What is the mechanics of a car jack?

Answer 1

There are a number of different types of "car jack" in use. One is hydraulic. They come in the shape of a bottle, or are built into a trolly (the "floor jack") or the like. By operating the handle, which is a lever (a simple machine), fluid is compressed and routed to an actuating cylinder. This results in lift. (More on the principles involved can be found in other questions on this forum dealing with hydraulics.) In a friction jack, the handle (that lever) lifts the load by simple mechanical amplification. The handle provides mechanical advantage. A lot of handle movement on a lever with the fulcrum right up against the load side of that lever will translate into lots of amplification. True, the load only moves a little bit, but that's the trade off; that's the way a lever works. We exchange the extended application of force on the handle to concentrate that force on the load and move it only a little bit. When we release the pressure on the handle, a "wedge" jams the load on the lift shaft so it can't go down. We can then reset our handle and apply another increment of force. Short or long strokes of the handle can be used. The original Volkswagons came with this kind of jack. Another type of jack similar to the friction jack is a ratcheting jack. It uses "steps" in the jack shaft and a little spring-loaded arm, lever or cam (a pawl) which swings into place to prevent the lift body from sliding back down the lift shaft after each lift increment. This jack "clicks" at each full stroke of the handle. A complete stroke must be made to lift the load to the next "step" on the jack shaft. This is a feature that makes this jack different from the friction jack. The handle-as-a-lever idea remains the same. In the scissors jack, a long screw (which is an inclined plane - another simple machine) is turned with a handle. On one end of the screw is a collar that fixes the screw in place but permits it to turn within it. On the other end is a nut, a threaded receiver for the operating screw. Turn the handle in the correct direction and the screw threads itself into the nut. This pulls the middle of an articulated frame together and forces the ends of the frame apart. On the ends is a base, wich is put on the ground, and a "pad" for use on a "hardpoint" on the underbody of the vehicle being lifted. There are a couple of other types of jacks (like an air jack that uses compressed air to inflate a bladder and lift a load) and also variations on the theme out there, but they are uncommon. Additionally, an electric motor, compressed air or hydraulics (or a combination of these) can be used to drive a friction or screw jack. We can use compressed air to drive a hydraulic jack so all we have to do is squeeze a little lever to admit compressed air to the primary side of the hydraulic system. This would be called an air over hydraulic jack. A look at any large automotive parts and accessory supplier's catalog (there are a number of them posted on the web) will allow an investigator to actually see the different types of jacks, though the operating mechanisms probably won't be visible. Hopefully this explanation and the pictures will permit said investigator to noodle out the way the jack works. Think it through and it should be something that can be understood without a lot of difficulty. Certainly a peek at the jack in your current car (or the family car) may be in order here.