Rope and Pulley Systems: Segment 6 - The Block and Tackle - 4:1 and 5:1.pds.m2ts

I'm going to talk about a block and tackle I think I said in the previous segment we look at three two ones but I want to do the block and tackle first and then we'll come back to that to me this is a block and tackle some might consider this to be a block and tackle also but to me this is what epitomizes the block and tackle system it's blocks of sheaves inside single housing that have independent sheaves that can help us do things this is reminiscent of the blocks that they used to use on the sailing ships they were either blocks with sheaves in wooden blocks literally and that was the block and the tackle block and tackle was the rope that ran through it in broader sense perhaps maybe any block and tackle constitutes a blocking tackle but not to me case you're not familiar with these things this is a basically a single pulley or it's a single housing that has two sheaves in it so it's like a double pulley and they can have more than that in it each roller or sheave can travel independent of the other and you can see that they'd have to be true because this rope is moving faster than this rope so they have to move at different speeds in order not to have rope dragging through them so we'll determine very quickly that there are a lot of similarities here or or just accept that there are and I want to next evaluate we'll apply our test for a simple pulley system to a block and tackle and see if it meets the requirements for a simple pulley system so if I pull it up all the moving pulleys and here's a moving pulley move toward the anchor that's requirement number one number two is that all the pulleys that move have to move at the same rate of speed toward the anchor well since we have basically a dual pulley here and there in one housing they do indeed have to move toward the anchor at the same speed so it is a simple pulley system we now know from our look at this that once we determine we have a simple pulley system we can calculate the mechanical advantage by simply counting the lines that support the load so we have one two three four again let me emphasize this because this will trip you up sometimes this line counting thing the ones that support the load only works in simple pulley systems you can't use that method in compound and complex pulley systems so we have to know it's a simple system and it is so if I pull here for every pound of force I put in here I'm going to have four pounds here because I have four lines here so my gain mechanical advantage if you want to call it is one two three four four two one every pound gives me four here a block and tackle is unique in another way - it's just so concise and compact it has one connection point at the anchor and yet we've got two pulleys connected it has one connection point at the load and yet we've got two pulleys connected there plus you can ramp this and I'm not going to do the full type ramp fold this thing up pack it in the bag and it'll stay just like that and you get ready to use it you take it out unfold it and unwrap it and it's ready to go now I think if you look at this being a four to one and a four to one that's probably not going to work very well with this and it ties up a lot of gear so a block and tackle has a lot of interesting advantages to it simple pulley systems because of the the way they're configured usually if not always can be flipped so I'm going to do that right now and we don't create something all that new let me know that's alright that's going to turn it around but I want but it does add a factor of mechanical advantage and the reason it does let's look at it we now have one two three four five lines supporting the load so we now have a five to one just flipping this thing around we now have a five to one an interesting thing happens at the load let's calculate what the force at the load is I think it's going to be a surprise let's do that with a hundred pounds let's just assume we've got a hundred pounds here and we have one two three four five lines supporting a load in a simple pulley system so we have 20 pounds in each line well let's look at that what do we have right here it is the sum of all the loads and the lines they add up to create the force here so we've got 20 40 60 80 we have 80 pounds right here so there's only 80 pounds pulling down on that from a hundred pound load well does that make sense how could you have something lighter here then you have here well the answer is I've got 20 of it right here 20 40 60 80 100 so I've got 20 pounds of upward force right here in addition to the 80 pounds right here so 80 plus 20 equals the load it's kind of tricked but that's the way it is and if you'll recall it's not maybe that strange when we had a simple pulley system with one pulley and it anchored here and we pulled here we found the load on the anchor was half the weight of the load because I had the other half so the two halves added so maybe not such a surprise after all in the next segment I'm going to drop back to the 3-1 systems we're going to look at a just a pulley three to one rope and pulley and we're going to look at a block and tackle three two one and we're going to look at a 0z rig is a pretty interesting pulley configuration you can probably do some similar things with ordinary pulley systems but the Z recall for some interesting advantages

Channel: Tanglerwr

Views: 778,046

Rating: 4.6967049 out of 5

Keywords: rope and pulley, mechanical advantage, pulley, pulleys, block and tackle, rope and pulleys

Id: sR6PTkwords

Length: 7min 7sec (427 seconds)

Published: Sun Sep 04 2011