Yellow Pages Basic Hydraulics

Oil Flow A hydraulic pump produces flow. Flow is the amount of fluid coming out of the pump. Pressure Pressure occurs when there is resistance to flow. Pascal's Law Pressure applied at any point upon a confined liquid is transmitted undiminished in all directions (Fig.1). This means that when more than one hydraulic cylinder is being used, each cylinder will pull or push at its own rate, depending on the force required to move the load at that point (Fig. 2). Cylinders with the lightest load will move fi rst and cylinders with the heaviest load will move last (Load A), if the cylinders have the same capacity. To have all cylinders operate uniformly so that the load is being pulled or pushed at the same rate at each point, control valves (see Valve section) must be added to the system (Load B). Force The amount of force a hydraulic cylinder can generate is equal to the hydraulic pressure times the “effective area” of the cylinder (see cylinder selection charts). Use the formula F = P x A to determine either force, pressure or effective area if two of the variables are known. Cylinder Oil Capacity The volume of oil required for a cylinder (cylinder oil capacity) is equal to the effective area of the cylinder times the stroke. Usable Oil Capacity The amount of hydraulic oil in the pump’s reservoir which can be used to activate one or more cylinders. Cylinder Speed Pressure applied at any point Cylinder speed is determined by dividing the pump fl ow rate by the cylinder effective area.

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