1 Introduction
For the precision round-table vertical grinder, it is necessary to ensure that the large-surface roughness of the workpiece to be ground is low and the precision is high. In addition to requiring a good grinding head, the working performance of the table is also required to be better. At present, in the φ1.6m precision round-table vertical grinding machines produced at home and abroad, the guide rails of the worktable basically adopt rolling guide rails. After investigation, the rolling elements are easily lost after being worn with high accuracy, the vibration resistance capability is not strong, and the grinding accuracy is high. In the plane, the roughness value is also not ideal. The hydrostatic guide rail, compared with it, has a smaller frictional resistance, long service life, good dynamic characteristics, good motion stiffness, a certain ability to absorb vibration, and high motion accuracy. Rolling guide rails are hardly comparable to hydrostatic guide rails, and the domestically produced static pressure system is not much different from the import of large-scale special plane rolling bearings. Therefore, we have adopted the hydrostatic guide rail in the development of the φ1.6m precision round-table vertical mill (this project is the original mechanical industrial science and technology development project in 1997). The following describes the design of the hydrostatic guide rail in this topic.
2 Determination of Hydrostatic Guide Way of Oil Supply
As far as the oil supply method is concerned, the hydrostatic guide rails are currently divided into two major categories of constant pressure and constant current fuel supply. In recent years, the machine tools produced by industrialized countries such as Germany, Japan, and the United States have adopted methods for supplying oil to hydrostatic guide rails in a way that does not use the same method. There are constant-current oil supply methods and constant-pressure oil supply methods. It may depend on the passing habits and the development of accessories for the fuel supply system.
Figure 1 shows that each of the two oil chambers share a throttle. The oil supply pressure of the pump is adjusted with a relief valve. The pressure is always controlled at a reasonable value, the so-called constant pressure type. Figure 2 shows each of them. The oil chambers have an oil pump full-flow oil supply, which is the so-called constant flow type. The two oil supply methods are compared as follows:
figure 1
figure 2
(1) Due to uneven work weight, limited rigidity of the basic parts, localized deformation due to clamping force, and difficulty in achieving extremely high and stable machining accuracy, roughness, and installation and commissioning of basic parts. As a result, the pressure of each oil chamber on the guide rail cannot be even. If an oil chamber reaches or approaches a certain oil pump pressure, static pressure cannot be established. With constant flow guides without relief valves, as long as there is enough flow, it is possible to keep the guide rails out of contact with each other and form pure liquid friction. The system has a large pressure reserve and strong overload capacity.
(2) Due to flying dust from the outside, some metals peeled off during operation, impurities precipitated in the oil, and some remaining dirt in the inner cavity of the base part will contaminate the oil, once the throttle is blocked, the oil of the constant pressure guide rail Loss of pressure in the chamber destroys static pressure. If constant-current static pressure is used, there is no throttle, that is, no clogging occurs, and the work safety and reliability are high, but the lubricating oil still needs precise filtration to prevent the rail from being damaged.
(3) When the oil pressure of the constant pressure type oil pump is higher than the pressure of the oil chamber, the pressure drop will be generated through the throttle, and there will be heat generation due to the pressure drop. To maintain the oil supply pressure, the overflow valve must overflow. Part of the overflow consumes both power and heat. As a result, the temperature of the oil rises, causing thermal distortion of the machine tool, reducing the precision of the machine tool movement, and may even prevent the static pressure guide from working properly.
(4) In terms of oil film stiffness, the stiffness of a constant-current static pressure system is worse than that of a feedback throttle system in a constant-hydrostatic pressure system, but it is much better than a fixed throttle system.
Based on the advantages and disadvantages of the above two, it is reasonable to choose a constant current fuel supply method. At the same time, based on a machine tool plant in Hubei that used constant-current hydrostatic guide rails for more than 20 years, none of the machine tools failed due to oil supply. Therefore, the oil supply for the hydrostatic guide rail of the grinder adopts the 1WZS04-type constant head flow divider manufactured by a certain machine tool factory in Hubei Province, and the schematic diagram thereof is shown in FIG. 3 . The operating principle, performance and parameters of the constant flow divider, and the safety at work are not described here.
1 - Motor 2 - Flywheel 3 - Fine Filter 4 - Variable Pump 5 - Fine Filter
6—Relief valve 7—Constant flow divider 8—Pressure relay
9—Pressure Gauge 10—Base Oil Chamber 11—Workbench
image 3
3 Design and Calculation of Hydrostatic Guides
(1) Workbench and base design of 1.6m round table vertical grinder Main technical parameters 1 Workbench diameter: 1600mm
2 working table speed: 0.8 ~ 32r/min
3 maximum grinding diameter: 1800mm
4 Workbench weight: W1=18kN
5 table magnetic chuck weight: W2=12kN
6 maximum weight of workpiece: W3=60kN
(2) The design of the hydrostatic guide rail is shown in Fig. 4. The outer diameter of the guide rail is 1200mm and the inner diameter is 1000mm. Ten oil chambers are evenly distributed. Except for the oil return of each oil chamber, a radial oil return groove is also provided. There are two functions of the oil return tank, which can be used as internal return oil passages, or as pressure cut grooves between oil chambers to avoid pressure interference. Inside and outside there is a 1.5mm high wall, so that the guide rail is always soaked in the lubricating oil, so as not to bring air into the oil chamber during the rotation and lose pressure, if the oil system fails, suddenly stop, the rails still maintain oil lubrication, No dry friction will occur. The oil chamber is opened on the base and the guide rails of the worktable are inlaid with zinc-aluminum-copper alloy rail plates. In order to preload the hydrostatic rails, the worktable to the guide rails is 375mm high, so that it is sufficient to withstand the rigidity.
(3) Hydrostatic guide calculation 1 Oil chamber pressure Calculate the size of the oil chamber as shown in Fig. 5. The dashed line indicates the effective area Ae for each oil chamber to withstand pressure:
Figure 4
Figure 5
An oil chamber pressure at no load:
One oil chamber pressure at full load:
2 Selection of hydrostatic guideway mechanical oil It is well known that the dynamic viscosity of mechanical lubricants is related to the temperature. If the viscosity varies greatly with the temperature difference, the flow rate changes greatly. This is unfavorable to the constant flow hydrostatic guide rail, and it is necessary to constantly adjust the variable displacement pump. The following manual checks the dynamic viscosity of two mechanical lubricants to compare.
10# mechanical lubricants:
Μ10°C=44.64×10-3 Pa·s, μ40°C=14.29×10-3 Pa·s
30# mechanical lubricants:
Μ10°C=250×10-3 Pa·s, μ40°C=42×10-3 Pa·s
From the above, it can be seen that 10# mechanical lubricant μ10°C/μ40°C=3.124 times and 30# mechanical lubricating oil: μ10°C/μ40°C=5.952 times. Obviously, the dynamic viscosity of the 10# mechanical lubricating oil varies little with the temperature difference. Therefore, use 10# mechanical lubricant.
3 oil chamber flow calculation:
Q=pλh3/(12μ)
P——pressure in the oil chamber (Pa);
h - oil film thickness (m);
λ - throttle coefficient;
μ—Lubricating oil viscosity (Pa·s).
Take h=9×10-5m.
Total flow at full load at 10°C:
Qεμ10°C=10Qμ40°C=6L/min
Total flow at full load at 40°C:
Qεμ40°C=10Qμ40°C=19L/min
YBX type variable displacement pump is used for oil supply. The rated flow rate is 25L/min. Calculated according to the shear film power consumption, if the tank capacity is greater than 600L, the oil temperature rise is only about ≤ 20 °C, enough to maintain safe and reliable work.
4 Shear power calculation of oil film:
When the table is rotated, the oil is sheared between the guide rails and power must be consumed. Since the hydrostatic guide has no direct metal contact, there is no friction loss. Therefore, only the relative speed of the table rail and the base rail, the oil is sheared, the shear force is proportional to the lubricating oil dynamic viscosity, area, relative speed, inversely proportional to the gap, its formula:
F'=μAv/h
Shearing moment: M=F'r
Power consumed by cutting: N=1.075×10-7μΑr2n2/h
In the formula: A—The actual contact area between the hydrostatic guide rail and the table guide rail on the base (the oil chamber and the radial oil return groove are not counted), A=(602-56.62+53.42-502)π+3.2× 3.4×2×10-5×10×10=2066.32cm2=2.06632×10-1m2;
R—the radius of the geometric center of the guideway width to the center O of the guideway, r=(60-50)÷2+50=55cm=0.55m;
n - maximum speed of the table (r/min).
5 oil film stiffness calculation:
J=F/e
Where: F - load e - the displacement amount calculated from the original load state F = W1 + W2 + W3 = 90000N
e=0 to h=0 to 9×10-5m=0 to 90μ
∴Jmin=90000/90=1000N/μ
4 Conclusion
Because of the series of characteristics of the hydrostatic guide rails, combined with the use of a constant flow oil supply method, the grinding machine has a small frictional resistance, reliable operation, good dynamic characteristics, strong vibration resistance, good motion stiffness, high motion accuracy, and therefore a grinding machine. Cutting workpieces with high precision.
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