4130 masts

[John Brosnahan]

A
>Also, are there any opinions, pro and con, to using either a larger 
>diameter and/or thicker wall? One would think that either one would 
>increase the PSI rating. But I am finding that not to be true. In some 
>cases the PSI decreases. Can anyone explain the the math on this? Or is 
>it a function of the percentage of elements in the alloy? This would 
>seem to be the logical explantion. I have found 2" x .25" 4130 with PSI 
>ratings from 110,000 up to 168,000.


Too early (6:00 a.m.) to get into the deep math but thicker wall is ALWAYS
stronger and larger diameter is ALWAYS stronger.  Any decrease in
strength MUST be due to a change in the material itself if the material
dimensions were increasing.

But the increase in strength goes up much faster for an increase in
diameter than it does for an increase in wall thickness.  Of course
I realize that a lot of things are standardized on 2 inch OD masts--
but if one wants to put up some serious stacks it would be much better
to go to a 3 inch diameter mast than it is to go to a 2 inch mast with
1/2 inch wall.

A quick example at the extreme limits to better make my point.

Conditions:

90,000 psi yield 4130 mast
20 ft long with 10 ft above top of the tower thrust bearing
1 antenna of 20 square ft at the top of the mast (to make it simple)

2 inch diameter mast with 1/2 inch wall will survive 80.1 mph and
it will weigh 160 pounds

3 inch diameter mast with 1/8 inch wall will survive 81.9 mph and
it will weigh 74 pounds

and for comparison with the "typical" situation 
2 inch diameter mast with 1/4 inch wall will survive 68.3 mph and
it will weigh 93 pounds


>From the perspective of weight one can INCREASE from a weight of 
93 pounds to a weight of 160 pounds with 2 inch masts in order to
go from 68 mph to 80 mph.  But one can DECREASE in weight from
93 pounds to 74 pounds by going from a 2 in x 1/4 wall to a 3 in x 1/8
wall while still gaining the survivability from 68 mph to 82 mph.
Lighter and stronger--quite a deal and this stuff is sold by weight!


OK--just to head off any questions about a 3 inch 1/4 wall mast.
Same conditions as above
3 inch diameter mast with 1/4 inch wall will survive 108 mph and
it will weigh 147 pounds

160 pounds of 2x0.5 wall will get you 80 mph but 147 pounds of
3x0.25 will get you 108 mph

You can see why I LIKE 3 inch masts.  (I used 90,000 psi, which is 
at the low end of 4130 ratings, just to be conservative--any increase
in the psi rating will gain performance, of course.)

73  John  W0UN

BTW  Just trying to get back into the good graces of K7LXC after 
my N-word debacle ad nauseum

John Brosnahan
La Salle Research Corp.
24115 County Road 40
La Salle, CO 80645 USA

voice 970-284-6602
fax   970-284-0979
email broz@csn.net


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