Like spreaders, cutters have a mouth that opens and closes. However, cutters are more like big chompers that bite through metal and other vehicle materials. If you've ever seen this device in action, you know that it can snap a car-door post like a twig in a few seconds. As the pressure comes down on the door post, the cutters just snap right through it.
Cutters typically have an aluminum-alloy housing with forged, heat-treated steel blades. The piston and piston rod are often made of heat-treated alloy steel. The cutters are used to cut or shear through materials such as sheet metal and plastic. Most often, they are used to cut through automobiles and other vehicles to free trapped passengers. Like the spreader, it can run off a gasoline-driven power unit. Jaws of Life systems can also be powered electrically, pneumatically or hydraulically.
Instead of arms, the cutter has curved, claw-like extensions that come to a point. Just like in the spreader, hydraulic fluid flows into a cylinder, placing pressure on a piston. Depending on the side of the piston that force is exerted on, the claws either open or close. When the piston rod is raised, the claws open. As the piston rod lowers, the claws of the cutter come together around a structure, such as a car roof, and pinch through it.
Cutters come in different sizes, but let's look at the Hurst Jaws of Life ML-40 model as an example. This particular model gives the operator:
- 12,358 pounds (60 kN) cutting force at the blade center
- 22,455 pounds (99.9 kN) cutting force at the notch
- 4.25-inch (10.8-cm) cuts
If you understand the operation of the spreader and cutter, the ram is going to seem about as complex as a pair of scissors (if scissors had hydraulics, of course). The ram is the most basic type of hydraulic system: It's just a matter of using hydraulic fluid to move a piston head inside a cylinder to extend and retract a piston rod. If you look at some heavy construction equipment, like a backhoe loader, you'll notice that rams are used to control the boom arm.
In the image above, you can see that the piston rod that extends outside the cylinder is actually moved by a piston head inside the cylinder. There is fluid on both sides of this piston head, fed by two different hoses. If the force is greater on the blue side, the piston moves to the left; if it is greater on the orange side, the piston moves to the right. All you have to do to change the direction of force is stop pumping oil to one side and start pumping it to the other.
The ram's function is to push apart sections of the car (or other structure). For instance, a rescue worker can place a ram on the door frame and extend the piston to push the dashboard up, creating enough space to free a crash victim.
"This is what we call a dash bow up, where the steering wheel or part of the dash has come down on the patient. We would actually take that [ram] and roll that dash forward," said David Price of the Bay Leaf Volunteer Fire Department.
Hydraulics play an important part in many of the machines around us, but none may be as vital as the equipment known as the "Jaws of Life." These devices have been called upon to save thousands of lives in situations where a few seconds could mean the difference between life and death.
For much more information, check out the links on the next page.