Take the 2-minute tour ×
Home Improvement Stack Exchange is a question and answer site for contractors and serious DIYers. It's 100% free, no registration required.

I've built the following murphy bunk bed:

Open Position of Murphy Bunk Bed

It rotates around the circles (the fulcrum) into a closed position:

Closed Position of Murphy Bunk Bed

I decided to buy gas springs (the retracting kind) to assist in closing the bed. Using the measurements from the picture,

Measurements to compute force

from what I understand, the calculation would be:

force = 110*(35.8125)/11.4375 = 344.43

At this point, I was feeling pretty savvy and bought two springs that had 150 lbs of force, thinking this would be perfect. But I had a problem--instead of naturally falling open as I expected, it required all 200 pounds of me at the edge of the bed to get it to budge.

So where am I going wrong?

share|improve this question
    
I'm confused by your description. Is it stuck in the up position? In order for me to answer your question, you'll need to provide the total weight of each bunk, with mattress included and the weight of each support leg for the top bunk. Also provide the height the gas pistons are attached to the frame above the bed. This geometry is needed since it affects how much of the force is directed vertically and horizontally. –  Doresoom Jun 18 at 14:26
    
@Doresoom--Thanks for the questions. It is stuck in the up position. The gas springs have an extended length of 30.11 in, and 18.11 in when compressed. The top of the gas spring is attached 23.5 inches above the bed. As for the weight of each bunk/leg support--I'm not sure I can answer that without taking the whole bed apart and weighing each separately. Is that required? I just assumed the standard force calculation would work (which only requires one knowing the force required to lift it to the closed position). –  dfife Jun 18 at 14:32
    
That's not the issue--the top of the spring is actually set back behind a bit (toward the wall) so when it's closed, it is 18.11 inches. –  dfife Jun 18 at 14:56
1  
A bit late now, but for what it's worth Rockler (among others) have kits with gas springs preselected for the purpose. –  keshlam Jun 18 at 15:54
    
@keshlam--I did look into a Rockler kit a while ago and decided against it so I could do my own design. (But after having such a headache with it, it may have been better that way). I didn't think about just buying gas springs from them. Oh well. –  dfife Jun 18 at 16:13
show 2 more comments

1 Answer 1

up vote 5 down vote accepted

It's going to take 71.5 pounds of force pulling straight out from the wall in order to begin moving the bed downwards. The required force will actually increase a bit as the door begins to rotate and the tension from the gas springs becomes closer to perpendicular to the bed frame, and then the force required will begin to let off as you reach closer to the bed's horizontal position.

So even if you're putting all 200 pounds of your own weight into getting it started, a good portion of that force is going down through the bed frame to the hinge, which will just add more rotational friction and cause it to be more difficult to open.

If I have time later, I can try to recommend a different mounting point for the gas springs that will alleviate some of this problem. I can tell you right now that the more acute the angle the gas spring makes with the bed when stowed, the easier it will be to get it moving downward. Then you'll have more of your weight working for you as the bed passes through perpendicular with the gas spring.

enter image description here

Here's the force profile for various angles and bed weights. The point where it dips negative is when the force exerted by the gas springs is not enough for the bed to be lifted, and you'll need to assist it that much to stow it upright. (All you other engineers out there on DIY.SE, feel free to make sure I didn't make any stupid mistakes. The angle graph looks right to me, but then again, there's some pretty tedious trig to get that monster formula.)

enter image description here enter image description here

The weights listed in the graph are for actual bed weights - if you took the whole assembly off the wall and threw it on a freight scale, not the force you experienced while lifting up one side of it. The forces (colors) are the perpendicular forces required to pull the bedframe down into a useable position. Where they go negative is the area you'd have to lift the bed up to get it back in the stowed position. enter image description here

Since you've already bought the gas springs, the best option may be to change the geometry of how they're mounted instead of looking for a different load capacity. Here are some graphs that give the required pull down (rather sideways) force to open the bed and then the lift force required to stow the bed. Note that it's for an assumed 220 lb bed, since I ran out of dimensions to graph things on. Hopefully these plots will allow you to make a decision on what tradeoffs you want to accept if you rearrange the gas springs mounting point on the frame. (You'll also need to check if the gas springs have the range of motion required beforehand too.) The vertical offset and horizontal offset are measured from the bed's pivot point on the frame (distances h and a in the second sketch, respectively).

enter image description here enter image description here

share|improve this answer
    
Thanks for the answer, but I'm a bit confused. If it takes 71.5 lbs of force to get it going, why does my 200 lb body not make it move? –  dfife Jun 18 at 16:16
    
That amount of force has to be perpendicular to the wall, not straight down. Since you can't brace very easily against the bed itself (which you're trying to move), you'll have trouble even if you've got 3X that amount of force working for you in the form of your weight. Plus, the upper bunk needs the force applied at a height of 6', which makes it awkward for even a tall person, and downright impossible for someone short. –  Doresoom Jun 18 at 16:30
    
Makes sense. So...back to the original question--how hefty should my springs be? –  dfife Jun 18 at 16:35
    
One other piece of information--the MAX force required to lift the bed from the middle is 110 lbs. My scale seemed to max out at around 45 degrees. –  dfife Jun 18 at 16:47
1  
Really good answer and right. Buy a strap for the bed to pull it out easier. –  DMoore Jun 18 at 19:26
show 4 more comments

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.