A calculation method for contact pressure distribution of piston pin hole

Abstract

A method for calculating the contact pressure distribution of a piston pin hole in the technical field of internal combustion engines includes the following steps: cutting a three-dimensional geometric model into a three-dimensional quarter geometric model, projecting the quarter geometric model onto a two-dimensional plane, drawing a two-dimensional geometric model and performing meshing; meshing the three-dimensional quarter geometric model, sampling the thickness distribution, and obtaining a thickness distribution interpolation function; constructing a variable thickness finite element model based on the two-dimensional grid and the thickness distribution interpolation function; performing contact detection on the basis of the finite element model and constructing a variable thickness finite element contact model; iteratively solving the variable thickness finite element contact model; and extracting the contact pressure distribution after the solution converges. While ensuring the equivalence of the contact pressure distribution trend, the present invention greatly reduces the scale of problem solving and improves the calculation efficiency, and can be applied to quickly calculate the contact pressure distribution results of piston pin holes under different profiles.

Description

Technical Field

[0001] The invention relates to a contact pressure calculation method in the technical field of internal combustion engines, in particular to a piston pin hole contact pressure distribution calculation method which can ensure the equivalence of contact pressure distribution trends and greatly reduce the problem solving scale. Background Art

[0002] With the increase of engine power and speed, the gas pressure and inertia force on the piston are getting higher and higher, and the mechanical load and thermal load on the piston pin hole are also getting higher and higher. A large number of engineering applications have shown that diesel engine piston pin hole wear is one of the main forms of piston failure. An effective way to solve the above failure problem is to improve the material properties of the friction pair. However, with the continuous increase in the degree of engine strengthening, the material properties of the piston pin hole are close to the use limit under the action of high explosion pressure and thermal load. Therefore, under the premise of limited material properties, adjusting the surface structure of the piston pin hole to make the stress distribution on the pin hole surface uniform and reduce the stress amplitude is the key way to reduce the failure of the piston pin bearing. The surface structure of the piston pin hole is achieved by designing the pin hole profile. Evaluating the effectiveness of the above profile is a key link in the design process. At present, this type of analysis is mostly evaluated by using the three-dimensional finite element method based on engineering experience. Although this method can characterize the differences in the contact surface stress levels of different profile structures, due to the nonlinear contact between the piston pin hole and the piston pin, it leads to complex solutions and difficult convergence, and consumes a lot of computing time. Therefore, it is of great significance to simplify high-dimensional complex problems and ensure trend equivalence, reduce calculation costs, and conduct comparative finite element analysis of multiple structural schemes for adjusting pin hole profiles. Summary of the invention

[0003] In view of the shortcomings of the prior art, the present invention proposes a piston pin hole contact pressure distribution calculation method, which is not only suitable for quickly calculating the piston pin hole contact pressure distribution results under different profiles, but also more suitable for promotion to engineering applications.

[0004] The present invention is realized by the following technical scheme, which includes the following steps: S1, pre-processing before analysis, drawing a two-dimensional geometric model, meshing the two-dimensional geometric model, and obtaining a two-dimensional grid; S2, sampling the thickness distribution according to the three-dimensional model, and obtaining a thickness distribution interpolation function; S3, constructing a variable thickness finite element model based on the two-dimensional grid and the thickness distribution interpolation function; S4, performing contact detection on the basis of the variable thickness finite element model, and constructing a variable thickness finite element contact model; S5, iteratively solving the variable thickness finite element contact model; S6, extracting the contact pressure distribution after the solution converges.

[0005] Furthermore, in step S1 of the present invention, the pre-analysis pre-processing comprises the following steps:

[0006] S11, cutting the three-dimensional geometric model of the piston pin hole-piston pin contact pair into three-dimensional quarter geometric models;

[0007] S12, projecting the quarter geometric model onto a two-dimensional plane to draw a two-dimensional geometric model;

[0008] S13, meshing the two-dimensional geometric model, wherein the mesh is a quadrilateral mesh.

[0009] Furthermore, in step S2 of the present invention, the medium thickness distribution sampling includes the following steps:

[0010] S21, the three-dimensional quarter model mesh is exported to STL format with only surface mesh;

[0011] S22. Set sampling points on the two-dimensional plane onto which the three-dimensional model is projected. Draw a ray starting from the sampling point s and passing through the surface mesh. Let the distance from the first point to the second point be t. s1 , in mm; the distance between the third and fourth points is t s2 , in mm; add up the distances that the straight line passes through all solid surfaces to get the corresponding solid thickness at the sampling point The unit is mm.

[0012] Furthermore, in step S3 of the present invention, constructing the variable thickness finite element model includes the following steps:

[0013] S31, boundary condition setting: apply pressure boundary load on the top of the piston skirt grid, apply pressure boundary load on the contact area between the piston pin and the connecting rod small end; apply x-direction displacement constraint on the symmetric surface of the piston pin and the piston skirt;

[0014] S32, material property definition: set the Young's modulus and Poisson's ratio of the piston skirt and piston pin respectively;

[0015] S33. Define contact relationship: set the piston pin hole as the slave surface and the piston pin as the master surface;

[0016] S34. Establish a variable thickness finite element stiffness matrix. The variable thickness unit stiffness matrix of the quadrilateral unit is calculated by the following formula:

[0017] K e =∫∫B T DXD

[0018] Where B is the unit strain matrix; D is the unit elastic matrix; t is the equivalent thickness of the unit, in mm.

[0019] S35. After establishing the stiffness matrix of each object, the node equivalent load vector is calculated based on the applied pressure boundary load and the unit thickness.

[0020] The unit stiffness matrix has a unit thickness factor, and different units have different thicknesses, so the introduction of the thickness factor t makes the unit used a variable thickness plane unit. The thickness of each unit should be taken as the equivalent thickness, usually the average thickness of the nodes of each unit is taken as the equivalent thickness, that is, Where t i is the node thickness in mm, which is calculated by the thickness distribution interpolation function.

[0021] Furthermore, in the present invention, step S4 includes the following steps:

[0022] S41. Traverse all possible node-to-segment combinations between the predefined master and slave surfaces and find a point x on the slave surface. 2 Corresponding to the nearest projection point on a segment of the main surface

[0023] S42. Calculate contact gap The unit is mm, where The nearest projection point on the segment The normal unit vector of

[0024] S43, determine whether contact occurs, the normal contact constraint condition requires the contact gap g N ≥0, so when g N When <0 and the projection point is within the segment, it is considered to be in contact, and the node-to-segment combination in contact is added to the valid constraint condition set;

[0025] S44. Use the penalty function method to introduce the weak form contribution generated by the effective constraint condition set into the system potential energy balance equation and calculate the contact stiffness matrix;

[0026] S45. Assemble the contact stiffness matrix and the stiffness matrix without considering the contact constraint to obtain the system tangent stiffness matrix.

[0027] Furthermore, in step S5 of the present invention, the iterative solution is: solving the finite element equation by the Newton-Raphson iteration method, solving the new displacement increment Δu in the i-th Newton iteration i , in mm, and through u i =u i-1 +Δu i Update displacement in mm. If the residual is less than the set tolerance under the current displacement field, the calculation is considered converged.

[0028] Furthermore, in step S6 of the present invention, the contact pressure distribution is extracted as follows: after the solution converges, the normal contact pressure and Mises stress of all slave nodes on the slave surface are extracted.

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows: by establishing a two-dimensional variable thickness finite element contact model, the present invention greatly reduces the scale of problem solving and improves calculation efficiency while ensuring the equivalence of contact pressure distribution trends. The present invention can be used to quickly calculate the contact pressure distribution results of piston pin holes under different profiles, is more suitable for promotion to engineering applications, and can be better applied to the design of piston pin hole-piston pin contact pairs. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a flow chart of the present invention;

[0031] Figure 2 This is a schematic diagram of a two-dimensional model and a grid in an embodiment of the present invention:

[0032] Figure 3 Schematic diagram of an STL grid used for thickness sampling in an embodiment of the present invention;

[0033] Figure 4 This is a schematic diagram of thickness sampling in an embodiment of the present invention;

[0034] Figure 5 Schematic diagram of boundary conditions in an embodiment of the present invention. DETAILED DESCRIPTION

[0035] In order to make the contents of the present invention easier to understand, the technical scheme of the present invention is further explained below in conjunction with specific embodiments. The examples are only used to illustrate the present invention, but the present invention is not limited to the contents. The operation methods in the following examples without specifying specific conditions are usually carried out under conventional conditions or according to the product instructions. The chemical reagents without specifying the manufacturer are all conventional drugs that comply with national standards and can be purchased on the market.

[0036] Example

[0037] like Figure 1 As shown, the present invention is implemented by the following technical solution, and the present invention comprises the following steps:

[0038] S1, perform pre-analysis pre-processing, draw a two-dimensional geometric model, and mesh the two-dimensional geometric model;

[0039] S2, sampling the thickness distribution according to the three-dimensional model to obtain a thickness distribution interpolation function;

[0040] S3, constructing a variable thickness finite element model based on a two-dimensional grid and a thickness distribution interpolation function;

[0041] S4. Perform contact detection on the basis of the variable thickness finite element model to construct a variable thickness finite element contact model;

[0042] S5, iteratively solving the variable thickness finite element contact model;

[0043] S6. After the solution converges, extract the contact pressure distribution.

[0044] First, the three-dimensional geometric model of the piston pin hole-piston pin contact pair is cut into three-dimensional quarter geometric models. Then the quarter geometric model is projected onto a two-dimensional plane, a two-dimensional geometric model is drawn, and the two-dimensional geometric model is meshed, such as Figure 2 As shown, the grid is a quadrilateral grid.

[0045] Then, the thickness distribution is sampled according to the three-dimensional model to obtain the thickness distribution interpolation function. Figure 3 As shown, first export the 3D quarter model mesh to the STL format with only the surface mesh; then set the sampling point on the 2D plane where the 3D model is projected, and make a ray through the surface mesh starting from the sampling point, as shown in Figure 4 As shown, let the distance from the first point to the second point be t s1 , in mm, the distance between the third and fourth points is t s2 , in mm, by adding up the distances that the straight line passes through all solid surfaces, we can get the corresponding solid thickness at the sampling point. The unit is mm.

[0046] A variable thickness finite element model is constructed based on a two-dimensional grid and a thickness distribution interpolation function. A pressure boundary load is applied to the top of the piston skirt grid. The load size is equivalently calculated based on the cylinder pressure at the moment of maximum explosion pressure. A pressure boundary load is also applied to the contact area between the piston pin and the small end of the connecting rod. An x-direction displacement constraint is applied to the symmetric surface between the piston pin and the piston skirt, such as Figure 5 As shown. Set the Young's modulus of the piston skirt to 165MPa, the Young's modulus of the piston pin to 210MPa, and the Poisson's ratio of the piston pin and the piston skirt to 0.3. Set the piston pin hole as the slave surface and the piston pin as the master surface. Establish the variable thickness finite element stiffness matrix. The variable thickness unit stiffness matrix of the quadrilateral unit is calculated by the following formula:

[0047] K e =∫∫B T DXD

[0048] In the formula, B is the unit strain matrix, D is the unit elastic matrix, and t is the unit thickness in mm. The unit stiffness matrix has a unit thickness factor, and different units have different thicknesses, so the introduction of the thickness factor t makes the unit used a variable thickness plane unit. The thickness of each unit should be taken as the equivalent thickness, usually the average value of the thickness at the node of each unit is taken as the equivalent thickness, that is, The unit is mm, where t i is the node thickness in mm, which is calculated by the thickness interpolation function. After establishing the stiffness matrix of each object, the node equivalent load vector is calculated according to the applied pressure boundary load and the unit thickness.

[0049] Contact detection is performed on the basis of the variable thickness finite element model to construct a variable thickness finite element contact model. Traverse all possible node-to-segment combinations between the predefined master and slave surfaces to find a point on the slave surface. Corresponding to the nearest projection point on a segment of the main surface Calculate contact clearance The unit is mm, where The nearest projection point on the segment Normal unit vector; to determine whether contact occurs, the normal contact constraint requires the contact gap g n ≥0, so when g N When <0 and the projection point is within the segment, it is considered to be in contact, and the node-to-segment combination in contact is added to the effective constraint condition set; the weak form contribution generated by the effective constraint condition set is added using the penalty function method. Introduce the system potential energy balance equation, where n c is the number of effective contact points, is the normal force from the node, in N, A s is the length of the segment, in mm, ∈ N is the size of the penalty parameter, in N / mm; Calculate the contact stiffness matrix, where

[0050]

[0051] The contact stiffness matrix is ​​assembled with the stiffness matrix without considering contact constraints to obtain the system tangent stiffness matrix.

[0052] The finite element equations are solved by the Newton-Raphson iteration method. In the i-th Newton iteration, the new displacement increment Δu is solved. i , the unit is mm, and through u i =u i-1 +Δu iUpdate displacement in mm. If the residual is less than the set tolerance under the current displacement field, the calculation converges.

[0053] After the solution converges, the normal contact pressure and Mises stress of all nodes on all slave surfaces are extracted.

[0054] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various changes or modifications within the scope of the claims, which does not affect the essence of the present invention. In the absence of conflict, the embodiments of the present application and the features in the embodiments can be combined with each other arbitrarily.

Claims

1. A method for calculating the contact pressure distribution of a piston pin hole, characterized in that: The following steps are involved: S1, perform pre-analysis pre-processing, draw a two-dimensional geometric model, mesh the two-dimensional geometric model, and obtain a two-dimensional mesh; S2, sampling the thickness distribution according to the three-dimensional model to obtain a thickness distribution interpolation function; S3. Construct a variable thickness finite element model based on the two-dimensional grid and thickness distribution interpolation function; S4. Perform contact detection on the basis of the variable thickness finite element model to construct a variable thickness finite element contact model; S5, iteratively solving the variable thickness finite element contact model; S6. After the solution converges, the contact pressure distribution is extracted; The step S3 of constructing the variable thickness finite element model includes the following steps: S31, boundary condition setting: apply pressure boundary load on the top of the piston skirt grid, apply pressure boundary load on the contact area between the piston pin and the connecting rod small end; apply x-direction displacement constraint on the symmetric surface of the piston pin and the piston skirt; S32, material property definition: set the Young's modulus and Poisson's ratio of the piston skirt and piston pin respectively; S33. Define contact relationship: set the piston pin hole as the slave surface and the piston pin as the master surface; S34. Establish a variable thickness finite element stiffness matrix. The variable thickness unit stiffness matrix of the quadrilateral unit is calculated by the following formula: K e =∫∫B T DBtdxdy Where B is the unit strain matrix; D is the unit elastic matrix; t is the equivalent thickness of the unit, in mm; S35, after establishing the stiffness matrix of each object, the node equivalent load vector is calculated according to the applied pressure boundary load and the unit thickness; The step S4 comprises the following steps: S41. Traverse all possible node-to-segment combinations between the predefined master and slave surfaces and find a point x on the slave surface. 2 Corresponding to the nearest projection point on a segment of the main surface S42. Calculate contact gap The unit is mm, where The nearest projection point on the segment The normal unit vector of ; S43, determine whether contact occurs, the normal contact constraint condition requires the contact gap g N ≥0, so when g N When <0 and the projection point is within the segment, it is considered to be in contact, and the node-to-segment combination in contact is added to the valid constraint condition set; S44. Use the penalty function method to introduce the weak form contribution generated by the effective constraint condition set into the system potential energy balance equation and calculate the contact stiffness matrix; S45. Assemble the contact stiffness matrix and the stiffness matrix without considering the contact constraint to obtain the system tangent stiffness matrix.

2. The piston pin hole contact pressure distribution calculation method according to claim 1, characterized in that The pre-analysis pre-processing in step S1 comprises the following steps: S11, cutting the three-dimensional geometric model of the piston pin hole-piston pin contact pair into three-dimensional quarter geometric models; S12, projecting the quarter geometric model onto a two-dimensional plane to draw a two-dimensional geometric model; S13, meshing the two-dimensional geometric model, wherein the mesh is a quadrilateral mesh.

3. The method for calculating the contact pressure distribution of the piston pin hole according to claim 1, characterized in that The thickness distribution sampling in step S2 includes the following steps: S21, the three-dimensional quarter model mesh is exported to STL format with only surface mesh; S22. Set sampling points on the two-dimensional plane onto which the three-dimensional model is projected. Draw a ray starting from the sampling point s and passing through the surface mesh. Let the distance from the first point to the second point be t. s1 , in mm; the distance between the third and fourth points is t s2 , in mm; add up the distances that the straight line passes through all solid surfaces to get the corresponding solid thickness at the sampling point The unit is mm.

4. The method for calculating the contact pressure distribution of the piston pin hole according to claim 1, characterized in that The equivalent thickness of the unit is calculated as follows: Where t i is the node thickness in mm, which is calculated by the thickness distribution interpolation function.

5. The piston pin hole contact pressure distribution calculation method according to claim 1, characterized in that: The iterative solution in step S5 is: solve the finite element equation by Newton-Raphson iteration method, and solve the new displacement increment Δu in the i-th Newton iteration. i , in mm, and through u i =u i-1 +Δu i Update displacement in mm; If the residual is less than the set tolerance under the current displacement field, the calculation is considered to be converged.

6. The piston pin hole contact pressure distribution calculation method according to claim 1, characterized in that: The extraction of contact pressure distribution described in step S6 is as follows: after the solution converges, the normal contact pressure and Mises stress of all slave nodes on the slave surface are extracted.

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