The Influence of Mechanical Process on the Performance of Roots Vacuum Pump

2.1 Rotor Engagement Clearance Rotor engagement clearance directly affects the pumping speed and normal operation of Roots pumps. The gap is too small to withstand the test of the maximum allowable pressure difference, that is, the Roots pump can not guarantee the normal operation when the pressure difference is large; because the pressure difference is large, when the heat is severe, the thermal expansion will cause the rotor and the rotor to produce friction and give out abnormal noise. In severe cases, the rotor and rotor will jam. If the gap is too large or the gap is not uniform, the return gas from the pump outlet to the inlet will increase, the zero-flow compression ratio will decrease, and the pumping efficiency will decrease. The maximum allowable pressure difference and the zero-to-be-compressed ratio are two extremely important performance indicators that complement each other and restrict each other. Therefore, the Roots pump rotor meshing clearance is an extremely important critical issue. How to ensure uniform clearance at each meshing point? , And keep within a certain range, not too large or too small, must start from the rotor profile processing. In the past, most profiling processes were used. This method has low investment and is quick to take effect, but the error is large. For example, the center of symmetry center is offset, the symmetry of the rotor is up and down, and the rotor gains weight. Therefore, in order to ensure quality, intensive cultivation is necessary, but the production efficiency is reduced. . Although the profiling process has a large error, in the past, due to the limitations of the processing equipment, it has actually been used in the processing of the Roots pump rotor for several decades, and has made a great contribution. With the development of process equipment technology and the widespread adoption of CNC machine tools, profiling 1 processing has gradually withdrawn from the historical stage. High degree of automation, control flexibility, high precision, high production efficiency of CNC machine tools has emerged completely in a new process of rotor machining process.

2.2 Gear meshing and clearance gear is an important part of the Roots pump, it ensures the synchronous rotation and clearance of the two rotors, so the requirements for the gear are relatively high (six-level accuracy), the machining machine must be selected according to the requirements of the gear precision and Processing methods, the main control tolerances in current gear processing: public normal length, tooth tolerance, base section limit deviation and tooth direction tolerance, of course, the gear center distance deviation must also be controlled, it is related to the center distance between the two bearing holes of the pump side cover . If the machining accuracy of the gears is out of control, serious consequences will occur if the Roots pump is installed.

2.2.1 The gear clearance is too small After eliminating the reason for the center-to-center distance between the two bearing holes of the side covers, the gears should be fat and large. The consequence of this is a severe crushing and heating between the gears. Abnormal noise, this kind of noise is high-frequency noise, if its fundamental frequency carries on the frequency spectrum analysis to Roots pump noise, can discover the noise anomaly (篼) at the basic frequency of the gear noise from noise frequency spectrum, this is a kind of most Simple and most effective inspection method. In addition, it can also be found from the power meter that the power is too large and the external surface temperature of the gear box is skewed.

Found that this situation must replace the gear, or as the temperature rises, the gear expansion increases, eventually will be stuck.

2.2.2 Excessive Gear Clearance Of course, the reason for the center-to-center deviation of the two bearing holes of the side cover must be excluded first. Here, the gear is thin or small.

The consequence of this is that it is difficult to adjust the rotor clearance, and it causes an impact between the rotors, but this phenomenon is difficult to find during starting and running. Only when the pump is stopped, because the inertia of the active and driven rotors is not the same, The speed of the stop was inconsistent, resulting in an impact of two revolutions of ¥. This impact was very obvious. As the speed is reduced, the speed of the impact is gradually reduced, which can completely determine the excessive clearance of the pump gear.

This phenomenon must also be replaced gear, or with the increase in gear clearance, the impact will become more and more powerful.

2.2.3 Poor gear meshing This condition is generally caused by insufficient accuracy of the machine tool. There may be one or more tooth abnormalities, or all teeth may be in poor engagement. They generate anomalous noise, and the frequency of its noise is also calculated according to equation (1), except that the number of teeth z is substituted with the number of bad teeth 1, 2, and 3 and can be found from the measured noise spectrum.

This situation can be achieved by using a very fine emery coated on the gear to perform grinding. It will reduce the noise of the gear and you may wish to give it a try.

2.2.4 Gear shifting gears There are three kinds of positioning methods on the shaft, namely, key positioning, interference (hydraulic) positioning and expansion ring positioning. The key positioning used more in the past, but its keyway processing accuracy is more difficult to control, resulting in the key Too tight or too loose. Over-tight assembly is difficult, must be rework, too loose the gear is easy to shift; interference (hydraulic) positioning is the use of hydraulic pressure in the gear hole (generally 20-40Mpa) expansion, and then located in the shaft on the interference, this The most reliable and reliable positioning, but must be equipped with hydraulic tools, inconvenience to the user to repair and go out to repair the production plant, but the large Roots pump is also recommended to use this positioning method is better; expansion ring positioning method is convenient and reliable, but the key It is the processing of the expansion ring. In the past, it was the processing of each pumping plant itself, and it was often difficult to control the precision. Now there is a professional factory for production, and the quality can be guaranteed. The only problem that is likely to cause problems is the assembly process. For example, if the cleaning is not careful, the uneven installation force may cause unreliable positioning and cause the gears to shift.

The shift of the gear will cause the rotor gap to change, causing the rotor to bump into a noise. The frequency of this noise is generally 2n/60 (Hz). The rotor gap change will inevitably affect the pumping speed and will reduce the pumping efficiency.

2.3 side cover positioning error. The positioning of the side cover determines the position of the center of the two bearing holes on the center of the pump cylinder, which determines the position of the rotor in the pump cylinder. This position is extremely important. It determines the clearance between the rotor and the pump body and the rotor. The gap between the pump ensures the pumping performance and the reliability of the normal operation under high pressure difference. In terms of the clearance between the rotor and the pump body, it is generally desirable that it is uniform both up and down and left and right, but considering that the pump outlet temperature reaches 120-130C while the helium pressure difference is operating, and the rotor temperature is as high as 160-180C, the pump inlet is The temperature is not high, so the rotor and the pump inlet are susceptible to friction due to thermal expansion. For this purpose, the center of the rotor must be lowered when positioning the side covers. To increase the gap on the upper side to ensure the safety and reliability of high-pressure differential operation. If an error is located, it may happen that the rotor and the cylinder wall of the pump inlet are rubbed or even stuck.

S Radial between sub-pump cylinders H: Illustration '2.4 Clearance between rotor and side cover in longitudinal direction Clearance between rotor and side cover In high pressure differential, thermal expansion is severe due to high rotor temperature. Therefore, a proper clearance is maintained between the rotor (axial) and the side cover. In order to facilitate the control of the gap, one end is a fixed end, its gap is relatively small, and the other end has a relatively large gap at the movable end, and the rotor expands axially. Long can only extend to the active end.

This clearance is first ensured by the length (axial) dimensional accuracy of the machining pump cylinder and the rotor. During installation, the fitter adjusts the gap, and the fixed end must be fixed to ensure a reasonable clearance. The selected bearing must also be small in axial direction. There have been encountered a number of axial bearing series bearings up to 0.25mm, such as the use of such bearings, the fixed end and the active end of the gap can not be guaranteed, in the high pressure differential operation may be stuck expansion.

2.5 Rotor unbalance The vibration noise caused by the inertial force is mainly caused by asymmetry of rotor shape or casting process defects caused by machining errors (non-uniformity of internal structure, sand holes, air holes, etc.). On the one hand, the unbalanced inertial force causes the rotor to deform. If the deformation is too large, it may cause collisions between the rotors. On the other hand, it will act on the bearings, causing the vibration of the bearings to increase and transfer the speed to other components. The characteristic frequency of this excitation force is the rotation frequency and harmonics of each order.

2.6 Vibration and Noise of Bearings The most basic vibration of rolling bearings is the vibration of rolling elements. Even if the bearing geometry is completely correct, when radial loads are applied, due to the radial clearance (sometimes the clearance is still relatively large), the rolling elements during operation are Gradually entering and exiting the load zone, the central position of the inner ring of the bearing changes periodically, resulting in the passage of vibration at a characteristic frequency that is the product of the rolling element's revolution frequency and the number of rolling elements. In addition, due to poor processing and manufacturing, including the bearing inner and outer rings and rolling elements of the shape error, defects and surface waviness caused by vibration.

Because of the high rotation speed of the Roots pump, the vibration and noise of the bearing is particularly outstanding. Therefore, bearings with a high precision, small radial and axial clearance, and reliable quality must be selected.

7 Couplings have more error coupling types, but they are used more in plum blossoms on Roots pumps. Because the material used for plum-shaped elastic blocks is polyurethane, the texture is hard, so the plum blossom on the coupling body The distribution of holes is very uniform. If the indexing is not uniform during processing, the elastic block will abnormally press with the coupling body, causing noise and severe heat generation. To determine if this is the cause, just check that the coupling is hot and generally not hot.

Conclusion In addition to the performance parameters of the so-called Roots pump, the more important is the operational safety and reliability. If it can not guarantee normal operation, then what ultimate pressure, pumping speed and power are meaningless, the process is to ensure that each Item design indicators and operational safety and reliability requirements.

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