WATER MOTORS. The subject of hydraulic transmission of power is treated generally under Power Transmission {Hydraulic), and the present article is confined to water motors.
Hydraulic Lifts.— The direct-acting lift is perhaps the simplest of all machines using pressure-water, but as the height of the lift increases, certain problems in construction become exceedingly difficult to cope with, notably those due to the great increase in the weight and displacement of the rani. In fact, with a simple ram it is not possible to lift beyond a certain height with a given pressure and load. It becomes, therefore, necessary to balance in some way the varying displacement of the ram if economy is to be secured in the working: this is often done by the use of counter-weights attached to chains travelling over head sheaves, but this largely destroys the simplicity and safety of the direct-acting lift, and hence some form of hydraulic balancing is more satisfactory and more certain.
Fig. 1.—Hydraulic
Balancing.
In one form, shown in fig. 1, the lift cylinder is in hydraulic connexion with a pair of short cylinders placed one above the other, the pistons working in them being connected together by a common rod. Below the piston of the upper cylinder is an annular space E (surrounding the common piston rod) with a capacity equal to the maximum displacement of the lift-ram, while the corresponding annular area C of the piston of the lower cylinder is just large enough when subjected to the working water pressure to enable the work of lifting the net load to be done and any friction to be overcome. The area B of the top side of the upper piston is proportioned in such a way that when under the full water pressure the dead weight of the ram and cage is just balanced when the former is at the bottom of its stroke. With this arrangement the lift-ram and the two balance pistons are always in equilibrium, or, in other words, the ever-changing displacement of the lift-ram is automatically in balance. To work the lift, pressure-water is admitted to the annular space C above the lower of the two balance pistons (the space B above the upper one is always in communication with the pressure-water), and the combined pressure on the two pistons is sufficient to lift the cage, ram and load. As the ram ascends it apparently increases in weight, but this is balanced by the greater pressure on the two balance pistons as they descend, owing to the increase of the head of water acting on them. To allow the lift-ram to descend, the pressure-water in C above the lower balance piston is discharged through the exhaust into the drain, while that above the upper piston is simply pushed back into the pressure main. As an illustration of the economy of this system, it may be mentioned that in one lift having a 6-in. ram with a lift of 90 ft., the working load being 1 ton and the maximum working speed 180 ft. a minute, the quantity of pressure-water used per journey of 90 ft. was reduced from 109 to 2412 gallons by the use of this method of balancing.
Fig. 2.—Hydraulic
Balancing.
In another system of hydraulic balance (fig. 2) the ram A has an annular area so proportioned that when it is connected with the water in an elevated tank (usually placed somewhere in the roof of the building), the hydraulic pressure upon it just balances the weight of the ram and cage. Here again, since the intensity of the pressure on A becomes greater as it descends owing to the increased head, the apparent increase of weight of the lift-ram as it rises is automatically balanced; water from the high-pressure system is admitted down the hollow ram B and does the work of lifting the live load.
Since the introduction of deep-level electric railways in London and elsewhere, hydraulic passenger lifts on a large scale have bees brought into use for conveying passengers up and down from the street level to the underground stations.
Direct-acting Water Motors.—Owing to the difficulty of securing a durable motor with a simple and trustworthy means of automatically regulating the quantity of water used to the power needed at various times from the motor, not much advance has been recently made in the use of water motors with reciprocating rams or pistons. Probably the most successful one has been a rotary engine invented by Mr Arthur Rigg.[1]
Fig. 3.—Section of
Rigg's Water Engine.
In this engine the stroke, and therefore the amount of water used, can be varied either by hand or by a governor while it is running; the speed can also be varied, very high rates, as much as 600 revolutions a minute, being attainable without the question of shock or vibration becoming troublesome. The cylinders are cast in one piece with a circular valve, and rotate about a main stud S (fig. 3), while their plungers are connected to a disk crank which rotates above the point O, which is the centre of the main crank; O S being the crank length or half stroke of the engine, any vanation in its length will vary the power of the engine and at the same time the quantity of water used. The movement of S is obtained by means of a relay engine, in which there are two rams of different diameters; a constant pressure is always acting on the smaller of these when the motor is at work, while the governor (or hand-power if desired) admits or exhausts pressure-water from the face of the other, and the movements to-and-fro thus given to the two rams alter the position of the stud S, and thus change the stroke of the plungers of the main engine. Fig. 4 gives an outside view of a 30-H.P. engine capable of using water at a pressure of 700 lb per sq. in.; the governor is carried within the driving pulley shown at the right-hand end, while the working revolving cylinders are carried inside the boxed-in flywheel at the left-hand end, the relay cylinder and its attachments being fixed to the bed-plate in front of the flywheel. On a test one of these engines gave an efficiency or duty of 80%.
Water Wheels.—The Pelton water wheel (fig. 5) has proved a most successful motor when very high heads are available, heads of 2000 feet having been used occasionally. Such machines have been extensively employed in America, and have also lately been used in Great Britain, worked by the high-pressure water supplied in large towns.
The wheel carries a series of cups placed at equal distances around the circumference. A jet or jets of water impinge on the cups, the interiors of which are shaped in such a way that the jet is discharged parallel to its original direction. If the linear velocity of the cups in feet a second is V1, and the linear velocity of the jet is V2, then the velocity of the jet relative to the cup is V2−V1 feet a second, and if the whole energy of the water is to be given up to the cups, the water must leave the cup with zero absolute velocity. But its velocity relative to the cup, as it passes backwards, is −(V2−V1), and since the forward velocity of the cup is V1, the absolute velocity of the water is −(V2−V1)+V1, or 2V1−V2. This will become zero if V1 is 12V2, that is, if the linear velocity of the cup-centres is one-half that of the jet of water impinging upon them. The theoretical efficiency of the wheel would then be 100%. The actual efficiency of these wheels when used with high falls is from 80 to 86%; when used in connexion with high-pressure water in London an efficiency
- ↑ This engine was fully described in Engineering, vol. xiv, p. 61.
