Can anyone identify this type of scissor jack

It's congruent triangles (remember those).

As long as top and bottom are identical (but rotated 180 degrees) it should work. Changing lengths will just affect how far it can move.

Neil

For those wanting to study/measure/play with the SolidWorks model, there is a free viewer program available. Handy if you don't have a CAD system that can import the files.

Do I remember congruent triangles? No.

I've always been obtuse.

Dave

I'm astounded by how much discussion this has caused beyond discovering it's namesake, size and tolerable load.

Michael W

I think he means the short arm appears to give support to the RHS. It is perfectly possible to add a similar parallelogram to the LHS.

Now why didn't I think of that... doh

I don't think you can....

But the 'unsupported' LHS at the top is mirrored by the unsupported RHS at the bottom.

Neil

The parallelogram that ADY drew is needed to ensure both sides rise at the same rate, without the two short links if the table was unevenly loaded there would be a tendancy for the screw to pull in more on the lighter loaded side and tilt the table as that side would expand more.

I don't think the ration is massively important but it will affect the max and min heights.

J

.

Thank you, Neil ... that's exactly how I saw it.

... the shapes are 'equal and opposite'.

MichaelG.

Here for your Friday evening entertainment is a little video that shows that without the two short links the top can go all over the place, as soon as the links are added hey presto the top stays level. Can't be bothered with all that Lego or Mecano - stick it in the CAD.

Edited By JasonB on 18/11/2016 16:57:33

HI Dan

You can download Solidworks eDrawing viewer.

**LINK**

Cyril

Jason,

From my lego messing about, the ratio appears to affect the way the thing moves - e.g. if you look at 43 seconds in your video, the top surface has moved to the left of the bottom surface, and the middle pivots are tilted downwards on the left.

Sheer luck gave me a physical model where it moves more or less vertically, and the centre pivots stay horizontal, which allows a second parallelogram to be added without the thing locking. It is not perfect, and wouldn;t work without a tiny bit of flex, but it makes me wonder if there is a ratio which would allow this ?

Dan

Yes I found that and have been playing with the lengths. I started with 150mm long links and 50mm short which is what you see in the video but nearer 60mm short links seems better. Will play some more.

.

Dan,

I think now might be a good time to do that ^^^

... It's really rather good.

MichaelG.

It seems to have more than a little in common with Watt's parallel motion.

More bedtime reading, for those intrigued by this subject: **LINK**

http://www.mirrorservice.org/sites/gutenberg.org/2/7/1/0/27106/27106-h/27106-h.htm

MichaelG.

http://www.zen180103.zen.co.uk/Scissor.avi

Above is a short clip of an experiment I did to figure out a new free tool I got.

It's been a while since the last post on this thread, but at the time, I had no time.

This mechanism stays parallel at the top and the bottom because of its symmetry. The thing is that the screw-link does not necessarily stay level unless you position the links carefully. This may not matter if you don't mind the hand-wheel going wonky.

Anyhow I finally had time to sit down with this new software tool I got and give it a go. You will see that I had to anchor the screw and have the top and bottom fly freely. This is actually a limitation of the simulator, but it does not prevent a clear understanding of accurate and optimal link sizes.

The tool is straightforwardly called "Linkage". Its good for locomotive valve gears too. Although its still not perfect from a user interface perspective, it still seems like quite a leap forward from some of the valve gear simulators I have seen.

It's free and you can find it;

http://blog.rectorsquid.com/linkage-mechanism-designer-and-simulator/

Edited By Andy Ash on 18/12/2016 01:48:39

Andy,

That is a really useful thing - thanks very much for the link. Would you mind posting your linkage file somewhere so I can have a look at your model ?

Regards,

Dan

You can find the file at the link below.

Sadly I have been unable to make it so that the file will download, rather than open directly in the browser.

In any case, if you are careful the file is actually just plain text. You can copy the whole text from the browser, paste it into notepad and save it to a file of your choosing. The extension should probably be specific however. If you decide to call it "Scissors", then the filename should be "Scissors.linkage2". (Obviously without the quotes).

Notepad will try to call it "Scissors.linkage2.txt", if it does this just rename the file, with a normal file explorer window.

The actual linkage has not been defined accurately. I just wiggled the links until it looked O.K. I'm sure you can be more scientific. The positions of all the links can be controlled by co-ordinate, and the whole thing scaled metric or imperial.

You can't import a drawing from a CAD system easily, but you can export your link pivot positions directly to DXF or DWG so you can get accurate positions directly into 2D cad if that's what you need.

Have fun!

Edited By Andy Ash on 20/12/2016 00:42:38

The parallelogram section reduces the travel of the screw compared to the distance travelled by the lift at the expense of leverage hence the large hand wheel.

Regards
John