[TowerTalk] Thrust Bearing, etc: more answers from UST calcs

[SPWoo]

Regarding failure points: I've seen photos of two fallen US Tower retractable towers and both of them failed at the very bottom.  The problem with designing a retractable tower such that it will fail at the top is that of cost.  The bottom section would have to be super massive and I wonder if the tower will be price competitive.

Regarding tower wind loading: US Tower specifies wind loading in terms of round members although they don't always make that clear.  On my documents they didn't make it clear.  OTOH antenna manufacturers like to expresstheir wind loading numbers in terms of flat members.  Therefore you need to divide the antenna wind loading number by 0.6 and compare that number to the published tower wind loading figure.  Also, the tower wind loading includes all accessories such as mast, rotor, coax cable, etc.  A mast of 2" OD and 15' length has a wind loading of 2.5 sq. ft.  This is often overlooked by hams.

Regarding wind survivability of retractable towers: my local county building dept. requires the tower to withstand a 3-second gust of 122mph.  The only way a retractable tower could survive that high of a wind speed is when it's fully retracted.  My tower (US Tower TX-472) will survive a wind speed of 122mph with a 15.6 sq. ft antenna wind loading when fully retracted.  Since my tower is motorized I leave it fully extended only when I'm on the radio.  When I'm done operating it I leave it half-way or fully retracted.  In a nut shell a retractable tower is very safe as long as the operator uses common sense.  If one doesn't have a motorized tower then he needs to go by the wind speed rating when the tower is fully extended.  By where I live a non-motorized crank up wouldn't be practical nor safe.

 
Best Regards,
Jonathan Woo, W6GX
(970) 646-1711