The fork frame is a crucial component in tumble washing machines, serving to connect and support both the inner and outer drums. During operation, the inner drum rotates at high speeds. However, due to the uneven distribution of clothes, it inevitably subjects the drum to significant eccentric loads, which are then transferred to the fork frame. This becomes even more pronounced in machines with a drying function, as the aim is to enhance the spinning effect for better drying performance. Consequently, this creates higher eccentric loads, increasing the requirements for the strength and stiffness of the fork frame.

The fork frame also plays a vital role as a supporting component, and it is integral to the assembly of various other parts within the washing machine. As a result, its design must not only meet strength and stiffness standards but also the assembly requirements with the machine’s core components. Traditionally, the design process followed a passive approach, where the team would refer to foreign products for design inspiration, determine the structural and dimensional parameters, and proceed with trial production. If the prototype didn’t meet requirements, the entire design process would be repeated, which significantly increased the product development cycle and cost.

1. Structural Analysis of the Fork Frame

The fork frame’s functional and assembly requirements were considered using CAD/CAE/CAM integrated software (UGⅡ). The initial 3D conceptual design of the fork frame was created, followed by the transfer of the model to MSC/MARC Finite Element Analysis (FEA) software for further structural evaluation. The fork frame is primarily a thin-walled shell structure with three arms, and a large central boss. The finite element model was developed by discretizing the entire structure and applying boundary conditions that align with real-world scenarios.

Two analysis models were created, as shown in Figure 11-10. Each model used three types of elements: 3D four-node shell elements. The resulting centrifugal force was applied as surface force on the corresponding faces, simulating the working conditions of the fork frame under load.

The analysis revealed that the frame’s strength and stiffness met the requirements. However, stress concentration was observed at several critical points:

• Near the connection between the central boss and the thin shell
• At the bottom and sidewalls of the arms, where structural and dimensional variations caused a significant concentration of stress, reaching a maximum equivalent stress of 89.5 MPa.

Regarding deformation, the maximum displacement occurred along the outer perimeter of the thin shell opposite each arm, with the largest deformation being 0.8mm.

Additionally, to facilitate the installation of electrical components, a 40mm cutout was proposed in the frame, as shown in Figure 11-10b. A separate analysis (using Model 2) was conducted to verify the feasibility of this modification. The results indicated that the stress distribution and magnitude of stress remained unchanged, but the deformation at the cutout increased significantly, reaching 1.21mm — an increase of 51%. This deformation exceeded the stiffness requirements, suggesting that this modification would not be feasible.

2. Improved Fork Frame Design and Rapid Prototyping

Following the structural FEA analysis and assessment, several deficiencies were identified in the initial design of the fork frame:

1. The transition radius between the central boss and the shell was too small, leading to severe stress concentration.
2. The thickness of the arms was larger on the sides compared to the bottom, where the stress and deformation were minimal, which was an inefficient use of material.
3. Although four reinforcing ribs were designed on the outer side of the central boss, the ribs’ height was insufficient for the strength demands.

Based on these findings, the fork frame design was improved. The updated design addressed the stress concentrations and structural weaknesses, ensuring the fork frame’s strength and rigidity would meet operational requirements.

However, the assembly relationships with several other washing machine components remained complex. Therefore, a rapid prototyping technique was used to create a wooden prototype of the fork frame, which was then tested for assembly. This allowed for quick validation of the design in the actual machine, significantly saving on time and costs associated with traditional prototype testing.

Conclusion

The case of the washing machine fork frame illustrates the value of structural optimization design and rapid prototyping in product development. By leveraging CAD/CAE/CAM integration and finite element analysis, the design process moved from a passive trial-and-error method to a more efficient and proactive approach. This resulted in an optimized product design that met the required performance standards and reduced development costs and time.

The combination of structural analysis and rapid prototyping not only enhanced the strength and rigidity of the product but also provided valuable insights into how design modifications affect assembly and performance, ultimately leading to a faster and more cost-effective product development cycle.