A thorough study of standing waves in hydraulic lines is carried out using transmission line theory. Special emphasis is attached to the representation of standing wave patterns of different systems by three dimensional graphs. From this study a new experimental test method has been developed for evaluating the fluid borne noise characteristics of pumps and other hydraulic components. The validity and accuracy of this test method, named the "tuned length method", was assessed by comparison with other existing methods. Unlike many other experimental methods, the tuned length method is capable of testing hydraulic components at low mean pressures. The "tuned length method" was used to evaluate the inlet fluid borne noise characteristics of three gear pumps and an axial piston pump. A purpose built pressurized reservoir was used to control pump inlet conditions. Large pressure fluctuations were measured at some positions in pressurized suction lines and were found to be similar to fluctuations measured in pump discharge lines. The levels of pressure ripple in a pump suction line were found to have a significant affect on the air borne levels generated by the hydraulic system. There is strong evidence of air release occurring in the pump inlet passageway under normal operating conditions. This accounts for the very low pressure ripples in the suction line of a normally aspirated pump. However, this does not affect the volumetric efficiency of the pump. When the pump inlet is pressurized air release is inhibited and the pressure ripple can be very large. An axial piston pump was tested when boosted by another piston pump and the inlet characteristics were evaluated. These characteristics are very similar to those obtained when the pump is supplied by a pressurized reservoir. In order to maintain low fluid borne noise in pump suction lines the mean inlet pressure must be kept as low as possible. The existence of air disolved in the fluid in small quantities is favourable as it prevents very low instantaneous pressures and hence limit the possibility of cavitation. The use of a pressurized reservoir is reccommended for this purpose, as long as steps are taken to avoid solution of additional quantities of air.
|Date of Award||1982|