Why do we use piston in a hydraulic car lift ?can we use something else instead?
With hydraulics, the piston is the most efficient approach. You can use a screw type device. but you would have to find some way to turn the screw, such as an electric motor. You can also use a lever and fulcrum. The lever would have to be pretty long.
It is the simplest and most effective solution. It gives a powerful, directed force. You could use a membrane or a bag that you pump full of water. These are often used in the open where a portable lift is needed, say at accidents, but are not as stable or as powerful as a hydraulic piston and barrel, because they cannot take the same level of pressure.
Using a piston allows design calculations for critical stresses in a pressure vessel to be applied to a particular region where the intensity of the stress is concentrated, and this allows for safety-conscious design while allowing for other less stress-vulnerable components to be designed using less material. Other than safety, there also may be maintainability issues involved which would dictate the lift mechanism's design. The 'form factor', even, of a single large vertical piston places the source of equipment failure in a single mechanism, solidly mounted below the lift in a location pretty much out of the way of a mechanic's normal maintenance activities. An overhead crane could do the same job and still be 'out of the way', but you could see for yourself why these weren't traditionally used. The auto frame needs to be supported at points directly on the frame, not on the body, and the single fixed support frame is the best solution to this situation (unless you're working on an exhaust system, of course). There isn't any reason why you couldn't machine a large screw to lift the vehicle instead of the piston, but you would use up so much material that you'd spend the rest of your years trying to pay for it. That's an economic issue, both for manufacturers and buyers.
