If you've read How Hydraulic Machines Work, you know that hydraulic equipment is based on a simple concept -- the transmission of forces from point to point through a fluid. Most hydraulic machines use some sort of incompressible fluid, which is a fluid that is at its maximum density.
Oil is the most commonly used incompressible fluid for hydraulic machines. However, the Jaws of Life equipment uses a phosphate-ester fluid, which is fire resistant and electrically non-conductive. At a crash scene, this type of synthetic fluid is favored over conventional oil.
In a simple hydraulic system, when a piston pushes down on the oil, the oil transmits all of the original force to another piston, which is driven up
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In a simple hydraulic system, when one piston is pushed down,
another piston is pushed up.
Click on the arrow for a demo.
The Jaws of Life equipment is some of the most unsophisticated hydraulic machinery, because there are very few parts involved in making the devices work. In the cutter and spreader, a portable engine pumps pressurized hydraulic fluid into the piston cylinder through one of two hose ports. A typical Jaws of Life machine uses about 1 quart of hydraulic fluid. An operator-controlled valve switch controls which port the fluid enters through. If it enters one port, the fluid forces the rod up and opens the arms of the spreader or blades of the cutter. The operator can then toggle the switch and cause the rod to retract, closing the arms or blades.
In the next two sections, we will take a closer look at the types of equipment that are generally referred to as the "Jaws of Life."