Study of the Stress–Strain State of the Structure of the GP-50 Support Bushing Manufactured by 3D Printing from PLA Plastic

Abstract

This article analyzes statistics on the failure of technological equipment, assemblies, and mechanisms of agricultural (and other) machines associated with the breakdown or failure of gear pumps. It was found that the leading causes of gear pump failures are the opening of gear teeth contact during pump operation, poor assembly, wear of bushings, thrust washers, and gear teeth. It has also been found that there is a problem related to the restoration, repair, and manufacture of parts in the conditions of enterprises serving the agro-industrial complex of the Republic of Kazakhstan (AIC RK). This is due to the lack of necessary technological equipment, tools, and instruments, as well as centralized repair and restoration bases equipped with the required equipment. This work proposes to solve this problem by applying AM technologies to the repair and manufacture of parts for agricultural machinery and equipment. The study results on the stress–strain state of support bushings under various pressures are presented, showing that a fully filled bushing has the lowest stresses and strains. It was also found that bushings with 50% filling and fully filled bushings have similar stress and strain values under the same pressure. The difference between them is insignificant, especially when compared to bushings with lower filling. This means that filling the bushing by more than 50% does not provide a significant additional reduction in stresses. In terms of material and printing time savings, 50% filling may also be the optimal option.

1. Introduction

The relevance of additive technology for the manufacture and restoration of agricultural machinery parts is primarily due to its time- and cost-efficiency. One of the most widely used products in agricultural engineering is hydraulic machinery, particularly gear pumps (GPs). Studies have shown that gear pumps (GPs) are most susceptible to wear and tear on components such as housings, bearing bushings, covers, shafts, and gears, which directly affect the efficiency and service life of the entire hydraulic system.

An analysis of failure statistics for technological equipment, assemblies, and mechanisms of agricultural (and other) machines related to the breakdown or failure of gear pumps was carried out at machine-building plants and repair bases of the agro-industrial complex of the Republic of Kazakhstan (AIC RK).

The analysis revealed that, from 2020 and 2025, more than 3000 pieces of equipment, units, and mechanisms of agricultural (and other) machines will require repair due to breakdowns and failures of gear pumps.

Table 1. Statistical data on types of GP failures.

No	Types of Failures	Causes of Failures	Percentage of Total Failures, %
1	Oil pressure fluctuations in the system	Gear teeth contact opening during pump operation	30%
2	Uneven operation of the unit or mechanism	Wear of the thrust bearing	25%
3	Oil leakage	Wear of bushings, thrust bearings, gear teeth, and other pump components	32%
4	Unit or mechanism shutdown	Poor pump assembly	13%
	Total number of failures of technological equipment, units, and mechanisms of agricultural (and other) machines related to GP breakdowns		100%

shows statistical data on the types of gear pump failures.

The table indicates that the share of GP failures due to oil leakage is 32%, primarily caused by the premature wear of GP parts, including bearing bushings, thrust washers, and gear teeth.

Figure 1 shows a photo of worn GP bearing bushings.

A pressing issue today is the restoration of parts and components that cannot be repaired using existing technologies. The lack of centralized repair and restoration facilities for servicing agricultural enterprises in Kazakhstan further exacerbates this problem. This problem is being solved most effectively by additive manufacturing (AM) technologies that have emerged in recent decades, which can play not only a technological but also an organizational role in machine repair. Additive technologies in repair will not only help restore parts but also enable manufacture of new ones. Parts of modern machine and equipment designs are characterized by the fact that they can be easily replaced with newly manufactured parts made from different materials and using alternative methods. This technology for repairing and restoring parts enables the proper service life of equipment at a relatively low cost.

The use of AM technologies in restoring worn parts is becoming a widespread and effective solution for ensuring the operational condition of technical equipment across various industries. In the agricultural sector, this is particularly relevant, as the use of AM technologies leads to a reduction in downtime of machinery and equipment due to technical issues and the time required to repair faults. It is worth noting that, in the design of agricultural machinery, the most vulnerable components are gear parts and rotating parts with specific mating surfaces. It is essential to note that the effectiveness of restoration is determined by ensuring adequate service life or resources between repairs.

Colleagues from the Belarusian Agrotechnical University have conducted similar studies yielding certain results in specific areas [1,2], particularly in the restoration of worn shafts using LENS technology. Traditionally, surfacing is used to restore shafts during repair, which often leads to shaft deformation. They used laser spraying, which helps to avoid this. Co-28Cr-4.5W powder was applied during spraying. The primary material is 9XC steel.

With LENS technology, different metals and alloys can also be used. The most important advantage is the simultaneous use of two or more materials. The equipment provides such a feed. Additionally, the technology enables the improvement of the workpiece’s surface quality. It is worth noting that, alongside the restoration process, a hardening process also occurs.

Study [3] emphasizes the significance of powder selection and mixing in laser powder bed fusion (PBF-LB/M), the most widely used 3D printing technology for metal parts. The authors developed a laboratory setup for mixing powders with minimal mechanical impact, which reduces degradation and ensures material stability during deposition. This is particularly important when restoring parts with high-performance requirements, such as bushings and bearings. The research [4] is devoted to a review of achievements in additive manufacturing of parts from cobalt and chromium alloys. These materials have high strength and wear resistance, making them promising for replacing or restoring bushings and bearings under high loads. LPBF, DED, and EBM technologies enable the production of strong yet complex parts with minimal defects.

The studies reviewed confirm the effectiveness and wide range of possibilities of AM technologies in the restoration and replacement of worn bushings, bearings, and other machine components. Modern approaches to powder mixing, temperature control, alloy selection, and thermal process modeling allow the creation of elements that are comparable or superior to the original in terms of quality. This is particularly relevant for agricultural machinery in Kazakhstan, where rapid and local restoration of equipment is required.

In [5], it is noted that laser powder bed fusion (PBF-LB/M) is the most widely used additive manufacturing (AM) technology for metal parts. The authors have developed a custom laboratory AM blender specifically designed to address these issues, with a focus on low-impact mixing to mitigate powder degradation. It should be noted that this study highlights the critical role of blender selection in AM and advocates further research to improve powder mixing methods to expand the capabilities and competitiveness of AM technologies.

In [6], a comprehensive overview of temperature-sensitive process parameters and their impact on the mechanical properties of manufactured parts is presented. Understanding how these parameters influence bond formation and material property changes enables optimization to maximize the potential of MEX-TRB/P-manufactured parts.

The mechanical properties of parts manufactured by MEX-TRB/P depend on the selection of a wide range of process parameters. The basic principle of MEX-TRB/P 3D printing is based on extrusion, which depends on the heating and cooling of materials. Heat distribution during MEX-TRB/P 3D printing is a complex process. The actual (coalescence) temperature is a determining factor that determines the bond quality formed and is the result of heat accumulation and loss during the deposition process. In MEX-TRB/P, the amount of heat available determines the bond quality of the formed material, which leads to changes in mechanical properties. Most materials have an optimal operating temperature at which these conditions are favorable for bond formation. The study [6] also notes that excessively high operating temperatures lead to a decrease in mechanical properties, which is likely due to changes in the material properties (or chemistry) that have not been widely studied.

An article [7] highlights the significant advancements in the AM of Co-Cr-based alloys, particularly their adoption in biomedical, aerospace, energy, and various industrial sectors. These alloys exhibit excellent mechanical, tribological, and corrosion resistance along with biocompatibility when processed via modern AM techniques, such as LPBF, DED, and EBM. However, achieving defect-free structures with optimal physical and mechanical properties remains a challenge. Innovations in alloy composition, process parameter optimization, and post-processing treatments hold the key to addressing these limitations.

The comprehensive overview and detailed analysis of Co-Cr additive manufacturing presented in this article offer valuable information for engineers, researchers, and manufacturers, stimulating the development of advanced Co-Cr-based alloys for next-generation applications.

Modeling additive manufacturing processes, such as wire arc additive manufacturing (WAAM), is becoming increasingly important for predicting the properties of materials and components before actual production [8].

The study [8] presents a finite element (FE) thermo-mechanical simulation of large-scale WAAM component production. The investigation focuses on various problems within the individual steps of the FE workflow, wherein ABAQUS influences the modeling of large-scale components. The investigations are founded upon a thermo-mechanically coupled FE model in ABAQUS 2020. For this purpose, several thermo-mechanical simulation models are set up to investigate meshing, element activation, and variations in process parameters.

WAAM processes demonstrate their technological advantages, especially in the context of large-scale components. The individual weld sizes used in the design of a component can vary significantly from its overall size, with the discrepancy reaching several orders of magnitude. When modeling WAAM, this discrepancy leads to a conflict in the geometric resolution of the modeled component. In this regard, the choice of element size is crucial, with smaller elements leading to a more accurate representation of the power distribution. It is essential to assess which variables play a significant role in determining the simulation result and to what extent simplifications can be applied. One such approach is to reduce the simulation size. In this approach, two-dimensional modeling is performed, where a section of the component is assembled using two-dimensional elements. In [8,9,10,11,12,13,14], simplification is achieved by modeling highly simplified geometries. The modeling performed in these studies illustrates the configuration of a wall consisting of separate welds that are superimposed on each other. This approach facilitates clear visualization of the heat source, enabling fundamental research on parameters to be conducted.

In [15], an analytical description of the volumetric additive manufacturing (VAM) process based on computed tomography is presented, with a focus on the influence of resin properties on product dimensions. The main issue addressed in this study is the estimation of the size limitation of objects produced using the VAM process, which is typically reported to be on the order of one centimeter. An analytical model has been developed to predict product size based on resin properties (penetration depth), bottle size (radius), and part formation time, yielding a significant correlation between these parameters. An analytical model for the VAM process has been developed to understand the relationship between geometric parameters and material properties, and it has been verified experimentally.

It is known [16] that additive manufacturing is the most commonly used technology for manufacturing complex products [17,18]. This technology stands out as a distinct group of technologies, the primary uniqueness of which lies in the production of a product using a layer-by-layer process based on 3D model data. Moreover, unlike traditional subtractive and molding technologies, AM does not use a blank, which significantly reduces waste.

The work [16] presents an experimental evaluation of the conditions for additively manufactured composites with continuous carbon fiber reinforcement for self-healing processes. Mechanical tests were conducted to evaluate the impact of heat treatment on the bending properties of composite samples. All samples were printed using the same printing parameters and materials. It was experimentally demonstrated that heat treatment enhances the interlayer properties of composites, leading to an increase in maximum load-bearing capacity and deformation resistance.

2. Materials and Methods

Research on the use of additive technology methods in part manufacture was conducted in the laboratory conditions of the Department of Technological Machines and Equipment (TME) at the S. Seifullin Kazakh Agrotechnical Research University (KATRU).

The TME Department laboratory features several types of 3D printers, including the Stratasys F170 3D printer (Stratasys Ltd., Eden Prairie, MN, USA), which is designed for precision printing with engineering plastics using FDM/FFF technology. This printer can be used to produce reliable accessories, prototypes, and machine parts. Figure 2 shows photos of the 3D printers available at the TME Department.

The most rapidly wearing part of the GP, the support bushings, was selected for computer modeling and subsequent printing on a 3D printer.

To manufacture various versions of support bushings on a 3D printer, the support bushings were modeled in CAD software for subsequent printing on a 3D printer and imported into MakerBot Print, a program designed for the MakerBot Replicator 3D printer. In MakerBot Print, the infill model density can be adjusted by changing the Infill Density parameter. The program allows one to view a cross-section of the part to assess the filling of its internal cavity.

The model imported into the MakerBot Print program was printed on a MakerBot Replicator 3D printer (Figure 3).

The MakerBot Replicator printer uses the FDM method. Fused deposition modeling (FDM) is a layer-by-layer 3D printing method in which a thermoplastic material (e.g., PLA, ABS, PETG) is heated and extruded through a nozzle, then layered to form the finished model.

Investigation of the Stress–Strain State of Support Bushings Under Various Pressures

The digital model was created in SolidWorks 2024 (Figure 4), and investigations were conducted to determine the values of stresses and strains that arise under various pressures on the printed 3D model.

The filling of the internal cavity of the part was determined by measuring the cross-section of the model printed on a 3D printer (Figure 4).

In this study, the boundary between the layers of printed PLA is treated as completely “bonded”. In SolidWorks Simulation terms, this is implemented by assigning a global bonded contact (no separation, no sliding). This contact causes the finite elements of adjacent layers to share common nodes, which eliminates any relative movement or separation between them. In essence, the model treats the multi-layer structure not as a set of separate plates but as a single, monolithic orthotropic body in which the interlayer strength and stiffness are identical to those of the material itself.

The outer wall thickness of the model was 1 mm. For the model with 25% filling, the inner wall thickness was 0.6 mm, and for the model with 50% filling, it was 1.5 mm.

In the bushing model, pressure was applied to its outer surface, and the seating end was fixed only against translational displacements to reproduce the actual installation of the part in the housing. The interlayer boundaries were considered completely “bonded”, and the contact with the shaft was friction-free, reflecting the presence of an oil gap. The material was specified as linearly elastic orthotropic, and calculations were performed in the elastic domain. The mesh was formed by second-order tetrahedra with automatic refinement in critical areas. The pressure is applied to the outer surfaces of the part, and fixation is carried out on the inner cylindrical surface (Figure 5). Before conducting the study, a calculation grid was applied to the model (Figure 5).

The bushing is manufactured using 3D printing from PLA plastic. The physical and mechanical properties of the material are presented in

Table 2. Physical and mechanical properties of the material.

Material	Density, kg/m3	Modulus of Elasticity, N/mm2	Poisson’s Ratio	Shear Modulus, N/mm2	Thermal Conductivity Coefficient, W/m·K
PLA plastic	1240	3000	0.35	1500	0.16

.

3. Results and Discussion

The filling of the part’s internal cavity can be assessed by examining the cross-section of the printed part.

The filling degree of the model’s internal cavity, as printed on a MakerBot Replicator 3D printer, affects the printing time (Figure 6).

The time difference between 0% and 100% filling is 171 min, which is more than double the printing time (Figure 7).

During numerical modeling, two characteristic pressure levels were set, reflecting the actual operating conditions of the GP series. The first, “low” range—1–3 MPa—corresponds to the contact pressure in the hydrodynamic clearance of the support bushings and fully complies with the GOST 19027-89 [19] requirements, which regulates the operation of low-pressure gear units with pressures up to 4 MPa. The second, “high” level—15 MPa—is accepted as the practical maximum for GP-10–GP-100 agricultural pumps, whose rated pressure is 16 MPa, with a permissible short-term increase to 20–21 MPa. This choice of ranges allows both idle (or intermediate) modes and regular loaded operation of the hydraulic system to be covered, ensuring the correctness and applicability of the design results to real operating conditions.

The deformation and stress of a hollow bushing with an internal space under various pressures are shown in Figure 8 and Figure 9. Under a pressure of 1 MPa, the hollow bushing deforms by 1.9 mm, under a pressure of 2 MPa by 2.8 mm, and under a pressure of 3 MPa by 3.5 mm (Figure 8).

In a bushing with a hollow interior, a stress of 146 MPa occurs at a pressure of 1 MPa, 188 MPa at a pressure of 2 MPa, and 218 MPa at a pressure of 3 MPa (Figure 9).

Other results for the deformation and stress of the hollow bushing under various loads are summarized in

Table 3. The occurrence of stresses and associated deformations in a bushing with 25% filling at various pressures.

No	Pressure, MPa	Stress, MPa	Deformation, mm
1	1	14.8	0.045
2	6	44.5	0.13
3	5	74.3	0.23
4	8	118	0.36

,

Table 4. Stress and associated deformation of a bushing with 50% filling at various pressures.

No	Pressure, MPa	Stress, MPa	Deformation, mm
1	1	4.2	0.012
2	3	12.5	0.038
3	8	33.4	0.098
4	12	50.1	0.14

and

Table 5. The occurrence of stresses and associated deformations in a bushing whose internal space is fully filled, under various pressures.

No	Pressure, MPa	Stress, MPa	Deformation, mm
1	1	2.5	0.008
2	5	12	0.043
3	12	31.1	0.1
4	15	38.9	0.13

and illustrated in the corresponding graphs (Figure 10 and Figure 11).

The study results showed that the hollow bushing experiences the most significant stresses and deformations under identical pressure. This is explained by the absence of internal material that could redistribute the pressure (Figure 8 and Figure 9).

A bushing with 25% filling shows a significant reduction in stresses and deformations compared to the hollow bushing but remains relatively susceptible to pressure (Table 3).

A bushing with 50% filling shows an even greater reduction in stresses and deformations, indicating an increase in structural rigidity (Table 4).

A fully filled bushing has the lowest stresses and deformations, indicating maximum stiffness and resistance to mechanical pressure (Table 5).

A critical analysis of the calculated data showed that the load-bearing capacity of the bushing is determined by the degree of internal filling. Hollow and 25% filled bushings are unsuitable: their calculated stresses exceed the static strength of PLA several times. At 50% filling, the stress drops to 50 MPa and only slightly falls short of the material limit, so such a bushing is only suitable for short-term static tasks; dynamic loads and pressure pulsations will quickly put it out of action. A fully filled design maintains stress within 39 MPa (Figure 10 and Figure 11), providing a minimum acceptable safety margin; however, this strength is only maintained up to a temperature of ~40 °C. At this temperature threshold, the modulus and limit of PLA drop significantly, and the bushing loses its rigidity. Thus, PLA bushings can only serve as a temporary, economical solution at low temperatures and in static conditions; for long-term operation, dynamic loads, or warmer operating environments, either a full PLA infill with reinforced walls or a switch to durable engineering polymers (PETG, PA-12, PC) or metal is required.

Figure 12 shows photos of the assembly process of the GP with bushings made of plastic printed on a MakerBot Replicator 3D printer.

When checking the dimensions of the support bushings, no deviations from the specified dimensions in the drawing were found. The assembly of the support bushings was performed with high precision. Currently, work is underway to assemble a special test bench for testing the GP with support bushings made of PLA plastic.

4. Conclusions

Research conducted at agricultural enterprises in Kazakhstan has revealed a problem with the repair and manufacture of parts for agricultural machinery and equipment. Due to the lack of centralized repair and restoration facilities, it is impossible to carry out repair and restoration work, as well as technical maintenance of agricultural machinery and equipment, in a timely and high-quality manner. This problem can be solved by using AM technologies for the repair and manufacture of parts for agricultural machinery and equipment.

The load-bearing capacity of 3D-printed PLA bushings is determined by two key factors: the degree of internal filling and the operating temperature. The study results on the stress–strain state of support bushings under various loads showed that the hollow and 25% filled variants are unsuitable, as their calculated stresses exceed the static strength of the material many times over. A 50% filled bushing maintains stresses near the PLA limit only in a quasi-static mode. Still, it quickly fails under vibration and pressure pulsations, as the fatigue strength of PLA is significantly lower than its static strength. A fully filled bushing demonstrates a minimally acceptable safety margin in static conditions, but this margin is only maintained up to ~40 °C; further heating sharply reduces the stiffness and strength of PLA.

As a result, PLA bushings can only be considered as a quick and cost-effective solution for short-term repair of hydraulic components at low temperatures and without dynamic loads. For long-term or dynamic operation, as well as at temperatures above 40 °C, either the filling and wall thickness should be increased to achieve a practically continuous structure or PLA should be replaced with more stable engineering polymers (such as PETG, PA-12, or PC) or metal parts.

It is planned to conduct tests of the GP with plastic support bushings in both laboratory and production conditions to compare the computer modeling results.