept made the assumption that you would require it like lifelike movement, which appears to be... the distal (tip) and middle move together no matter how hard you try and move them independently, therefore they can be oscillated by a single actuator, the Proximal you are able to move independently regardless of the middle and distal position.

if you are happy with only one actuator, you need to decide on the finishing angles of the finger when gripping, as these angles will be the ratios of your pulley. for example if you wanted it to close right into a clenched fist.

Image 2
(Distal to Middle) - aprox 30Deg
(Distal to Proximal) - aprox 30Deg
(Proximal to hand) - aprox 30 Deg

A clenched fist for example
Distal to middle - Aprox -30deg
Middle to Proximal - aprox 100deg
proximal to hand - aprox 80 deg

So my next concept..... (Some how i feel i am assisting in a Product design A level or something) and I'm not entirely certain a mechanical design Forum is an except-able Harvard reference)

In my mind this isn't the most elegant of design, a tad clumsy. for the same effort, you could place a servo motor at each joint, all driven off the same signal. all dependent on the clamping force required., the black stitched lines represent the pulley belts / wire rope.

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