[TowerTalk] Tilt-over Tower/Rotator Question

[John Brodie]

Thanks to all for responding to my post.  I am now assured that the Alfa
Spid RAK rotator will safely handle the forces imposed on it when the tower
is in the horizontal position with the beam et al hanging off the end.

John VA7XB for VE7SAR

 

From: John Brodie [mailto:brodiejb at shaw.ca] 
Sent: Thursday, April 20, 2017 8:18 AM
To: 'towertalk at contesting.com'
Subject: Tilt-over Tower/Rotator Question

 

I just joined this group and can imagine my question has come up before.
Our club has a tilt-over crank-up tower on a trailer which we are using in a
semi-permanent location at our club station.  The top of 3 tower sections is
very narrow with about 5 in. between vertical legs with a thin plate and
sleeve-type bearing welded on top of the 3 legs.  A 2 in. diameter pipe
extends about 4 ft. down from the top bearing to a lower bearing and plate
and the pipe is free to turn.  A TH7 beam (weighing 75 lb) is mounted on the
pipe close to the top sleeve bearing.  The spacing of the verticals leaves
no room for a rotator mounted inside the tower so normally the beam is
turned with a rope.  Our plan is to shorten the pipe, restrain it from
turning, then install an Alfa Spid RAK rotator on the shortened pipe with
the beam and VHF antenna on a second pipe immediately above the rotator.
Our concern is that when the tower is lowered to a horizontal position for
beam maintenance etc., the weight of the antenna and anything  mounted above
it will place a large bending moment on the rotator and damage it (we have
no doubt about the ability of the rotator to withstand forces when in the
normal vertical position).  We have seen this concern expressed before, and
note that others have constructed a "cage" to house the rotator above the
top of the tower and thereby provide some bearing support above the rotator.
The RAK appears to be very robust so I would appreciate informed opinions
about the necessity to do this (and how best to do it). Thanks in advance
for any advice.

John VA7XB for VE7SAR  (Surrey ARC ) 

Email: sarc at ve7sar.net