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## Erecting Long Masts

 To: Erecting Long Masts k6ll@juno.com (David O. Hachadorian) Thu, 19 Dec 1996 15:45:29 PST
 ```On Wed, 18 Dec 1996 19:18:51 -0700 Terry Dunlap writes: >I am curious as to the need to have so much mast buried in the tower. >Wouldn't the rotator be just as isolated from side forces with say >half as >much embedded mast as long as the intermediate support was still >there? I >understand about erring on the side of caution but isn't this huge >overkill? Interesting question, here are some numbers. The way to figure this is to regard the bearing at the top of the tower as the fulcrum of a lever. All of the bending moments above the bearing must be equal in magnitude to the bending moments below the bearing. Here are several different ways to use a 20 foot mast, with 22 square feet of antennas concentrated 2 feet below the top of the mast, 71 mph wind (20 psf). This is for a stiff, non- bending mast. a. 2 ft rotor to bearing, 16 ft bearing to antenna. bending moment = 22x20x16 = 7040 ft-lbs. side force on rotor = 7040/2 = 3520 pounds. b. 5 ft rotor to bearing, 13 ft bearing to antenna. bending moment = 22x20x13 = 5720 ft-lbs. side force on rotor = 5720/5 = 1144 pounds. c. 10 ft rotor to bearing, 8 feet bearing to antenna. bending moment = 22x20x8 = 3520 ft-lbs. side force on rotor = 3520/10 = 352 pounds. Once you have those numbers, you can determine whether they are within the bending moment specifications for your mast and tower top, and within side force specifications for the rotor. Dave, K6LL k6ll@juno.com -- FAQ on WWW: http://www.contesting.com/towertalkfaq.html Submissions: towertalk@contesting.com Administrative requests: towertalk-REQUEST@contesting.com Problems: K7LXC@contesting.com Sponsored by Akorn Access, Inc & KM9P ```
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