A method is given for calculating pressure transients in an axial piston hydraulic pump. Some theoretical predictions are given of the effect of port timing and the effect of introducing restricting grooves at the ends of the kidney ports in the valve plate and suggestions are made of the effects of these parameters on noise emission; comparative measurements of noise are then quoted that support the general arguments. A parallel shot is recommended as the best compromise for the restrictor groove geometry to give good results over the widest range of operating conditions, including reverse rotation. Finally, mention is made of the use of a relief valve in the port plate for noise reduction.
The limitations of sound wave theory, and the work of Earnshaw and Riemann are first examined in relation to the propagation, under frictionless conditions, of waves of finite amplitude. The theory of pressure equalization in a semi-infinite pipe, due to Aschenbrenner, is extended to cover discharge from a cylinder into a pipe of the same bore, or to atmosphere. The effects of friction are examined and the accepted mathematical solution of the analogous electrical transmission-line problem is applied to the case of small-amplitude waves. Friction in relation to waves of finite amplitude is examined, and a hyperbolic law defining the effects of friction is deduced. The arguments developed are applied to discharge from a cylinder through an orifice of smaller diameter. The experimental apparatus consisted of a number of cylinders of 2-inch bore, and of lengths from 1 foot to 40 ft. 6 in., from which air, at gauge pressures from 140 lb. per sq. in. to minus 25 inches of mercury, was discharged either directly to atmosphere or through a pipe of 2-inch bore and up to 81 feet in length. Indicator diagrams were recorded at points spaced along the cylinder and pipe, and corresponding diagrams were also obtained, using a cylinder 4 feet long and 4 inches in diameter in conjunction with a pipe of 2-inch bore. From the general agreement between the experimental and the theoretical investigation, it is concluded that the processes of discharge, under the conditions considered, are capable of full explanation on the basis of accepted physical laws. Experimental confirmation is obtained for the theoretical treatment put forward by Giffen for discharge from a cylinder through an orifice of smaller diameter, and quantitative data are given in regard to velocities of propagation, pressure amplitudes, attenuation, etc., for the range of conditions covered.
The paper describes an investigation of the effects of piston-excited pressure waves in the plain induction pipe of a small, high-speed, single-cylinder air compressor. For a range of pipe lengths and diameters, compressor speeds and delivery pressures, the observed compressor throughput and driving torque are compared with those obtained with no intake pipe fitted. Based on classical laws of wave motion but using an experimentally obtained steady-flow characteristic for the inlet valve, a theoretical treatment is developed to describe the pressure pulsations in the inlet port and cylinder and to predict the change in airflow resulting from the fitting of any particular intake pipe. Experimentally recorded indicator diagrams compare satisfactorily with those predicted theoretically and similar agreement is obtained between the experimental and theoretical values of airflow. An 18 per cent increase in airflow is reported.
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