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.
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!
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.
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
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.
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.
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.
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.
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.
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