From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Design description of a hydraulic transmitter, focusing on the fluid dynamics and construction of its components.
| Identifier | ExFiles\Box 156\4\ scan0091 | |
| Date | 2nd September 1938 guessed | |
| 8 radii of curvature of the inner boundary of the axial cross-section of the hydraulic circuit and the radius of curvature of its outer boundary. It will be seen that in the constructions illustrated in Figs. 19 and 20, any tendency of the stream to leave dead liquid along the leading duct faces is effectively counteracted in the radial portion of the ducts, since in this portion they can be made to converge not only in their axial depth, but also in their width to the requisite extent, while in the curved portion, which in the construction shown in Fig. 20 begins practically at the radius of the duct outlets, such tendency is completely absent, since in this portion the liquid is decelerated angularly by deflection backwards relatively to the ducts. The design is therefore effective in removing the cause of unsteadiness of flow similar to that shown in Fig. 17 and 18. As regards its efficacy to remove also the other cause of unsteadiness which has been mentioned, i.e., that which divergence of the vane faces of a centrifugal pump constitutes per se, independently of angular motion of the wheel, a series of tests, the model wheel being at rest, has been carried out in the laboratory, the results of which are of interest and show conclusively the serious effect of divergence, whether in a straight or in a curved path. The tests were made by the well-known Reynolds method of coloured bands, which enables direct visual observation of the nature of the flow. The liquid medium used in the transmitter is common engine lubricating oil. For convenience, however, the tests were carried out with water at 20 deg. C., and from the values obtained the corresponding values were reduced for the oil medium, the kinematic viscosity of the latter, at the temperatures obtaining in the transmitter under normal working conditions, being taken at ten times that of water at 20 deg. C. The nature of the motion of the liquid at different velocities of flow within ducts of three different conformations was observed and was recorded photographically. Figs. 21, 22 and 23 are reproductions of photographs showing the effect of the angular disposition of opposite faces in a duct (Fig. 23) of the type used in the driving element of the transmitter now being considered, as compared with the effect of the angular disposition of opposite faces in a duct (Fig. 21) of an ordinary hydraulic coupling and in a duct (Fig. 22) of an ordinary centrifugal pump. A glass disc, through which the photographs were taken, forms one of the faces which limit each duct in depth, the opposite face being in each case formed by an inverted metal cone of an angle of 6 deg. Clear water at 20 deg. C. was delivered to each duct in a steady stream, a thin stream of coloured water at the same temperature being simultaneously introduced by means of an injector. The liquid was drawn through each duct at gradually increasing velocities, the tests being directed to determine the minimum velocity of liquid which can flow per unit time, under conditions of streamline or non-sinuous motion, through the ducts of three stationary pump wheels of a given diameter, each consisting of Fig.8. Fig.9. Fig.11. SECTION AT A Fig.12. SECTION AT B Fig.13. SECTION AT C Fig.14. SECTION AT D Fig.15. SECTION AT E Fig.10. Fig.16. (6474.E) ENGINEERING 5 radial planes from their inlets up to a point near to the major radius of the circuit and are curved backwards from the latter point towards their outlets, so that the forward velocity of the liquid flowing between the vanes is reduced before it has reached the outlet. The ducts of the driving element are constructed on the principle of the gradually convergent nozzle, a principle which was heretofore held to be inapplicable in the construction of centrifugal pumps. The principle of the gradually convergent nozzle is adhered to in the construction of the ducts of all the elements constituting the hydraulic circuit. The profile of the vanes of the driving element is such that the vane outlets impart to the outflowing liquid a component of velocity backwards relatively to such outlets of a magnitude varying substantially as the speed of relative rotation of the driving and driven elements. The receiving ends of the vanes of all the elements are of bulbous formation, so that unavoidable discrepancies of rotational velocities do not result in serious shock. The profile of the vanes of the driven element is appropriate for efficient functioning for transmission at the ratio of 1 to 1, when the reaction vanes are idling, and for transmission at a variable higher ratio when the reaction vanes are in operation. The vanes of the reaction element are disposed substantially in the line of axial flow, but their inlets are at a radius slightly larger than their outlets. General arrangement.—A general arrangement of the transmitter for use in an automobile is illustrated in Fig. 3. This shows a construction in which the driven element and the reaction element are of the single-stage type. This construction is suitable only for vehicles having a particularly favourable power-to-weight ratio. For cases where starting torque of a high order is required, as, for example, in an automobile of average power-to-weight ratio, or in a heavy lorry, the driven element and the reaction element are of the two-stage type. Seeing that the fundamental characteristic of the transmitter under consideration relates to its driving element, and that the construction of this element remains the same whether the transmitter is of the single-stage or of the double-stage type, for the sake of clarity and brevity, the simpler construction shown in Fig. 3 has been selected for illustration of the general principles which govern the design. In the arrangement shown in Fig. 3, p is the centrifugal pump or driving element; t is the turbine or driven element, and r the reaction element. The centrifugal pump is carried by the driving shaft a, the turbine is carried by the driven shaft b, the reaction element is free to rotate idly when it is inoperative, i.e., during transmission at the ratio of 1 to 1. For the purpose of performing its normal function of deflecting the liquid in the direction of primary motion during transmission at higher ratios than 1 to 1, the reaction element is arranged to become anchored to the stationary part c through pawls d arranged in a dash-pot at e, which pawls, when the reaction torque exerted on the reaction element falls below a predetermined value, become disengaged from the stationary anchorage and allow the reaction element to run free. Fig. 1 is reproduced from a photograph showing a face view of the driving element, and Fig. 2 is a similar view of the assembly of the driven element and reaction element showing at the hub, the vanes and pawls of the reaction elements. The general arrangement illustrated calls for no further comment, the operation of the machine differing from the usual only in that, by virtue of the nozzle and duct characteristics, which have already been mentioned, and which are described in greater detail later, the liquid within the transmitter flows in a steady Fig.3. ENGINEERING | ||