Understanding Hydraulics This is the beginning of a series of articles aimed at providing general knowledge and helping the layman understand the principles of industrial hydraulics. This can help save your organization many hours (dollars) of downtime and non-productivity while making you look really good to your supervisor. Most hydraulic equipment problems are the result of poorly specified and/or ill-adjusted components. With a solid understanding and application of the information presented here and in following articles, many common issues can vanish. My intent, however, is not to encourage anyone to adjust components or modify circuits beyond their knowledge or capability. Please see Warning at the end of the article. I've included the math formulae primarily for "thinking about it" purposes, but they are certainly effective to calculate actual values. Math is the native language of fluid power, after all.
OK,
The best place to start is the beginning. These first-principles provide the foundation for the articles that will follow, and may seem a bit "unattached" on their own. We have to start somewhere! The full value of this initial intel will become obvious in the context of later articles.
Terminology
Actuator - A mechanical device for moving or controlling a load; a cylinder or motor, for example.
Area - The total amount of surface on a flat plane. Usually measured in square inches.
Force - The measured amount of pressure applied to a given area. The weight of the load (pushing or pulling).
Flow - The movement of oil in a circuit from high-pressure to low-pressure along the path of least resistance.
GPM - Gallons Per Minute. A measure of fluid flow.
Power - The measure of the rate at which work is done over time. Usually expressed in Horsepower or Watts.
Pressure - A measure of force on a unit of area. Measured in pounds per square inch (PSI).
Pressure Drop - The difference in pressure between two points in a system. Pressure is lower downstream.
Advantages of Hydraulics
• Hydraulics is more efficient than an equivalent mechanical system.
• A hydraulic system is generally small and compact but with very large output force.
• Changing the actuator to a different size easily changes the amount of force output.
• Hydraulics is versatile; a single power unit can operate many actuators.
• Most components can be mounted in any position.
• Hydraulic systems are by nature expandable due to modular, component-based design.
• Force limiting is a natural feature to hydraulic systems.
• Stalling the actuator does no harm to the actuator or the system.
• A hydraulic actuator can be started and stopped abruptly without damage to the components.
• Hydraulics provides infinitely variable speed by flow adjustment.
• Both linear and rotary motion is possible. Cylinders provide linear motion. Motors provide rotary motion.
• Abruptly reversible force direction is another desirable feature of fluid power systems.
Fluid Power Rules-of-Thumb
By applying the following basic principles of fluid power, nearly all hydraulic problems become easy to solve.
• Do your own thinking.
• The conclusions you come to must make sense to you.
• Don’t trust anyone (including your instructor).
• Use sketching or pictures to clarify your thinking.
• Think of hydraulic oil as a solid.
• Hydraulics is not a source, but a transmitter of power.
• Hydraulics multiplies force by sacrificing distance.
• Series loads are additive.
• Hydraulics is made up of two primary aspects: Pressure and Flow.
• Oil pressure determines force.
• Oil flow determines speed.
• A pressure drop must exist for oil to flow.
• Pressurized oil always flows from high pressure to low pressure.
• Pressurized oil always takes the path of least resistance.
• Pressurized oil takes the shape of its container.
• Pressurized oil flowing through a line creates friction.
Area
Area may be found by the following formula: Area = Diameter Squared x 0.7854
Double the Diameter = Quadruple the Area. Quadruple area means quadruple force.
See figure 1, Area at bottom of article.
Pressurized oil in a sealed container exerts force. The amount of force is a function of the amount of pressure and the area it works against. Area = Force / Pressure
Pressure
See figure 2, Hydraulic Lever at bottom of article.
Pressure is created by resistance to flow.
Pascal's Law - Pressure applied on a confined fluid is transmitted undiminished in all directions, and acts with equal force on all equal areas, and perpendicular to them.
Pressure is measured in PSI (Pounds per Square Inch). This refers to the number of pounds of force applied to an area measuring one inch by one inch square.
Oil pressure determines force. Increasing pressure increases force.
Hydraulic Pressure may be found by the following formula: Pressure = Force / Area
Pressurized Fluid:
• Flows from high pressure to low pressure
• Always finds the path of least resistance
• Takes the shape of its container
Force
Hydraulics multiplies force by sacrificing distance. See figure 3, Force on Closed Container at bottom of article.
The amount of force exerted by a hydraulic cylinder can be calculated by the following formula:
Force = Pressure x Area
hint: A good aid to remembering this formula, F = PA is Father = PA. Rearrange as needed to find Pressure or Area.
Think of hydraulic oil as a solid.
Hydraulics is not a source, but a transmitter, of power.
Flow
Fluid always flows along the path of least resistance.
Oil flow determines actuator speed.
A pressure drop must exist for oil to flow. See figure 4, Flow, Friction, & Heat at bottom of article.
Pressurized oil flowing through a line causes friction. This energy is dissipated as heat.
Heat and Horsepower
Horsepower is a measure of the energy required to lift 33,000 lbs., one foot in one minute, or to move 550 pounds 1 foot in 1 second. Equal to 746 watts. Think of 7-1/2 100-Watt light bulbs at the same location. Lots of heat!
Energy can be neither created nor destroyed, but may be transformed. In a hydraulic system, energy which does not do work is transformed into heat. Energy not used for producing work is transformed to heat.
Horsepower (HP) is the product of Flow and Pressure: HP = (GPM x PSI) / 1714
1 HP is approximately equal to 1 GPM at 1500 PSI
Next in this Series
Fluid Line Sizing
Reading & Writing Schematics
Basic Common Circuits
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Contributor's Note
Warning: While fluid power is a relatively straightforward science, tremendous forces are generated. There are many situations in which it is prudent to call in an expert, preferably a Certified Fluid Power Specialist. The information contained here can help you to determine when that is necessary. If at any time you don't feel confident in making an adjustment or modifying a circuit - DON'T DO IT!
Always observe all relevant precautions and follow all safety procedures including lockout/tagout.
If you make a mistake with electricity, you can kill yourself. If you make a mistake with fluid power, you can kill someone else. This is frowned upon.
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