lifting mechanism - how to keep level?

[Lude123]

Hi,

attached is a quick sketch of the side and front view of a lifting mechanism.

the plate is supported by 4 arms mounted in between 2 walls.
Black is the original position and gray is the mechanism in lowered position.

My problem is that the rear of the plate drops down when lowering and the plate won't stay level (outlined in red)
Due to the plate swinging 'through'the arms and the tight fit between the walls I don't see how i could connect the arms to make them stay parallel at all times.

Is there any way i can achieve this?

thanks!

 

[drawoh]

Lude123,

How level does this thing need to be?

I take it this is a large weldment with sloppy tolerances, and that you are powering the lifting arms on the right. When the front link is horizontal, I would expect there to be lots of play at the rear. Even if you are willing to machine all the large components, you need clearances to manage thermal expansion.

The simplest fix to me would be to power both sets of arms.



--
JHG

 

[EdStainless]

Is the distance between the top 2 pivots the same as the bottom two?
If so then try a bar between the two middle pivots.
There are special linkages that will do this.
Do you have an old Mechanics handbook (like Marks), there should be a section on linkages.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[drawoh]

EdStainless,

I thought about a connecting link between the bars. I don't think it will work. The top structure is a connecting link, and it does not work.

What might work would be a connecting link attached 90[°] away from the connection to each hub.

--
JHG

 

[Lude123]

Hey guys thanks for the replies.

This is a construction made from wood. So tolerances are sloppy.

I have 2 gas struts on the rear arms (right ones in the sketch) and consider mounting 2 more for the front ones to make lifting and lowering easier.
I want to lift this mechanism by hand so no power lifting possible.

I considered a link but since it swing through the link would have to connect to the inside of one arm and outside of the other.
I don't think that would really work very well.

I don't quiet understand what you mean with a connecting link 90deg away from the connecting hub.
Could you explain or sketch?

 

[hendersdc]

I agree with Ed, adding a link between the two middle pivots will make this 2X four bar linkages in series. This will ensure that each set of arms stays parallel to each other whereas their relative angle is currently uncontrolled.

As drawoh said though, tolerance stack up and clearances will limit how parallel the plates will be to each other, how much is needed?

All that said, a scissor lift design would have probably been easier. That way you wouldn't need to guide the upper plate to prevent lateral movement like you do with this design.

 

[Lude123]

I needed the lower position to be further back than the upper position. That's why I didn't chose the scissor lift even though it probably would have been the better choice.

It doesn't have to be 100% level. I just need it to not 'fall' in the back when moving it up or down.
I'm not sure how to link it since the mounting points will collide with the link.

 

[rb1957]

very simple, beguilingly simple, diagram.

show the starting and ending positions.

show the plate widths properly … are they same ?

why double links ? (maybe because of the starting/finishing geometry we don't know)
why not single links, one end pinned, the other sliding in a (horizontal) track; one link pinned to each plate.
offset the links so they don't clash.

post a better drawing …
if you want better answers.

another day in paradise, or is paradise one day closer ?

 

[Lude123]

rb1957 I'm not sure if you are trying to troll me or just didn't read my post.

there is only one plate. the black is showing starting position, the gray end position.

 

[drawoh]

Lude123,

Something like this...

--
JHG

 

[Lude123]

drawoh,

yea that should work, thank you.

 

[TheTick]

Adding a horizontal link across the "knees" will keep level, but it won't prevent movement in X-direction (horizontally).

Also, what is driving the lift action?

Done a bunch of these...