BY BILLY LEACH JR.
The hydraulically powered rescue tool was introduced to North American rescuers around 1970. Generally, a complete rescue system includes a power unit, spreader, cutter, ram, combination tool, and assorted accessories.
Power unit. The hydraulic power unit usually includes the following components: a hydraulic fluid reservoir, hydraulic fluid, a hydraulic pump, a pump power source, directional and relief controls, and hoses and connectors. The hydraulic power unit operates the hydraulic pump to supply pressure and flow to the hoses, directs the hydraulic fluid flow to the selected outlet, stores a quantity of hydraulic fluid, and provides a means of relief from system overpressure.
The correct hydraulic fluid is necessary for proper operation. Keep the fluid clean and free from contaminants. Check the tool’s fluid level often, and change it as directed by the manufacturer, purging the hoses of old fluid. Contaminants such as debris can clog screens, filters, and valves, thus affecting system operation.
In a common power unit configuration, a gasoline (photo 1) or electric (photo 2) motor powers the pump, which is submerged in hydraulic fluid. The pump pressurizes the hydraulic fluid in the hoses until the fluid finally reaches the rescue tool. To create the force necessary to perform the work, adequate fluid volume, pressure, and flow are needed. A common formula to determine the force supplied is F (force) = P (pressure) × A (area). For example, a 10 inch/pound pressure applied over a 10-square-inch area would equal 100 pounds of force. This formula is the basis for hydraulic rescue tool systems; the larger the area (piston) and the greater the pressure, the greater the force supplied.
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Pump. Generally, hydraulic rescue tool systems usually use either radial piston or axial piston pumps. A reciprocating piston and two check valves move the hydraulic fluid. As the piston withdraws from the piston cavity, it creates a vacuum that draws hydraulic fluid past the intake check valve. When the piston is pushed back into the piston cavity, the intake check valve closes and the hydraulic fluid is pressurized until it is slightly higher than system pressure. Hydraulic fluid is then forced past the discharge check valve into the system. The discharge check valve closes once no hydraulic fluid is moving past it.
A piston pump may include one or more pistons and check valves, depending on the pump’s size. The piston units may be arranged radially or axially. If arranged radially, the pistons are driven by an eccentric that is attached to the motor shaft. The number of piston units depends on the amount of flow required from the pump and engine rpm.
Axial piston pumps are actuated by a motor-driven cam plate. The cam drives the pistons down, and the springs return the pistons on the intake portion of the stroke. These pumps generally operate in two stages: high-flow/low-pressure and low-flow/high-pressure. The stage used depends on what the tool needs for the situation. For faster operation, the first stage is used when the tool meets less resistance. An example is when a tool is spreading minor amounts of sheet metal. When the tool meets greater resistance, the second stage is engaged to provide higher pressure with less flowfor example, when force is needed to open doors and cut objects.
Do not exceed the working pressure of the lowest rated component in the system. Use hydraulic pressure gauges to indicate pressures within the system. Test hydraulic systems for both static pressure and fluid flow with an accurate test gauge. The results should be within the manufacturer’s specifications; if not, have an authorized service technician evaluate the system (photo 3).
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Valves.Valves control the hydraulic circuits. They can divert the flow of hydraulic fluid from one area of the system to another; increase, reduce, or maintain the pressure; and start, stop, or reduce hydraulic fluid flow.
Some power units may be able to operate two or more tools simultaneously, two tools intermittently, or just a single tool. Generally, this depends on the pump configuration and the hydraulic fluid reservoir’s capacity.
Hose. To transmit the fluid under pressure, it is pumped through hoses and connectors to the tool. These hoses are generally nonconductive reinforced plastic or rubber, varying in length and color. The hoses themselves carry a working pressure and burst pressure rating, expressed as a ratio 4:1 means the burst pressure is four times the working pressure. Make sure you fully tighten hydraulic couplers; loose couplers will act as a partial or complete restriction, allowing little or no hydraulic fluid flow. Do not use the hydraulic hoses to lift, carry, or move any hydraulic component. Avoid sharp bends or kinks in hydraulic hoses. If pressure is applied to a bent or kinked hose, the hydraulic fluid flow restriction will cause severe backpressure, which may lead to internal damage of the hose lining and cause premature hose failure. Usually the remainder of the system is protected with check valves that minimize backpressure. Hoses and couplers deserve special care and attention, especially cleaning. Closely inspect the hoses and couplers for physical, thermal, and chemical damage. In photo 4, the hose itself has swelled at the coupling, indicating that the inner liner has failed because of mechanical damage from persistent flexing.
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Friction.Whenever something moves, friction is inevitable. If hydraulic fluid moves from one point to another in a system, a certain amount of inefficiency exists because of friction. Friction, change in direction, and restriction result in a loss of energy in the form of heat. Loss of energy is indicated by a reduction in system pressure, and all reductions are additive. For example, it may take five pounds per square inch (psi) of pressure to pump fluid through 10 feet of hose, another 10 psi for a change of the direction 90°, and 25 psi for a restriction created by a coupling. With the hydraulic fluid moving through this system at a constant rate, the pressure at the end of the system will be 40 psi less than at the beginning.
Hoses have two considerable restrictionsthe couplings attached to each end. A hose with a ¼-inch internal diameter may have couplings with a 1⁄8-inch internal diameter. The area of the 1⁄8-inch-diameter opening is one-fourth the area of the ¼-inch opening. Therefore, the hydraulic fluid must suddenly quadruple in velocity when moving through the smaller opening. This results in transference of energy, usually resulting in a loss of pressure within the system. Avoid excessive heat, which will soften packing and seals and result in leaks.
RESCUE TOOL TYPES
Spreaders. The heart of spreaders is a piston inside a cylinder. The hydraulic pump builds pressure within the system, which moves the piston, which in turn moves the spreader arms. The control valve regulates the spreader arms’ movement (photo 5). This is a directional valve, which allows fluid to flow through, opening or closing the spreader arms. Compare the surface areas of the actual piston head. If forcing fluid with the larger/flat side (spreading), the pressure is much greater than forcing fluid from the opposite end (closing/pulling) (photo 6). You can attach various tips to the spreader arms to perform various functionse.g., spreading or pulling.
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Cutter. A cutter’s operation is similar to that of the spreader in that hydraulic fluid forces a piston to move the cutter blades through linkages connected to both blades. Various cutting blade styles (e.g., curved, straight) are available. Keep the cutter blades and the center bolt clean, and regularly inspect them for wear (photo 7). Ensure that the cutter bolt is torqued correctly according to the manufacturer’s specifications. In photo 8, damage from incorrect torque and debris physically damaged this cutter center bolt and caused scoring on the bolt and blade, necessitating replacing the center bolt and blades.
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Rams.Rams are sophisticated hydraulic jacks with a traveling piston enclosed in a cylinder; they may be either single or double acting. A single-acting ram employs hydraulic power to extend the piston, generally relying on a spring to retract it. A double-acting ram employs hydraulic power to extend and to retract the piston. Photo 9 is a cutaway of a hydraulic ram displaying the inner components.
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The ram’s force depends on the hydraulic working pressure multiplied by the piston’s surface area. Rams have the least structural stability when fully extended, either pushing or pulling. At full extension, the piston has little surrounding support, making it vulnerable to overstressing, especially in the area of the stop ringa machined part that prevents the piston from extending totally from the ram body. Off-center loading of the ram puts considerable force on the piston and seals and occurs when the piston and load are not in axial alignment, such as in a dash roll technique.
Hydraulic tools will provide reliable continuous service if you regularly inspect and maintain them according to a schedule, such as the one outlined in Figure 1. Figure 2 is a list of common troubleshooting problems and solutions.
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BILLY LEACH JR. has been active in emergency services since 1976, combining career and volunteer experience. He is the developer of and senior presenter for Big Rig Rescue, an extrication program focusing on incidents involving heavy trucks. He has presented at numerous fire/rescue training seminars. Leach is coauthor of the book Big Rig Rescue on heavy truck anatomy and extrication.
