【正文】 critical highpressure hydraulic systems where contamination and fluid quality are difficult to control. The performance of the fluid’s antiwar additive is generally very important with vane pumps. Piston Pumps As with all hydraulic pumps, piston pumps are available in fixed and variable displacement designs. Piston pumps are generally the most versatile and rugged pump type and offer a range of options for any type of system. Piston pumps can operate at pressures beyond 6000 psi, are highly efficient and produce paratively little noise. Many designs of piston pumps also tend to resist wear better than other pump types. Piston pumps operate at a normal fluid viscosity range of 10 to 160 cSt. Gear Pumps There are two mon types of gear pumps, internal and external. Each type has a variety of subtypes, but all of them develop flow by carrying fluid between the teeth of a meshing gear set. While generally less efficient than vane and piston pumps, gear pumps are often more tolerant of fluid contamination. 1. Internal gear pumps produce pressures up to 3000 to 3500 psi. These types of pumps offer a wide viscosity range up to 2200 cSt, depending on flow rate and are generally quiet. Internal gear pumps also have a high efficiency even at low fluid viscosity. 2. External gear pumps are mon and can handle pressures up to 3000 to 3500 psi. These gear pumps offer an inexpensive, midpressure, midvolume, fixed displacement delivery to a system. Viscosity ranges for these types of pumps are limited to less than 300 cSt. Hydraulic Fluids Today’s hydraulic fluids serve multiple purposes. The major function of a hydraulic fluid is to provide energy transmission through the system which enables work and motion to be acplished. Hydraulic fluids are also responsible for lubrication, heat transfer and contamination control. When selecting a lubricant, consider the viscosity, seal patibility, base stock and the additive package. Three mon varieties of hydraulic fluids found on the market today are petroleumbased, waterbased and synthetics. 1. Petroleumbased or mineralbased fluids are the most widely used fluids today. The properties of a mineralbased fluid depend on the additives used, the quality of the original crude oil and the refining process. Additives in a mineralbased fluid offer a range of specific performance characteristics. Common hydraulic fluid additives include rust and oxidation inhibitors (Ramp。C. 3. Collect the temperatureviscosity characteristics of the lubricant in use. The ISO viscosity rating system (cSt at 40186。C) is remended. Viscosity is 32 cSt at 40186。C. 4. Obtain an ASTM D341 standard viscositytemperature chart for liquid petroleum products. This chart is quite mon and can be found in most industrial lubricant product guides or from lubricant suppliers. 5. Using the viscosity characteristics of the lubricant found in Step 3, start at the temperature axis (xaxis) of the chart and scroll along until you find the 40 186。C line, track upward until you find the line corresponding to the viscosity of your lubricant at 40186。C and mark the intersection point (dark blue line, Figure 3). 7. Connect the marks by drawing a line through them with a straight edge (yellow line, Figure 3). This line represents the lubricant’s viscosity at a range of temperatures. 8. Using the manufacturer’s data for the pump’s optimum operating viscosity, find the value on the vertical viscosity axis of the chart. Draw a horizontal line across the page until it hits the yellow viscosity vs. temperature line of the lubricant. Now draw a vertical line (green line, Figure 3) to the bottom of the chart from the yellow viscosity vs. temperature line where it is intersected by the horizontal optimum viscosity line. Where this line crosses, the temperature axis is the optimum operating temperature of the pump for this specific lubricant (69