[TowerTalk] Aluminum mast

[Bruce Ferratt]

Jim-
Thanks for your email.
The mast is 1 1/4" OD.  Solid, round.  Side writing on the states it is ASTM 
6061-T6511 grade aluminum.  It was $66.  Weighs about the same as the seet 
6' mast that came with the tower.  I just don't like mounting a rotor right 
in the open section of Rohn25g,... it's gotta be lower inside the tower.
The beam is a Mosely TA-33M Tri-bander.  Longest element is 26ft---13ft each 
side of boom.  Boom is 14ft long.
The beam and stacked vertical 2m/440 will probably weigh under <40lbs at the 
most!
Also,... your formula,....
1 1/4" OD *4 (to the 4th power) minus 0 ID (zero) (my mast is solid),... is 
this correct?  The mast is solid,...there is no ID,...please recalculate the 
formula with the details I have provided.
73
Bruce
K4BOF

>From: Jim Lux <jimlux at earthlink.net>
>To: "Bruce Ferratt" <uglyatc at msn.com>, <towertalk at contesting.com>
>Subject: Re: [TowerTalk] Aluminum mast
>Date: Wed, 15 Mar 2006 06:14:13 -0800
>
>At 07:56 PM 3/14/2006, Bruce Ferratt wrote:
>>Hello and thanks for letting me post a question.
>>How strong is a solid aluminum mast compared with a walled/hollow mast?
>
>almost all the strength for a round member comes from the outside.
>
>Consider bending.. the maximum stress is at the edges
>For torsion, the derivation is a bit trickier, but it's the same
>
>So, to answer, for the same weight of tubing (which is how you buy it, by 
>the pound), the tube is stronger than the rod.
>
>it goes as od^4-id^4
>
>>I secured a solid, round 12' aluminum mast 6061-T6511 grade.  What is the 
>>likelihood of bending when loaded with a TM-33A and VHF 2m/440 vertical at 
>>a rated 70mph wind load in Northern Virginia?  8ft of mast will be exposed 
>>above the point of the tower peak (Rohn 25G), with about 4 ft inside the 
>>tower to rotor plate.
>
>
>That must weigh a bit!...  You forgot an essential dimension: the diameter 
>of the mast, and the cross sectional area of the antennas.  Ballparking, a 
>load of about 300-350 lbs at the end will bend a 2" solid bar 8ft long.
>
>There is an old QST/QEX article around that goes through the calculations, 
>and I think there is a spreadsheet that accompanies it, either on the ARRL 
>site, or maybe K7LXC's site?
>
>Jim
>
>