Jack/W6NF wrote:
> Roger (K8RI) wrote:
>> Is there a good description of the mechanics for a crank up mast on the
>> web? I'm looking at three sections or more. I've found a couple of
>> basic designs, but I haven't be satisfied with the safety aspect and
>> they were basically just crank up poles. Standard pipe (schedule 40)
>> might or might not be strong enough so some sections could be schedule
>> 80. In addition to the raising cable(s) there need to be guides to
>> prevent each section from rotating inside the next larger size.
We used to build stuff like this in the effects business.. (not usually
for long life, though...). We used HDPE collars as bushings between
sections (which also gives you a place to pass the cables between
segments.. that is, the OD of the inner tube is 1/4" to 1/2" less than
the ID of the outer tube, at a given joint. Notches and keying keeps it
all aligned (not to mention the tension in the cables.
A
>> single cable, although easy to route has to support the entire weight so
>> it should have some sort of latching mechanism.
You can approach this two ways:
1) The cable has sufficient design margin that it's stronger than other
aspects of the system, so why bother latching (unless you need
rigidity). The cable life isn't much affected by being under tension
(think guy cables, as an example)
2) Multiple hoist lines, each one of which is strong enough to support
the system with appropriate design margin.
3) Some sort of electrically operated latching system (solenoid actuated
bolts, for instance.
(a single pulley in the
>> base of each section with a single on the outside at the top of the
>> section) It'd be even nicer if each section could be raised and lowered
>> independently of the rest which of course would mean a winch mounted at
>> the top of the previous section which would add overall weight and a
>> larger wind profile.
or multiple cables, one for each raise/lower section. There are some
clever schemes with the cable drums (which are all at the bottom) where
it pays out first one, and then when it gets to the end of that cable,
the next drum starts turning, etc. A matter of some cams and cogs, as I
recall.
Ideally the entire mast would rotate. Also the
>> *innards* need to be accessible if the cable breaks or hangs up.
>>
> Hi, Roger:
>
> I've never built one but we have quite a few 30-foot crankup masts at
> work that are made from thick-wall square aluminum material. The lower
> part of the mast is probably 6-inch outside dimension. I believe these
> are commercially available but there is nothing on the masts to identify
> the manufacturer. Neither have I ever paid any close attention to
> details of the way the cable and crank assembly are configured.
Force 12 makes aluminum crank up masts with square tubing.
>
> The square material obviously solves the rotation problem and may be a
> solution if the material is not cost prohibitive.
>
Material cost is driven mostly by the weight of the metal, so for the
same weight, square and round tubing cost about the same. For a given
weight, round is slightly stronger (because it can be larger in diameter
for the same cross sectional area of the metal).
By the way, you might want to think pneumatics and/or hydraulics.
Pneumatics are a handy way to actuate locking pins, etc. because the
"control cable" is just a fairly small tube (no electrical, lightning,
etc. issues)
Pneumatics are also a good way to get the mechanical "push", if you can
figure out a way to get a decent piston seal (if you have latches, you
can have a lot of blowby loss in the pneumatics.. you push it up, latch
it, and turn off the pump). We used HDPE collars in square tubes with
square cross section O-rings.
Hydraulics are another interesting way. Most commonly, you build
something like a 6,8,10 or more part block and tackle arrangement that
is driven by the cylinder (inexpensive <$100 ag cylinder) so you get 16
feet of cable pull for a 2 foot throw on the cylinder. I notice that
the portable climbing wall folks do this in reverse for the auto belay
system.. a spring loaded cylinder with a check valve and bleed... as you
go up the cylinder expands, if you fall, the load compresses the
cylinder and it slowly contracts, limited by the size of the bleed.
Hydraulic (or pneumatic) motors and winches can also be used.
One of the more clever self erecting masts I saw had a cable that
wrapped in a spiral around the mast, so the actual support was the
cable, one turn stacked around the other.
There are also a variety of cable/rod extensible masts (think of the
ones used on Shuttle to extend the outboard antenna for SRTM), but they
tend to be fairly complex.
Jim
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