A method for predicting frictional starting noise of an engineering plastic under grease lubrication

Abstract

The application discloses a kind of engineering plastics friction start noise prediction methods under grease lubrication.First, the pin disc sample of the engineering plastics friction pair to be measured is prepared, and the grease to be measured is applied at the contact interface of the pin disc sample;Then, the pin disc sample to which the grease to be measured is applied is tested on a friction and wear testing machine to obtain the transient friction coefficient time curve of the start-up stage;And the pin disc sample to which the grease to be measured is applied is tested on a friction and wear testing machine to obtain the Stribeck curve;Finally, according to the transient friction coefficient time curve of the start-up stage and the Stribeck curve, it is predicted whether the current engineering plastics friction pair to be measured will produce friction noise under the current working condition of the grease to be measured.The application method comprehensively covers the transient energy burst and the double instability mechanism of steady-state negative damping, effectively and accurately predicts whether the engineering plastics friction pair produces friction start noise.

Description

Technical Field

[0001] This invention belongs to the field of tribological testing technology, specifically relating to a method for predicting the frictional start-up noise of engineering plastics under grease lubrication. Background Technology

[0002] Engineering plastics and their composites are widely used in core transmission components (such as gears, guide rails, and bearings) of high-end equipment such as precision machinery, automobiles, and medical devices due to their excellent self-lubricating properties, high specific strength, and low manufacturing cost. However, as applications evolve towards high-end, quieter operation, engineering plastic friction pairs are highly susceptible to "stick-slip" dynamic instability induced by interfacial tribological behavior during startup or extremely low-speed operation, resulting in transient frictional noise. This unsteady-state startup noise not only severely degrades the acoustic quality of the product but is also typically accompanied by accelerated interfacial wear and shortened fatigue life.

[0003] Currently, industrial practice mainly relies on trial-and-error methods, which involve repeatedly replacing grease or component materials in actual transmission assemblies. This is not only time-consuming and costly but also highly unpredictable. In academic research, mainstream methods heavily depend on multiphysics monitoring and complex numerical simulations. For example, acoustic emission sensors or high-frequency vibration accelerometers are used to capture the energy release from microscopic protrusion fractures. This dynamic monitoring method requires extremely expensive specialized acoustic equipment and cumbersome signal processing procedures, which completely fails to meet the practical needs of industry for large-scale, rapid, and low-cost screening of transmission materials.

[0004] Furthermore, existing research has shown that the frictional noise induction mechanism of engineering plastics is highly dependent on the specific material pairing system. Relying solely on a single conventional macroscopic mechanical characteristic or a single tribological testing method often leads to serious underestimation or misjudgment because it ignores the potentially complex excitation mechanisms under different material contact systems.

[0005] Therefore, there is an urgent need for an efficient evaluation method that can comprehensively and accurately predict the starting noise of different engineering plastic friction pairs without relying on complex high-frequency acoustic equipment, using only conventional tribological parameters. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a method for predicting the frictional start-up noise of engineering plastics under grease lubrication. The method proposed in this invention breaks away from the diagnostic failure problems of single-mechanism methods when facing different material pairings and the noise prediction methods that rely on traditional macroscopic mechanical parameters or simple empirical trial and error. By comprehensively implementing a "dual-criteria" test of low-speed start-up and continuous step acceleration, it achieves accurate identification of noise risks in any engineering plastic friction pair.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] S1: Prepare the pin-disc sample of the engineering plastic friction pair to be tested and apply the grease to be tested at the contact interface of the pin-disc sample.

[0009] S2: On a friction and wear testing machine, a starting test is performed on a pin-disc sample with the grease to be tested, and the transient friction coefficient time curve during the starting phase is obtained.

[0010] S3: Perform continuous step acceleration tests on the pin disc sample with the grease to be tested on the friction and wear testing machine to obtain the Stribeck curve;

[0011] S4: Based on the transient friction coefficient time curve and Stribek curve during the startup phase, predict whether the engineering plastic friction pair under test will generate friction noise during startup under the current grease condition.

[0012] The engineering plastic friction pairs to be tested include metal-engineering plastic friction pairs or pure engineering plastic friction pairs.

[0013] S2 includes:

[0014] A constant low-speed start-up test was conducted on a friction and wear testing machine on a pin disc sample with the grease to be tested applied. The sliding speed in the constant low-speed start-up test was 1 mm / s to 5 mm / s.

[0015] S3 includes:

[0016] The discrete velocity nodes of the continuous step acceleration test are distributed in the velocity range of 1 mm / s to 500 mm / s. Each velocity node needs to be maintained for a set time to obtain the steady-state dynamic friction coefficient corresponding to each discrete velocity node. The Stribek curve is formed by the steady-state dynamic friction coefficients corresponding to several discrete velocity nodes in the range of 1 to 500 mm / s.

[0017] S4 includes:

[0018] Extract the initial maximum static friction coefficient μ from the transient friction coefficient time curve. s and steady-state dynamic friction coefficient μ k The steady-state dynamic friction coefficient μ k The closest value to the initial maximum static friction coefficient μ s The steady-state dynamic friction coefficient will be the initial maximum static friction coefficient μ. s and steady-state dynamic friction coefficient μ k The difference is denoted as the difference between the dynamic and static friction coefficients, Δμ, where Δμ = μ s -μ k ;

[0019] Extract the friction coefficient-relative velocity slope K of the Stribek curve in the low-speed range, K=dμ / dv, where μ is the friction coefficient and v is the relative velocity.

[0020] If the difference between the dynamic and static friction coefficients is less than or equal to the preset threshold δ (i.e., Δμ≤δ) and the slope K of the Stribeck curve in the low-speed range is always greater than 0 (i.e., K>0), then the engineering plastic friction pair under test will not generate friction noise when starting under the current grease condition. If the difference between the dynamic and static friction coefficients is greater than the preset threshold δ (i.e., Δμ>δ) or the slope K of the Stribeck curve in the low-speed range is less than or equal to 0 (i.e., K≤0), meaning there is a negative slope or zero slope segment, then the engineering plastic friction pair under test will generate friction noise when starting under the current grease condition. It is necessary to change the type of engineering plastic friction pair or grease under test according to actual needs.

[0021] If Δμ>δ and K≤0 are satisfied simultaneously, it is determined that there is a mechanism coupling effect between transient energy release and steady-state negative damping in the friction pair system, and it is defined as an extremely high start-up noise risk level.

[0022] The nominal contact compressive stress range of the pin disc specimen on the friction and wear testing machine is set to 4 MPa to 30 MPa.

[0023] Compared with existing technologies, the beneficial effects of the present invention are as follows:

[0024] (1) A comprehensive screening mechanism without blind spots has been constructed: This invention does not presuppose a specific noise mechanism of the material under test, but performs two tests simultaneously or sequentially on all friction pairs: transient (extracting the difference between dynamic and static friction coefficients) and steady-state (extracting negative slope). As long as any instability mechanism is triggered, an alarm can be accurately triggered, effectively avoiding the omission of a single evaluation standard and greatly improving the accuracy of noise prediction.

[0025] (2) Standardized testing methods and strong engineering applicability: This invention does not require complex microscopic analysis. It only requires performing standard low-speed start-up and continuous step acceleration tests on a conventional friction and wear testing machine to achieve accurate judgment of noise tendency.

[0026] (3) Great potential for widespread application: This invention bridges the key gap between macroscopic tribological characteristics and interfacial dynamic instability behavior, providing an extremely accurate and practical engineering tool for the directional material selection and lubrication optimization of various engineering plastic transmission components. Attached Figure Description

[0027] Figure 1 This is a time curve of the transient friction coefficient under a certain lubricating grease working condition in Embodiment 1 of the present invention.

[0028] Figure 2 This is the steady-state Stribeck curve extracted under the corresponding operating condition in Embodiment 1 of the present invention.

[0029] Figure 3 This is a time curve of the transient friction coefficient under a certain lubricating grease working condition in Embodiment 2 of the present invention.

[0030] Figure 4 This is the steady-state Stribeck curve extracted under the corresponding operating condition in Embodiment 2 of the present invention.

[0031] Figure 5 This is a time curve of the transient friction coefficient under safe operating conditions in Embodiment 3 of the present invention.

[0032] Figure 6 This is the steady-state Stribeck curve extracted under the corresponding safe operating condition in Embodiment 3 of the present invention.

[0033] Figure 7 This is a flowchart of the method of the present invention. Detailed Implementation

[0034] To make the objectives, technical mechanisms, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are for illustrative purposes only and are not intended to limit the scope of protection of this invention.

[0035] Macroscopic frictional start-up noise in engineering plastics is caused by high-frequency vibrations of the system resulting from interfacial "stick-slip" dynamic instability. This invention proposes a method for predicting frictional start-up noise in engineering plastics under grease lubrication, which reveals two independent and parallel physical mechanisms inducing this instability:

[0036] First, the transient strain energy release mechanism. At the moment of low-speed start-up, if the initial maximum static friction coefficient μ at the interface... s Significantly higher than the subsequent steady-state coefficient of dynamic friction μ k A large difference in the coefficient of friction (Δμ) injects excessive elastic strain energy into the mechanical system during the "sticky" phase. At the instant of the "stick-motion" transition, this energy is released rapidly, directly exciting macroscopic structural vibrations. The threshold δ set in this invention represents the critical difference in the coefficient of friction at which the released strain energy is sufficient to overcome the intrinsic structural damping of the system and generate audible noise.

[0037] Second, the steady-state velocity weakening mechanism. According to classical tribodynamics, if the coefficient of friction decreases with increasing velocity during steady-state continuous sliding (i.e., the Stribeck curve exhibits a negative slope, K≤0), this introduces a "negative damping" term into the system's calculus. When this interfacial negative damping exceeds the viscoelastic structural damping of the polymer material itself, the system will continuously absorb energy, eventually leading to steady-state sliding instability and continuous sharp noise.

[0038] like Figure 7 As shown, the method for predicting frictional starting noise of engineering plastics under grease lubrication proposed in this invention includes the following steps:

[0039] S1: Prepare the pin-disc specimen of the engineering plastic friction pair to be tested and apply the grease to be tested at the contact interface of the pin-disc specimen; the nominal contact compressive stress range of the pin-disc specimen on the friction and wear testing machine is set to 4MPa to 30MPa.

[0040] S2: On a friction and wear testing machine, a starting test is performed on a pin-disc sample with the grease to be tested, and the transient friction coefficient time curve during the starting phase is obtained.

[0041] Specifically, it includes:

[0042] A constant low-speed start-up test was conducted on a friction and wear testing machine on a pin disc sample with the grease to be tested applied. The sliding speed in the constant low-speed start-up test was 1 mm / s to 5 mm / s.

[0043] In one feasible implementation, in order to accurately capture the sudden change in static friction peak within a very short time, the sampling frequency of the transient friction coefficient time curve is set to ≥1000Hz.

[0044] S3: Perform a continuous step acceleration test on the pin disc sample with the grease to be tested on a friction and wear testing machine to obtain the Stribek curve.

[0045] The specific operational procedures and physical mechanisms for continuous step acceleration testing are defined as follows:

[0046] (1) Discretization of velocity nodes: Unlike the conventional continuous linear acceleration mode, this invention divides the set target sliding test range (1 mm / s to 500 mm / s) into several discrete velocity step nodes.

[0047] (2) Forced steady-state maintenance: After running to each discrete speed node, the friction and wear tester must maintain a constant relative sliding speed for a set duration (e.g., 5 minutes). The physical purpose of forced constant speed maintenance is to eliminate the transient hydrodynamic pressure change and extrusion film effect caused by rapid acceleration, and to ensure that the contact interface between the engineering plastic and the grease reaches sufficient thermodynamic and interfacial rheological equilibrium.

[0048] (3) Steady-state data extraction: Discard the fluctuation data of the speed jump transition stage, and extract only the average dynamic friction coefficient of the constant speed maintenance stage as the only steady-state characteristic value under the discrete speed node.

[0049] By obtaining the steady-state dynamic friction coefficients corresponding to step nodes at different speeds through continuous step acceleration tests, a Stribek curve that truly represents the steady-state dynamic evolution of the interface is then fitted and constructed.

[0050] S4: Based on the transient friction coefficient time curve and Stribek curve during the startup phase, predict whether the engineering plastic friction pair under test will generate friction noise during startup under the current grease condition.

[0051] Specifically, it includes:

[0052] First, extract the initial maximum static friction coefficient μ from the transient friction coefficient time curve. s and steady-state dynamic friction coefficient μ k The steady-state dynamic friction coefficient μ k The closest value to the initial maximum static friction coefficient μ s The steady-state dynamic friction coefficient will be the initial maximum static friction coefficient μ. s and steady-state dynamic friction coefficient μ k The difference is denoted as the difference between the dynamic and static friction coefficients, Δμ, i.e., Δμ = μ s -μ k .

[0053] Next, extract the friction coefficient-relative velocity slope K from the Stribek curve in the low-speed range, K=dμ / dv, where μ is the friction coefficient and v is the relative velocity.

[0054] If the difference between the dynamic and static friction coefficients is less than or equal to the preset threshold δ (i.e., Δμ≤δ) and the slope K of the Stribeck curve in the low-speed range is always greater than 0 (i.e., K>0), then the engineering plastic friction pair under test will not generate friction noise when starting under the current grease condition. If the difference between the dynamic and static friction coefficients is greater than the preset threshold δ (i.e., Δμ>δ) or the slope K of the Stribeck curve in the low-speed range is less than or equal to 0 (i.e., K≤0), meaning there is a negative slope or zero slope segment, then the engineering plastic friction pair under test will generate friction noise when starting under the current grease condition. It is necessary to change the type of engineering plastic friction pair or grease under test according to actual needs.

[0055] The low-speed range of the Stribeck curve is a well-defined concept, representing an operating range with clear tribological significance and numerical boundaries. In tribodynamics theory, this low-speed range corresponds to the initial stage of the transition from boundary lubrication to mixed lubrication. During this stage, the interfacial hydrodynamic effect is not yet fully established, and the frictional behavior is strongly influenced by the coupling effect of microscopic roughness peak contact and the rheological properties of the lubricating grease. This is the core region inducing negative damping instability.

[0056] In specific engineering practice and the standardized testing of this invention, the low-speed range is the relative sliding linear velocity of 0~100mm / s. Extracting the friction coefficient-relative velocity slope K within this range can most sensitively capture the negative damping (K≤0) characteristics of the system caused by the velocity weakening effect, thereby achieving accurate prediction of the starting noise of friction pairs such as pure engineering plastics.

[0057] If Δμ>δ and K≤0 are satisfied simultaneously, it is determined that there is a mechanism coupling effect between transient energy release and steady-state negative damping in the friction pair system, and it is defined as an extremely high start-up noise risk level.

[0058] The preset threshold δ is not a single fixed constant, but a critical abrupt change value determined by the equivalent structural stiffness and intrinsic damping of the friction and wear testing machine or the actual transmission component.

[0059] Specifically, the preset threshold δ can be determined in the following two ways:

[0060] (1) Relative volatility calibration method: In order to eliminate the difference in absolute friction force caused by the change of normal load, δ can be defined as the steady-state dynamic friction coefficient μ k The percentage threshold.

[0061] (2) Absolute experience limit method: Operators can collect several greases with known actual noise performance to conduct pin-disc friction and wear tests as benchmark tests, obtain benchmark thresholds, and use them as thresholds for subsequent mass screening of new materials and new grease formulations.

[0062] The method proposed in this invention is described in detail below through three typical embodiments. Through extensive benchmark testing, the preset threshold δ in all three embodiments was set to 0.01.

[0063] Example 1

[0064] The system under test is a friction pair between a 65Mn metal pin and a polyoxymethylene (POM-H) plastic disc under grease lubrication.

[0065] The specific steps of the method are as follows:

[0066] S1: Prepare standard specimens of 65Mn metal pin and POM-H plastic disc, and apply EM-30L grease to the contact interface. The contact compressive stress is set to 19.10MPa.

[0067] S2: Perform a constant low-speed start-up test of 2.44 mm / s on the standard sample using a friction and wear testing machine, with the sampling frequency set to 1000 Hz. Record as follows: Figure 1 The transient friction coefficient curve shown is as follows. Figure 1 A sudden change in the static friction peak value appeared within the dashed box.

[0068] S3: Perform continuous step acceleration test, with discrete nodes set between 2.44 and 488 mm / s. Each node is held for 5 minutes to collect the steady-state dynamic friction coefficient. The Stribeck curve is plotted as follows: Figure 2 As shown.

[0069] S4: Under this condition, the interface exhibits a large static friction peak, which then drops sharply to the steady-state dynamic friction range. The calculated difference between the static and dynamic friction coefficients, Δμ, is 0.15, which is significantly greater than the preset threshold of 0.01. Within this range, K>0, and no negative slope is observed. Overall judgment: Although the system does not exhibit a steady-state speed weakening effect (K>0), the detection of Δμ far exceeding the threshold triggers the first instability criterion. It is determined that this friction pair will inevitably generate starting noise under this grease lubrication.

[0070] Therefore, the friction pair formed by the 65Mn metal pin and the POM-H plastic disc generates frictional starting noise, which is dominated by the difference between the transient dynamic and static friction coefficients. The method of this invention effectively avoids potential omissions that may result from relying solely on the Stribeck curve.

[0071] Example 2

[0072] The system under test is a friction pair between a polyamide (PA66) plastic pin and a polyoxymethylene (POM-C) plastic disc under grease lubrication.

[0073] The specific steps are as follows:

[0074] S1: Prepare standard samples of PA66 plastic pins and POM-C plastic discs, apply GLY 801 grease, and set the contact compressive stress to 6.08 MPa.

[0075] S2: A low-speed start-up test was performed at 2.44 mm / s, with a sampling frequency of 1000 Hz. The transient friction coefficient curve is shown below. Figure 3 As shown.

[0076] S3: Perform continuous step acceleration tests (2.44 to 268.4 mm / s), and plot the Stribeck curve as shown below. Figure 4 As shown.

[0077] S4: Under this condition, the transient friction coefficient curve transitions extremely smoothly, with no obvious static friction peak, and Δμ is close to 0 (Δμ≤δ). Relying solely on traditional static testing would incorrectly classify the system as silent. The Stribeck curve shows a significant friction coefficient slippage across the low-speed range, subsequently remaining flat, with negative and zero slope segments (K≤0) within the low-speed range. Although the system does not exhibit a sudden drop in transient friction coefficient, the strong velocity weakening effect triggers the second instability criterion. It is determined that the system will generate significant noise during startup due to the system's negative damping effect.

[0078] Therefore, the frictional starting noise generated by the friction pair between the polyamide (PA66) plastic pin and the polyoxymethylene (POM-C) plastic disc is dominated by the steady-state velocity weakening effect. The method of this invention compensates for the blind spots caused by relying on single dynamic and static friction tests for pure engineering plastic systems.

[0079] Example 3

[0080] For the PA66 and POM-C friction pair in Example 2, PG-54 grease was selected.

[0081] After applying PG-54 grease to the sample of the PA66 and POM-C friction pair, the same test procedure was performed again. Figure 5 The transient friction coefficient curve under this lubrication condition shows no static friction peak (Δμ≤δ). Figure 6 The Stribeck curve shows a continuously rising positive slope trend (K>0) across the entire speed range.

[0082] Under the current operating conditions, the friction pair simultaneously satisfies the conditions of no excessive elastic strain energy release (Δμ≤δ) and positive system damping (K>0). Neither criterion is triggered. Therefore, it is determined that the friction pair is absolutely safe to start and will not generate any macroscopic friction start-up noise. It can be used as a reliable selection scheme for practical engineering.

[0083] Therefore, the method of the present invention can make a noise-free determination, thereby screening out the safe operating conditions corresponding to each friction pair.

Claims

1. A method for predicting frictional starting noise in engineering plastics under grease lubrication, characterized in that, Includes the following steps: S1: Prepare the pin-disc sample of the engineering plastic friction pair to be tested and apply the grease to be tested at the contact interface of the pin-disc sample. S2: On a friction and wear testing machine, a starting test is performed on a pin-disc sample with the grease to be tested, and the transient friction coefficient time curve during the starting phase is obtained. S3: On a friction and wear testing machine, a pin-disc sample with the grease to be tested is subjected to a continuous step acceleration test to obtain the Stribek curve; S4: Based on the transient friction coefficient time curve and Stribek curve during the startup phase, predict whether the current engineering plastic friction pair under test will generate friction noise during startup under the current grease condition. S4 includes: Extract the initial maximum static friction coefficient μ from the transient friction coefficient time curve. s and steady-state dynamic friction coefficient μ k The steady-state dynamic friction coefficient μ k The closest value to the initial maximum static friction coefficient μ s The steady-state dynamic friction coefficient will be the initial maximum static friction coefficient μ. s and steady-state dynamic friction coefficient μ k The difference is denoted as the difference between the dynamic and static friction coefficients; If the difference between the dynamic and static friction coefficients is less than or equal to the preset threshold δ and the slope K of the friction coefficient-relative velocity of the Stribeck curve in the low-speed range is always greater than 0, then the engineering plastic friction pair under test will not generate friction noise when starting under the current grease condition; if the difference between the dynamic and static friction coefficients is greater than the preset threshold δ or the slope K of the friction coefficient-relative velocity of the Stribeck curve in the low-speed range is less than or equal to 0, then the engineering plastic friction pair under test will generate friction noise when starting under the current grease condition.

2. The method for predicting frictional starting noise of engineering plastics under grease lubrication according to claim 1, characterized in that, The engineering plastic friction pairs to be tested include metal-engineering plastic friction pairs or pure engineering plastic friction pairs.

3. The method for predicting frictional starting noise of engineering plastics under grease lubrication according to claim 1, characterized in that, S2 includes: A constant low-speed start-up test was conducted on the pin disc sample with the grease to be tested applied on the friction and wear testing machine. The sliding speed in the constant low-speed start-up test was 1 mm / s to 5 mm / s.

4. The method for predicting frictional starting noise of engineering plastics under grease lubrication according to claim 1, characterized in that, S3 includes: The discrete velocity nodes of the continuous step acceleration test are distributed in the velocity range of 1 mm / s to 500 mm / s. The steady-state dynamic friction coefficient corresponding to each discrete velocity node is obtained, and the Stribek curve is formed by the steady-state dynamic friction coefficients corresponding to several discrete velocity nodes in the range of 1 to 500 mm / s.

5. The method for predicting frictional starting noise of engineering plastics under grease lubrication according to claim 1, characterized in that, The nominal contact compressive stress range of the pin disc specimen on the friction and wear testing machine is set to 4 MPa to 30 MPa.

Images