THE PPC EXPERT GUIDE TO: FLUID POWER PRINCIPLES

Part I: Fluid Power Fundamentals

Presented by Progressive Power & Control, Indiana’s trusted fluid power specialists.

A. Defining Hydraulics

1. Hydraulics is a means of transmitting power. It may be used to multiply force or modify motions.
2. Pascal’s Law: Pressure exerted on a confined fluid is transmitted undiminished in all directions, and acts with equal force on all equal areas and at right angles to them.
3. Pressure Drop: There must be a pressure difference across an orifice (restriction) to cause fluid flow through it. If there is no flow, there is no pressure drop​ ​

1. Work is force acting through a distance.

Example: Work (in-lbs) = Force (lbs) × Distance (in.).

​2. Power is the rate of doing work.

1. One Horsepower (HP) is defined as 550 ft.lbs/sec or 33,000 ft.lbs/min.

Horsepower Formulas (Input/Output):
​

• Pump Output H.P.:

1. Hydraulic Oil serves as a lubricant and is practically non-compressible.
2. Compressibility: Hydraulic oil will compress about $\text {0.4%}$ at 1,000 PSI and $\text {1.1%}$ at 3,000 PSI(at 120 degrees F).
3. Atmospheric Pressure: Atmospheric pressure equals 14.7 PSIA at sea level. Pressure gauge readings (PSIG) do not include atmospheric pressure unless marked “PSIA”.
4. Oil Weight: The density of hydraulic oil is approximately 55 to 58 lbs/cu.ft across the common viscosity range.
5. Pressure from Height: Pressure at the bottom of a one-foot column of oil is approximately 0.4 PSI.

B. Pump and Flow Principles

1. Pumps Do Not Create Pressure: A pump creates flow, not pressure. Pressure is caused by resistance to flow.
2. Fluid is Pushed: A fluid is pushed, not drawn, into a pump. Fluid enters due to external pressure pushing it.
3. Path of Least Resistance: Fluid always takes the path of least resistance.
4. Cylinder Speed: The speed of a cylinder depends on two parameters: its size (piston area) and the rate of flowinto it. Pressure exerted on the cylinder is not a factor.

C. Pipe and Hose Sizing​

1. Friction Losses: Friction losses (pressure drop) in a pipe vary directly with velocity. The faster the fluid moves, the more pressure loss there is due to friction. 
2. Pipe Diameter: Actual inside diameter (I.D.) of standard pipe is usually larger than the nominal size quoted. A standard conversion chart should be used. 
3. Tubing Size: Steel and copper tubing size indicates the outside diameter. To find the actual I.D., subtract two times the wall thickness from the quoted size. 
4. Hydraulic Hose: Hydraulic hose sizes are usually their nominal inside diameters, given by a dash number showing the number of sixteenth-inch increments.