One problem is metal fatigue. There will be small movement of the cable
around the sheave which will stress cycle the steel strands. Over time
the strands can reach their fatigue limit and break.
There is also significant differential stress in a cable strands around
a sheave. The fact that the cable gets a small wave set in at that
point shows the stretch or slippage of some strands vs others in the bundle.
Any bend or clamp on a cable or knot in a rope significantly reduces the
How much depends on clamping technique, knot type, radius of bends,
Every cable/rope has elasticity so if the force varies there is movement
over the sheave. Halyards (wire or rope) in sailboats do fail at the
top sheave if the wear is not spread out. Of course the cyclical load
variation is significantly higher in this situation.
On 12/2/2011 8:24 AM, VE6WZ_Steve wrote:
> What is a "Fixed flaw" ????
> I really do not mean to be difficult Steve, but exactly what is the science
> behind this??
> Do you have a reference?
> Where did you get this information exactly?
> In my documentation, US tower has no reference about not stopping the tower
> in the same place.
> Are cables not meant to bend?
> It seems to me the cable is permanently "set" on the drum when its down and
> I don't see this as a problem.
> Sent: Friday, December 02, 2011 9:12 AM
> Subject: Re: [TowerTalk] Which type of cable for a crank-up tower?
>>> I do not wish to hi-jack this thread, but over the years I have seen
>> same comment repeated about a cable "retaining a set" if left in the same
>>> Why is this a problem???
>> Because it puts a fixed flaw in the cable. It won't take many moving
>> cable cycles for it to break so I'd say it's a potentially fatal problem.
>>> I have had my US tower for 12 years, it rests in the same place when
>> and stops in the same place when up.....have not noticed any problems.
>> Does it matter if the cable stops in the same place all the time?
>> Umm, yes. I don't recommend it because of the set that the cable takes
>> - see above. I highly recommend NOT having it stop at the same spot every
>> time. Vary the resting spot (up and down) and you'll spread the wear over
>> bigger area, thus decreasing the wear and increasing the reliability of
>> the installation.
>> Steve K7LXC
>> TOWER TECH -
>> Professional tower services for hams
>> TowerTalk mailing list
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