Engineering help please

[j king]

I have a big project I am building for home. It's an automotive scissor lift that sits down flush with the top of the concrete when its down.

The problem is I am copying a few design bits from different manufactures and want a pivot piece that helps the scissor raise from being completely flat so it's not stressing the unit trying to Greg it to open. Please bear with my descriptions as I don't really know what stuff is called.

Here is a pic of such a lift. The part that the cylinder rod pushes against has a roller on the back side and the unit pivots on the main center pin to help lift open.

I must be dumb and dumber's brother because I don't see what keeps the pivot from continuing to travel about the center pin until the ram comes into contact with the center pin.

Hope this makes sense...I don't see anything that stops the rotation of the pivot.

Can someone explain this?

Thanks. Jim

[Rustybolt]

The length of the stoke of the cylinder.

[j king]

Another shot.

Rusty bolt, You mean it won't hit if rod is short enough? I still see the pivot rotate towards the rear and continue since it would be the easiest movement and wouldn't lift the scissors until the rod and pivot pin interfered. As I see it the rod and pin would get more clearance as the scissor opens to max.

[ironmonger]

While not answering your question, an alternative method is used to help raise the platform on man-lifts. Some of these units use a small 'helper' cylinder that is positioned vertically. It has a 12" stroke or so and just sits extended while the load is in the air. on the way down it helps as well.

I never traced the hydraulic circuit, but it seem that you could just pipe it in parallel with the main lift cylinder. The main cylinder would present a large load and allow the helper cylinder to extend until it reaches it's limit at which point the main lift cylinder takes over.

paul

[schoolie]

Look at the first picture in the photo album (the one looking at the front of the Tahoe up in the air). It looks like there's a plate welded between the two sides of the scissor leg. The pivot piece with the roller hits that plate once it rotates a certain amount, stopping it's rotation and causing the cylinder to act directly on the scissor leg.

[j king]

This has to be very compact.the one pictured fold up to a little over 4 inches laying flat.Mine is going to be 5 inches but in a pit.

[j king]

Schoolie. I saw that but it can't be. The flat height is the same so you wouldn't be gaining anything if that was so.Some shots you see the pivot beyond the scissor arm by an inch or so.

It can't be that complicated.lol

[schoolie]

Schoolie. I saw that but it can't be.l

I'm pretty sure it can be, but I've been wrong before...

The flat height is the same so you wouldn't be gaining anything if that was so.Some shots you see the pivot beyond the scissor arm by an inch or so.

Not sure what you mean by this, but I think you're saying the pivot will be inline with the scissor arm when it's collapsed? I'm thinking it will end up just slightly misaligned from the arm, with the end the cylinder attaches to higher than the roller end. That way when the cylinder starts to extend it forces the pivot up, and the roller down, lifting the platform.

The shots you see the pivot piece an inch or so past the scissor arm (all of them, I believe) show the pivot at the end of its travel. At that point, the cylinder might as well be attached directly to the scissor arm. The pivot's only there to get the platform off the ground. Without the pivot, the total height of the lift would have to be higher to start with the cylinder not in a direct line with the arms.

At least that's what I think...

[Davek0974]

Interesting project.

One question, what secures the lift when raised?

Surely there must be a mechanical or electro-mechanical lock or latch or the car will drop suddenly if a pipe or seal blows or pressure is lost????

[Paul Alciatore]

They have a large, flat plate welded over the opposite side of the pivoting piece. It is completely hidden in the photos above, but can be clearly seen here in the first photo:

http://www.atlasautoequipment.com/products/kwik-bay-7000#gallery

Also, there appears to be a rack like safety device on both sides. This can be seen in the sixth photo in the gallery above.

The leverage on the welds on that flat plate is going to be tremendous. Perhaps the most highly stressed part in the whole design. If you do this yourself, be very careful with this weld. If it fails, you are on the safety racks. With only 13 positions, the load can fall 1/13th of the lift height, gathering momentum before they take effect. Wham! If they then fail, the whole house of cards comes down.

Not the best design. I don't like it.

[j king]

Paul. I'm putting a lock rack in the center of the arms and will have larger teeth to engage. Stole a design from a different company.

[Rosco-P]

When all is said and done, is the cost of the material for this type of lift going to be less than that of a twin post above ground lift? Or is the primary criteria that it be out of sight and out of the way when not in use?

[j king]

#1 i wanted it floor flush so not to take up any space and bang doors into #2 I wanted to get maximum lift height since it will be in the floor so 4.5 inches lost there.# 3 because I wAnt a project and have a hydraulic unit and lots of steel. I don't have the height for a regular 2 post lift and again they are room hogs. Jim

[darryl]

I don't like the design either. It's too complicated, and likely prone to jamming because it's independent. I think it's a safety hazard also, and there's too much stuff in your way to have to work around.

But to answer the original q- the bottom of one scissor drags or rolls inwards as the lift is being operated. There would likely be a limit to how far that can move, so that would then define the maximum height.

A vertically mounted 'helper' cylinder directly under the pivot pin could help raise the platform initially. That cylinder would be plumbed in parallel with the lift cylinders, and since it would have a maximum extension limit, it simply goes that far and stops, allowing the lift cylinders to keep receiving fluid.

In any event, it would seem that you are willing to alter or create the cement floor to accommodate whatever system you install. If that's the case, and you're willing to fabricate, I can see a couple of alternate systems. I think I would prefer four corner posts rising straight up, although it would require a method of synchronizing them. I haven't done anything like this, so it's only my 02 worth.

[Stern]

See this sort of design on scissor lifts, but as stated there is no work room under it (and I sure as hell wouldn't get under something like that without multiple safety lockouts. I have contemplated putting something similar in my garage but decided it just wasnt worth the cost to buy, or the risk to make. If you do make it, just be safe and add as much stuff to it as you can to be there "just in case" the hydraulics fail, as while dropping a few inches may not seem like much, but it can cause a lethal head blow, not to mention pinching off a few digits (one of the reasons you are not allowed to EVER go under the scissor lift platform when its up (for service) unless you have two huge locking bars in place and the platform dropped onto them.