BASICS OF HYDRAULICS
The question “What is hydraulics ?” can be answered in the following way.
Hydraulics is the transmission and control of forces and motions through the medium of fluids. Short and simple.
Hydraulic systems and equipment have wide-spread application throughout industry.
– machine tool manufacturing
– press manufacturing
– plant construction
– vehicle manufacturing
– aircraft manufacturing
– injection molding machines
Prerequisites that hydraulics requires of the user and serviceman:
– knowledge of the basic physical laws of hydrostatics and hydrodynamics
– knowledge of the symbols of hydraulic control elements
– knowledge of hydraulic circuit diagrams
– knowledge of the maintenance of a hydraulic system
Hydraulic to Electrical Analogy
Hydraulics and electrics are analogous, because they both deal with flow, pressure and load. The components in each type of circuit perform similar functions and therefore can be related, a few examples are listed below:
Various forms of energy are converted to accomplish mechanical movement in the injection molding machine. Electrical energy is converted to mechanical energy, which in turn is converted to hydraulic energy to operate and control the moving components of the machine. The hydraulic energy is converted to mechanical energy to achieve the final desired result, which may be “mold clamping pressure” or “material injection”. The figure above summarizes the energy conversions for an injection molding machine. Click on the thumbnail for a larger view.
Pascal’s Law states that a pressure acting on a confined fluid is transmitted equally and undiminished in all directions. In the figure below, a 10 pound force acting on a 1 square inch area generates a pressure of 10 pounds per square inch (psi) throughout the container acting equally on all surfaces.
This principle is important to remember, that the pressure in any portion of an hydraulic system is equal throughout that system. This statement is valid with the omission of the force of gravity, which would have to be added, according to the fluid level. Due to the pressures that hydraulic systems operate at, this smaller
amount need not be considered e.g. a 32 foot head of water approximately equals 14.5 psi. (a 10 meter head of water approximately equals 1 bar.)
Force Transmission in Hydraulics- click thumbnail.
Area and Force
As the clamp piston is moved forward during the clamp close function, the pressure developed acts upon the clamping piston which has a certain size or area.
A basic formula in hydraulics states that pressure multiplied by area to which that pressure is applied equals force. i.e. pressure x area = force
p x A = F
The formula can be manipulated to calculate any one of the three variables p, A or F, if any of the other two variables are known.
p x A = F
F / p = A
F / A = p
Hydraulic pressure is generated when a flowing fluid meets resistance which is generally related to the load that is being moved.
A force is applied via the lever to produce system pressure (p = F/A or F = p x A).
If more force is applied, the system pressure rises until the load moves, if the load remains constant the pressure will increase no further. The load can therefore be moved if the necessary pressure is generated. The speed at which the load moves will be dependent upon the volume of fluid which is fed to the load cylinder. For example, as the mold is opening or closing, the pressure generated in the system represents the resistance of the toggle lever to movement. Adding to that resistance would be the weight (i.e. mass) of the mold and toggle lever and also the friction between the toggle lever bushings and the tiebars. When the two mold halves touch and the toggle begins to straighten out, the increasing pressure
represents that which is required to stretch the tiebars in the generation of a particular clamp force. Similarly when injecting material into the mold the pressure generated in the injection system represents the resistance of the injection ram to movement. Adding to that resistance would be the mass of the injection ram and screw, the friction between all moving components and the resistance of the plastic melt as it is forced quickly into the mold cavity.
In order to safeguard the system, pressure relief valves are installed. The valves serve to limit the amount of pressure that can develop in the hydraulic system since the various hydraulic components are expensive and they are subject to pressure limitations before failure occurs. One characteristic of fluid flow that is important to note here is that flow occurs always in the path of least resistance. Pressure would continue to rise in the circuit consistent with the load being
moved. The pressure relief valve is always set to allow flow to travel through the relief valve well before pressure rises above safe levels and causes damage to the system and its components. In other words, the path of least resistance is employed here to safeguard the system after the other movements have taken place.
An extremely important concept to understand about pressure relief valves is their pressure override characteristics. Pressure override is the difference between the pressure at which the relief valve just starts to crack open and the pressure at the full open position. For direct acting pressure relief valves this pressure differential can be as high as 30% and proportional pressure relief valves range from
10% – 20%.
Another important concept to keep in mind is that of pressure intensification. This law of hydraulics is often forgotten when troubleshooting hydraulic circuits.
For example, if two pistons of different size are connected by a rod, the pressure existing on the smaller area will always be greater. This principle also applies to the cap side and the rod side of a normal double acting piston.
If P1 = 1,000 psi and A1 = 10 square inches, then F1 = 10,000 pounds of force.
If F1 = 10,000 pounds of force and if A2 = 5 square inches, then P2 = 2,000 psi.
Speed in Hydraulics
The speed of a hydraulic component can be calculated based on the formula below:
For example, given the conditions below the injection piston, therefore the screw, will move at 3.85 inches per second. However, this speed will not be possible if the pressure relief valve opens.
Click on thumbnail above
As well as understanding the concept of speed in hydraulics, it is also important to have some insight into flow characteristics. For example, the drawing below shows that when oil is flowing through different diameter pipes an equal volume flows in an equal unit of time. If that is true and if the shaded quantity Q1 equals
the shaded quantity Q2, then velocity V2 must be greater than velocity V1.
Click on thumbnail above
As the diameter of the pipe decreases, the flow rate will increase. Specifically, if the pipe diameter decreases by one half in the direction of oil flow, the cross sectional area will decrease by four times, and visa versa. Oil flow velocity through different pipe sizes can be calculated using the formula:
The same gallons per minute will have to travel 4 times faster through the smaller pipe.
Another important concept in hydrodynamics is how fluids flow based on certain critical flow speeds or as the result of meeting restrictions to flow such as bends in the pipe or system components.
One goal in the initial design of hydraulic power transmission systems is to encourage laminar flow as much as possible since an increase in turbulence will increase flow resistance and hydraulic losses as well. The diagram below illustrates the concept of turbulent flow.
Although turbulent flow is wasteful in most hydraulic applications, it is desirable to have turbulence in the oil flow as it travels through the heat exchanger for cooling purposes. If turbulence exists as the oil flows through the heat exchanger, more of the oil molecules come into contact with the heat exchanger cooling tubes and more efficient cooling is the result.
One of the main advantages of hydraulic based systems is that the oil flow direction is easily controlled. The drawing below shows a piston being extended, held stationary and then retracted, simply by changing the position of a directional valve. Even though the drawing is simple in nature, it still demonstrates the principle involved in directional control. In addition to simple directional control valves, we also employ proportional directional control valves on some machines to control the clamp opening and closing function.