daj349

you might need to do some reading on pneumatics. I like the project idea but it could be very dangerous if not done correctly. Here are a few things you should consider:

-A safety factor for the amount total force the cylinders will produce. If you're lifting a 3000lb car with 3300lbf then it won't be vary stable when it's off the ground and take a bit of time to lift up. Air is compressible, unlike hydraulic fluid, and the cylinders are gonna have some give without a large difference between their force and the car's weight. I'd say you should go with at least 4000lbf total, especially if someone is gonna be under this thing. You also need to consider the weight distribution of the car. Not every cylinder will be loaded equally by the car.

A 2.5" bore air cylinder is the internal bore which experiences the driving pressure. The shaft diameter will be much less. You need to make sure that the shaft diameter and material is strong enough to not fail in bending, compression, or shear. This can be calculated using the shear and tensile strengths of the shaft material along with the dimensions. Sounds harder than it is but you should also consider a large safety factor here. Similar material strength calculations should be done for however you mount the cylinders to the car.

Yes, you will need a single acting cylinder with a return spring.

Bimba is a big name in air cylinders, I've never had an issue with their stuff

The force the cylinder produces is the air pressure multiplied by the area of the bore. (pi/4)*(bore)^2*(pressure) 2.5" bore is closer to producing 4.9 times the line pressure, I wouldn't round up on this..

I would run a regulator in front of your cylinders too so they receive consistent pressure. Your compressor isn't gonna give a constant pressure at it's maximum rated pressure especially when you try to fill the cylinders. Shoot for 60 or 70% of what your compressor puts out. Also, this will let the system work consistently with any compressor that runs above what you set your regulator at.

You need to control the cylinder speed with flow control values or you're gonna crack the concrete when you put full pressure to a depressed cylinder that has no opposing force. It would be like a jack hammer. haha

Flow rate metering values or equal line lengths between cylinders would also be a good idea. This would help with actuate at similar rates which would make the car more stable as it rises and put less bending force on the cylinder shafts. Bending forces on a linear actuator are bad news.

The cylinders also need to be equidistant from the ground. The shaft needs to be perpendicular to the ground when raised or the shaft will experience bending forces.

That's all I can think of off the top of my heads. Good luck with the project, I wanna see what you come up with!

EDIT
Also, you can use the butterfly valve you bought to trap pressure in the cylinders after they are aired up. That way the car won't fall if there is a failure in the compressor or the feed line. (someone tripping over it and pulling it out, etc.)