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Pulley System Design for lifting platform bed

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Since I am not an engineer, I am looking for recommendations on how to design/configure a pulley system that would assist me with lowering and lifting a "Lori Bed" into place. This is a Murphy-type bed that was designed without the installation of pneumatic hardware. The "legs" are curved so that the platform can be rocked up into place. I am imagining that a pulley could be installed over the wall frame and whatever additional configuration recommended would pull the foot of the bed up into place. I have attached 2 images that show the bed lowered for use and stowed away. I'm guessing the platform with mattress weighs approximately 100-125 lbs. Thank you in advance.

Bed Down Photo.JPG

Bed Up Photo.JPG

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  • 5 months later...

Pulleys are a very simple form of mechancial advantage. 

They work on the equation Work Done = Force x Distance (WD = Fs) Note: in equatons "distance" is 's' for "spacial displacement"

If you increase the distance (by using additional pully wheels) you increase the work done for the same amount of force used. 

I have given the following advice completely ignoring resolved forces and moments - which would make things easier...effectively, I have based everything on "worst case".

I grabbed this image from the "school physics" website ( https://www.schoolphysics.co.uk/age11-14/Mechanics/Forces in motion/text/Pulleys_/index.html ) 

schoolphysics ::Welcome::

The first item there is no change.

The second has two cables, therefore the length is doubled, using the equation and re-jigging for the force, F = WD/s - the work done would be the weight of the bed and the distance required to raise it. 

Say it has a mass of 100kg and has to be raised by 2m. In Newtons that would be roughly 1000N of work over 2m

1000N x 2m = 1000 N of input required over 2m

If we double the length over which the force is applied, that would work out to be 1000 (x 2) / (2 x 2)  = 500 Newtons of force required to raise the bed on a 4m draw.

On the third wheel, there are three lengths of cord drawing the bed up, so that would be 6m of draw for the 2m rise...
1000 (x 2) / (2 x 3) =  total giving 333.3N of force to raise the bed on a 6m draw

On the fourth, there are 4 cables so - as before - 1000 (x 2) / (2 x 4) = 250N on an 8m draw

On the image above, the final image has 6 pully wheels, wired to give 6 lengths for the draw. Using the same formula, 1000 (x 2) / (2 x 6) = 166.67 N. the length of the cord drawn out in this case will be 12 metres.

For a bed, this last detail above is critical to be fully aware of. 12 metres of cable is an enormous amount. You have to be very careful that there is no risk of someone being accidently (!) tied up in the pully system - this could have catastrophic, tragic results should someone become inadvertently tangled in their sleep - it has happened in the past!

I do hope this helps.

Good luck

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  • 3 months later...

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