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Re: [TowerTalk] Which type of cable for a crank-up tower?

To: VE6WZ_Steve <>
Subject: Re: [TowerTalk] Which type of cable for a crank-up tower?
From: Grant Saviers <>
Date: Fri, 02 Dec 2011 11:24:06 -0800
List-post: <">>
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 
breaking strength.

How much depends on clamping technique, knot type, radius of bends, 
etc.  YMMV.

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.

Grant  KZ1W

On 12/2/2011 8:24 AM, VE6WZ_Steve wrote:
> Steve:
> 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.
> Steve.
> --------------------------------------------------
> From:<>
> Sent: Friday, December 02, 2011 9:12 AM
> To:<>;<>
> 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
>> this
>> same comment repeated about a cable "retaining a set" if  left in the same
>> position.
>>>   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
>> down,
>> 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
>> a
>> bigger area, thus decreasing the wear and  increasing the reliability of
>> the installation.
>> Cheers,
>> Steve    K7LXC
>> Professional tower services for  hams
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