First I'm just wondering if anyone out there has ever used a block and
tackle system to achieve high cable acceleration? My M.E. senior
design team is proposing using a block and tackle mechanism in
"reverse" to accelerate a cable from 0 to 250 mph in 2 seconds. The
proposed system uses hydraulic actuators (rams) to push apart a large
structure of sheaves (pulleys). The mechanical advantage the rams see
would be on the order of 1/100th, or you could say the cable end of
the block and tackle system would see a mechanical advantage of 100.
Secondly I would really appreciate some input on cable selection.
Synthetic, steel? We like the idea of synthetic because of its light
weight characteristics, but are not sure how it might react in the
machine.
Thanks for the help
Josh
http://jyw.tacorp.net/usseries5/laundry/launcher.html
http://www.realcoasters.com/element/cable.htm
http://home.comcast.net/~ok3wirebob/Catapults.html
http://www.islandone.org/LEOBiblio/SPBI1SL.HTM
(down to reference to cable catapult)
I think you'd be better off with the synthetic, and probably need composite
pulleys. Depends on what the cable is driving...
David A. Smith
Don't forget to consider all of the stretch in the cabling, and all of
the potential energy which is thereby stored there. And that energy
will get released. You may be designing a giant sling!
Mitchell Timin (former M.E.)
--
"Many are stubborn in pursuit of the path they have chosen, few in
pursuit of the goal." - Friedrich Nietzsche
Don't forget to take account of the mass of the pulleys etc.
Design depends upon what mass you are trying to accelerate.
Obviously you need very low friction bearings, especially in the high speed
pulleys.
I would suggest synthetic cable as it may be lighter. One other area to
consider is the angular velocity of the cable as it passes around the
pulleys.
John
I think that this principle is used in a fairground "sling shot" ride which
shoots a person strapped into a cage vertically upward,then lets them bounce
around till it stops.Its fairly spectacular
Hello Josh
what is the G force on a cable as it tries to run round a pulley at 250 MPH,
with small pulleys the G force is huge, with large pulleys the inertia of
the pulleys is huge...
If you have 100 wraps the cumulative friction in the system will put a huge
tension on the cable.
how is this cable slowed up from 250 MPH ?
--
Jonathan
Barnes's theorem; for every foolproof device
there is a fool greater than the proof.
To reply remove AT
Sounds dangerous to me.. What are you trying to accelerate? Synthetic would
be my choice, but when you stop pushing the pulleys (or when you get
excessive velocity on that last sheav) things are going to get messy
quickly...
AC carrier catapults are simply accelerating a shuttle (with aircraft
attached) down a slotted pipe using steam as the driving force... New ones
will use Magnetic induction.
Al...
MPH?
of
huge
I thought aircraft catapults where like pop guns, powered by the ship's
steam.
I don't think cables are used except to connect the aircraft to the block,
with no use of a pulley at all.
--
Jonathan
Barnes's theorem; for every foolproof device
there is a fool greater than the proof.
To reply remove AT
You will have to look at the elasticity of the synthetic cable. It will
most definitely stretch as you push the rams apart, and then (hopefully)
relax as it gets things up to speed. That stored energy can be
significant. Nylon comes to mind as something to avoid. It will stretch to
150 percent of its original length before it snaps (not neatly). This
stretch has to be considered as a shock load.
I don't know what you are trying to accelerate, but moving much of anything
up to 250 mph in 2 seconds is going to take a good bit of tension. Assume
that you will have to put in 4 to 10 percent more energy per sheave to
account for friction. Since this is about a 10 g acceleration, the rope
strength should be in the order of at least 100 times the weight of whatever
you are going to accelerate (including the weight of the rope itself). If
there is any slack in the rope, then there can be some jerks well in excess
of 10 g's, which would mean that you will want to increase your rope specs
accordingly. Usually you use a rope that is 5 to 10 times stronger than
your load to compensate for wear and tear, knots, sudden loading, and other
problems. I would guess that most natural fibers are out.
I would strongly consider using a steel cable on this setup. Kevlar, or
some other low stretch, high strength rope might be a good idea. I presume
you've already figured out how to guard spectators from a loose rope if
anything should break.
Would it be more efficient to use a geared system and an electric motor?
Even with a lot of sheaves, you are looking at some very large rams.
Michael
OK -- in 30 odd years of engineering, having designed wire rope and sheaves and
handling systems, written some fiber rope use papers, and consulted to wire
rope mfg and tackle makers:
Not outside of behind the blast wall.
Be very damn careful - it can be extremely dangerous. To get an idea,
calculate how much energy/power you are putting in, and see how much steel it
can lift how high/how fast/etc. . Scary as hell.
What will almost certainly happen in the cable is beyond the area in which most
professors work - it's the energy realm, not the force realm. It's energy
density engineering, like in explosives, not force engineering which assumes
energy equalization.
Basically, in that realm, the energy cannot disperse inside the material
fast enough, and the cables explode where the energy gathers/waves cross.
In the wire rope design books there are some guides for using wave breakers
in cable, and some equations for explaining why.
In short, the wave in many high rate/high speed applications will reflect off
the curved sheave and double back, soemtimes several times - if you are lucky,
it will only birdcage. If you are unlucky, it will break and the ends and
pieces will move out at velocities which remove body parts before your nerves
will know they are gone.
. I did a seminar at MIT many years back in that energy area with several of
the ME profs, and got a lot of startled looks and excited curiosity at this
"new" concept.
It's not new - it's just not something anyone should be doing unless you have
a test area with a blast wall.
Second, if any pin goes - the yo-yo will walk very very fast - and at high rate
inputs, it likely will -
the sheaves will walk up the wall, across the ceiling, and stick in your
back (or lower).
From experience - the other engineer thought I was paranoid when I made us
all stay out of the test room and behind the second wall - and the steel sheave
took a high speed walk around the (empty) shop at very high speed.
Have a cover on the blast wall, or the discus (disci? ) will show you why it
was a weapon of war.
try a different project....
check on cable dynamics, check on your
most
There was a dramatic example of this on TV a few weeks ago. It's a new TV show
where they build wild contraptions (I forget the name, but not Monster Garage
or Junkyard Wars). The task that week was to build a guillotine that could chop
a car in half. They used a 6000 lb blade, and another pair of dropping weights
that used cable running over pulleys to pull the blade down.
It worked, but in the final run, where they put the full weight on and raised
them to the top, the cables failed as the weights jerked to a stop. They
snapped where each cable run through the eye of the weight (1500 lbs as I
recall), and snapped with such force that each exploded in a shower of sparks
like a Hollywood special effect.
It was quite amazing. They replayed it many times and it was fascinating to see
a cable fail in such a huge burst of energy.
DT
Actually, we use something like this all the time in the special effects
end of the movie industry. To get speed, we always use gas, nitrogen
from a high pressure tank, dumped thru a large orifice valve into a long
stroke cylinder, typically 5" or 6" in diameter. We're usually
accelerating something on the order of 100's of pounds and need the
power. Usually the pullers are rigged 10 to 1 or less.
Good luck- Paul
g3_josh wrote:
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