The tendency of an object to fly outwards from the axis round which it is rotating has been applied to diverse uses. Pumps, purifiers, separators and governors have been devised to work on this principle of centrifugal force, which has its applications in countless engineering activities
CASTING IRON PIPES by a centrifugal process in the Stanton Ironworks, near Nottingham. Molten metal poured from a ladle runs down a trough into a cylindrical mould which is revolving about its own axis. Centrifugal force causes the molten metal to form a layer on the inside of the mould, where it quickly solidifies and whence it is withdrawn as a pipe.
IN the articles of constitution of the Institution of Civil Engineers there occurs the following remarkable sentence, attributed to Thomas Telford, the founder President. It defines civil engineering as “the art of directing the great forces of power in Nature to the use and convenience of man”.
The engineer has harnessed water power, steam pressure and electrical energy to do his bidding. In addition to these forces, there is the force which causes a body rotating round an axis to endeavour to fly outwards — centrifugal force.
Scientifically controlled and properly applied, this force can be used, by the engineer to achieve all kinds of results. In Biblical times the most famous instance of the application of centrifugal force was the sling with which the diminutive David slew the giant Goliath. The stone in the sling was whirled round and round until sufficient force had accumulated to give the stone its requisite velocity in the direction of the target when one strand of the sling was released.
The effect of centrifugal force can be shown in the laboratory by an interesting experiment. If a thick rubber ring, fitting tightly over a pulley, is rotated at high speed by a motor it will expand to four or five times its original diameter, touching the pulley at one point only. If the ring is then pushed off the pulley, it will roll along the floor for a considerable distance, gradually decreasing in diameter as it rolls.
An even more striking demonstration is provided by substituting a loop of chain for the rubber ring. When the motor is run up to full speed, this chain loop will expand into a circle. If the chain is pushed off the pulley when running at speed, it will run along the ground in the manner of a rigid hoop for a short distance, after which it will become irregular in shape and finally collapse.
Another interesting example of the application of centrifugal force is afforded by an instrument known as the bolas and used by the Argentine gauchos, or cowboys, on the rolling pampas of their country. The bolas consists of three heavy balls which are held together by three leather thongs of equal length, and it is used for the purpose of catching ostriches or run away cattle and horses.
The gaucho holds one of the balls in his right hand and whirls the other two round his head. When they have attained sufficient speed he lets them go in the direction of the quarry. They spin round in the air under the action of centrifugal force, and are aimed so that they become twined round the legs of the animal at which they are aimed. Properly aimed, this device will bring any animal to the ground. It is another example of the skilled application of centrifugal force.
STANDING WAVES caused by vibration in a rubber disk rotating at high speeds. The rotating parts of a steam turbine are virtually disks, which carry heavy centrifugal loads. A rotating steel disk is liable to vibrate at certain critical speeds and develop waves, though not to the same extent as the rubber disk shown in these illustrations.
In the early development of the steam engine the need soon arose for an efficient governing device. The centrifugal governor devised by James Watt has undergone many modifications since his day, but remains the same in its principles. It consists essentially of two heavy balls, which are attached to a pair of arms, hinged at the top end to a rotating shaft. As the speed of the shaft increases the balls tend to fly outwards, due to the centrifugal force imparted to them. By a simple system of levers this movement of the balls is translated into a vertical movement, which can be used for controlling the steam inlet valve of an engine.
For the slow engine speeds of Watt’s days this form of governor was adequate for the purpose. For higher speeds, however, it suffers from one grave defect. This is known as “hunting”. As the speed of the engine increased, the action of the balls rising caused the steam valve to close , the speed immediately dropped and the valve opened again. This caused another spurt of speed, which was again corrected by the governor. At high speeds this hunting effect is undesirable, and only by the most careful research and experiment has it been possible to avoid it.
Overspeed Safety Device
A modern form of governor is known as the Whitehead governor. It works on the same principle as Watt’s, but the rate of movement of the working is carefully controlled by a special arrangement of springs. In other words, the rate of upward movement of the arms connected to the balls is controlled within fine limits by a carefully designed spring.
It is claimed that this adjustment is so fine and accurately proportioned that the speed of an engine can be controlled to within 1½ per cent of the speed required. There is nothing intricate about this. It comes about merely by the proper application of natural laws. By reason of the close control which can be obtained there is no hunting. It has been found in practice that on some engines on which it has been fitted, the engines run slower on light load than on heavy load.
Another interesting example of the application of this type of governor is afforded by that fitted to automatic electric standby plant. This is known as a centrifugal switch, and is so designed that it can come into action when the speed of the motor driving it falls below 5 per cent of its normal value.
The construction of this switch is similar to that of an engine governor, fitted with spring-loaded weights which tend to fly outwards with increase of speed. Another switch of this type is also fitted to the engine, so that it can detect high and low speeds. When the engine is cranked to about 200 revolutions a minute, electrical contacts of the ignition system are opened and these are closed again by the same switch when the engine has attained about three-quarters of its normal speed.
Another type of electrical switch which has many applications is the overspeed safety device. This consists of a pin which is let into a rotating shaft and kept in position by a spring. The pin itself is connected to one pole of an electrical circuit, and when the speed becomes so great that the centrifugal force on the pin overcomes the resistance of the spring, the pin flies outwards and makes contact with a fixed ring connected to the other pole of the same circuit. When contact is made, an alarm bell rings or a warning light is lit, so that the plant can be shut down without delay or danger.
A difficult problem is caused by centrifugal force set up in the rotating parts of steam turbines. These generally run at such high speeds that the disks, which loads, are rate which of the disk, centrifugal force on each ounce of metal near the periphery of the rotor may amount to as much as five or six hundredweight. The problem is further complicated by vibration effects and the subject has for a number of years been the special study of research workers, who have discovered much about the rotation of disks at high speed.
SECTIONAL DIAGRAM of a centrifugal oil purifier showing the principal working parts. The external appearance of this type of purifier is shown below.
A turbine rotor consists essentially of a number of disks carried on a shaft, on the periphery of which are fixed a large number of vanes upon which the steam impinges. These carry heavy centrifugal liable to vibrate at a depends upon the shape the load due to the vanes and various other factors.
The calculation of this vibration becomes so complicated that it is necessary to carry out tests upon the disks to obtain accurate results. In common with other turbine manufacturers, the English Electric Company has built for this purpose a special machine in which the vibrations of the disk are recorded by an ingenious magnetic arrangement.
It has been discovered that a rotating disk tends to develop trains of waves, which travel round the disk, either in the direction of motion or against it. This was a most important discovery, because it showed that, if a force of the same frequency as these waves acted upon the disk, there would be grave danger of the disk breaking up, because of the enormous forces set up within its structure, even more serious vibration, also due to the effect of centrifugal and other forces, is brought about when these waves travel backwards at the same rate as the disk. As a result of careful tests these different forms of vibration are now thoroughly understood, and centrifugal force has an important bearing on this subject.
One of the most outstanding examples of the applications of centrifugal force is provided by the centrifugal pump. This simple and highly efficient apparatus consists of a number of curved vanes which are spaced equally, in the same way as the spokes of a wheel, and are cast together in the form of a circular disk. This disk, or impeller, as it is called, is keyed to an axle and is rotated inside a spiral shaped casing.
The water or liquid to be pumped enters at the centre of the impeller, the vanes of which are curved in the same direction as the direction of rotation in which the pump works. Thus, the water is whirled round within the casing and the centrifugal force imparts a high velocity. When the water leaves the pump it is therefore at a high pressure, due to this velocity, and the curvature of the vanes is carefully designed to give a certain discharge from the pump at a certain definite speed.
In a well-designed centrifugal pump it is found that about 75 per cent of this centrifugal force is converted into pressure in the pipe line. As a centrifugal pump will not suck air, it is necessary to prime it, or fill it with water beforehand. Unless this is done, the pump will not- operate. As a general rule, centrifugal pumps will not pump above a height of 300 feet with a single impeller. If it is required to pump above this height, several impellers are used, and the pump becomes known as a multi-stage pump. The liquid travels from one impeller to the next, with consequent increase in pressure.
Centrifugal pumps are widely used by the engineering industry. They range from the gigantic installations used for filling and emptying docks to the small portable pumping sets which can be used for so many different purposes. Multi-stage pumps for waterworks service have been built up to sizes which require steam turbines of as much as 3,200 horse-power to run them at the required speed. Special pumps are available which can deal with water containing a large proportion of solids, the impellers being designed in such an ingenious manner that these solids pass right through them without doing any damage.
CENTRIFUGAL OIL PURIFIER. Lubricating or transformer oil which has become dirty is fed into a rotating bowl, as shown in the diagram above. Centrifugal force separates the dirt from the oil because of the differing specific gravity.
The action of the fluid flywheel is similar to that of a centrifugal pump, operation depending entirely upon the proper application of centrifugal force. The fluid flywheel is used for the transmission of power from an engine or motor to the machine which it is required to drive. It is of simple construction, the driving and the driven members consisting of a cylindrical body in which are cast a number of semicircular passages, equally spaced round the circumference. These two members are enclosed in liquid, and are kept filled with it.
When the driving member is rotated, the action of centrifugal force causes the liquid in the passages to fly to the outside edge and thus into the passages of the driven member. As soon as a certain speed is reached, there is hydraulic connexion between the two members. In other words, the driven member rotates as does a water turbine.
The fluid flywheel has a large number of applications, and one of the best known is in the transmission system of certain makes of car. It is used also for transmitting the power of an excavator, and here it is of great advantage if the excavator bucket should come up against a, boulder or obstruction. When this happens, the driving member merely continues to rotate and carries the liquid round with it. No shocks are transmitted to the driven member, which remains stationary.
The Delavaud System
Another important application of centrifugal force is in the casting of iron pipes, and it is used also for other forms of casting work. It was in 1914 that the resourceful engineer, Sensaud de Lavaud, conducted in Brazil a number of experiments which had as their object the evolution of an efficient system for the centrifugal casting of iron pipes. Several inventors had tried to do this before, but the Delavaud system, as it is called, proved the most successful.
This method was such a radical departure from those which had been in vogue for more than 200 years that it was some time before it was accepted as standard engineering practice. The machines are. remarkably simple, and an excellent example of the plant is that installed at the Stanton Ironworks, near Nottingham. The mould is of cylindrical shape, the inside diameter of the cylinder being equal to the outside diameter of the pipe. The mould revolves on its own axis and is supported upon a number of rollers, the molten metal being fed into the mould through a trough. Centrifugal action causes the molten metal to adhere to the side of the mould.
In a few seconds the metal solidifies and contracts from the mould, after which the pipe is removed without any difficulty.
It is claimed that this method makes a casting which has a much denser structure than would be obtainable by other methods. The effect of centrifugal action is to reduce the possibility of blowholes, or bubbles of air, being formed. Furthermore, the metal does not require to be so thick as in a pipe cast in a vertical sand mould and much time is saved.
The same kind of process is used also at Stanton for casting concrete pipes. In this instance the steel reinforcement for the pipe is inserted into a cylindrical mould, after which the mould is rotated and the concrete placed inside it. This also makes a pipe having a dense structure, and many thousands of concrete pipes are now made in this way. Another process consists of lining the inside of cast-iron pipes with concrete by centrifugal action. Such a lining is of great value where the pipes have to take water or liquid which contains acids or other harmful substances. The concrete lining is highly resistant to the effects of tuberculation.
CENTRIFUGAL LOADER for grain and similar material. The grain comes from the store down a trough and on to the revolving drum of the loader. The stream of grain flies off the drum by centrifugal force and can be directed with even distribution into wagons.
Centrifugal force is used in the foundry in the operation of a machine known as a sandslinger. When a mould is being prepared for a casting to be poured, it is necessary to ram tightly the sand of which it is composed. For generations this operation has been done by hand, but it requires skill and hard work.
Modern foundry work is largely mechanized, and a machine for compacting moulding sand has been devised. The machine itself consists of a short conveyer belt mounted on a framework. At the end of the conveyer there is a rotating shaft with a projection on it. This shaft is mounted at right angles to the conveyer, and it and the conveyer are driven by an electric motor. Sand is fed on to the conveyer, and when it has reached the end of the belt it is flung downwards into the mould by the projection on the rotating shaft. The centrifugal force imparted to the separate grains of sand which are thrown off in this manner is sufficient to compact them thoroughly in the mould.
A machine of this type will fling the equivalent of 1,500 handfuls of sand a minute into the mould, a speed of operation many times greater than would be possible by manual labour.
In the chemical and manufacturing industries centrifugal force is used to a great extent for the purpose of drying wet substances and for separating one substance from another. The machines used for these purposes are known as centrifuges and are mounted on vertical or horizontal shafts. The advantage of the horizontal type is that it can be easily discharged. It is necessary only to have a horizontal scraper, which is placed in the cylinder alongside the rotating layer of dried or separated material, which can then be run off from the machine.
This form of separator is used not only for drying wet material, but also for separating different types of vegetable oils and various chemical substances. For example, in one factory a centrifugal separator handled 2,000 tons of starch weekly in an operating time of 135 hours. In another instance, some 71 tons of vegetable oil were handled every hour. Centrifuges are used for a large number of purposes.
The cream separator is another mechanism which depends for its action upon centrifugal force. This again is a simple apparatus, consisting merely of a number of conical shaped disks closely packed together on a vertical shaft which is rotated at high speed. The mixture of milk and cream is poured into a bowl above the shaft, which is hollow, and the heavy cream is flung off by centrifugal action through a spout at the side.
A similar mechanism to the cream separator is the centrifugal oil purifier. In this, dirty lubricating or transformer oil is fed into a rotating bowl, which turns round at high speed. The centrifugal force has the effect of separating the dirt from the oil, because they are of different specific gravity. This form of oil separator has many different applications, one of the most important being on board ship.
For Clarifying Beer
Portable units of this type of machine can be obtained for treating up to 600 gallons an hour of oil. Similar machines are Used for such purposes as the clarification of paint and varnish, the purification of olive oil, fish oil, or vegetable oil, the extraction of moisture from tar, the separation of yeast from starch and the refining of such substances as petrol, kerosene and mineral lubricating oils.
Centrifuges can be made so that the liquids under treatment are not in contact with the air. These machines are suitable for clarifying beer and other fermentable liquids, and for the clarification of liquids which are highly volatile. When these are exposed to the air there is a grave risk of fire, and they are liable to evaporate quickly.
In modern mining practice, the separation of feebly magnetic ores from the matrix is accomplished by a combination of magnetism and centrifugal force. The material is granulated and fed into a hopper, from which it flows into a roller rotating in a strong magnetic field. The roller becomes magnetized by induction and, while the non-magnetic particles are thrown off by centrifugal action, the magnetic particles are held for a short time on the roller. When they leave the roller, they describe a different trajectory from that of the non-magnetic particles. The two streams are then intercepted by separate chutes. By the use of a number of rollers in series it is possible to effect further subdivision of the different particles.
One of the most remarkable examples of centrifugal force is its application to the separation of dust in the flue gases from the boilers of large power stations. It is just as important to remove the dust as it is to remove the acids from flue gases. It has been proved that this dust, of which a large proportion is extremely fine, carries the acids and deposits them on buildings and stonework. If the dust is removed before the smoke issues from the chimney, there is little chance of acids being carried away from the chimney stack. Tarry substances, too. are not so likely to be carried away.
One efficient form of centrifugal dust collector is due to the researches of van Tongeren, a Dutch engineer of Heemstede, North Holland. The design is the outcome of much aerodynamic research into the flow of fine particles in suspension. This is a highly technical matter, and the ultimate solution of the many problems involved a large amount of mathematical calculation, to say nothing of the practical experiments carried out.
SPEED DETECTION BY A GOVERNING DEVICE is the application of centrifugal force embodied in this electrical switch, designed for controlling automatic electric circuits. Spring-loaded weights open or close electrical contacts according to the speed at which the weights revolve.
A common form of dust collector, used in a great many industries, is known as a “cyclone separator”. It consists of a conical-shaped casing into which the dust is drawn by a suction fan. The effect of this conical shape is to separate by centrifugal action the particles of dust from the gas containing them. It has been known for many years that gas flow in this type of apparatus has been influenced by a remarkable phenomenon known as “double eddy current”. When any gas or air passes along a curved path, this double eddy current is set up. For a long time this important feature was not taken into consideration.
Recently, however, the need has arisen for a dust separator which will be able to extract particles of almost microscopic size from flue and other gases. The effect of the eddy current is to hamper the proper use of centrifugal action, due to the swirling action set up in the mixture of gas and particles. Another curious effect is caused by these eddy currents. In the lower conical portion of the separator an upward current of gas is created, and a kind of “waterspout” effect is brought into play. This causes gas and particles to rise inside the separator, a condition which is inefficient.
Thus the path of an individual dust particle in this apparatus is somewhat complicated, being due to a combination of centrifugal action and eddy currents. In the van Tongeren system of dust separation, these eddy currents are turned to useful account in a cyclone separator.
The flue or other gases from which dust has to be removed are introduced into the separator through the inlet pipe into what is known as the primary separator, arranged horizontally round the chimney. The casing of this primary separator is made in a volute curve, so arranged that deflection of the gas current towards the inner part is avoided. It is in this primary separator that the eddy currents are used to good advantage, for the dust will concentrate not only against the outer wall but also on the bottom and under the cover of the device.
Through slots and pockets the dust is carried by another gas current into the secondary separator, which has certain special characteristics. The chimney is provided with an opening for one half of its circumference. The opening is provided with a number of inclined vanes and the space between them can be regulated by a simple lever mechanism. Through these openings the cleaned gases enter the chimney. The regulation of the vanes has a marked effect on the efficiency of the apparatus.
Gases which do not pass between the vanes into the chimney pass through a duct and mix once again with the main stream. Dust particles which are not carried to the slot and the pockets in the first swirl round the casing are removed by successive swirls. Finally, all dust particles are precipitated from the gases and collected in the secondary cyclone. The clean gases are thus passed through the outlet pipe of the cyclone to the chimney, where they mix with the main current of the remaining clean gases for discharge into the atmosphere.
SANDBLASTING by centrifugal force. In this machine abrasive material is fed into a device known as a “Wheelabrator”. This consists of a patented wheel, driven by a motor of 15 horse-power, which throws, by centrifugal action, the abrasive against the material to be cleaned or surfaced. The work is continuously turned over, to expose fresh surfaces, on an endless conveyer. The spent abrasive is intercepted and returned to the feed.
