I put four on my milling machine. When you adjust the feet, it's really easy to strip the teeth off the cast alloy ratchet. Then it's useless. It really needs a bearing between the foot and the screw thread, because it's so hard to turn that when you put an extension bar in the hole provided you strip teeth. The only way to get them to work is to lift the load up a bit to take the pressure off the foot (in my case a 400kG milling machine) and then try to adjust the foot. Good idea, but poor execution