Evaluating Interference Fit Performance in Thin-Walled Stainless Components

Finite element analysis was employed to simulate the local stress experienced by a thin-walled component during interference fitting, allowing for the determination of the equivalent stress (σv) in the part. The analysis focused on identifying the locations of plastic deformation that occurred during the press fitting process. Additionally, the assembly structure was optimized to minimize issues such as bending and tearing during interference fitting, ultimately improving the yield rate.

 

1 Introduction

Interference fit is a critical assembly process in modern machinery. Components that rely on interference fit must be capable of withstanding significant axial forces and impact loads. The effectiveness of the assembly process directly influences the overall performance of the product. However, several factors can affect assembly quality, including manufacturing errors in the parts, the assembly techniques used, and the precision of the assembly itself. These factors often hinder the connection components from meeting the actual requirements, particularly when using materials that undergo significant elastic deformation.

Excessive pressure during assembly can negatively impact the assembly effectiveness, causing slight relative movement at the connection surfaces. This movement can lead to wear, which in turn compromises the stability and durability of the product.

Stainless steel is commonly used in fields such as aerospace, automotive manufacturing, and precision instruments. While stainless steel offers excellent mechanical properties and corrosion resistance, its high hardness and elastic modulus can lead to considerable stress and deformation during interference fits. Additionally, its relatively high coefficient of thermal expansion can exacerbate dimensional changes during hot and cold fitting processes, increasing the difficulty of assembly.

Due to product requirements, stainless steel components are typically thin-walled, making them vulnerable to deformation under external forces. Excessive or uneven force during interference fits can result in plastic deformation or even fracture. Moreover, the stringent precision requirements for the mating surfaces of thin-walled parts mean that even minor deformations or damages can significantly affect assembly quality and performance.

To address these challenges, this paper analyzes and explores the interference fit process for thin-walled stainless steel parts. This analysis not only aims to enhance assembly accuracy and reliability but also provides valuable technical support for mechanical manufacturing and maintenance in related fields.

 

2. Analysis of the Interference Fitting Process

The dimensions of the non-standard part are illustrated in Figure 1. This part is utilized in the insert tray of a tool coating equipment. Since only the cutting edge needs to be coated during the coating process, it is essential for the tool neck and shank to fit tightly within the stainless steel cylinder. A loose fit can lead to uneven coating due to inconsistent internal and external pressure.

During operation, the thin-walled stainless steel cylinder experiences significant wear. To ensure interchangeability, a press-fitting process is typically employed. This part is created by press-fitting a ratchet end to a 1.5mm thick, thin-walled stainless steel cylinder. It features a hole-based interference fit connection, where the cylinder serves as the containing part and the ratchet end boss is the contained part. The boss height is 1mm.

Due to the interference between the two components, according to elasto-plastic theory, the ratchet end experiences a pressure (F) directed upwards during the press-fitting process. When the pressure between the mating surfaces exceeds a threshold (Fe), the pressure on the cylinder surpasses the elasto-plastic limit pressure. As a result, the surface of the cylinder undergoes plastic deformation and gradually expands outward. Once the boss is fully pressed into the cylinder, the contact surface enters a state of plastic deformation. The region of elastoplastic deformation is depicted in Figure 2, with the plastic deformation area defined as b1≤r≤b2. It is important to note that a smaller wall thickness results in a larger deformation region.

 

During the press-fitting process of this CNC component, there is currently no effective inspection method available; the surface quality can only be evaluated after the press-fitting is completed. To address this, finite element analysis is utilized to simulate the local stresses experienced during the interference fit of the thin-walled part. This analysis helps to identify the locations of plastic deformation during the press-fitting process, which is crucial for enhancing the yield of this component.

 

3 Finite Element Analysis of Interference Fitting

3.1 Establishing the Press-fitting Process Model

To reduce the time required for finite element analysis and to accurately represent the stress distribution in the cylinder during the press-fitting process, the model of the press-fitting machine is simplified. Only the components in contact with the ratchet end and the cylinder are retained, while other parts are constrained by positional conditions. The simplified model is illustrated in Figure 3.

During the pressing process, the press head of the machine applies a downward force ranging from 300 to 450 N to the ratchet end, with a displacement of 1 mm and an interference fit of 0.01 mm. Both the ratchet end and the cylinder are made from AISI-304 stainless steel. A small slip is allowed, using a penalty function-Coulomb friction type with a friction coefficient of 0.2.

The model is divided into 120,000 meshes, as shown in Figure 4. This simplified model is imported into finite element simulation software for analysis, which is conducted over 40 steps with a total computation time of 4 hours.

 

3.2 Post-processing of Simulation Results

The post-processing results indicate that when the ratchet end is subjected to a force in the -Z direction, stress concentration occurs at the contact surface between the ratchet end and the cylinder. The stress distribution at the beginning of the press-fitting process is illustrated in Figure 5. As the ratchet end is pressed in, plastic deformation takes place at the contact area with the cylinder, leading to slight curling in the elastic region outside the cylinder. This behavior aligns with the deformation region and state described by elastoplastic theory. Excessive stress in the deformation region results in an unreliable state of the cylinder during press-fitting, as well as poor surface quality. The stress distribution after press-fitting is depicted in Figure 6, while Figure 7 shows the plastic deformation of the contact surface material. Finally, Figure 8 illustrates the state of the cylinder at the contact point after the press-fitting process.

 

 

3.3 Product Structure Optimization

Simulation results indicate that excessive stress at the contact point between the ratchet end and the cylinder can lead to significant plastic deformation of the cylinder. To address this issue, the geometry of the boss has been modified. The optimized design of the ratchet end is illustrated in Figure 9. The boss features a 2° taper, which serves as a guiding CNC turning components. This design facilitates the automatic alignment of the ratchet end, thereby postponing the onset of plastic deformation and ultimately enhancing the product yield.

3.4 Post-processing of the product

The finite element analysis of the optimized model indicates that when a force is applied to the ratchet end in the -Z direction, no immediate plastic deformation occurs at the contact surface between the ratchet end and the cylinder. Figure 10 illustrates the pressure distribution at the beginning of the optimized press-fit process. As the ratchet end is pressed into the cylinder, minor plastic deformation occurs at the contact area, which reduces stress and effectively addresses the issue of excessive stress, resulting in improved surface quality. The stress distribution following the optimized press-fit is shown in Figure 11, while Figure 12 displays the plastic deformation of the contact surface material after the optimized press-fit. Finally, Figure 13 depicts the condition of the contact area of the cylinder after the optimized press-fit.

 

 

4. Experimental Verification

Based on the results of the finite element analysis, the part was remanufactured and press-fitted. After the press-fitting process, no burr marks were observed on the surface of the cylinder. The prototype stainless steel cylinder is depicted in Figure 14. Additionally, press-fitting was conducted on cylinders made from other materials using the same method, and no significant defects were noted. Prototypes of these cylinders, made from alternative materials, are shown in Figure 15.

 

5. Conclusion

In summary, a mechanical model was established, and finite element analysis was utilized to identify areas of plastic deformation in non-standard parts during the press-fitting process. Additionally, certain uncertainties related to these non-standard parts were optimized, resulting in an improved yield rate. This approach can assist in selecting and optimizing assembly processes for non-standard parts, serving as a reference to enhance product quality and reduce production costs.

 

 

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