Linear Actuators, Screw Jacks or cables and winch?

Question

Complete novice looking for a simple "yes, you can do that" or a "don't be daft" answer Anything in addition to one of these would be a complete bonus.

I'm thinking about at using four 12v or 24v linear actuators to raise and lower a queen size mattress and bed frame in a camper truck I'm designing. Total load would be less than 50kgs (bed would always be empty when moving). The height it needs to lift from the floor is 1700 - i.e. I want 1700mm clearance when the bed is in the raised position (there will be benches and a table below and I will eat there and work there so I just need seated headroom). I actually need the bottom of the bed frame when it is "down" to be around 750mm off the ground (table height). I guess this means that the stroke will be 950 mm then.

Not sure with actuators how this will all move at the same speed. That's another question I haven't asked yet. I have seen this done but I could not remember the video - the guy actually explained how he controlled this.

Happy to consider absolutely any mechanism - cables and winch, screw jacks - really open to suggestions. Needs to have minimum physical effort as my other half has damaged shoulders and needs to be able to operate when I'm not there.

A little more info to clarify. The vehicle will be a Mercedes Unimog U1700L. Tray dimensions are approx 4050MM (L) X 2375MM (W) X 500MM (H). The area I will be working with will be approximately the same (less wall thickness - probably 29mm (25 + 2 x 2mm walls) all round. I'm planning on making the entire box myself with composite fiberglass panels or possibly with walk-in freezer panels. Still in the early planning stage at the moment.

Answer 1

The main problem is the space which is at a premium inside the camper truck.

TL;DR: probably your best bet would be the linear actuator/screw jack with a scissor lifting mechanism.

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Linear actuator at the length of 1700mm would probably be a no-no. The length is just too much, especially if you subject it to compression. The only way --IMHO-- you would be able use them in your scenario (i.e. camper truck) is with a mechanism similar to what is used to scissor lifts (See below). However, you will still need some clearance.

Figure: scissor lift mechanism source

More precisely, you could use the following mechanism (inverted sitting at the top of your cabin).

Figure: scissor lift mechanism (sourcehandling.com)

The mechanism in the figure is for 3 tonnes and its hydraulic powered, and its intended so that the hydraulic jacks are in compression at the high load (hence the big diameters). In your case, it would be inverted, so the high load would be when the mattress is being raised (so pulling instead of pushing), which means that you could use smaller size actuators.

power screw/screw jacks while in most cases would be a really nice solution (self locking etc), however you'd still need the space. I.e. you might have four powerscrew running from the top to bottom inside the camper truck which is less than ideal. Unless you use a solution as mentioned above (i.e. a scissor mechanism). See image below

Figure: scissor jack (sourcehandling.com)

Finally, the winch/winch is another viable one, although probably the most messy. You could have a configuration with two shafts on the ceiling of the camper truck. Each shaft can have two cables. You could use a right angle gear box with two outputs (one for each shaft). However, the cables might not tighten the same, and it may be wobbly.

Answer 2

I will throw this out there but it should be the most compact but is also probably the most invasive. It would consume the least space underneath the bed:

Four large acme lead screw pillars that the bed frame rides on. You probably want those lead screws to run all the way to mountings on the ceiling for stability and safety unless these lead-screws are unreasonably.

You will still have the issue that the if driven independently the motors will not synch up without electronic controls so you can choose whether to install one or whether to gang up the four lead-screws and drive them with a single large motor.

You will need brief access to a lathe at some point to turn the ends of the acme screws smooth and provide a shoulder so it can rest inside bearing mounts at both ends, and also to mount a gear or sprocket so they can be driven.

Using a high thread pitch acme screw will allow the use of less powerful motors.

Answer 3

I think the simplest mechanism for this will be a winch which pulls 4 cables each of which supports 1 corner of the bed. You can stabilize the bed in the low position by hooking it to the floor with additional cables and then raising it slightly for tension.

If you've got the space you can just use a regular winch and pull parallel with the ceiling, then attach 4 cables where the hook goes and run those to each of the 4 corners using pulleys.

Here's a terrible diagram: edit: it should probably have 2 anchors not 1 otherwise it can twist