Calculating tension in a twin left and right hand lead screw

[KristianSilva]

Hi,

I'm working on a material handling design and in the process of sizing a trapezoidal lead screw that has twin left and right threads (is there a formal name for this arrangement??).

The twin lead screw performs a similar function to that seen in a self-centring vice (

https://www.youtube.com/watch?v=0kFUfX1m5aI),

i.e, it is being used to self-centre and clamp a work piece.

To size an appropriate lead screw, I am calculating the total tension in the lead screw which will be the 2x the force F (see diagram), plus the additional tension produced when a torque is applied to the hand-wheel. To calculate the tension produced when a torque is applied to the hand-wheel I have used the below equation taken from Mechanical Design Engineering Handbook pp697 by Peter R.N. Childs:

Where:
T = torque
P = load/tension
dp = pitch dia
L = thread lead
alpha = thread angle

I understand this method would be accurate for single/ordinary lead screw, e.g., considering a jack or one half of the twin lead screw in this application, however I am unsure how the addition of an opposing thread will effect the tension produced when a torque is applied to the hand-wheel?

Looking at the application and deriving the equation I am thinking that I need to double the thread lead used in this equation as this governs the thread lead angle which has now effectively doubled, doubling the inclination angle of the plane which the load acts on, however I am uncertain and would welcome some guidance.

Hope this makes sense, let me know if it doesn't!

Kristian

 

[zeusfaber]

Formal name? It's got a lot in common with a turnbuckle.

 

[handleman]

Really? The tension in the screw is twice the force? How do you figure?

Lead angle is not doubled. But torque is now distributed over two nuts, so is therefore halved at each nut (neglecting friction etc)

 

[hydtools]

Create a free body diagram from the hand wheel to plane through the shaft located midway between the clamping faces. You will see that tension force equals F.

Ted

 

[KristianSilva]

Handleman, based on the diagram I have said that the tension on the lead screw caused by the force is 2 X F because I have two forces acting in opposite direction, apologies for this not being clear, I probably should have labeled each of the forces as 1/2F.

Yes, that makes more sense when simplifying and considering only half of the lead screw where the load at each nut becomes 1/2P

Ted, Thank you, my digram should be ammended so that there is a force acting in each direction is F+1/2P and then the tension in the screw is 2F +P.

Thanks both

 

[handleman]

Dude you totally missed it. It doesn't matter what the crap you label the forces. The tension in the rod is NOT double the force on the clamp face or whatever you are calling it. Draw some FBDs. Go take Statics again.

 

[KristianSilva]

WOW, I really did totally miss it.

Think I best take statics again...

Thanks!