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Design Focus Of Oil-free Scroll Vacuum Pump
- Aug 15, 2018 -

1. Selection of vortex lines

 As a kind of volumetric pump, the oil-free scroll vacuum pump can achieve the purpose of vacuum pumping through the volume change, which is determined by the scroll profile.  In this way, the design of vortex profile becomes the most basic problem in the design of oil-free vortex vacuum pump.

 The advantages and disadvantages of the judgment profile are mainly considered from the aspects of small volume, high volumetric efficiency, smooth operation and easy guarantee of machining accuracy.  Currently, the main types of profiles used are single profile, modified profile, combined profile and general profile.

Profile mainly includes base circle involute, regular polygon involute, line segment involute, semicircle involute, Archimedes spiral, algebraic spiral, variable diameter base circle involute, envelope profile, etc.  At present, all the mathematical models of forming commercial oil-free double-sided scroll vacuum pump profiles in the world are circular involute.  This is due to the following reasons:

First, starting from the definition of effective volume, comparing the advantages and disadvantages of several different types of curves, the results show that the effective volume ratio of the involute of a circle is 1, the involute of a regular quadrangle is 0.9, the involute of a regular hexagon is 0.95, the involute of a regular octagon is 0.972, and the involute of a line segment is 1.023.  The effective volume of the involute of a circle is relatively high;

 Secondly, the oil-free double-sided scroll vacuum pump using this type of profile has a more compact structure and good working characteristics;

 Thirdly, the involute of a circle is easy to process and can be developed. this is of great significance for the use of high-speed and high-efficiency special planes for large-scale ultra-precision machining.

2. Optimization of scroll parameters

 The main components of the oil-free scroll vacuum pump are the moving scroll and the stationary scroll. the moving scroll moves in a plane relative to the stationary scroll under the drive of the crankshaft, so that the moving scroll and the stationary scroll enclose a closed crescent cavity to complete the process of gas suction, compression and removal.  Proper optimization of the key parameters of the scroll profile of the moving and stationary scroll is conducive to the simplification and compactness of the whole machine structure and is convenient for production and processing.

If the scroll is characterized by structural parameters, the main parameters that make up the scroll are as follows: they are:

 Base circle radius a, vortex pitch p, vortex wall thickness t, vortex line start angle theta, and vortex wall height H..

 Where p = 2π a, t = 2aθ.

 Generally speaking, in order to facilitate measurement and design, we usually first determine the three parameters of pitch p, wall thickness t and height h, and then calculate the base circle radius a and vortex line starting angle θ, thus establishing the vortex line equation.  For the oil-free double-sided scroll vacuum pump, in order to ensure the compact and efficient structure, the above three parameters are often optimized.  The specific method of optimization will not be described in this paper. from the point of view of machining, the reasons and results are as follows:

1 ) height (h)

 Under the condition of constant gas transmission, increasing the h value is beneficial to reducing the diameter of the scroll, reducing the length of the leakage line of the axial gap and improving the volumetric efficiency.  However, with the increase of h, the cutting edge must be lengthened, which means that the requirements for machine tools and cutting tools are increased, which increases the processing difficulty of scroll plates and is not easy to ensure the processing accuracy of scroll plates.

 ( 2 ) wall thickness (t)

 The selection principle of t value is to ensure a certain strength and rigidity, to meet the requirement that the deformation under a certain working condition should not be too large. the wall thickness of scroll is usually 3 ~ 4 mm, and the wall thickness is too thin, which affects the strength and machining accuracy.  Too thick increases weight and increases wear and vibration of moving parts.

 ( 3 ) pitch( p)

 Generally, the value of { p - t } is used to characterize the groove width between the scroll walls. the larger { p - t } is, the larger the diameter of the tool that can be used, which means that the better the rigidity of the tool is, the easier it is to ensure the machining accuracy.  Similarly, the size of { p - t } must also be limited because it also affects the diameter of the entire scroll.  In fact, the selection of { p - t } should try to select an integer value, which is beneficial to the selection of standard tools and measuring tools.

3. Revision of vortex profile

 The modification of scroll profile to a certain extent is beneficial to the improvement of the performance of the whole machine.  Due to cutter interference and other reasons, it is very necessary to correct the profile of the head of the vortex profile.  Since the head is located at the beginning of the vortex line respectively, it is easy to produce stress deformation and tool let-off phenomenon. it is very necessary to thicken this area in order to reduce stress deformation and thermal deformation under certain working conditions.  The correction here can indeed greatly reduce the power consumption, reduce the invalid exhaust volume inside the exhaust hole, improve the stability of the exhaust process, and prevent the occurrence of large airflow pulsations.


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Rainbow Zhuo

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