Looking for feedback on a lift bed power lift mechanism

Question

I built a king size lift bed. I want to add a powered lift mechanism to raise and lower it to access the storage area under it. In order to get the height I want when the bed is raised, I'll need about 20° of lift, which will raise the foot of the bed up to about 46".

I have a pair of linear actuators I would like to use to do the lifting. Each has 12" of travel and is rated for 200 pounds. Currently, it requires about 85 pounds of lift to raise the bed. The base of the bed is 14" high, and the lift needs to be inside the bed, so it cannot be higher than 14" when the bed is lowered. Those are my only real constraints. The bed frame itself is made from 1x2" 14ga steel tube, and the frame supporting the mattress is 3/16 2"x3" steel angle.

My plan is to have a pair of scissor type lifts, one on each side. The lower part of the lift would be connected to the base via a pivoting rod, and the upper part of the lift would be similarly connected to the bed frame. The actuator would press on the joint on the lifts to raise and lower the bed. The actuator would be aligned so it is even with the center of the lift when the lift is down. This will allow the most direct force from the actuators when the mechanical advantage of the lift is lowest.

If the legs on the mechanism are 13.75" between pivot centers, it would be 12" high when closed, and have a vertex angle of 64°. When the actuator fully extends, it should raise to 27.5". By placing these lifts about 45" from the head of the bed, it should have the desired result.

So, are my numbers correct and am I on the correct path, or is there an easier way to accomplish my goal? See image below for a visual on what I am thinking.

I have or have access to a fairly complete welding and machine shop, so I can do a reasonable amount of fabrication.

Answer 1

If this doesn't work out, check out the "6 bar dump truck mechanism" on page 12 of this link

Answer 2

The general idea is worth working out. Looks like the max loading, and worst case for leverage is when the device is almost closed -- so draw it up also in that configuration, and do the analysis when it is "almost closed".

General comment: you should do two types of analysis. (1) actually lifting the thing (2) will it survive a extra loading by careless people, kids, etc.

The 85lb load, translated to where the top linkage of the scissor pushes up on the bed, is magnified by the leverage ratio in proportion to (total horizontal length)/(horiz. length from pivot to lift contact point). Looks a little under 3:1? (Show that dimension....). So that's 200+ lb. vertical force. (total both sides)

Then that vertical force at the lift contact point, has to be translated into a force along the line of the linkage, which will be greater than the vertical force. Divide the vertical force by the cosine of the angle of top linkage from vertical.

Finally, from that, and the angle of the two linkages in the scissor mechanism, will pretty much determine the load on the line of the actuator itself. When the angle betw. scissor linkages is large (well more than 90 deg) then you have good mechanical advantage.

In the low position, the angle between the two linkages may be less than 90 degrees, in which case the actuator is at a mechanical disadvantage.

Increasing that angle-between-linkages-when-load-is-down might be needed, don't know. Introducing another linkage rotating around a pivot point may be necessary (posted link in another answer)... If you have a welding shop, many possibilities!!

All this assumes frictionless bearings too, so add a fudge factor for that.

Anyway for now, draw it in the closed position and see what you get.