Force calculation of rope used in block and tackle system


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Hi all,

I need to build a system to lift something heavy, for the moment I most likely will use this design https://en.wikipedia.org/wiki/Block_and_tackle#/media/File:Pulley3a.svg with 4 pulleys.

Let's say I need to lift 600 lbs weight in total, I buy 4 sheaves and 1 rope for this purpose. Each sheave has 500 lbs safe working load, and rope has 250 lbs safe working load.

I think my sheave should be strong enough, but I am not very positive about the rope in this case. If my understanding is correct the force I need to pull the 600 lbs weight is 150 lbs, does it mean as long as the rope is strong enough for 150 lbs it should do the work? Or the rope should be strong enough for entire 600 lbs weight?

Thanks!
 
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Hi,

https://en.wikipedia.org/wiki/Block_and_tackle

I wouldn't trust a rope of 150 lbs breaking strength on the set up you describe but the safety factor of the 250 lb breaking strain in the same set up should be fine but I'm no expert; its over 50 years ago I was involved in such calculations. There are many factors to consider regarding angle of lift etc; I suggest taking time to do a bit of research on the web; whatever you do don't ever stand beneath a suspended weight and I'm definitely not in any way responsible. Why not just buy a chain hoist they are cheap enough second hand?

https://www.ebay.co.uk/itm/Heavy-Duty-1-Ton-Chain-Hoist-Load-Lifting-Block-Tackle-Engine-Winch-Pulley-Tool/302518363134?_trkparms=aid=555017&algo=PL.CASSINI&ao=1&asc=20151005190540&meid=32b326ebfdd74e438978e8eeb049a8c9&pid=100505&rk=1&rkt=1&&itm=302518363134&_trksid=p2045573.c100505.m3226

Kind regards, Colin.
 
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Hi,

I've a bit more time this morning so can add a bit more. Lifting heavy loads can be highly dangerous so personal safety and safety of others is paramount also if something heavy suddenly drops it can cause a great deal of collateral damage.

Consideration is needed regarding not only the lifting capacity of the rope/sheaves these being well up to he job but what are the sheaves suspended to or anchored from; this support too needs to bear the full weight as do any slings; ropes; eye bolts and hooks etc that are used to secure the load to the sheaves.

https://www.safetyliftingear.com/categories/lifting-equipment?gclid=CjwKCAjwio3dBRAqEiwAHWsNVWZPXhfMhVPA3MAoOutv1wy6WYTNjDrPtiXBAZk42e-Sex4E4TpeaRoCAHMQAvD_BwE

I'm a mechanical engineer having served my apprenticeship down a deep coal mine where assorted equipment was used daily to move heavy loads; wire ropes; chains; winches all manner of couplings and all these had to comply with current safety regulations so I would never encourage any novice to play around with such things without firstly doing a lot of research; all it takes is a simple mistake to have serious consequences.

Without more details of why you need to use rope and sheaves if you can use a chain hoist this is what I would use or even an hoist with a wire rope; sheaves need a very long rope?

Won't a rope and sheaves suitable for lifting such a weight cost a great deal more than a chain hoist? Just a few ideas.

Kind regards, Colin.
 
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Hi,

I've a bit more time this morning so can add a bit more. Lifting heavy loads can be highly dangerous so personal safety and safety of others is paramount also if something heavy suddenly drops it can cause a great deal of collateral damage.

Consideration is needed regarding not only the lifting capacity of the rope/sheaves these being well up to he job but what are the sheaves suspended to or anchored from; this support too needs to bear the full weight as do any slings; ropes; eye bolts and hooks etc that are used to secure the load to the sheaves.

https://www.safetyliftingear.com/categories/lifting-equipment?gclid=CjwKCAjwio3dBRAqEiwAHWsNVWZPXhfMhVPA3MAoOutv1wy6WYTNjDrPtiXBAZk42e-Sex4E4TpeaRoCAHMQAvD_BwE

I'm a mechanical engineer having served my apprenticeship down a deep coal mine where assorted equipment was used daily to move heavy loads; wire ropes; chains; winches all manner of couplings and all these had to comply with current safety regulations so I would never encourage any novice to play around with such things without firstly doing a lot of research; all it takes is a simple mistake to have serious consequences.

Without more details of why you need to use rope and sheaves if you can use a chain hoist this is what I would use or even an hoist with a wire rope; sheaves need a very long rope?

Won't a rope and sheaves suitable for lifting such a weight cost a great deal more than a chain hoist? Just a few ideas.

Kind regards, Colin.
Thanks for your input, chain hoist is definitely a good idea!
 

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