Hydraulic Basics

Objectives

Explain basic fluidic principles.
Demonstrate the relationships between pressure, area, and force.

Flow

Flow is the general movement of fluid.

Flow has two components to consider: flow rate and flow velocity.

20 GPM

75.7 LPM

Flow rate is the movement of a specific volume of fluid in a set amout of time. Flow rate is typically measured in U.S. gallons per minute (gpm) or litres per minute (lpm), using a flow meter.

Flow velocity is the distance a specific volume of fluid travels in a set amount of time.

The flow velocity is not measured directly, but is instead calculated using the flow rate and the cross section area of the hose.

Flow velocity is directly dependant on flow rate and hose size.

If we change the flow rate of the pump but leave the hose size unchanged, we can change the flow velocity of the fluid.

If instead we keep the pump size unchanged, but change the hose size we have the same effect.

As flow velocity increases, heat also increases.

This is due to friction.

Friction is caused by the fluid molecules rubbing against the inside surface of hoses and pipes.

Don't take this too literally — it's just for fun.

Laminar Flow

We imagine that fluid flows as a single mass but in reality that is not the case.

At low velocities, fluid flows in distinct separate parallel layers.

Each of these layers is moving at a slightly different rate.

This state is known as laminar flow.

Turbulent Flow

As the velocity of a fluid increases, tiny imperfections in the surface of the flow conductor (hose or pipe) disturb the flow path.

This creates a chaotic state rather than the organized layers of laminar flow.

This turbulent flow (due to friction) causes an increase in heat.

Turbulent flow is evident anywhere in a hydraulic system where bends and restrictions occur.

Keeping hoses and fittings large helps to minimize this effect.

Pascal's Law

Pascal's law states that any pressure exerted on a confined fluid is transmitted with equal force in every direction.

But this is true only as long as the fluid is trapped.

Pressure

Pressure is created either by resistance to flow, referred to as dynamic pressure, or by the potential energy of an object being affected by gravity, known as static pressure.

Static pressure is present when fluid wants to flow but cannot. Gravity is trying to pull this cylinder rod down, but since the valve is closed the fluid in the cylinder is unable to escape.

This trapped fluid gains energy due to the force pulling the cylinder rod down. This energy is the pressure value shown on the gauge.

On the other hand, dynamic pressure is tied to the kinetic energy of a fluid.

Thus as the resistance to flow increases, the pressure increases.

When fluid flows through a restriction there is a pressure drop due to an energy conversion (friction causing heat).

Bernoulli's Principle

Because the total energy of a system must remain constant, Bernoulli's principle states that if there is a decrease in kinetic energy (fluid velocity) there must be a proportional increase in potential energy (pressure).

Surface Area

Surface area is the total exposed area of a solid object.

In hydraulic systems we are concerned with the surface area of components that interact with the fluid.

The surface area of a component can have dramatic effects on the work that the system is capable of!

The FPA Triangle

There is a direct mathematical relationship between the force that a hydraulic system is capable of transferring, the pressure of the system, and the surface area of the component being driven.