Cables between pylons always dip but surely a lot of money could be saved if they were pulled taut.
In: Engineering
Any rope or wire of any material that stretches to any amount will have some sag in them. You would need a materai that do not stretch in any way to have a straight wire but not material like that exist.
So all wires will have some slack and the amount depend on the length between the pylons and the amount tension and strength of the wire. The strength and mass of the wire is also very important.
You also need margins so the force of the wind or even ice that form on the cables will not result in a cable tha
So the design is a composite between strength of the materia vs the cost and the cost of the pylons.
Use the calculator at [http://eguruchela.com/math/Calculator/cable-sag-error](http://eguruchela.com/math/Calculator/cable-sag-error) and 300 feet length, 32 ft/s^2 in force perpendicular. With a wire from [https://www.engineeringtoolbox.com/wire-rope-strength-d_1518.html](https://www.engineeringtoolbox.com/wire-rope-strength-d_1518.html) take a 1/4 inch wire with a mass of 0.11lb/ft.
If you use the Minimum Breaking Strength of 5480 lb_f the sag is 7 feet and you use a extra 0.4 feet wire. If you use more force you cant be sure that it will hold and you haven’t margin for wind
At Safe Load of 1100 lb_f the sag is 35 feet and the wire is 11 feet longer (3%)
The number are for steel wires but they are bad conductor. So power lines use Aluminium-conductor steel-reinforced cable that have lower strength compared to mass so you will have more sag. If you add more steel the wire can have less sag but cost more so you optimize cost vs extra cable needed and we get what is used.
Cables expand and contract when they get hot/cold, they also are blown on by the wind and have birds land on them.
A taut rope is way more likely to snap, so it would cost them more long term in material upkeep than it would to add a foot of slack into the system.
They’re pulled as taut as they can be.
There’s a couple reasons for the slack.
1. any cable, no matter how tight you pull it, will have SOME slack, that’s just physics.
2. Imagine they were in fact pulled as tight as physically possible without breaking, like guitar strings from pole to pole. This would cause constant stress on the pylons, or telephone poles, or whatever they’re strung between, which could lead to bending, breaking, or other structural damage over time (imagine an old guitar neck that’s been warped cause it was left strung for years in
storage). It’s much easier to re string cable, than it would be to repair/replace the pylons themselves.
3. Also, having no slack means there’s no wiggle room for unforeseen circumstances. High stormy winds, seismic activity, heavy snowfall, vehicle collisions; any number of things could cause strain on the stretched cables. Without any slack the % of times the cables would snap and detach from the pylons would skyrocket, and you’d have downed live power lines, which are super unsafe.
Imagine if every time a car struck a telephone pole, the power lines hanging above snapped and came tumbling down with the collision. If that could be avoided most of the time by introducing a little extra cable between them, seems like a pretty easy choice to make.
One big reason is thermal expansion. Normally this effect is small, but when you’re talking miles of cable, a big change in temperature can make a noticeable impact on the length of a cable. On hot days when there’s a lot of load cables tend to sag a bit more, it’s something utility companies have to monitor to avoid damage.
If you made the cable super tight, especially considering most of this kind of maintenance work is usually done in the summer when it’s warm, when the cold causes the cables to shrink in the winter it can lead to damage or snapping.
It also makes the cables a bit more resilient in general. If the cable is super tight and something like a heavy branch falls onto it, it’ll snap right off. If it’s got some wiggle room that force is spread out a bit more and the cable might rock or shake but is less likely to snap entirely.
Lots of good answers here. Let’s see how brief I can get: The straighter the wire, the more tension. The more tension, the bigger the wire has to be. So your straighter wire is some amount *shorter*, but quite a bit *more expensive*.
Special application: An electric railway needs the wire to be pretty straight to accomodate the wiper arms on the cars. Instead of just pulling the wire tight, they use two wires: a moderately strong one hanging in the traditional curve, with a much lighter one hanging from that on short support wires.
Fun fact: The shape of a wire suspended at the ends looks like a parabola, but isn’t quite; it’s called a *catenary*, and it’s described by the hyperbolic cosine function.
Ropes and cables can only transmit forces along their length. To hold their own weight (plus ice, rain etc.) they need some vertical component (i.e. slack). Even then the forces along the rope/cable and its attachment point are much greater than the weight.
You could probably pull down the walls of your room by mounting a really tight and strong rope from one wall to the other and then loading it with your own body weight.
For pylons there is probably a sweet spot between ground clearance, cable length and cable stress. You could pull them tighter (to require less cable and have more ground clearance) but then you’d need stronger cables and pylons. You could allow more slack (to allow for weaker and cheaper cables and pylons) but then you’d have less ground clearance.
Take a piece of rope, pull it between your fists. It’s pretty easy to pull it taut and perfectly horizontal.
Take that same rope, and add a 5 pound weight in the middle. Try and pull it taut and perfectly horizontal.
It’s science you can try!
In addition to what others have said, pylons move. The wind causes them to sway slightly an the sun can sometimes heat up one side more than the other. This causes a slight bend in the structure.
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