In this paper, we apply Group Technology (GT) to analyze the characteristics of parts used in cycloidal drives. The cycloidal drive, a type of cycloidal speed reducer, is a planetary gear transmission with few teeth difference. It is renowned for its compact structure, small volume, and high transmission ratio, making it widely used in machinery across mining, construction, lightweight industry, and other fields. As a serialized product, the cycloidal drive comes in numerous specifications and varieties. Manufacturers often receive orders with multiple specifications but small quantities per order, leading to low production efficiency and high costs. To achieve mass customization production—that is, to accomplish small-batch order tasks with the efficiency of mass production—we have designed a part classification and coding system specifically for cycloidal drive components. This system aims to enhance production planning, process standardization, and resource utilization.
Based on the information description methods used for part features, part classification methods can be broadly divided into visual inspection, functional name analysis, production flow analysis, and coding classification. Visual inspection and functional name analysis rely on information from part drawings and part names, suitable for classifying a small number of parts. Production flow analysis primarily utilizes information from process documents represented by codes. Coding classification is based on part features described by a part classification and coding system. Coding classification can be seen as a computerized version of visual inspection. In this work, we adopt the coding classification method. The core of this approach lies in capturing the essential attributes of cycloidal drive parts through a structured code, facilitating similarity grouping for design retrieval, process planning, and cell formation.
To design an effective classification and coding system for cycloidal drives, we first conduct a detailed feature analysis of typical parts. Part features are generally categorized into functional features, structural features, and process features. Functional feature information is primarily derived from assembly drawings. For those unfamiliar with the product’s function, it can be challenging to discern functional features from part drawings alone, as the same structural feature may serve different functions—for example, an external cylindrical surface could be a support surface or a guide surface. Structural feature information comes from part drawings, which provide a complete depiction of the part’s geometry. Process features, on the other hand, are obtained from part process documents. Functional and structural features can be considered inherent or intrinsic features of a part, while process features are non-inherent or extrinsic, as different process methods can achieve the same technical requirements.
We take two key components of the cycloidal drive—the cycloidal gear and the output shaft (or intermediate shaft)—as examples for feature analysis. Their functional roles are critical to the operation of the cycloidal drive. The cycloidal gear acts as a planet gear; its central inner hole is hinged with the crankshaft (eccentric shaft), and its external cycloidal teeth mesh with the sun gear (pin gear) to achieve the few-teeth-difference planetary transmission. The output shaft (or intermediate shaft) receives the decelerated rotary motion from the cycloidal gear via evenly distributed holes on its larger end that are hinged with corresponding holes on the cycloidal gear. Its inner holes and outer diameters serve support functions or transmit motion to the next stage.
Structural features of parts can be divided into shape features and material features. Shape features reflect the geometric and topological characteristics, while material features describe the intrinsic properties. The structural features of the cycloidal gear and output shaft are summarized in Table 1.
| Feature Category | Cycloidal Gear | Output Shaft (or Intermediate Shaft) |
|---|---|---|
| Geometric Shape Features | Short epicycloid teeth, inner cylindrical hole | Multiple external cylindrical surfaces, inner hole |
| Dimensional Accuracy & Surface Roughness | High accuracy (e.g., IT7) and low roughness (e.g., Ra 0.8 µm) on teeth and bore | Moderate accuracy (e.g., IT8) and roughness (e.g., Ra 1.6 µm) on journals |
| Form and Position Tolerance | Generally tight, especially for tooth profile and location | General requirements for concentricity and runout |
| Material Type | Alloy steel (e.g., 20CrMnTi) | Medium carbon steel (e.g., 45 steel) |
| Raw Blank Form | Flat cylindrical forged blank | Cylindrical forged or bar stock |
| Material and Surface Properties | High strength, high hardness, good wear resistance after heat treatment | Good comprehensive mechanical properties after quenching and tempering |
Process features encompass the manufacturing steps required to produce the part. For the cycloidal gear, typical processes include heat treatment (normalizing, quenching, low-temperature tempering) and machining operations (rough turning, finish turning, gear milling, gear grinding). For the output shaft, processes involve heat treatment (quenching and tempering) and machining (rough turning, finish turning, internal grinding). These process characteristics are crucial for grouping parts with similar manufacturing requirements, a key advantage of applying GT to cycloidal drive production.
The design of the classification and coding system for cycloidal drive parts involves several steps. First, we define the types of codes: name code, shape code, and auxiliary code (non-shape feature code). The name code standardizes part names based on function and shape. The shape code represents the structural shape of the part. Auxiliary codes generally include main basic dimensions, accuracy (including surface roughness), material category, heat treatment, blank type, etc. These codes together form a comprehensive descriptor for each part in the cycloidal drive assembly.
We considered two schemes for the coding system structure. Scheme one uses a chain structure where all code bits are arranged sequentially without hierarchical grouping. Its advantage is simplicity, but it may not efficiently handle complex part descriptions. Scheme two employs a semi-tree structure where the overall shape code serves as the first digit, and the name code and auxiliary codes are semi-dependent on the overall shape code. This allows for greater information capacity, as shape codes are closely related to the overall shape, while auxiliary codes remain independent. After comparative analysis, we selected scheme two because it facilitates coding by organizing features from major to minor and better accommodates complex parts. For complex parts, which are often main or special-purpose components with low similarity, a name code provides unique descriptive power that shape codes alone may lack.
Based on analysis of various domestic and international coding standards, we finalized the basic structure of the part classification and coding system for cycloidal drives, as shown in Table 2. The system comprises 14 code digits, each capturing specific part attributes. The code for a part can be represented as a vector: $$C = (c_1, c_2, c_3, \ldots, c_{14})$$ where each $c_i$ is a digit from 0 to 9, and the entire code encapsulates key features for GT applications.
| Code Digit | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Content | Overall Shape | Name Code | Shape Feature & Machining Code | Material | Heat Treatment | Raw Blank Form | Accuracy & Roughness | Dimension Code | ||||||
| Details | e.g., rotational, non-rotational | Standardized part name | External shape | Internal shape | Plane/surface machining | Auxiliary holes | Additional features | Material grade | Heat treatment type | Blank type | Primary accuracy grade | Location of accuracy | Major dimensions | |
The overall shape code (first digit) is designed with digits 0–9 representing categories such as 0 for solid cylindrical parts, 1 for bar/rod parts, 2 for plate parts, 3 for block/frame parts, 4 for box/housing parts, and 5–9 for other specialized forms common in cycloidal drives. The name code (second digit) uses digits 0–9 in a semi-tree relationship with the overall shape code. For instance, under overall shape code 0 (solid cylindrical), name code 1 might denote “cycloidal gear,” while under overall shape code 1 (bar/rod), name code 1 might denote “output shaft.” This linkage ensures that the coding system reflects both geometry and function, crucial for identifying parts in the cycloidal drive context.
Shape feature and machining codes (digits 3–8) are detailed based on part category. Rotational parts are subdivided into general rotational and special rotational classes. For general rotational parts, digit 3 encodes external shape characteristics, as partially shown in Table 3.
| Code Digit 3 Value | Meaning |
|---|---|
| 0 | Smooth external cylinder, no steps |
| 1 | Unidirectional step or shoulder |
| 2 | Bidirectional steps |
| 3 | Four or more external diameters |
| 4 | Non-circular cross-section |
| 5 | Split or segmented design |
| 6–9 | Other complex features (e.g., threads, grooves) |
For special rotational parts, which include components like eccentric shafts or crankshafts in cycloidal drives, digit 3 encodes axis geometry, as partially shown in Table 4.
| Code Digit 3 Value | Meaning |
|---|---|
| 0 | Straight axis, no deviation |
| 1 | Bent or curved axis |
| 2 | Axis with perpendicular offsets |
| 3 | Axis with交叉 (crossed) arrangements |
| 4 | Axis with parallel offsets |
| 5–9 | Other special configurations |
Digits 4 through 8 for rotational parts respectively describe internal shape and machining, plane and curved surface machining, auxiliary hole processing, and other supplementary features. For non-rotational parts (e.g., housing, covers), digit 3 indicates categories like bar/rod, plate, block/frame, or box/housing, and digits 4–8 have similar interpretations as for rotational parts but tailored to planar geometry.
Material feature codes (digits 9–10) are designed using two two-digit matrices: one for cast blanks and another for non-cast blanks (e.g., forgings, bar stock). This approach considers the correlation between material type and raw blank form. For example, a matrix for cast materials might include codes like 01 for gray cast iron, 02 for ductile iron, etc., while for forged materials, codes like 11 for low-carbon steel, 12 for alloy steel, etc. The coding can be expressed as a mapping function: $$M = f(\text{material grade}, \text{blank type})$$ where $M$ is a two-digit code from 00 to 99, covering typical materials used in cycloidal drive manufacturing.
Heat treatment feature code (digit 11) uses digits 0–9 to encode various heat treatment processes. For instance, 0 denotes no heat treatment, 1 denotes stress relieving, 2 denotes normalizing, 3 denotes quenching and tempering, 4 denotes case hardening, 5 denotes induction hardening, and 6–9 denote other specialized treatments like nitriding or cryogenic treatment. This digit helps group parts with similar thermal processing requirements, optimizing furnace scheduling in production.
Raw blank form code (digit 12) is based on enterprise standards for metal materials and statistical analysis of cycloidal drive parts. Ten feature items are defined, represented by digits 0–9. For example, 0 denotes standard parts or outsourced components, 1 denotes castings, 2 denotes forgings, 3 denotes bar stock, 4 denote weldments, 5 denote sintered parts, and 6–9 denote other forms like extrusions or composites. This code supports inventory management and process planning for blank preparation.
Accuracy and roughness feature codes (digits 13–14) capture the most critical surface finish requirements. The system considers only primary surfaces with important accuracy and roughness. Accuracy and roughness grading is aligned with machine tool capabilities: low accuracy and high roughness (e.g., Ra 12.5 µm) can be achieved by planing or milling; medium accuracy and low roughness (e.g., Ra 1.6 µm) require fine turning or grinding; ultra-high accuracy demands super-finishing methods. Digit 13 encodes the primary accuracy and roughness grade, while digit 14 indicates the location of that accuracy on the part. Both digits use 0–9, with correlation to convey rich information. Partial encoding for digit 13 is shown in Table 5, and for digit 14 in Table 6.
| Code Digit 13 Value | Meaning | Typical Roughness Range (Ra in µm) | Typical Tolerance Grade (IT) |
|---|---|---|---|
| 0 | Very low accuracy, very high roughness | > 25 | IT13–IT16 |
| 1 | Low accuracy, high roughness | 12.5 – 25 | IT11–IT12 |
| 2 | Medium accuracy, medium roughness | 6.3 – 12.5 | IT9–IT10 |
| 3 | Medium accuracy, low roughness | 1.6 – 6.3 | IT7–IT8 |
| 4 | High accuracy, very low roughness | 0.4 – 1.6 | IT5–IT6 |
| 5 | Very high accuracy, ultra-low roughness | 0.1 – 0.4 | IT3–IT4 |
| 6–9 | Special or super finishes | < 0.1 | IT01–IT2 |
| Code Digit 14 Value | Meaning |
|---|---|
| 0 | All surfaces have similar accuracy |
| 1 | Internal rotational surfaces (bores, holes) |
| 2 | External rotational surfaces (shafts, journals) |
| 3 | Planar surfaces (faces, plates) |
| 4 | Curved surfaces (gears, cams) |
| 5 | Auxiliary holes (tapped, drilled) |
| 6 | Keyways or splines |
| 7 | Threads |
| 8 | Complex combined surfaces |
| 9 | Other specific features |
Dimension feature codes (implicitly covered in earlier digits, but we can dedicate digits for size ranges) are represented using three additional digits (though not explicitly in the 14-digit structure, they can be part of auxiliary codes). For clarity, we can consider that main dimensions—such as outer diameter or width, length, and inner diameter or height—are encoded within shape or separate digits. In practice, dimensions can be categorized into ranges. For example, outer diameter $D$ might be coded as: $$ \text{Code for } D = \begin{cases} 0 & \text{if } D < 20 \text{ mm} \\ 1 & \text{if } 20 \leq D < 50 \text{ mm} \\ 2 & \text{if } 50 \leq D < 100 \text{ mm} \\ \vdots & \vdots \end{cases} $$ Similar schemes apply for length $L$ and inner diameter $d$. This allows grouping parts by size for machine tool selection and fixture design.
The entire coding system enables efficient retrieval and grouping of cycloidal drive parts. For instance, to find parts similar to a given cycloidal gear, one can compute a similarity measure. Let $P_1$ and $P_2$ be two parts with codes $C_1 = (c_{11}, c_{12}, \ldots, c_{1,14})$ and $C_2 = (c_{21}, c_{22}, \ldots, c_{2,14})$. A weighted similarity score $S$ can be defined as: $$ S(P_1, P_2) = \sum_{i=1}^{14} w_i \cdot \delta(c_{1i}, c_{2i}) $$ where $w_i$ is a weight reflecting the importance of the $i$-th feature (e.g., higher for shape codes, lower for auxiliary codes), and $\delta(c_{1i}, c_{2i})$ is 1 if $c_{1i} = c_{2i}$ or 0 otherwise. This facilitates computer-aided part family formation for cellular manufacturing.
In implementation, this classification and coding system for cycloidal drives has been preliminarily applied in manufacturing enterprises, showing positive results. It reduces part variety proliferation, standardizes process plans, and improves production responsiveness. By encoding features systematically, the system supports design reuse—where existing designs for similar cycloidal drive parts can be retrieved and modified, reducing lead times. In process planning, parts with similar codes can share standardized operation sequences, tooling, and setups, lowering costs. For production scheduling, grouping parts with similar codes into batches enhances machine utilization and reduces changeover times.
Furthermore, the system adapts well to the evolving needs of cycloidal drive production. As new materials or processes emerge, the coding tables can be expanded without overhauling the structure. For example, additive manufacturing blanks could be incorporated into digit 12. The semi-tree structure allows flexibility while maintaining consistency. The repeated emphasis on cycloidal drive components throughout the coding ensures that the system remains focused on this application, though the principles are generalizable to other gear systems.
In conclusion, the application of Group Technology through a tailored classification and coding system offers a robust solution to the challenges of small-batch, high-variety production in cycloidal drive manufacturing. By analyzing functional, structural, and process features, we designed a 14-digit code that captures essential part characteristics. The system’s semi-tree structure balances detail and simplicity, enabling efficient part family formation for mass customization. Future work could integrate this coding with CAD/CAM systems or extend it to include supplier information for supply chain optimization. Ultimately, this approach enhances competitiveness for manufacturers of cycloidal drives by transforming traditional job-shop production into streamlined, GT-based operations.