Over 300 years ago, Blaise Pascal, a French scientist, discovered the principle behind hydraulics. He found due to the fact that liquid cannot be compressed, when pressure was put on a confined liquid the liquid transmitted the same pressure in all directions. If a container holding liquid has an outlet for the liquid to escape, the liquid will try to escape at a pressure equal to the pressure exerted into the container.
Forklift Hydraulics Operation
Forklift hydraulic systems, using cylinders, operate similarly. The hydraulic fluid is pumped from a reservoir or sump, into a system of tubing. The pressure with which this fluid moves is equal to the flow introduced by the pump. This flow is measured in gallons-per-minute (GPM). Since the liquid will not compress, the pressure introduced by flow is harnessed to do work.
The cylinders are sealed tubes with a rod inserted. As the hydraulic lines feed pressurized fluid into the tube, the rod is pushed out as the fluid seeks the path of least resistance. The amount of work the rod can perform is roughly equal to the GPM of the fluid pump, minus pressure lost to resistance.
Resistance is introduced because the fluid must run through hoses and connections on various hydraulically powered components. Each hose loop and connection introduces resistance, and as resistance to the pressure builds, energy is lost as heat.
- In most forklifts, the sump (tank) is integrated into the frame. Since the fluid is heated by resistance from the system, this design enables the frame to act as a heat sink – a cooling system. The heat in the oil is drawn off by the entire truck.
- The pump produces a constant, even flow of hydraulic fluid to supply the control valve. Most forklifts use a gear-type pump that consists of a pair of rotating meshed gears which push the fluid in a direction opposite their rotation.
- The control valve is the brain of the system. It controls where the fluid is directed by using spools. It also controls the amount of fluid directed into a system line.
- A relief valve protects the entire system in case of pressure overload or other trouble. For example, when a cylinder reaches the end of its travel, pressure in the system will raise. The spring-loaded relief valve senses excessive pressure build-up and opens, allowing oil to flow back into the sump.
- Return lines return fluids to the sump. An electric truck has two hydraulic pumps, one for upright lift, tilt and auxiliary, plus a separate pump for the power steering cylinder.
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