[TowerTalk] Mast bonding

Eric Gustafson n7cl@mmsi.com
Fri, 7 Apr 2000 08:48:51 -0700

Hi Mark,

I fought with this one for a while too.  Unfortunately,
regardless of what you do to bypass the thrust bearing with a
linear conductor, a substantial fraction of the strike current
will be carried through the bearing.  Bonding strap inductance is
hard to avoid given that the structure has 360ish degrees of
differential rotation between the pieces.  This is not all bad.
If the bearing is in good shape, and is running under sufficient
load, it is easily capable of carrying the current without
significant damage.  And, this is the _BEST_ place for the mast
to tower connection to be established for both lightning survival
and tower-as-antenna considerations.

Here is what I'd do:

1.  Go to the local industrial bearing supply house and get a
    real sealed tapered roller thrust bearing of the proper size
    that is rated for exposed outdoor use.  This bearing will
    have a grease fitting on it somewhere and a grease plug to
    remove so that forcing grease into the assembly doesn't blow
    the seals away from the inner race.  Properly greased, the
    resistance of the contact between the bearing parts will
    remain relatively stable (and low) over time.

2.  Replace the TB3 with this bearing.

3.  Adjust the mast and rotor attachments so that _ALL_ (or
    almost all if the rotor is a type that requires some vertical
    load for proper internal alignment) of the vertical load is
    supported by the thrust bearing.  Under light loads, only
    three rollers will be making contact.  Under higher loads,
    race deformation will permit more contact points to be
    established.  However, the real important point is to never
    let this bearing to run at near zero vertical load.  It is
    the contact pressure on the three main load carrying rollers
    that will permit the corrent to flow without generating
    enough localized heating to microweld pits into the races at
    the contact points.

4.  Bond the feedlines that traverse the bearing station to the
    mast above it and the tower below it.  You can skip the mast
    connection if you want if the feedline shield is bonded to
    the boom of the antenna.  But don't skip the tower
    connections.  One near the top and one near the bottom where
    the cable leaves the structure.

73, Eric  N7CL

>From: "Mark ." <n1lo@hotmail.com>
>Date: Fri, 07 Apr 2000 08:39:24 EDT
>Greetings Gang;
>I would like your comments on an idea to bond my mast to my
>tower.  I have seen some posts in the past referring to the
>vulnerability of rotors to damage from lightning strike current
>that comes down the mast and through the rotor housing.  My
>tower is R25 with a flat top plate and a Rohn TB-3 thrust
>bearing.  The mast is 6061-T6 aluminum, 2" od.  The TB-3, with
>its three centering bolts, doesn't necessarily present a solid
>bonded connection to the tower frame.
>Well, thunderstorm season is just about upon us here in
>Virginia, so I need to get cracking (zzzzZZZZT!).
>Here's the plan I have so far:
>I bought 2, 48 in long, #4 gauge battery cables, having crimped
>lugs on each end (not post clamp ends).  I will place a band of
>stainless steel foil around the mast, just below the top plate,
>and clamp the top lug of each cable to the band using two
>stainless hose clamps.  I will peen the lugs lightly with a
>hammer as I tighten the clamps, to maximize the contact patch,
>using antioxidant paste on all mating surfaces (under the band,
>too).  To protect the antioxidant from washout, I will tape the
>joint thoroughly.  At this point, the cables are hanging
>straight down, parallel with the mast.  Next, I will clamp the
>bottom ends to two separate tower legs in the same fashion as
>the top ends.  I will have to choose the bottom clamp point
>carefully to leave enough slack for the cables to wind around
>the mast during rotation.  This should present a gently curved
>path of discharge from the mast to the tower legs, 2 to 3 feet
>above the rotor position, with no abrupt change of direction.
>Is this similar to how any of you are accomplishing this?  Please
>pass along your comments and ideas.
>   Gloucester, VA

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