2.1. Computational modelling
In this study, the PrePoMax open-source pre- and post-processor for the finite element method (FEM) [
36] was used to analyse the wear behaviour of rolling/sliding contacting mechanical elements (i.e. mating gear flanks). The solver used by PrePoMax is the open-source CalculiX FEM solver [
37] that does not support the wear analysis by itself, so the wear analysis was implemented into the PrePoMax. The linear Archard wear model [
38] for predicting the volume of worn material was chosen for this purpose. The model was originally developed based on experimental results of cyclic wear of metal specimens under dry contact conditions. The model is represented by the following equation:
where
is the volume of worn material,
is the dimensionless wear coefficient,
is the normal contact force,
is the surface hardness in contact, and
represents the sliding distance. To account for changes in the geometry of the contact surfaces during wear cycles, the aforementioned model, which neglects the geometry change, can be divided into multiple steps when used in numerical analyses using the FEM. In a multi-step numerical procedure, where the depth of wear needs to be calculated for each wear cycle [
39], Eq. (1) can be expressed as follows:
where
represents the depth of wear in the
i-th wear cycle,
denotes the normal contact pressure, and
represents the differential sliding distance. The total wear depth
over
wear cycles is then computed by the Eq. (3), summing up the wear depths of individual cycles. Such an approach enables the wear analysis to be independent of the model geometry, boundary conditions, and loading.
The multi-step method implemented in PrePoMax is based on the following assumptions: (i) the entire wear process can be divided into multiple repetitive wear cycles, (ii) wear occurs only on the softer surface in the contact pair (slave surface), (iii) the change in contact surface geometry during one wear cycle is negligible and (iv) the dimensionless wear coefficient is constant.
For each wear cycle, a finite element model specific to that cycle needs to be prepared. The model must accurately capture the evolving contact conditions during the cycle, requiring a nonlinear contact analysis solved in multiple increments. For an accurate result, the number of increments must be large enough to result in a smooth movement of the contact pressure over the finite element mesh of the contact surfaces.
The wear calculation for the cycle begins after the nonlinear contact analysis is completed. All wear parameters are computed only at those slave surface finite element nodes that were in active contact with the master surface during the wear cycle. The normal contact pressure
at the node
and the nodal relative sliding distance
are determined from the resulting contact fields. The wear depth at node
is then computed using the expression:
where the material properties
kj and
Hj are taken at node
j. Processing of each wear cycle concludes with the calculation of the change in the contact surface geometry due to wear, determined by the wear displacements at the nodes calculated from the wear depth. The node wear displacements due to wear in each cycle
are calculated by a scalar product:
where where
represents the node’s outward normal vector. The node’s outward normal vector is determined as an average value of the normal vectors of the FEs sharing the node
(
Figure 1). At the start of the next wear cycle, the wear displacements computed in the previous cycle are used to update the mesh. Thus, the simulation of the new wear cycle is carried out on a mesh where the wear from the previous cycles is considered
Applying the wear displacements to the mesh changes only the position of the surface nodes and thus deforms the shape of the finite elements, making all surface elements on the wear surface a little thinner. If the accumulated wear displacement is large enough, this might lead to unusable finite element shapes. This is especially problematic when a dense mesh is used on the contact surfaces to obtain an accurate contact pressure field since even a small wear displacement will deform a small finite element beyond being usable. To overcome this problem, local or global remeshing of the domain can be applied, or a method for mesh redistribution can be used. In PrePoMax software, a boundary displacement method (BDM) [
40] for mesh redistribution was introduced. This is a two-step procedure where the surface nodes are first moved, and then new positions of the internal nodes are computed. From many different approaches to the BDM, the positions of the internal nodes were determined based on the solution of the linear elastic solid body problem. This was the most convenient approach since it can be solved with the FEM method on the existing mesh. The method was integrated into PrePoMax by an additional BDM simulation step that is added after each wear cycle. In this simulation step, the mesh from the previous wear step is taken, uniform elastic properties are assigned to all elements, and a prescribed boundary condition is applied to all nodes on the surface. In the nodes where the wear occurs, the value of the prescribed displacement vector is equal to the wear displacement
and equal to
for all other nodes. After the BDM simulation step completes, the computed nodal displacements for all nodes from the BDM step are used to deform the mesh before the next wear step is started.
Running multiple wear steps one after the other and deforming the mesh after each step can lead to the occurrence of high-frequency oscillations (roughness) of the mesh (see
Figure 1), which results in an irregular contact pressure field. This causes an irregular wear displacement field, which additionally enhances the roughness of the contact surfaces and renders the results of the wear analysis unusable. A mesh smoothing algorithm on the wear displacement field was introduced to remove the high-frequency oscillations from the mesh. The smoothing algorithm is based on the Laplacian mesh smoothing procedure [
41], where the new position of the node
is computed from the positions of its neighbouring nodes
as:
where
equals the number of node neighbours. Mesh smoothing is applied after the wear displacement field is computed and before the BDM step is prepared. The effect of the smoothing step can be increased by running the smoothing algorithm multiple times, which can be defined by the user. All these features make wear analyses applicable and robust for a large variety of problems, and the user can prepare the wear analysis without additional coding using only the built-in user interface. This makes the implementation usable for many engineers since all these features are already available in the latest release version of the PrePoMax v1.4.0, available online [
42].
2.3. FE model of gear pair
The geometry of the gear pair is shown in
Figure 2, where the pinion is shown in grey, and the support gear is shown in green. Here, only one tooth of the gear and three teeth of the pinion are presented. It was due to the smaller stiffness of the pinion material (POM) if compared to the gear material (steel). The analysis of one load cycle was split into three simulation steps, as shown in
Figure 3: static step (contact initialisation), wear step (computation of wear) and BDM step (mesh redistribution). In the simplified model presented in
Figure 2, the reference points RP1 and RP2 have been defined in both axes of the gear pair. Here, RP1 was connected rigidly to the gear shaft, with the surfaces of the gear marked in red. Additionally, at this reference point, a torque of
T = 1.6 Nm was applied acting to the pinion tooth, and a boundary condition was imposed, which allowed the pinion to move only about its axis. The point RP2 was connected rigidly to the pinion surfaces marked in blue. At this reference point, a boundary condition was prescribed first in the static step, which prevented displacement and rotation in all directions of the coordinate system. As sliding was required for the wear calculation procedure, the boundary condition in RP1 in the second step (Wear step) required a rotation around the pinion axis of ρ = 0.285 rad. The angle of rotation was prescribed in such a way that the gear only reached the position where the gear started to return to the already worn surface.
The computational model implemented into the PrePoMax software (see
Section 2.1) was then used to analyse the wear behaviour of mating gear flanks. The numerical simulation of gear meshing consisted of three separate simulation steps, which followed one after the other. This simulation then repeats to account for the selected number of load cycles. Simulating each load cycle in multi-thousand load cycle analysis would require too much computational effort. At the same time, during one load cycle, the geometry of the model does not change significantly; thus, multiple load cycles can be merged into one simulation cycle that updates the mesh.
Archard’s model implemented in the PrePoMax software is limited to a 3D wear calculation, as it calculates the volume of wear. However, as the pinion and gear analysed in this study do not vary in thickness, it would be reasonable to simplify the model to 2D, but this was not possible. Therefore, a reduced thickness of 0.25 mm was assumed to reduce the computational time, and consequently, the load (torque) has also been reduced proportionally. Furthermore, an additional boundary condition has been added to both gears to prevent the gears from moving in the thickness direction. To best approximate the 2D model, a finite element mesh with prismatic finite elements was created using the boundary layer function, as shown in
Figure 4. The overall model was meshed with a global mesh size of 1 mm. However, for better accuracy, the model around the contact was meshed with a local mesh size of 0.05 mm, which resulted in 3300 parabolic prismatic finite elements. The model used a linear overclosure-pressure contact relation with stiffness
K = 105 N/mm
3 and coefficient of friction μ = 0.25 mm.
Two computational models have been considered in the numerical simulation. In the first model (Model 1), a wear calculation after 10,000 load cycles was determined. First, for the purpose of comparison with VDI 2736, the 10,000 load cycles were calculated in one simulation cycle, which means that the geometry was not updated during the calculation. The purpose of this computational model was to compare the results with the VDI 2736 calculation, which does not consider the geometry change. In the second model (Model 2), 10,000 load cycles were used again. However, if compared to the first model, the number of cycles was divided into ten simulation cycles, meaning that after every 1,000 load cycles, the geometry was updated. The wear results were thus transferred directly to the finite element mesh, and the wear was recalculated on the updated geometry.
2.4. Analytical approach according to VDI 2736
Following the calculation according to VDI 2736, a wear behaviour can be obtained for the tooth flank profile of the polymer gear. The calculation is position-dependent, as the wear is different at each point due to the involute shape of the tooth. Here, the wear starts to decrease from the tip of the tooth up to the pitch point. Due to the involute shape of the tooth, sliding dominates at the starting engagement point, then the tooth rolls at the pitch circle, and later, towards the root of the tooth, sliding increases gradually again, and rolling decreases. Therefore, in the pitch point region, it can be observed that the wear is theoretically zero. Considering this fact, the local wear of the gear flank can be obtained as follows:
where
bw is a common face width,
NL is the number of load cycles,
ζ is the specific local sliding of gear flanks,
kw is the wear coefficient, and
Fn is the normal force acting on the tooth flank
where
Td is the nominal torque,
dw is the pitch circle diameter, and α
t is the pressure angle in the transverse section. It follows from Eq. (8) that the normal force
Fn is a constant throughout the engagement line of gear flanks. However, its tangential and radial components change, resulting in a combination of sliding and rolling of gear flanks. When sliding is involved, a specific local sliding
ζ appears, which should be considered when determining the local wear using Eg. (7).
Figure 5 shows the sliding conditions in an arbitrary mating point Y before pitch point C. The circular velocities in an arbitrary mating point Y (
vy1=
ry1⋅ω
1 and
vy2=
ry2⋅ω
2), which are acting perpendicular to the lines O
1Y and O
2Y, can be distributed in the directions of common normal (
vny1 and
vny2) and common tangent (
vty1 and
vty2). Here, the normal velocity components must be the same, i.e.
vny1 =
vny2. Furthermore, the tangential components
vty1 and
vty2 can be obtained from the triangles O
1T
1Y and O
2T
2Y and the hatched triangles.
The sliding velocities of gear flanks in an arbitrary mating point Y (
vgy1 for a pinion and
vgy2 for a gear) are defined as a difference between the tangential velocity components
vty1 and
vty2 (
vgy1 =
vty1 −
vty2;
vgy2 =
vty2 −
vty1). From triangles O
1T
1C and O
2T
2C and with consideration O
1C =
dw1/2 and O
2C =
dw2/2, it follows:
Because the sum (ω1 + ω2) is a constant value, it is clear from Eq. (9) that the sliding velocity vgY only depends on the distance of point Y from the pitch point C. When an arbitrary mating point Y coincides with pitch point C, the sliding velocity equals zero.
Based on the sliding velocity
vgy1 in an arbitrary mating point Y, the specific local sliding of gear flanks
ζ is defined as [
44]:
Following the calculation procedure according to the VDI 2736, the wear behaviour of gear flanks is position-dependent due to different sliding velocities along the engagement line of mating gears. It is clear that the sliding velocities and, consequently, the wear of gear flanks are the highest at the starting and ending engagement points and then decrease toward pitch point C.