Pneumatics or Hydraulics?

8 bar is plenty for 300 KG. 5000 pound cars are lifted with 8 bar shop air.

http://www.engineersedge.com/mechanics_machines/scissor-lift.htm

Only if you use an air over hydraulic system. A 1/4" of movement in a hydraulic cylinder will eliminate all pressure with the valves closed. A pneumatic cylinder will be a cannon waiting to go off.

Once the load is raised into position the lift should be locked and the air pressure released or else the lift will rise further if you remove the weight.

using air over hydraulic uses a large diameter cylinder full of oil that connects to a cylinder that lift the load. Once the load is positioned and valve is shut off and the air can be released and the cylinder will stay where it is.

In car lifts they usually place a pin or stop on the lift to take the weight and reduce the air pressure. T lower the car you must re-pressurize the lift to move the weight off the through pins that now hold the car weight so they can be removed and the lift lowered. This is done is case a drop in pressure allows the car to descend on the mechanic...the mechanical stops prevent this.

There are many variations on this type of lift, so read all about the various aspects before you build it.

Often they are used as small dock loaders for lorries and you can inspect one.

It is also possible you can buy one for less than the cost of trying to re-invent the wheel from a loading lift maker. The manufactured one will be cheaper, it will look better as well.

One possibility is to use air to create some kind of "hovercraft", for want of a better word, that can be slid underneath the load and inflated to the point where the load can be moved. 8 bar would considerably exceed the pressure required for such a device.

You do not state at what height or distance you need to lift the load, although the meantion of scissors lift gives a clue that it will be some way off the floor.

If the distance is not too great and the floor area is free from obstacles then you could allow a pnuematic hose trail. Depending upon the height then, there are various lifting devices on the market such as air skates or bags from the likes of Vetter gmbh (Germany). However, based on the limited information to hand and the fact that you already consider a scissors lift, why not go for basic hydraulic version?

This could be totally independant of trailing hoses or electric cables by use of a hand pump. This connects directly into a single acting hydraulic ram. Pump it up to the required height and allow its own weight to return when finished. There are numerous manufacturers of this type of equipment worldwide. I would not even consider trying to build your own when you could probably buy one from Ebay e.g. unless this is a project, in which case it is basically like asking you to re-invent the wheel.

Hope this helps...

The link is an example of a lift table.

http://search.harborfreight.com/cpisearch/web/search.do?keyword=lift+table&Submit=Go

I've built several fixtures that sit on the table, for the disassembly/repair of 600kg subassemblies. these fixtures allow 1 person to do heavywork w/out forklifts or bunches of grunt work.

Yes, $139 for a 500 pound lift table. I am not sure I would trust a costly piece to a made in china lift table. but even a high quality one would only be $300-500, much cheaper than re-inventing the wheel.

www.engineersedge.com/mechanics_machines/scissor-lift.htm - 11k -

These guys posted a diagram which helps and solicit your interest.

The cap end of a hydraulic or pneumatic cylinder with a pilot operated check valve connected directly to the port with a suitable nipple presents good integrity. However I would never place myself or my bird dog under or in the way of a 300 kg load propped up by a fluid circuit without a mechanical stop.

Unless your lift distance is great you can lift 300kg (662 lb) at 7 bar (101 psig) directly with a single 3 1/4 or 4" bore suspended from a gantry or boom on wheels for positioning. Another approach might be direct lift between a pair of 63 mm bore rodless cylinders outside the load.

The gentleman who felt a pneumatic cylinder was a potential bomb threat has been watching too many horror movies. Pneumatic cylinders hurl logs that weigh 300 kg from conveyor to saw bed etc. all day, all year in all weather and never explode.

I was assuming a slight lack of expertise on the part of the poser. A pneumayic cylinder lifting an unconfined load is subject to a disasterous load shift and rapid extension. It's not an explosive hazzard, it can, however, be a finger eater.

You've got enough pressure to move the load, but your mention of

'positioning` is troublesome. Due to the nature of an air powered lift,

you've essentially got a spring support. Your load can 'bounce` as stresses

are applied. Also the stored energy in the air system is released a lot more

rapidly under conditions of failure which poses a safety hazzard, especially to

the untrained.

I believe that as a scissors lift gains height the force required to lift the fixed load decreases. If that is correct you will experience one of the difficult characteristics of compressed air. As the load decreases on an air cylinder, the speed increases. The same applies the other way. This is a result of the "critical backpressure ratio". This is a rather obscure term, but responsible for the difficulties in getting the results intended. The ratio for compressed air is 49%. This means that as long as the load pressure is less than 49% of the available, or regulated pressure; the load will move quickly. At 49%, and above, the load slows more as 100% is approached. The problem I see in this application is a gradual increase in the load velocity as the scissors lift extends. If this is ok, then a flow control can be placed in the end of the cylinder that is exhausting as the lift rises. This causes the exhaust air to be metered out of the cyinder. It will be set to limit maximum velocity as the lift approaches it's upper limit. The downside of this is slow speed when starting. You also question safety. This is the most important consideration. There should be a mechanical lock which automatically engages as the full up position is reached. In addition, a pilot operated check valve can be installed in the end of the cylinder which is pressurized to extend the lift. This valve locks air in the cylinder until the retract end of the cylinder is pressurized to lower the lift. These are common, and radily available from a fluid power product distributor. Regarding sizing the cylinder; you need to know the lowest pressure available to the lift, and still be able to lift, the maximum force from the cylinder when lifting the heaviest load. This has to do with angles, and friction. I can't help you there. Maybe the lift manufacture can advise you of this number. Remember that at a given pressure, and flow control valve setting; the lighter the load, the higher the velocity, and the heavier the load the slower. When reaching near 100% of the load it will move very slow, and would jerk. Be careful. You also need to know the time to extend, and the stroke of the cylinder. This is most of the information you need to know. If you aren't discouraged at this point let me know, and I'll give you the rest. There is an air driven pump which automatically reciprocates which would pump oil into a directional control valve, and on to the cylinder. It would allow more controlability, and are available from several manufacturers. If you are buying a scissors lift with the hydraulic cylinder included; why not buy a selfcontained hydraulic power unit? I would still suggest a pilot operated check, and mechanical lock to insure the lift would not fall unexpectedly. Keep children away from this. They manage to do the enexpected.