Hydraulics basics

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Hydraulics basics

The term hydraulics is used to specifically describe fluid power circuits that use liquids—especially formulated oils—in confined circuits to transmit force or motion.

Hydraulic circuits:

Hydraulic brakes

Power steering systems

Automatic transmissions

Fuel systems

Wet-line kits for dump trucks

Torque converters

Lift gates

Pascal’s Law

Pressure applied to a confined liquid is transmitted undiminished in all directions and acts with equal force on all equal areas, at right angles to those areas.

Fundamentals

Hydrostatics is the science of transmitting force by pushing on a confined liquid.

In a hydrostatic system, transfer of energy takes place because a confined liquid is subject to pressure.

Hydrodynamics is the science of moving liquids to transmit energy.

We can define hydrostatics and hydrodynamics as follows:

Hydrostatics: low fluid movement with high system pressures

Hydrodynamics: high fluid velocity with lower system pressures

A column of air measuring 1 square inch extending 50 miles into the sky would weigh 14.7 pounds at sea level.
If we stood on a high mountain, the column of air would measure less than 50 miles and the result would be a lower weight of air in the column.
Similarly, if we were below sea level, in a mine for instance, the weight of air would be greater in the column.
In North America, we sometimes use the term atm (short for atmosphere) to describe a unit of measurement of atmospheric pressure.
Europeans use the unit bar (short for barometric pressure).

Forces are push or pull effort.

The weight of one object placed upon another exerts force on it proportional to its weight.

If the objects were glued to each other and we lifted the upper one, a pull force would be exerted by the lower object proportional to its weight.

Force does not always result in any work done.

If you were to push on the rear of a parked transport truck, you could apply a lot of force, but that effort would be unlikely to result in any movement of the truck.

The formula for force (F) is calculated by multiplying pressure (P) by the area (A) it acts on.

F = P x A

Pressure Scales

There are a number of different pressure scales used today but all are based on atmospheric pressure. One unit of atmosphere is the equivalent of atmospheric pressure and it can be expressed in all these ways:

1 atm = 1 bar (European)

= 14.7 psia

= 29.920 Hg (inches of mercury)

= 101.3 kPa (metric)

However, each of the above values is not precisely equivalent to the others:

1atm = 1.0192 bar

1 bar = 29.530 Hg = 14.503 psia

10 Hg = 13.60 H2O @ 60° F

Torricelli’s Tube

Evangelista Torricelli (1608–1647) discovered the concept of atmospheric pressure.

He inverted a tube filled with mercury into a bowl of the liquid and then observed that the column of mercury in the tube fell until atmospheric pressure acting on the surface balanced against the vacuum created in the tube.

At sea level, vacuum in the column in Torricelli’s tube would support 29.92 inches of mercury.

Manometer

A manometer is a single tube arranged in a U-shape used to measure very small pressure values.

It may be filled to the zero on the calibration scale with either water H2O) or mercury (Hg), depending on the pressure range desired.

A manometer can measure either push or pull on the fluid column. Examples:

Crankcase pressure

Exhaust backpressure

Air inlet restriction

Absolute Pressure

Absolute pressure uses a scale in which the zero point is a complete absence of pressure.

A pressure gauge has as its zero point - atmospheric pressure.

A gauge therefore reads zero when exposed to the atmosphere.

To avoid confusing absolute pressure with gauge pressure

Absolute pressure is expressed as: psia.

Gauge pressure is usually expressed as: psi or psig.

Flow

Flow is the term we use to describe the movement of a hydraulic fluid through a circuit.

Flow occurs when there is a difference in pressure between two points.

In a hydraulic circuit, a device such as a pump creates flow.

A pump exerts push effort on a fluid.

Flow rate is the volume or mass of fluid passing through a conductor over a given unit of time. Flow can be measured in two ways velocity and flow rate.

An example would be gallons per minute (gpm).

Flow rate determines the speed at which a load moves
A constant flow rate will result in a lower velocity when the diameter increases
A constant flow rate will result in higher velocity when diameter is decreased
The velocity of in a hydraulic line is inversely proportional to its cross sectional area

Fluid velocities are generally desirable to reduce friction and turbulence in the fluid. The same analogy can be used for hydraulic cylinders:

An equal flow rate - a small cylinder will move faster than a larger cylinder

Objective is to increase the speed the load moves

Decrease the sectional area of the cylinder
Increase the flow to the cylinder (gpm)

Objective is to slow the speed at which the load moves

Increase the size sectional area of the cylinder
Decrease the flow to the cylinder (gpm)

Measuring Flow

Flow can be measured in two ways: velocity and flow rate. The velocity of the fluid in a confined circuit is the speed at which the fluid moves through it. It is measured in feet per second (fps). Flow rate is the volume of fluid that passes a point in a hydraulic circuit in a given time. It is measured in gallons per minute (gpm).

Flow Rate and Cylinder Speed

Given an equal flow rate, a small cylinder will move faster than a larger cylinder. If the objective is to increase the speed at which a load moves, then:

Decrease the size (sectional area) of the cylinder.

Increase the flow to the cylinder (gpm).

The opposite would also be true, so if the objective were to slow the speed at which a load moves, then:

Increase the size (sectional area) of the cylinder.

Decrease the flow to the cylinder (gpm).

Therefore, the speed of a cylinder is proportional to the flow to which it is subject and inversely proportional to the piston area.

Pressure Drop

In a confined hydraulic circuit, whenever there is flow, a pressure drop results.

Again, the opposite applies. Whenever there is a difference in pressure, there must be flow.

Should the pressure difference be too great to establish equilibrium, there would be continuous flow.

In a flowing hydraulic circuit, pressure is always highest upstream and lowest downstream. This is why we use the term pressure drop.

A pressure drop always occurs downstream from a restriction in a circuit.

Flow Restriction

Pressure drop will occur whenever there is a restriction to flow.

A restriction in a circuit may be unintended (such as a collapsed line) or intended (such as a restrictive orifice).

The smaller the line or passage through which the hydraulic fluid is forced greater the pressure drop.

The energy lost due to a pressure drop is converted to heat energy.

Work

Work occurs when effort or force produces an observable result.

In a hydraulic circuit, this means moving a load.

To produce work in a hydraulic circuit, there must be flow.

Work is measured in units of force multiplied by distance, for example, in pound-feet.

Work = Force x Distance

Energy

There are many forms of energy, which simply means the capacity to perform work. In a hydraulic circuit the objective is to transfer energy. The hydraulic circuit transfers energy from one form to another and form one point to another. The idea is to accomplish this as efficiently as possible and not waste too much by transferring it to heat. In a typical hydraulic circuit, mechanical energy is required to drive a hydraulic pump to create flow and kinetic energy potential in the fluid. Fluid under pressure is the potential energy of a hydraulic circuit. The term PRIME MOVER used to describe the machine that creates the mechanical energy required to power a hydraulic pump

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