[TowerTalk] Slipp-Nott

[Grant Saviers]

There are a number of problems with pinning -

Shear pins in many situations - my backhoe auger, lawn mowers, outboard 
motors, etc are not required to handle frequent reversing loads, so a 
little slop in the fit isn't much of a problem.  In a antenna rotator, 
if the fit isn't tight to start with, the back and forth wear and wind 
oscillation can destroy the fit and increase the shock loads on the system.

The problems getting a tight fit shear pin to hole are -

1. drilling a hole straight thru the center axis of a round tube usually 
takes stiff fixturing, a sharp split point drill or center drill or 
pilot hole and ability to center within a few thou.
2. none of this is easy/possible on a tower
3. masts are usually "chrome molly", 4130 or A513, hardened enough that 
a cobalt drill is nearly mandatory and continuous lubrication is 
desireable.  It takes a lot of thrust force to drill high strength steel 
even with a split point drill.
4. a hand held drill will not yield an "on diameter" or round hole so 
there will slop between the pin and the hole leading to problems
5. a hand (tapered) reamer is an "iffy" way to ream alloy steels, it 
might work, and likely will be the end of said reamer.  A machine reamer 
in a hand drill of any power will likely be a painful experience to 
operator and reamer.

If the mast can be drilled, clamped in a drill press or mill , then a 
proper hole can be drilled and reamed if needed.

As previously noted, a shear pin needs to have a yield stress lower 
("softer") than the material it is in.  Since many rotators have 
aluminum clamps, that is unlikely unless a grade 1  fastener is used and 
maybe not then.  Dowel pins are really hard (HRC60) and drill blanks 
(HRC62/64) likewise, much harder than any mast, harder than Grade 9 
fasteners, and brittle.  Common drill rod (W-1 & O-1 alloy) is usually 
fully annealed (about a grade 2 fastener) as procured and can be 
hardened after machining and is rarely used to make drills.

A tapped hole has some advantages, but then an accurate round hole is 
needed and a tap that can tap harder alloy steel.

Some of the aftermarket clamp systems with more contact area than most 
stock rotators provide are the way to go IMO, if the array needs it.

Another factor worth mentioning is that clamping systems relax a bit 
after initial tightening.  So, re-torquing after a few days and or 
temperature cycles can make a big difference in the actual clamping force.

Grant KZ1W





On 12/20/2013 4:26 PM, Charlie Gallo wrote:
> On 12/20/2013 Roger (K8RI) on TT wrote:
>
>>   Drill rod is great
>> for this
> Great for being on size, not so great alloy wise (depending).  Another thing that works is what they call "Turned, ground and polished" (TGP) shafting.  Other options are "dowel pins" and "drill blanks"
>
> Another option is to drill/ream the mast, and have threaded hole in the rotor (or use a nut) and have a "Shoulder Bolt" - which is a bolt with a long, smooth area of a larger diameter than the threads
>