Pretension wire with horizontal load

[jzcrouse]

I have a wire used for cutting cheese that gets pushed horizontally into the wire repeatably. Naturally the wires break as the wear. Currently, we just tighten the wire until it seems good but I am unsure how much the pretension in the wire influences the rate the wire breaks. I was looking to find the optimal wire tension to cause less broken wires. We typically have 90 psi pushing the cheese with .2 in wire. Its a steel wire so youngs modulus is 29E6 psi

 

[Bystander]

.2 in wire.

You meant 0.020"? How long? What sort of anchor geometry?

 

[Baluncore]

Welcome to PF, sorry about the delay in reply.
Where does the wire break? At the terminals or at the point that last cuts the cheese?

This problem is more complex than it first appears. That is because the cutting force is perpendicular to the wire, while the tension is along the wire. The wire is loaded across the width of the cheese so ideally, it will form a parabolic curve while cutting. The wire can be approximated by a circular arc as it cuts the cheese. If the additional length of wire to form the arc comes from stretch, then the wire will tend to break more often. To get a long life from the wire it must start slack and form a significant arc through the cheese while it is cutting.

To prevent breakage at the ends, the ends should attach to a small round pulley that can align with the wire direction as tension causes the arc to form. In order to begin the cut straight and level there needs to be some initial tension in the wire. A gentle spring at one end wire mount is sufficient to provide the initial tension. Once the cheese is in contact with the horizontal wire, the spring deforms as the wire is pulled out, until the wire end reaches a pre-set solid stop. The length of wire is then long enough to form a significant arc that can cut the cheese without breaking. At the end of the cut, the spring should return the wire quietly to the straight starting line.

If the wire arc is too long, then the wire may deviate from the horizontal while cutting. It is a compromise that will allow you to use the thinnest wire possible that will cut with minimum force and last a long time.