Capacitance method measuring device and method for thickness of lubricating oil film between mechanical friction pairs based on dynamic correction model

Abstract

The invention discloses a capacitance method measuring device and method for the thickness of a lubricating oil film between mechanical friction pairs based on a dynamic correction model, and belongs to the technical field of electrical measurement. Wherein the point contact ball disc testing machine comprises a point contact friction pair formed by a conductive disc and a steel ball; the conductive disc is electrically isolated by arranging insulating rubber pads on the upper surface and the lower surface of the conductive disc, and the steel ball is electrically isolated by arranging an insulating ceramic ball bearing in a transmission mechanism where the servo motor is connected with the steel ball. The two independent servo motors respectively drive the steel ball and the conductive disc to rotate, so that test working conditions at different speeds can be realized. According to the invention, the capacitance distribution in the Hertz contact area can be effectively calculated, the problem of measurement errors caused by peripheral capacitance interference of the Hertz contact area in a traditional capacitance method is solved, and accurate measurement of the oil film thickness between common metal friction pairs in engineering is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrical measurement, and in particular relates to a capacitance method for measuring the thickness of lubricating oil films between mechanical friction pairs based on a dynamic correction model. Background Art

[0002] Maintaining a lubricating oil film of a certain thickness between mechanical friction pairs to separate the two moving surfaces is an effective means of reducing friction and wear. The formation of this lubricating oil film mainly depends on the fluid dynamic pressure effect generated by the relative motion of the friction surfaces, and its thickness is usually in the order of tens of nanometers to submicrometers. Measuring the thickness of the lubricating oil film between friction pairs is a key technology for evaluating their lubrication status and lubrication effect. The thickness of the lubricating oil film between friction pairs is one of the most difficult parameters to monitor. Optical interferometry is one of the most accurate and widely used oil film thickness measurement methods. However, this method requires that at least one of the two contacting surfaces to be measured is made of transparent materials (common materials such as optical glass, sapphire, etc.). However, most mechanical friction pairs are made of metal, not transparent materials. Therefore, it is difficult to directly apply this method to the measurement of the lubricating oil film thickness of the lubrication interface of real mechanical friction pairs. The capacitance method is one of the most successful methods for measuring oil film thickness in the early days. It is based on the parallel plate capacitor theory and infers the oil film thickness by measuring the capacitance value between the two friction pairs. However, the interaction area between the two friction pairs consists of the interior of the Hertzian contact area and the periphery of the Hertzian contact area (entrance / exit area, both sides). Figure 4 Therefore, the capacitance value measured between the two friction pairs includes not only the capacitance corresponding to the oil film inside the Hertz contact area ( ), also includes the capacitance outside the Hertzian contact area ( ), that is, the capacitance at the inlet / outlet and both sides. Moreover, the capacitance outside the Hertz contact area varies significantly with the oil film thickness and the geometry of the friction pair, resulting in the total capacitance between the friction pairs measured by the traditional capacitance method ( ) and the actual capacitance corresponding to the oil film inside the Hertzian contact area, which causes measurement errors and reduces the accuracy of oil film thickness measurement. Summary of the Invention

[0003] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0004] A capacitance-based measurement device for lubricating oil film thickness between mechanical friction pairs based on a dynamic correction model comprises a point-contact ball-on-disc tester and a capacitance acquisition system. The point-contact ball-on-disc tester comprises a point-contact friction pair consisting of a conductive disc and a steel ball. The conductive disc is electrically isolated by disposing insulating rubber pads on its upper and lower surfaces, and the steel ball is electrically isolated by providing an insulating ceramic ball bearing in a transmission mechanism connecting a servo motor and the steel ball. Two independent servo motors drive the rotation of the steel ball and the conductive disc, respectively, to achieve test conditions at different speeds.

[0005] The capacitance acquisition system includes a Hertz contact area composed of a conductive disk and a steel ball, a rated resistance , data acquisition card and computer; the capacitor equivalent to the Hertz contact area generated by the contact between the conductive disk and the steel ball is the capacitor to be measured, the first output terminal and the second output terminal of the data acquisition card are connected to the conductive disk and the steel ball respectively, and a rated resistor is connected in series between the conductive disk and the first output terminal , forming a voltage output system; the first input terminal and the second input terminal of the data acquisition card are respectively connected to the conductive disk and the steel ball, forming an electrical data acquisition system; the data acquisition card is connected to the computer.

[0006] A capacitance method for measuring the thickness of a lubricating oil film between mechanical friction pairs based on a dynamic correction model, and a capacitance device for measuring the thickness of a lubricating oil film between mechanical friction pairs based on a dynamic correction model, comprising:

[0007] Step 1: Output an excitation voltage to the Hertz contact area formed by the conductive disk and the steel ball through the data acquisition card. , excitation voltage Through the rated resistance The voltage is transmitted to the friction pair consisting of a conductive disk and a steel ball; in response, the data acquisition card collects the voltage change in the Hertz contact area and obtains the equivalent capacitor charging curve of the Hertz contact area;

[0008] Step 2: Fit the equivalent capacitor charging curve obtained during a measurement process to obtain the capacitor charging time and the maximum voltage across the capacitor ;

[0009] Step 3: The maximum voltage across the capacitor obtained in step 2 , calculate the resistance value of the equivalent capacitor ;

[0010] Step 4: Calculate the resistance of the equivalent capacitor obtained in step 3. Calculate the total resistance of the circuit in the measurement system ;

[0011] Step 5: Measure the total resistance of the circuit in the system based on the value obtained in step 4. and the capacitor charging time obtained in step 2 , calculate the capacitance of the measurement system during one measurement ;

[0012] Step 6: Measure and calculate background capacitance ;

[0013] Step 7: Conduct the test under the selected working conditions, set the rotation speed of the conductive disc and the steel ball at the contact point, and start the conductive disc and the steel ball to rotate respectively under the action of the drive motor, collect the charging curve, and measure the capacitance of the system during the measurement process obtained in step 5. and the background capacitance obtained in step 6 Get the total capacitance of the equivalent capacitor inside and outside the Hertz contact area ;

[0014] Step 8: Calculate the dimensionless velocity parameters of the Hamrock-Dawson oil film thickness empirical formula according to the working conditions selected in step 7. , dimensionless load parameters and dimensionless material parameters ;

[0015] Step 9: Calculate the dimensionless velocity parameter of the oil film thickness calculated in step 8. , dimensionless load parameters , dimensionless material parameters and ellipticity ratio Factor, the selected test conditions factor;

[0016] Step 10, according to step 9 Factor and the total capacitance of the equivalent capacitor inside and outside the Hertzian contact area obtained in step 7 , calculate the capacitance inside the Hertzian contact area ;

[0017] Step 11: The capacitance inside the Hertz contact area obtained in step 10 Calculate the average lubricating oil film thickness value in the deformation area inside the Hertz contact zone .

[0018] The present invention has the following beneficial effects:

[0019] The present invention provides a capacitance method for measuring the thickness of the lubricating oil film between mechanical friction pairs based on a dynamic correction model. The invention provides an insulating ceramic ball bearing in a transmission system connecting the steel ball and the motor to insulate the steel ball from the motor. The conductive disc and the ball disc are insulated from the base of the experimental table by arranging insulating rubber pads on the upper and lower surfaces of the conductive disc. The capacitance method is combined with the capacitance charging and discharging principle to measure the thickness of the lubricating oil film between mechanical friction pairs. The factor dynamically corrects the capacitance distribution inside and outside the Hertz contact area (especially the dynamic correction of the influence of the capacitance distribution outside the Hertz contact area on the capacitance inside the Hertz contact area). It can effectively calculate the capacitance distribution inside the Hertz contact area, solve the measurement error problem caused by the interference of the capacitance outside the Hertz contact area in the traditional capacitance method, and realize the accurate measurement of the oil film thickness between metal friction pairs commonly used in engineering. It can also be used to capture the local damage and reconstruction process of the lubricating oil film in the lubrication micro-area, and clarify the lubrication state (complete oil film thickness) inside the metal friction pair. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 Schematic diagram of the point contact ball-on-disc testing machine and the conductive and insulating treatment of the point contact friction pair of the conductive disc and the steel ball;

[0021] Figure 2 This is a schematic diagram of the circuit connection of the ball-disk point contact friction pair;

[0022] Figure 3 is the equivalent circuit model of the Hertz contact area under EHL (elastohydrodynamic lubrication) state;

[0023] Figure 4 Schematic diagram of capacitance distribution inside and outside the Hertz contact area of ​​the friction pair;

[0024] Figure 5 This is the charging curve collected during a measurement process. DETAILED DESCRIPTION

[0025] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only intended to illustrate the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0026] The present invention discloses a capacitance method for measuring the thickness of a lubricating oil film between mechanical friction pairs based on a dynamic correction model, comprising: a point-contact ball-on-disc testing machine and a capacitance acquisition system; the point-contact ball-on-disc testing machine comprises a point-contact friction pair consisting of a conductive disc and a steel ball; the conductive disc is electrically isolated by arranging insulating rubber pads on the upper and lower surfaces of the conductive disc; and the steel ball is electrically isolated by arranging insulating ceramic ball bearings in a transmission mechanism connecting a servo motor and the steel ball; two independent servo motors respectively drive the rotation of the steel ball and the conductive disc, thereby realizing test conditions at different speeds.

[0027] The capacitance acquisition system includes a Hertz contact area composed of a conductive disk and a steel ball, a rated resistance , data acquisition card and computer; the capacitor equivalent to the Hertz contact area generated by the contact between the conductive disk and the steel ball is the capacitor to be measured, the first output terminal and the second output terminal of the data acquisition card are connected to the conductive disk and the steel ball respectively, and a rated resistor is connected in series between the conductive disk and the first output terminal , forming a voltage output system; the first input terminal and the second input terminal of the data acquisition card are connected to the conductive disk and the steel ball respectively, forming an electrical data acquisition system. The data acquisition card is connected to the computer.

[0028] The present invention uses a point contact ball-on-disc testing machine to measure the thickness of the lubricating oil film inside the Hertzian contact area of ​​the ball-on-disc (a local deformation area formed on the contact surface when two elastic solids contact each other under normal load). Among them, a conductive disc made of metal materials such as tungsten carbide and steel contacts a steel ball to form a point contact friction pair. Lubricant is added between the conductive disc and the steel ball to form a lubricated contact area (the lubricated contact area is equal to the Hertzian contact area. The surfaces of two solids (conductive disc and steel ball) that bear loads and move relative to each other are effectively separated by a layer of continuous or partially continuous lubricant (oil, grease lubricating film, etc.), thereby significantly reducing the friction and wear of the contact area. This area includes the inside of the Hertzian contact area and the periphery of the Hertzian contact area (entrance area and exit area). The rotation of the conductive disc and the steel ball is controlled by two independent drive motors to simulate the lubrication and contact state of rolling bearings and gears in engineering under different working conditions and the relative sliding and rolling motion forms. The conductive and insulation treatment process of the point contact friction pair in the point contact ball-on-disc testing machine is as follows: Figure 1 shown.

[0029] In the measurement system, a data acquisition card is used as a voltage signal output device and a feedback signal receiving device. Figure 2 As shown, the first output terminal (such as ao0) and the second output terminal (such as ao1) of the data acquisition card are connected to the conductive disk and the steel ball respectively, so as to apply an excitation voltage signal to the Hertz contact area; wherein, a rated resistor is connected in series between the first output terminal (such as ao0) of the data acquisition card and the conductive disk. , used to delay the capacitor charging process. At the same time, the first input terminal (such as ai1) and the second input terminal (such as ai8) of the data acquisition card are connected to the conductive disk and the steel ball respectively to collect the voltage signals at both ends of the Hertz contact area.

[0030] Use USB (Universal Serial Bus) data line to connect the data acquisition card to the PC (computer) end, and control the data acquisition card excitation voltage through the program written in LabVIEW (laboratory virtual instrument engineering platform, a program development environment) software on the PC end The output of the signal, at the same time, the voltage signal fed back from the acquisition end (first input end and second input end) of the data acquisition card is displayed and stored in real time, the excitation voltage The settings and recording of the changes in voltage value (feedback voltage signal) during the charging process of the Hertz contact area are all achieved by controlling the data acquisition card on the PC.

[0031] The capacitance method measuring device for the thickness of the lubricating oil film between mechanical friction pairs based on the dynamic correction model proposed in the present invention is an improved point contact ball-on-disc testing machine, such as Figure 1 As shown in the figure, the present invention insulates the steel ball from the motor using insulating ceramic ball bearings. Insulating rubber pads are placed on the upper and lower surfaces of the conductive disc to insulate the conductive disc from the base of the point contact ball-on-disc tester. Electrical signals from the conductive disc are then conducted away by placing conductive copper sheets on the surface of the disc.

[0032] The capacitance method for measuring the thickness of the lubricating oil film between mechanical friction pairs based on a dynamic correction model proposed in the present invention specifically includes:

[0033] Step 1: Output an excitation voltage to the Hertz contact area formed by the conductive disk and the steel ball through the data acquisition card. , excitation voltage Through the rated resistance The voltage is transmitted to the friction pair consisting of a conductive disk and a steel ball; in response, the data acquisition card collects the voltage changes in the Hertz contact area and obtains the equivalent capacitor charging curve of the Hertz contact area.

[0034] like Figure 2 , the data acquisition card controlled by the PC's LabVIEW software outputs an excitation voltage , excitation voltage Through the rated resistance The transmission is to the Hertz contact area composed of the conductive disk and the steel ball (including the inside and outside of the Hertz contact area). The equivalent circuit diagram of the Hertz contact area in the EHL (Elastohydrodynamic Lubrication, a key concept describing the lubricating film formation mechanism in high-load point / line contact motion pairs (such as gears and rolling bearings)) state is shown as follows Figure 3 As shown in the figure, in response, the voltage across the equivalent capacitor in the Hertz contact area begins to change. The data acquisition card collects the voltage changes in the Hertz contact area and uses the PC's LabVIEW software to display and store the equivalent capacitor charging curve of the Hertz contact area in real time during the measurement process.

[0035] Step 2: Fit the equivalent capacitor charging curve obtained during a measurement process to obtain the capacitor charging time and the maximum voltage across the capacitor .

[0036] The equivalent capacitor charging curve obtained during a measurement is fitted using data processing software such as MATLAB (commercial mathematical software used in data analysis, signal processing, control systems, etc.) or Origin (scientific plotting and data analysis software). The fitting formula is the zero-state response of a first-order RC circuit (resistance-capacitance circuit): ,in, The transient voltage value collected in real time at the contact interface between the conductive disk and the steel ball during the charging process is the vertical axis; The horizontal axis represents the continuous charging time from the start of charging to the current sampling time during a single measurement. and is the value to be fitted. is the time constant, which reflects the speed of charging time; is the steady-state voltage across the equivalent capacitor after charging is complete.

[0037] Step 3: The maximum voltage across the capacitor obtained in step 2 , calculate the resistance value of the equivalent capacitor .

[0038] Since the current flowing through the capacitor is the same as the current charging the resistor, the resistance value can be calculated according to the following formula :

[0039] ;

[0040] After charging is complete, = ,Will Substituting into the above formula, we can get the resistance value of the equivalent capacitor .in, is the voltage applied to the friction pair consisting of the conductive disk and the steel ball by the LabVIEW software through the data acquisition card, also known as the excitation voltage; the definitions of other variables are the same as above.

[0041] Step 4: Calculate the total resistance of the circuit in the measurement system , the calculation formula is as follows:

[0042] .

[0043] Step 5: Measure the total resistance of the circuit in the system based on the value obtained in step 4. and the capacitor charging time obtained in step 2 , calculate the capacitance of the measurement system during one measurement , the calculation formula is as follows:

[0044] .

[0045] Step 6: Measure and calculate background capacitance , which is used for subsequent correction. Among them, the background capacitance is mainly the inherent capacitance of the test wire. Completely separate the conductive disk and the steel ball, repeat steps 1, 2, 3, 4, and 5, and obtain the capacitance of the measurement system when the conductive disk and the steel ball are completely separated as the background capacitance. , that is, when the conductive disk and the steel ball are completely separated, the background capacitance is equal to the capacitance of the measurement system.

[0046] Step 7: Calculate the total capacitance of the equivalent capacitor in the Hertzian contact area , conduct the test under the selected working conditions, collect the charging curve, and calculate the capacitance obtained in step 5. and the background capacitance obtained in step 6 Get the total capacitance of the equivalent capacitor (inside and outside the Hertzian contact area) , the calculation formula is as follows:

[0047] .

[0048] Step 8: Calculate the three dimensionless parameters in the Hamrock-Dowson oil film thickness empirical formula (point contact elastohydrodynamic lubrication oil film thickness formula) based on the working conditions selected in step 7: dimensionless velocity parameter , dimensionless load parameters and dimensionless material parameters , which is used in subsequent calculations to determine the capacitance inside the Hertzian contact area.

[0049] Dimensionless velocity parameter The specific calculation formula is: ,in, is the dynamic viscosity of the lubricant (Pa·s), is the entrainment speed set during the experiment (the speed at which the lubricating oil is brought into the contact area by the two surfaces at the entrance of the contact area when the conductive disk and the steel ball are running). The specific calculation formula is: ,in, and are the linear velocities of the conductive disk and the steel ball at the contact point (m / s); is the comprehensive curvature radius along the direction of motion velocity in the contact between the conductive disk and the steel ball (m), which is equal to the radius of the steel ball; is the equivalent elastic modulus (Pa) of the friction pair of the conductive disc and the steel ball. The specific calculation formula is: ,in: 、 are the elastic modulus of the friction pair materials of the conductive disc and the steel ball (Pa); 、 are the Poisson's ratios of the friction pair materials of the conductive disk and the steel ball, respectively.

[0050] Dimensionless load parameters The calculation formula is: , is the normal load at the contact point between the conductive disk and the steel ball (N).

[0051] Dimensionless material parameters The calculation formula is: ;in, is the viscosity-pressure coefficient of the lubricant (Pa·s -1 ).

[0052] Step 9: Calculate the dimensionless velocity parameter of the oil film thickness calculated in step 8. , dimensionless load parameters , dimensionless material parameters as well as Factor (ellipse ratio, for the contact circle formed by the conductive disk and the steel ball, the ellipse ratio is 1), the selected test conditions are obtained Factor; the calculation formula is as follows:

[0053] .

[0054] Step 10, calculated according to step 9 Factor and the total capacitance of the equivalent capacitor inside and outside the Hertzian contact area calculated in step 7 , calculate the capacitance inside the Hertzian contact area , the calculation formula is as follows:

[0055] .

[0056] Step 11, according to the formula , calculate the interior of the Hertz contact zone (the "Hertz contact zone" is the area where elastic deformation occurs when the conductive disk and the steel ball contact each other, as described by the Hertz theory of elastic mechanics; such as Figure 4As shown, the Hertz contact area includes the internal deformation area of ​​the Hertz contact area (the equivalent capacitance is ) and the area outside the Hertz contact area that does not produce elastic deformation but is covered by the lubricating oil film: the inlet area and both sides (the equivalent capacitance is ), outlet area (equivalent capacitance is ), is the average lubricating oil film thickness value in the deformation area inside the Hertz contact zone; is the dielectric constant in vacuum, and its specific value is 8.854187817×10 -12 F / m, is the relative dielectric constant of the lubricant, is the contact area of ​​the friction pair of the conductive disk and the steel ball (m 2 ), the specific calculation formula is: , where π is the ratio of the circumference of a circle to the circumference of a circle, is the Hertz contact radius, and its specific calculation formula is: , where b is the radius of the steel ball and the other parameters are the same as above.

[0057] The technical solution of the present invention is described below in conjunction with embodiments:

[0058] Take the friction pair composed of a conductive disk and a steel ball as an example. Figure 1 As shown in the figure, a conductive disc and a steel ball rotate around their respective central axes. An axial load of 30 N is applied to bring the conductive disc and the steel ball into contact. The resulting contact area of ​​local elastic deformation is called the inner region of the Hertzian contact zone. 2 mL of polyalphaolefin (PAO) synthetic lubricant is evenly distributed on the raceway between the conductive disc and the steel ball as the lubricating medium. The lubricant used is a mixture of 40% PAO10 and 60% PAO40, with a viscosity of 180 mm at 40°C. 2 / s. Under the entrainment effect of the movement of the two surfaces, a lubricating oil film of a certain thickness is formed in the contact area. The thickness of this oil film is the measurement object.

[0059] The experimental environment temperature of this embodiment is 20±1℃, the relative humidity is 50%±5%, the conductive disk is made of WC (tungsten carbide), and the surface roughness is (Arithmetic mean deviation of the profile) is 20 nm, the steel ball diameter is 25.4 mm, the G5 grade accuracy, and the steel ball surface roughness is Less than 14 nm. Rated resistance in series with the measuring circuit The resistance is 100 kΩ, the excitation voltage The excitation voltage is set to 0.2 V, the sampling frequency of the data acquisition card is set to 250 kHz, and the single test time is set to 10 ms. The data acquisition card is controlled by LabVIEW software on the PC to apply the excitation voltage. , while collecting the transient voltage across the equivalent capacitor in the Hertzian contact area formed by the conductive disk and the steel ball The data collected is transmitted to the PC via a USB cable for real-time display and storage. The specific measurement steps include:

[0060] Step 1: Use the control program on the PC of the point contact ball-on-disc testing machine to control the rotation of the conductive disc and steel ball, respectively, at a speed of 100 mm / s. Simultaneously, a load of 30 N is applied to the friction pair using weight loading.

[0061] The data acquisition card is controlled by LabVIEW software on the PC to output a 0.2 V excitation voltage ( At this time, the capacitor equivalent to the Hertz contact area formed by the conductive disk and the steel ball begins to charge, and the transient voltage across the two ends is The data acquisition card collects the voltage changes and displays them on the LabVIEW software. The collected equivalent capacitor charging curve is as follows: Figure 5 The horizontal axis in the figure is t (time), and the vertical axis is the transient voltage value collected in real time at the contact interface between the conductive disk and the steel ball during the charging process. (Voltage). is the excitation voltage amplitude applied by the data acquisition card controlled by LabVIEW software. is the steady-state voltage across the equivalent capacitor after charging is complete. ΔU and Δt are the change in voltage over a certain period of time and the duration of the voltage change process, respectively.

[0062] Step 2: Import the capacitor charging curve collected in step 1 into the Origin data processing software for nonlinear fitting. The fitting formula is: ,in Corresponding to the y-axis, Corresponding to the x-axis, the fitting result is and The numerical value of .

[0063] Step 3, according to the formula (When the capacitor is fully charged, = ), calculate the inner and outer Hertz contact area formed by the conductive disk and the steel ball, which is equivalent to the resistance value of the capacitor .

[0064] Step 4: According to the formula , calculate the total resistance of the circuit in the measurement system .

[0065] Step 5, according to the formula , the capacitance of the measurement system during a single measurement is obtained .

[0066] Step 6: Measure and calculate background capacitance , which is used for subsequent corrections. The background capacitance is the inherent capacitance of components such as the test leads in the capacitance method test system. Completely separate the conductive disc and steel ball, and repeat steps 1, 2, 3, 4, and 5 to obtain the measured capacitance when the conductive disc and steel ball are completely separated. This capacitance is the background capacitance, i.e., the capacitance measured when the conductive disc and steel ball are completely separated.

[0067] Step 7: Calculate the total capacitance of the equivalent capacitor inside and outside the Hertzian contact area , the capacitance obtained according to step 5 and the background capacitance obtained in step 6 , the total capacitance of the equivalent capacitor is obtained , the specific formula is: .

[0068] Step 8: Calculate the dimensionless parameters in the Hamrock-Dowson oil film thickness empirical formula (point contact elastohydrodynamic lubrication oil film thickness formula) according to the working conditions selected in step 1.

[0069] U is a dimensionless velocity parameter, and its specific calculation formula is: ,in, is the dynamic viscosity of the lubricant (Pa·s), is the entrainment speed set during the experiment. The specific calculation formula is: , and are the linear velocities of the conductive disk and the steel ball at the contact point (m / s); is the comprehensive curvature radius along the direction of motion velocity in the contact between the conductive disk and the steel ball (m); is the equivalent elastic modulus (Pa) of the friction pair of the conductive disc and the steel ball. The specific calculation formula is: ,in: 、 are the elastic moduli of the two friction pair materials (Pa); 、 are the Poisson's ratios of the two friction pair materials respectively.

[0070] is a dimensionless material parameter, and its calculation formula is: ;in, is the viscosity-pressure coefficient of the lubricant (Pa·s -1 ).

[0071] is a dimensionless load parameter, and its calculation formula is: , is the normal load at the contact point between the conductive disk and the steel ball (N).

[0072] Step 9, calculate Factor: Dimensionless velocity parameter based on the oil film thickness calculated in step 8 , dimensionless load parameters , dimensionless material parameters as well as Factor (ellipse ratio, for the contact circle formed by the conductive disk and the steel ball, the ellipse ratio is 1), calculate the Factor, the specific calculation formula is:

[0073] .

[0074] Step 10: Calculate the internal capacitance of the Hertzian contact area , the calculation formula is as follows:

[0075] .

[0076] Calculated according to step 9 Factor and step 7 , the capacitance value inside the Hertz contact area is obtained , the specific formula is .

[0077] Step 11, according to the formula The average lubricating oil film thickness value of the internal deformation area of ​​the Hertz contact zone is calculated , where is the dielectric constant in vacuum, and its specific value is 8.854187817×10 -12 F / m, is the relative dielectric constant of the lubricant, is the contact area of ​​the ball-disk friction pair (m 2 ), the specific calculation formula is: , where π is the circumference of a circle and a is the Hertz contact radius. The specific calculation formula is: , where b is the radius of the steel ball, the specific value is 12.7 mm, and the other parameters are the same as before.

[0078] The capacitance method for measuring the thickness of lubricating oil film based on the dynamic correction model of the present invention is particularly suitable for analyzing the nanometer-scale lubricating oil film thickness of mechanical contact pairs such as rolling bearings and gears under elastic hydrodynamic lubrication (EHL) conditions. By dynamically correcting the capacitance distribution inside and outside the Hertz contact area ( factor), effectively separating the capacitance distribution outside the Hertzian contact area, solving the measurement error problem caused by the interference of the external capacitance in the Hertzian contact area in the traditional capacitance method, and achieving accurate measurement of the oil film thickness.

[0079] The above descriptions are merely embodiments of the present invention and are not intended to limit the scope of the present invention. Any equivalent structure or equivalent process transformation made using the contents of the present invention description and drawings, or directly or indirectly applied to other related system fields, are also included in the scope of protection of the present invention.

[0080] The contents not described in detail in the specification of the present invention belong to the prior art known to those skilled in the art.

Claims

1. A capacitance method measuring device for the thickness of lubricating oil film between mechanical friction pairs based on a dynamic correction model, characterized in that: include: A point-contact ball-on-disc tester and capacitance acquisition system. The point-contact ball-on-disc tester includes a point-contact friction pair consisting of a conductive disc and a steel ball. The conductive disc is electrically isolated by placing insulating rubber pads on its upper and lower surfaces, and the steel ball is electrically isolated by installing insulating ceramic ball bearings in the transmission mechanism connecting the servo motor and the steel ball. Two independent servo motors drive the rotation of the steel ball and the conductive disc, respectively, to achieve test conditions at different speeds. The capacitance acquisition system includes a Hertz contact area composed of a conductive disk and a steel ball, a rated resistance , data acquisition card and computer; the capacitor equivalent to the Hertz contact area generated by the contact between the conductive disk and the steel ball is the capacitor to be measured, the first output terminal and the second output terminal of the data acquisition card are connected to the conductive disk and the steel ball respectively, and a rated resistor is connected in series between the conductive disk and the first output terminal , forming a voltage output system; the first input terminal and the second input terminal of the data acquisition card are respectively connected to the conductive disk and the steel ball, forming an electrical data acquisition system; the data acquisition card is connected to the computer.

2. A capacitance method for measuring the thickness of the lubricating oil film between mechanical friction pairs based on a dynamic correction model, used in the capacitance method for measuring the thickness of the lubricating oil film between mechanical friction pairs based on a dynamic correction model as claimed in claim 1, characterized in that: include: Step 1: Output an excitation voltage to the Hertz contact area formed by the conductive disk and the steel ball through the data acquisition card. , excitation voltage Through the rated resistance Transmitted to the friction pair consisting of a conductive disc and a steel ball; In response, the data acquisition card collects the voltage change of the Hertz contact area and obtains the equivalent capacitor charging curve of the Hertz contact area; Step 2: Fit the equivalent capacitor charging curve obtained during a measurement process to obtain the capacitor charging time and the maximum voltage across the capacitor ; Step 3: The maximum voltage across the capacitor obtained in step 2 , calculate the resistance value of the equivalent capacitor ; Step 4: Calculate the resistance of the equivalent capacitor obtained in step 3. Calculate the total resistance of the circuit in the measurement system ; Step 5: Measure the total resistance of the circuit in the system based on the value obtained in step 4. and the capacitor charging time obtained in step 2 , calculate the capacitance of the measurement system during one measurement ; Step 6: Measure and calculate background capacitance ; Step 7: Conduct the test under the selected working conditions, set the rotation speed of the conductive disc and the steel ball at the contact point, and start the conductive disc and the steel ball to rotate respectively under the action of the drive motor, collect the charging curve, and measure the capacitance of the system during the measurement process obtained in step 5. and the background capacitance obtained in step 6 Get the total capacitance of the equivalent capacitor inside and outside the Hertz contact area ; Step 8: Calculate the dimensionless velocity parameters of the Hamrock-Dawson oil film thickness empirical formula according to the working conditions selected in step 7. , dimensionless load parameters and dimensionless material parameters ; Step 9: Calculate the dimensionless velocity parameter of the oil film thickness calculated in step 8. , dimensionless load parameters , dimensionless material parameters and ellipticity ratio Factor, the selected test conditions factor; Step 10, according to step 9 Factor and the total capacitance of the equivalent capacitor inside and outside the Hertzian contact area obtained in step 7 , calculate the capacitance inside the Hertzian contact area ; Step 11: The capacitance inside the Hertz contact area obtained in step 10 Calculate the average lubricating oil film thickness value in the deformation area inside the Hertz contact zone .

3. The capacitance method for measuring the thickness of the lubricating oil film between mechanical friction pairs based on a dynamic correction model according to claim 2 is characterized in that: In step 2, when fitting the equivalent capacitor charging curve obtained during a measurement, the fitting formula is the zero-state response of the first-order RC circuit: ； in, The transient voltage value collected in real time at the contact interface between the conductive disk and the steel ball during the charging process is the vertical axis; Indicates the continuous charging time from the start of charging to the current sampling time during a single measurement, which is the horizontal axis; and is the value to be fitted; is the time constant, reflecting the speed of charging time; is the steady-state voltage across the Hertz contact area after charging is completed.

4. The capacitance method for measuring the thickness of the lubricating oil film between mechanical friction pairs based on a dynamic correction model according to claim 3 is characterized in that: In step 3, the resistance value of the equivalent capacitor The calculation formula is: ； After charging is complete, = ,Will Substituting into the above formula, we can get the resistance value of the equivalent capacitor ;in, is the excitation voltage applied to the friction pair consisting of the conductive disk and the steel ball through the data acquisition card.

5. The capacitance method for measuring the thickness of the lubricating oil film between mechanical friction pairs based on a dynamic correction model according to claim 4 is characterized in that: Step 4, The calculation formula is as follows: ； Step 5, The calculation formula is as follows: 。 6. The capacitance method for measuring the thickness of the lubricating oil film between mechanical friction pairs based on a dynamic correction model according to claim 5 is characterized in that: Step 6 includes: completely separating the conductive disc and the steel ball, repeating steps 1, 2, 3, 4, and 5, and obtaining the capacitance of the measurement system when the conductive disc and the steel ball are completely separated as the background capacitance. .

7. The capacitance method for measuring the thickness of the lubricating oil film between mechanical friction pairs based on a dynamic correction model according to claim 6 is characterized in that: In step 7, the test is carried out under the selected working conditions, the charging curve is collected, and the capacitance of the system is measured according to the measurement process obtained in step 5. and the background capacitance obtained in step 6 Get the total capacitance of the equivalent capacitor inside and outside the Hertz contact area ; The calculation formula is as follows: 。 8. The capacitance method for measuring the thickness of the lubricating oil film between mechanical friction pairs based on a dynamic correction model according to claim 7 is characterized in that: In step 8, Dimensionless velocity parameter The calculation formula is: ,in, is the dynamic viscosity of the lubricant, is the entrainment speed set during the experiment; is the comprehensive curvature radius along the direction of motion speed when the conductive disk and the steel ball are in contact, which is equal to the radius of the steel ball; is the equivalent elastic modulus of the friction pair of the conductive disk and the steel ball; Dimensionless load parameters The calculation formula is: , is the normal load at the contact point between the conductive disk and the steel ball; Dimensionless material parameters The calculation formula is: ;in, is the viscosity-pressure coefficient of the lubricant.

9. The capacitance method for measuring the thickness of the lubricating oil film between mechanical friction pairs based on a dynamic correction model according to claim 8, characterized in that: In step 10, the capacitance inside the Hertz contact area The calculation formula is as follows: 。 10. The capacitance method for measuring the thickness of the lubricating oil film between mechanical friction pairs based on a dynamic correction model according to claim 9, characterized in that: Average lubricating oil film thickness value in the deformation area inside the Hertz contact zone The calculation formula is as follows: ； Where, is the dielectric constant in vacuum, is the relative dielectric constant of the lubricant; is the contact area of ​​the friction pair of the conductive disc and the steel ball, and the calculation formula is: , where π is the ratio of the circumference of a circle to the circumference of a circle, is the Hertz contact radius, and the specific calculation formula is: , where b is the radius of the steel ball.

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