Thanks for that info Grant:
I think I understand your explanation.
Cycling stress on the wire rope is understood. That is always a factor with
rope rolling over a sheave I suppose.
Differential stress around the sheave causing a "set" as the strands maybe
migrate in the bundle makes sense, and I now see how that could cause a
problem.
I suppose my problem was that my tower is going up and down twice a day
about 5 days a week ( I always leave my tower nested) so the wire rope is
rolling over the sheaves all the time, and its was not obvious what the
difference is if it stops on a sheave.
My desire is not to be "nit-picky" here, but I own one of these and I don't
want a failure.
The practicality of stopping the tower up, and down, in a different spot all
the time is a bit challenging to say the least for me (twice a day, 5 days a
week).
I suppose I'm also somewhat confounded about why, if this were a problem, US
tower would build these units with limit switches on the winches that force
a stop in the same place all the time.
The fact is, I have had this tower for 15 years, (HDX-589mdpl) it has always
been stopped both up and down at the same spot, and I have had no problem.
I have re-cabled the tower twice and replaced a few of the sheaves which
showed wear. The removed cables always looked in very good shape, but I
replaced as per US tower schedule.
Maybe I'm almost out of time and a failure is imminent, but I inspect my
tower weekly and see no obvious problem with the cables. Maybe I cant see
the problem?
Since taking measure to prevent a this potential problem is not minor, I
just want to be sure the "issue" about a "cable set" is not just tower-talk
folklore base on a "gut feeling" from someone.
de steve ve6wz
--------------------------------------------------
From: "Grant Saviers" <grants2@pacbell.net>
Sent: Friday, December 02, 2011 12:24 PM
To: "VE6WZ_Steve" <ve6wz@shaw.ca>
Cc: <towertalk@contesting.com>; <K7LXC@aol.com>
Subject: Re: [TowerTalk] Which type of cable for a crank-up tower?
> 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:<K7LXC@aol.com>
>> Sent: Friday, December 02, 2011 9:12 AM
>> To:<towertalk@contesting.com>;<ve6wz@shaw.ca>
>> 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
>>> TOWER TECH -
>>> Professional tower services for hams
>>>
>>> _______________________________________________
>>>
>>>
>>>
>>> _______________________________________________
>>> TowerTalk mailing list
>>> TowerTalk@contesting.com
>>> http://lists.contesting.com/mailman/listinfo/towertalk
>> _______________________________________________
>>
>>
>>
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>
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>
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