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How to choose hydraulic oil based on a hydraulic pump? And how to choose hydraulic oils with different viscosities? First, we should take a look at what the viscosity of hydraulic oil is. Viscosity is an important property of hydraulic oil. Viscosity refers to the property whereby, when a liquid flows, the internal frictional resistance within the liquid prevents relative movement between its layers. Viscosity is the unit used to measure the extent of this property. Viscosity is divided into three types: dynamic viscosity, kinematic viscosity, and relative viscosity. Dynamic viscosity is a definition with a very clear physical meaning, but it is rarely used in practical applications. It represents the relationship between a liquid’s internal friction force and the velocity gradient, and it is extremely difficult to measure. Kinematic viscosity, which is easier to understand, is the ratio of dynamic viscosity to density; it is used as a parameter to indicate the viscosity of hydraulic oil. The hydraulic oil grades we refer to are all determined based on kinematic viscosity, which holds great practical significance. Relative viscosity, in simple terms, refers to the specific viscosity of hydraulic oil as measured in a particular hydraulic pump; it is relatively easy to measure and is determined using an Engler viscometer. Therefore, when choosing hydraulic oil of different viscosities, we need to make the selection based on kinematic viscosity and relative viscosity. In fact, hydraulic pumps come with a specified viscosity of hydraulic oil suitable for their use at the time of manufacture, and this viscosity is the kinematic viscosity. Ultimately, whether the oil is suitable for use with the hydraulic pump also depends on the relative viscosity. This is because relative viscosity is the most accurate measure of viscosity in hydraulic pumps; this viscosity is affected by various factors and thus differs from dynamic viscosity. Therefore, we need to understand the various factors that affect changes in relative viscosity in order to select the appropriate hydraulic oil. The main factors affecting viscosity changes are temperature and pressure. Pressure is easier to manage, as its level can be controlled fairly easily; as long as there is no overload, there are generally no problems, unless the air pressure at the location of use differs from that in normal areas. Other than that, pressure is relatively much easier to control. Temperature, on the other hand, is different: the higher the temperature, the lower the viscosity. There are many factors that affect temperature; varying dimensions, different seasons, and the temperature in the working environment can all result in significant differences. The different operating times of hydraulic pumps, the varying speeds of hydraulic cylinders, as well as different precision requirements for operations, also lead to varying temperature demands. Therefore, it is best to install a thermometer to monitor the temperature regularly in order to ensure operational precision. Generally speaking, unless there are special precision requirements, maintaining the hydraulic oil temperature between 15-65°C ensures normal operation. Therefore, if our operating conditions are very special, we cannot choose the hydraulic oil solely based on the specifications given for hydraulic pumps; instead, we need to take into account both the relative viscosity and the dynamic viscosity as criteria for selection.