There's certainly a possibility that more frequent maintenance can increase
the probability of failure, but, like many observations based on statistics,
the numbers can be misleading. For example, sometimes post-maintenance
failures would have happened anyway, sooner or later (due to worn out or
faulty parts that were not fixed or repaired during maintenance, or due to
additional stresses caused by the maintenance that accelerated an otherwise
inevitable failure.)
On the other hand, it would be foolish to replace crank-up tower cables and
pulleys too often. The procedure itself can be dangerous, non-related parts
can be damaged, and costly mistakes can be made. However, one problem is
that it may be very difficult to detect when replacement is required: parts
of the cable and pulley may not be visible without going through the
disassembly required for replacement. You can go with the tower
manufacturer's recommendation on how often to replace the cables, but that
could be overly conservative. After all, their instructions have to cover
the worst case, like installations near salt water (U.S. Tower says cables
should be replaced every three years -- I wonder how many people actually do
that!)
I think the only sensible alternative is a thorough inspection on a regular
basis (every 6 months, minimum), at which time a light-to-moderate
lubrication should be done (uh, I guess that's what K7LXC said...) I don't
think there's a hard and fast rule, but I suspect people in relatively
moderate climates can go five years between replacements. Weighing the
effort, money, and potential for error of this procedure, this interval
doesn't seem too short. The alternative could be a lot worse.
73, Dick, WC1M
-----Original Message-----
From: alsopb@vance.net <alsopb@vance.net>
To: towertalk@contesting.com <towertalk@contesting.com>
To: <towertalk@contesting.com>
Date: Sunday, February 01, 1998 3:03 PM
Subject: [TowerTalk] Failures due to preventative maintenance?
>Guys,
>
>The "pulley replacement" issue lead me to the following general
>questions:
>
>Can imprudent (imprudent = different from manufacturer recommendations
>or ?) or frequent periodic maintenance actully increase the risk of
>failure?
>
>I'm not completely from the school of thought that "If it ain't broke
>don't fix it." However, I do think that maintenance when not needed
>can actually increase the failure probability. (The probablity of
>screw-up is greater than zero, the failure probability is fixed-- if
>nothing near failure was actually replaced/repaired. . The combined
>probability of failure could thus increase after maintenance !
>
>It seems that many recommendations for maintenance are offered as
>"experience" and not backed up by statistics proving that a benefit
>actually results. I'm not saying ignore experience-- just quantify it.
> I'd like to know things like component mean-time-to-failure etc or see
>that something is really wrong before judging that maintenance was
>needed. Would you replace a 10,000 hour light-bulb every 100 hours to
>feel better?
>
>I offer the following (unquantified) observation made my my dad 40 years
>ago. The commercial airplanes he flew had the greatest failure rate of
>components just after periodic inspections and preventative maintenance.
> These were performed by those presumably highly skilled in the art and
>who did it all the time.( Any others out there with similar
>experiences?) In reality, how practiced are we hams on most tower
>maintenance items? Some are some aren't. Which are you? Are you
>sure you are truly adding benefit when undertaking "needed"
>maintenance-- or are you creating problems?
>
>73
>Brian/K3KO
>Vance Net Inc
>End Of Message
>
>
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