Hello Dave,
Great advice.
I use this clamp on top of my bearings:
https://www.dxengineering.com/parts/dxe-sds-300p
Along with the additional backing plate:
https://www.dxengineering.com/parts/dxe-sdsbp-300
This clamp when properly tightened - does not slip
73
Tim K3LR
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf Of
Leeson
Sent: Sunday, June 22, 2025 12:56 PM
To: Tom Hellem; towertalk
Subject: Re: [TowerTalk] Thrust Bearing Concerns
Two thoughts: I can't find the reference just now, but I recall an awful
years-ago tower accident in which a ham was injured and trapped on his
tower when the thrust bearing bolts broke and the loaded mast fell onto
his hands. So I'd be concerned about any thrust bearing setup that
relied only on the clamping bolts.
I have had good luck with adding one or more clamps on the mast just
above the bearing, typically heavy U-bolts or machined bolted clamps.
There may be other more formal clamps that can perform this function.
Once I loaned my tower to some visiting hams to put a temporary contest
Yagi on my mast, and when they left they didn't know to replace the
clamps; the swooshing noise of the mast slipping down ended with the
boom of my own antenna hitting me in the head and shoulder. Luckily it
was cold and I had a thick hat on, or it could have been very much
worse. So if you use this approach, be sure to check that the clamping
is tight before you do anything else on the tower. I use two, just for
the extra redundant reliability.
Caution and plenty of your own research is strongly recommended.
Dave, W6NL/HC8L
On 6/22/25 5:00 AM, Tom Hellem wrote:
> Looking for some input on the DX Engineering TB-400 thrust bearing.
>
> Here is the scenario: Rohn 55 tower supporting a large 6 meter EME
> array, in an "H" configuration. 4 yagis , each with 50' boom, and the
> main boom is also 50 '. The total weight is approx. 700 lbs. The
> rotor is one of Kurt's prop pitches, the small version (9,000:1). I
> believe the max thrust that can be handled by this rotor is 500 lbs.
> or so.
>
> We have purchased the DXE-TB400 thrust bearing to be installed on the
> BPL55G flat top. The hope is that this bearing can be used to support
> the entire weight of the array, thereby eliminating any thrust load
> on the rotor.
>
> Problem is, the documentation that comes with the bearing does not
> specify a max thrust load for it, nor does it give a torque
> specification for the three set screws that tighten against the
> mast. Furthermore, the upper half of the bearing is cast aluminum, so
> the threads for the 3 3/8" stainless set screws are aluminum. My
> gut feeling is that the aluminum threads are going to strip out
> before we can get enough torque on them to prevent the mast from
> slipping down and putting the entire weight of the array onto the
> prop pitch rotor.
>
> I am in no way suggesting that the bearing is not a good quality
> part. But I am concerned that we are asking too much of it. Another
> concern is that the bearing doesn't come with a grease fitting. This
> setup is located near the coast of Oregon, they get a lot of rain
> there.
>
> Anybody have suggestions how we can improve this setup?
>
> All input is appreciated.
>
> Tom Hellem K0SN _______________________________________________
>
>
>
> _______________________________________________ TowerTalk mailing
> list TowerTalk@contesting.com
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