Powder metallurgy manufacturing processes constitute a scientific discipline employing metallic and non-metallic powders as raw materials, utilising pressing, sintering and various subsequent treatment techniques to produce metal components. This technology enables the manufacture of high-performance powder metallurgy products at comparatively low cost. The application of powder metallurgy techniques in gear production significantly reduces manufacturing expenses.

This thesis, grounded in the principles of powder metallurgical forming and sintering mechanisms, primarily investigates the development and application of high-performance iron-based powder metallurgical helical gears.

Research was conducted in the following specific areas:

Study of high-performance iron-based powder metallurgical helical and spur gears. Based on the pressing forming principles for powder metallurgical products with complex structures, different mould design methods for powder metallurgical helical gears were investigated. By establishing mathematical models for helical gear process parameters, mould design was conducted to enhance strength and precision. Research into high-performance, low-cost iron-based powder metallurgical gear materials.

I. Comparative performance study of helical and spur gears.

1. During meshing, the principle governing straight gear tooth engagement is as follows: tooth engagement – pure involute rolling for force transmission – tooth disengagement – transfer of force transmission to the next pair of teeth.

2. From the above, it is evident that gear meshing is inherently intermittent. Factors such as manufacturing tolerances and installation errors in involute gears accentuate this discontinuous transmission characteristic.

3. The adoption of an involute profile aims to achieve smooth transmission and constant gear ratios. However, due to manufacturing tolerances, abrupt engagement and disengagement of teeth can induce momentary impact forces within the mechanism.

4. The engagement principle of helical gears: They exhibit non-impact sudden engagement and disengagement. Within each engagement-disengagement cycle, the engagement and disengagement of each pair of teeth occurs progressively. Unlike spur gears, they lack the engagement impact phenomenon, resulting in smooth operation and facilitating the maintenance of a constant transmission ratio.

5. The force-bearing surface of spur gear teeth extends along the entire tooth width in the axial direction. In helical gears, while the force-bearing surface is distributed axially, it does not span the full tooth width. Under identical conditions of module, tooth count, and material, helical gears experience lower stress than spur gears.

Advantages of helical gears:

1) Superior meshing performance. Smooth transmission, low noise, minimal impact.

2) High overlap ratio. Each tooth shares the load, enabling high-speed, heavy-duty operation with smooth performance.

3) Compact structure. Requires fewer equivalent teeth.

II. Mould Design and Forming Principles

1. Mould design depends on product shape, size, density, height, and machine selection. To ensure dimensional consistency in mass-produced parts, moulds must possess high service life and precision. Selecting appropriate mould materials, heat treatment processes, and machining techniques for mould components is crucial for producing high-quality moulds. The die and core should be selected for excellent wear resistance, with a heat treatment hardness of HRC 60 or above, such as cemented carbide YG15, YG8, high-speed steel W18Cr4V, ASP60, or SKH-9. Upper and lower punch dies should be chosen for impact resistance and adequate wear resistance, with a heat treatment hardness of HRC 56-60, such as alloy steel GCr15, 9CrSi, SKD11, or 3V.

2. Powder metallurgy forming refers to the process where loose metal powders or mixtures (powder bodies) are loaded into steel dies. Under die pressure, the powder body is compressed, held under pressure, and subsequently released to form a compact with defined shape, dimensions, density, and strength. The compact is then ejected from the female die.

III. Comparative Study of Raw Materials

Through process trials on materials with different compositions—Fe-2.0Cu-0.8C, Fe-1.5Cu-0.6C-1.0Ni, and Fe-1.5Cu-0.6C-1.75Ni-0.5Mo—the pressing and sintering properties of various iron-based powder metallurgy gear materials were investigated, along with the influence of alloying elements on the sintering performance of these materials.

Development of High-Performance Iron-Based Powder Metallurgical Helical Gears. Building upon the aforementioned research, powder metallurgical helical gear products were developed through material design, process optimisation, and mould design. The resulting gears successfully passed bench testing, demonstrating significant social and economic benefits.