I suppose there is some need to be concerned about lightning protection for
a thrust bearing. However, I think this problem is being blown out of
proportion. There may be some on this list that have suffered from bearing
damage but I have never had a problem or heard of one from my ham friends.
I did have a rotator zapped by lightning. Even though there have been four
observed direct lightning hits on my tower in the past two years, the only
damage has been the rotator and a 2M vertical at the top that was blown to
bits. When the rotator was replaced the bearing (TB3) was checked and found
to be in excellent condition. It was installed in 1999.
73, Keith NM5G
[mailto:firstname.lastname@example.org] On Behalf Of Jim Lux
Sent: Tuesday, December 12, 2006 9:53 PM
To: email@example.com; 'Bill Turner'; firstname.lastname@example.org
Subject: Re: [TowerTalk] Which thruse bearing?
At 01:41 PM 12/12/2006, Gary Schafer wrote:
> > -----Original Message-----
> > From: email@example.com
> > firstname.lastname@example.org] On Behalf Of Jim Lux
> > Sent: Tuesday, December 12, 2006 4:05 PM
> > To: Bill Turner; email@example.com
> > Subject: Re: [TowerTalk] Which thruse bearing?
> > At 08:51 AM 12/12/2006, Bill Turner wrote:
> > >I've had towers for years but never used a thrust bearing. Now I
> > >want to. In comparing the Rohn TB-3 and the Yaesu GS-065 I see the
> > >Rohn sells for about three times the price. Is it three times better?
> > >
> > >This may sound odd, but the reason I want a thrust bearing is to
> > >provide a lightning path directly from mast to tower rather than
> > >having the strike go through the rotator to the tower. In other
> > >words, I want to use the thrust bearing to ground the mast as
> > >opposed to having the mast free-floating. Does that make sense?
> > If you use a ball or roller bearing, the lightning might destroy the
> > bearing, because the contact area is very small. (actually it might
> > just flashover the entire bearing)
> > If your only concern is lightning, why not mount a wide low
> > inductance electrode close to the mast. A clearance of 0.1" will be
> > jumped at a few kV, and will have very low impedance once the arc is
> > lit. Any pitting or erosion will occur on the electrode and the
> > mast, as opposed to inside a bearing.
> > I'm thinking about something like a piece of 1"x1/8" copper bar
> > stock with the 1" direction parallel to the axis of the mast. The
> > end of the bar stock is spaced a small distance from the mast. You
> > can bolt it to the tower, or, use the ever popular exothermic
> > welding to connect it to a down conductor.
> > Jim, W6RMK
>Keeping a constant space between the bar and the mast may be a problem
>as the mast moves with wind etc. But the bigger problem is that the
>voltage across the gap needs to rise high enough for an arc to start
>which means that the voltage across the bearing must also rise that
>high. I would think that the bearing would be the first to arc because
>of the closer spacing in it.
You've got a bearing there to position the mast from radial motion. If the
copper bar is only a few inches above the bearing, there shouldn't be much
slop. 0.1" will arc over at 3-5 kV, so the question is will enough current
flow through the bearing to damage it before the voltage rises high enough
to arc over.
If you put an insulating shim between mast and bearing (say, a
0.001-0.004 thick layer of almost anything) would prevent current flow
through the bearing. Maybe insulating bushings on the bearing mounting
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