It is well known that hydraulic noise can change as a system warms up. That change can be a factor for misperception of mechanical failure, because noise can play an important role as a signal that indicates abnormal operation. It is therefore important to understand the behavior of hydraulic pressure ripples that are a source of hydraulic noise in operating conditions, and how they change in relation to the temperature of the hydraulic oil.
This study has investigated the ripple behavior that results from temperature change in simple hydraulic systems, using mathematical models that took thermal properties into account. Physical properties of the oil and the speed of sound in the oil have been defined as temperature-related variables in the mathematical models. The physical properties that should be used in the mathematical models have been obtained directly from the oil manufacturer. In contrast, the speed of sound in the oil has to be obtained from the isentropic tangent bulk modulus of the oil in an actual operating condition. That has been determined from the specific volume ratio of entrained air to the oil and the isentropic tangent bulk modulus of the only oil. The thermal properties of the speed of sound in the oil have been determined from the thermal characteristics of these variables, and it has been found that the speed of sound in the oil decreases with a rise in the oil temperature.
The mathematical models of pressure ripples have shown that there were three distinct phenomena resulting from the temperature change of the oil. The first is the change of wavelength. The second is the spatial dependence of the thermal characteristics of the pressure ripples. The third is the difference of the thermal characteristics of the pressure amplitude at the peak in spatial modes. These changes that result from the temperature variation tend to be large at higher frequency.