Mechanics / Pulleys Question
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themosquitoking
Guru
- Location
- Spain
If everything is balanced perfectly i reckon the load would lift fairly horizontally.
snorri
Legendary Member
- Location
- East coast, up a bit.
The rig you have sketched is precisely what I use to lift my bike off the ground for maintenance, as supplied by Lidl. It works well, but the bike seldom goes up horizontally although this is easily adjusted by lifting the lower end of the bike which then remains sufficiently stable for me to carry out most maintenance tasks.
I assume this is due to frictional losses in the pulleys, but if the load is not great a bit of instability may not matter, unless you are lifting a full fish tank!.
If you want to ensure the load remains horizontal throughout, I would suggest you secure one end of the rope above the LH end of the load. take the other end of the line and feed it throught the pulley on the LH end of the load, then up round the LH roof pulley and along to one of the RH pulleys, then back **into the other RH pulley, down to the pulley on RH of load, and back up and secure to the roof beside the RH pulleys having ensured the length of the bight of rope between the RH pulleys (at **) is sufficiently long to be within reach when the load is on the ground.
I assume this is due to frictional losses in the pulleys, but if the load is not great a bit of instability may not matter, unless you are lifting a full fish tank!.
If you want to ensure the load remains horizontal throughout, I would suggest you secure one end of the rope above the LH end of the load. take the other end of the line and feed it throught the pulley on the LH end of the load, then up round the LH roof pulley and along to one of the RH pulleys, then back **into the other RH pulley, down to the pulley on RH of load, and back up and secure to the roof beside the RH pulleys having ensured the length of the bight of rope between the RH pulleys (at **) is sufficiently long to be within reach when the load is on the ground.
Smurfy
Naturist Smurf
- Location
- Smurfette's Kitchen
If you assume the friction in the pulleys is negligible, the tension, T, in the rope is equal along the entire length of the rope. This means that each end of the load sees a lifting force of 2T. Unfortunately this means that if the attachment points are not equidistant from the load C of G, the load will tip up as there will be an unbalanced force. The amount of tipping will depend how bad the force imbalance is, and will only reach equilibrium once the load is completely off the ground. For small imbalances the tipping may be tolerable. But with all the weight at one end, one end will remain on the ground until the load has rotated almost 90 degrees.
Take a look at this pallet lifter. There are a number of holes for the crane hook/shackle, to allow for the lifting of pallets that do not have a central C of G. The user of this piece of equipment most probably chooses the correct hole by trial and error, rather than sitting down with a pen, paper and calculator!
Take a look at this pallet lifter. There are a number of holes for the crane hook/shackle, to allow for the lifting of pallets that do not have a central C of G. The user of this piece of equipment most probably chooses the correct hole by trial and error, rather than sitting down with a pen, paper and calculator!
Smurfy
Naturist Smurf
- Location
- Smurfette's Kitchen
The OP reads a bit like a school homework question! Do you have a son/daughter?
Archie_tect
De Skieven Architek... aka Penfold + Horace
- Location
- Northumberland
Wouldn't the left hand side pulley raise at half the height of the distance the rope travels on the RH pulley as the end is fixed to the ceiling not the platform?
[Edit: I think with 2 separate lifting pulleys they would only would lift at half the distance with a ratio of 2 rather than 4, but it will lift level providing you triangulate the load platform support cables from the bottom of the 2 lifting pulleys both supported from the ends of a lifting rod. To get a ratio of 4 you'd have to have the lifting pulley rigidly suspended from the others to keep it rigid and triangulate the cables supporting the platform from that pulley.]
[Edit: I think with 2 separate lifting pulleys they would only would lift at half the distance with a ratio of 2 rather than 4, but it will lift level providing you triangulate the load platform support cables from the bottom of the 2 lifting pulleys both supported from the ends of a lifting rod. To get a ratio of 4 you'd have to have the lifting pulley rigidly suspended from the others to keep it rigid and triangulate the cables supporting the platform from that pulley.]
It sounds a bit like one of the questions that is now asked when taking lateral thinking tests for some job applications.
thom
____
- Location
- The Borough
Here's one for you : if you were shrunk to be the size of a 2p piece and placed in a blender that was going to be turned on in 30 secs time, what would you do ?
Archie_tect
De Skieven Architek... aka Penfold + Horace
- Location
- Northumberland
Switch the electric off.
thom
____
- Location
- The Borough
You must have very long arms in relation to the rest of your body
Archie_tect
De Skieven Architek... aka Penfold + Horace
- Location
- Northumberland
Having long arms would be the least of my worries Thom- but in your world if I can be shrunk then I can also be Mr Tickle.
Night Train
Maker of Things
- Location
- Greater Manchester
With frictionless 'theoretical' pulleys the load would rise evenly.
In real life the cumulative friction would mean the right hand side would rise first. However, the mass of the load would have an effect if the force of gravity acting on the mass makes the forces of friction in the pulleys negligible. The load would then find its own equilibrium if it is hoisted slowly.
Hoisting quickly would not allow the load to settle and would still result in the right hand side remaining higher during the process of hoisting.
To ensure the load rises level you would have a single hook on the hoist attached to a multi leg sling that is centred over the CoG of the load, like this:
In real life the cumulative friction would mean the right hand side would rise first. However, the mass of the load would have an effect if the force of gravity acting on the mass makes the forces of friction in the pulleys negligible. The load would then find its own equilibrium if it is hoisted slowly.
Hoisting quickly would not allow the load to settle and would still result in the right hand side remaining higher during the process of hoisting.
To ensure the load rises level you would have a single hook on the hoist attached to a multi leg sling that is centred over the CoG of the load, like this:
