April 10, 2012

THE ADVANTAGES OF HYDRAULIC CYLINDER

Hydraulic power has been in use in various primitive forms for thousands of years. In the 19th century, though, with the onset of the industrial revolution with its accompanying growth in technology, the science of hydraulics has been developed and perfected to the advanced state we see today. In fact, hydraulic equipment is so commonplace that few people give much thought even to the impressive display of power and motion shown by a large hydraulic excavator, a road grader or a telescopic crane. Yet, hidden in the bowels of those machines is a form of technology without which we would hardly recognize our world.

 Above: Rugged powerful hydraulics are a common sight.


Why are hydraulic cylinders used in machinery? What advantages do they have over other forms of power? Are their disadvantages to using hydraulic equipment and how are these overcome by engineers and equipment users?

Hydraulic power is usually called Fluid Power by those in the industry because it harnesses the qualities of fluids to transmit its power and convert power into force and motion. In hydraulic equipment that fluid is usually hydraulic oil, though in some rare circumstances water is used. (Since compressed air and steam are also considered fluids, in that they are media that flow and transmit power, they are also lumped in as part of the science of fluid power, though their characteristics differ from those of hydraulic fluids.)

Fluid power is a form of power that is always a slave of another form of power. This means that hydraulic systems need a source of external power to run the hydraulic pump which pressurizes the oil. This external power source is usually an electric motor or a gas or diesel engine. It could be said therefore that fluid power really converts the power of electricity or combustible fuels into an easily controlled form of force and motion. It is with this idea in mind that many companies in the field of hydraulics refer to their area of expertise as "fluid power and motion control".
Fluid power is used by engineers and machine designers because it has a number of advantages over other forms of power transmission and control.


The 5 Advantages of Hydraulic Cylinders

1. Power Density
One of the primary advantages of fluid power is its incredible power density. This means that a very small hydraulic device can produce a very large amount of usable force for its relatively small size. This powerful device can be easily built to fit into the tight confines of modern machinery.

For example, a hydraulic cylinder with a diameter of only 1" can produce an output force of 900 lbs when pressurized by hydraulic fluid at 1000 psi (which is very typical in common hydraulic systems). In comparison, an electrical solenoid actuator of comparable diameter could only produce a linear force of perhaps 4 lbs. In addition, the hydraulic cylinder would be able to continually produce this large force output for the entire length of its bore. The electrical solenoid can produce its force for a short distance (1/8") after which its output force begins to rapidly drop off.

 Above: Even a small bore hydraulic cylinder can produce enormous forces.

The other type of linear motion actuator used in industry is an electric motor driven worm gear. The worm gear converts the rotary motion of the electric motor into linear motion. The force output of this type of actuator is much less than a hydraulic cylinder. In fact it is usually less than a hydraulic cylinder of the same physical size working at only 100 psi.

One reason that hydraulic actuators have such a large power density is because the actuator itself is a separate component usually mounted remotely from the hydraulic power supply unit itself. Although the actuator is very small and powerful, the hydraulic power supply unit is considerably larger but can be mounted some distance away from the work being done. The hydraulic power unit will usually consist of a large reservoir of oil, the hydraulic pump, the control valves and fluid filters, as well as the primary source of power, an electric motor or a gas or diesel engine. Hydraulic fluid under intense pressure is conveyed from the power unit to the actuators via hydraulic lines in the form of pipes, tubes, and/or hoses. This remote generation of fluid power is really the secret to hydraulic power.

In comparison, electric actuators usually have the electric motor closely coupled to the worm gear drive. In addition, a bulky gear reduction box is often required to reduce the shaft speed of the motor and boost the motors output torque. All of this combines to make an all electric actuator somewhat bulky while producing less linear force than its hydraulic counterpart.

2. Range of Motion or Stroke
The above power density comparison examples highlight another advantage of hydraulic actuators, the length of motion produced can be quite long. Small industrial hydraulic cylinders used in a clamping operation may only have an output motion (called stroke) of 1/4". Other hydraulic actuators may have strokes numbering several inches or several feet. In the heavy equipment industry, large hydraulic cylinders are produced that have power strokes of 20, 30 or even 50 feet.

Above: Hydraulic cylinders produce consistent power over long strokes.

The ultimate record may be held by the two massive hydraulic cylinders used in the "Lift Locks" on the Trent Canal in Peterborough, Ontario. In this massive pure water hydraulic system, hydraulic cylinders buried deep in the ground lift boats still floating in closed locks 98 feet vertically so they can continue their journey along the waterway.

Other types of industrial actuators often have limitations in producing the kind of force and motion accomplished so easily by hydraulic actuators.

A cable driven system, such as a winch, is often limited in stroke by the complex system of force multiplying pulleys. A long stroke cable actuator is usually used only in the pull direction and cannot reverse and also then push the load.

An electric motor driven worm gear drive is limited both in power and stroke. The power is governed by the size of the motor and the arrangement of teeth on the drive gear and worm gear. A worm drive actuator with an output force of several tons would require an enormous electric motor to drive it. In addition, a very expensive worm gear shaft would need to be machined that would accommodate the huge output force. A long stroke worm gear actuator suffers from harmonic vibration problems and also worm gear shaft sag. These inherent problems limit this style of actuator in output force, stroke and speed for most practical applications where hydraulic actuators excel.

Another form of motion that is achieved relatively easily by hydraulic cylinders is that of the telescoping actuator. A telescoping actuator is able to extend to a length several times longer than its fully retracted length. Telescopic hydraulic cylinders accomplish this by nesting several cylinder bodies inside each other. These multistage telescopic cylinders are available to produce extremely long strokes and incredibly large forces that electric actuators simply cannot achieve.

 Above: Large bore telescopic cylinders.

3. Ruggedness
A hydraulic cylinder is, for most applications, a very rugged device. If a hydraulic cylinder is overloaded so that it can not produce enough force to move a load, it simply stops. It will not burn out, overheat, or suffer ill effects from over exertion. It will keep pushing as hard as it can but it will never burn out.

This can not be said of electric devices such as solenoids or electric motors.

If the armature of an AC solenoid is held from fully stroking, it is kept in a state of high inrush current in its coil winding. If held for a long time, heat will continue to build up and eventually it will melt the insulation coating its wires. Coil burn out soon follows. The solenoid fails in service and must be replaced.

Similarly, an electric motor, when it stalls, or has the output shaft locked in position, enters into a state of very high current flow through its windings. This high current flow produces heat which in turn melts the insulation on its windings and the motor burns out. Although most large motors have built in heat sensors and current limiting devices to protect them from burnout, these in themselves will cause the actuator to fail in service until these protective devices are reset or the motor cools sufficiently. In the meantime, the machine is not running.

Hydraulic actuators do not suffer from these same problems. The hydraulic system is equipped with relief valves to prevent over pressurization. If a cylinder stalls in mid-stroke, the pressurized oil is redirected back to the reservoir through a valve that will automatically reset when the high pressure state passes. The motor producing the hydraulic pressure is thus protected if the actuator stalls. It will not burn out. In more complex hydraulic systems, pressure compensated pumps automatically adjust pressure and flow to meet the system demands thus protecting the motor from overloads. A hydraulic cylinder is thus always ready to go back to work after an overload.

4. Economy
A hydraulic cylinder is relatively easy to manufacture to the exact range and speed of motion required by the end user. Different stroke lengths simply involve cutting the right length of barrel for the piston to slide through and the right length of output shaft (piston rod). The other components of a hydraulic cylinder generally remain the same for different stroke lengths and speeds that will be required of the actuator. Common parts such as pistons, heads and end caps, rod bearings and seals can thus be mass produced with great economy. The output speed of the hydraulic cylinder is usually controlled by the control valves in the fluid circuit. The same cylinder can move at a wide variety of speeds.

Above: Hydraulic pistons easily mass produced with exact repeatability.

This is not so of electric actuators. Solenoids, as previously mentioned, are limited to very short strokes in the fractional inch range. Electric linear drives require custom matching of the gears to the general range of speed and thrust that the actuator will be asked to provide. A high speed electric actuator will have a coarse thread worm gear that will thus limit its output force. A low speed actuator will have a finer thread and produce more thrust but move slower.

Variable speed electric drives require expensive electronic control systems that are delicate and must be protected from harsh environments. In contrast, hydraulic actuators can have their speed controlled by a simple, rugged, inexpensive mechanical valves. Computer controlled hydraulic cylinders can have their speeds controlled by electronic servo valves that are more expensive but also retain a high level of durability.

In addition, the worm drive shaft of an electric actuator requires a complicated machining process to produce the long grooved shaft. This machining process must be customized for each total output stroke. For this reason, manufacturers of electric actuators are reluctant to customize. They will offer standard stroke lengths in common sizes. Anything outside of this range is subject to very high surcharges and set up fees.

5. Power Storage, Emergencies, and Failure Modes
Hydraulic systems have another advantage that gives them an edge in many applications. Hydraulic power is easily stored for large surges of power, emergency use, or use during power failures.

A certain industrial application may require the application of an enormous amount of power but for a very short period of time. That short period of time may involve 100's of electrical horse power. To supply it directly with an electric motor would require a very large motor, a very large power draw and a very large capital equipment cost.

That same task can be accomplished with a cleverly designed hydraulic system using a much smaller electric motor. The motor would work continually at a much lower horse power but working to store the hydraulic energy in devices called accumulators. Hydraulic accumulators store pressurized fluid volume for release when required. The end result is a system that will accomplish the sudden large amounts of work but at a much lower cost in a more compact machine.

The ability to store hydraulic pressure in event of an electric power failure or some other system failure, can be employed to the users advantage to design safety systems for failure modes. After a power failure, hydraulic systems can still close gates, release moulds, open doors, close gates, and so on. In aircraft systems, stored hydraulic pressure will enable the pilot to lower the landing gear even in the event of a total loss of power. Critical systems can even be equipped with hand pumps to provide the ultimate power back up.


What Weaknesses?

Hydraulic systems are not perfect or immune to failure. Yet, you will see hydraulic cylinders working in the most difficult environments on earth from the frozen wastes north of the artic circle to blistering hot sands of Arabian deserts. Obviously, they can be designed to meet these inhospitable conditions.

Apart from the rest of the hydraulic system, the key area of most hydraulic cylinders that needs to be protected is the rod gland where the piston rod extends out through the end of the cylinder.

The rod gland area provides a series of seals that keep the pressurized hydraulic fluid from leaking out even as it allows the piston rod to move freely back and forth. This area also has a seal, called a rod wiper, that prevents external contamination from entering the cylinder through the rod gland. Also installed in this part of a cylinder is the rod bearing, which along with the piston bearings, supports and guides the extended shaft . The rod bearing must be able to accommodate the load attached to the output shaft and any side forces applied to the shaft while it cycles back and forth.

Above: Cylinder rod glands showing seals and wipers.

This area is thus the most likely place where a cylinder will suffer failure usually due to seal failure, bearing wear, or ingesting contaminants. The surface of the rod that enters the rod gland must be kept smooth and clean in order to preserve the seals and the bearing. In especially harsh and dirty environments, special components such as metal rod scrapers or protective rod boots can be installed to protect the rod gland area.

The other common source of failures in hydraulic actuators is actually not from the cylinder itself but from the hydraulic hoses and fittings attached to it.

Hydraulic hoses must contain the high pressure fluids flowing from the pump to the actuator. These hoses usually consist of alternating layers of rubber and steel wire windings. The higher the system pressure, the more steel windings are required to retain the pressure in the hose. As a result, high pressure hydraulic hoses can be quite heavy. Although somewhat flexible, they may have a large bend radius and stiff enough to exert large side load forces on the stationary fittings to which they are coupled. Overtime, the hoses wear out and the fittings may break.

Above: Fittings and hoses must be installed to minimize wear.

Careful planning and design of the hose pathways and fitting connections will minimize the loads exerted on the hydraulic fittings and the unnecessary flexing and rubbing of the hoses. The hydraulic supply industry is very large and technically mature and as a result there is a wide variety of hose types and fitting styles to accommodate almost every conceivable application.

Each actuator application should be evaluated and a cylinder specified to meet the exact needs of that application. We invite our customers to discuss their specific actuator applications with our design engineers so that the cylinder provided will meet their needs and provide a long trouble free service life.


The Future of Hydraulic Cylinders

Although the world has seen incredible jumps in computer control and sensor technology, the world of hydraulics has evolved much more slowly. Yet, evolved it has. Hydraulic cylinders can be equipped with many innovative electronic controls to allow smooth integration into modern computer controlled systems.

For instance, hydraulic cylinders can be equipped with a variety of stroke limit sensors and digital or analogue motion feedback devices. These include end of stroke switches, linear potentiometers, and magneto-restrictive displacement transducers.

Above: A large hydraulic cylinder equipped with a high tech stroke sensor.

Although electric linear drives have made great advances in power due to recent leaps in the power density of electric motors, it is unlikely that this technology will soon replace hydraulics for the reasons listed above. Hydraulic cylinders are simply powerful, rugged, flexible and economical.


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