Another lift mechanism

[GlennD]

Hi Folks


I have this lift stage designed for a client. It's part of a larger machine and met all their requirements except the marketing people came back and want the whole machine smaller. Their engineers came back and wanted this stage to be shorter and this is the original design.

The top level is just a top with some stuff to do it's job on it. the next level is connected to the top with what I have been fond of calling knees of which there are 4 sets with the 3 pivots you see on the side view (left) and one set in each corner of the table. the next layer down is fixed and holds the bottom pivot of the knees. The lower level has 2 threaded rods operated by a hand crank with left and right hand threads to bring the lower pivot points closer together (they are actually cam followers trapped in a track) and the arms attached act directly on the middle part of the knee lifting and lowering the top.

The object of all this was to have a stronger mechanism and to have the top level remain parallel with the ground regardless of loading. The previous (someone elses) design was a scissor but with the sliding elements and an uneven load it would not remain level and was generally weak


I suggested to them that we do away with the bottom level and put the threaded rod through the middle bar that connects the middle pivot of the knees together in pairs. I'm having a tough time seeing how this is going to affect the design though and wanted some reassurance. for example whether the lifting action would be as strong and if the top would still remain level. this design relies on the tracks and bearings to transfer most of the force to, now it's going to be all on the threaded rod..

New lift mechanizm.JPG

New lift mechanizm a.JPG

[jboggs]

I have skimmed over this and I can't say I understand the details... but I can say this: it sounds like human safety could be involved. If that is the case then you better make sure you have a thorough enough understanding of all the details and ALL the forces that you could make a jury of non-engineers understand that you and your company were not negligent when some idiot overloads or misuses your device and gets hurt.

You are going to want a very detailed stress analysis of all potential failure points and very substantial factor of safety.

[GlennD]

as it's shown here the minimum FoS is 3.0 and goes up from there. it's not as big as it seems but 4 people could stand on it and thats what I used to decide a safety factor for it. The old designs that I say was not strong enough, well it actually was strong enough it just seemed weak so it was more of an image problem and all the collected tolerances and flexes made it that way. This is much more rigid I'm just not sure how removing the lower level is going to affect how the system works and if will still seem as rigid or if it will seem as floppy as the old version.

I'd much rather stay with what I want in there I am however trying to accommodate their list of wants.

Edit: attached is a photo of part of the same kind of lift mechanism I used on another machine before. essentially I would be removing the light grey links and the threaded bar would be going through the middle joint instead

New lift mechanizm b.jpg

[GlennD]

I actually had to make a model of this to see what the effects would be. The vertical motion is not constrained enough. which was the question that nagged me on this. I made other changes to the machine to make the total lift height to be less than 4" and now all this can be simpler because it won't be stuck so high in the air when it's up and I can put the lower sliding rails back in because there is room now and the thing will once again be stable.