Hydraulic Repair Guides

THE PPC EXPERT GUIDE TO: FLUID POWER PRINCIPLES

10/16/2025

Essential Formulas, Definitions, and Troubleshooting for Hydraulic Systems

Part I: Fluid Power Fundamentals

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

A. Defining Hydraulics

Hydraulics is a means of transmitting power. It may be used to multiply force or modify motions.
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.
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

B. Formulas for Force, Area, and Volume

Conversion Factor: To convert volume from cubic inches (cu.in.) to U.S. Gallons, divide the volume by 231.

C. Work and Power

Work is force acting through a distance.

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

2. Power is the rate of doing work.

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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.:

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Pump Input H.P. (Motor Size): To account for pump efficiency (since the pump is not 100% efficient ):

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Part II: Fluid and Component Behavior

A. Hydraulic Fluid Properties

Hydraulic Oil serves as a lubricant and is practically non-compressible.
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).
Atmospheric Pressure: Atmospheric pressure equals 14.7 PSIA at sea level. Pressure gauge readings (PSIG) do not include atmospheric pressure unless marked "PSIA".
Oil Weight: The density of hydraulic oil is approximately 55 to 58 lbs/cu.ft across the common viscosity range.
Pressure from Height: Pressure at the bottom of a one-foot column of oil is approximately 0.4 PSI.

B. Pump and Flow Principles

Pumps Do Not Create Pressure: A pump creates flow, not pressure. Pressure is caused by resistance to flow.
Fluid is Pushed: A fluid is pushed, not drawn, into a pump. Fluid enters due to external pressure pushing it.
Path of Least Resistance: Fluid always takes the path of least resistance.
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.

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C. Pipe and Hose Sizing

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.
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.
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.
Hydraulic Hose: Hydraulic hose sizes are usually their nominal inside diameters, given by a dash number showing the number of sixteenth-inch increments.

Part III: Troubleshooting Chart

Diagnose common system problems quickly using the table below. The checkmark (✓) indicates the most likely cause for each symptom.

Click the link below to access are full guide to diagnosing common system problems:

Full Toubleshooting CheatSheet PDF Here

Don' just Diagnose it, Solve it.
The troubleshooting chart is a great starting point, but diagnosing complex hydraulic issues requires expert precision. If you're experiencing pump noise, overheating, or erratic action, don't risk further damage.

Talk to a certified Specialist

This guide was compiled and verified by the expert team at Progressive Power & Control. We are committed to providing Indiana's industrial sector with reliable parts, systems, and technical knowledge.

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