The reducer is composed of modules, and we make all the modules combined to become the appearance of the reducer.
        The characteristic of the open system is: sufficient numbers and types of modules are connected to each other in a direct or direct way, the same type of modules can be repeatedly presented without restriction, and then an infinite number of combinations can be formed; but when the type and number of modules are limited When, the number of combinations is also limited. For example: coins, characters, power supply, pipelines, block gauges, disc springs and other systems; hanging transportation installation is a more complex open system, which consists of straight rails, curved rails, forks, underframes, hangers, The lifting frame and the tail board are constituted, and the above-mentioned modules can be selected appropriately to form a transportation line of different lengths and shapes.

　　The characteristic of the closed system is: a limited number and types of modules cannot be used or can only be used repeatedly for a limited number of times to form a module system with a limited combination of modules. The practice group should think about the compatibility between the modules, the requirements and the ability of processing, etc. Modular machine tools, desk lamps, etc. are all closed systems.
　　 The combinable number of closed system and open system modules under certain prerequisites can be calculated by the mathematical method of combination and display. Suppose the number of system types is N, the total number of system modules is M, the total number of modules required for each product is K, the number of identical modules required for each product is Z, and the number of combinations of system modules is C (not counting the sequence) , The display number of the system module is A (thinking group merges to distinguish the display order of differences), and the comprehensive combination number is P.
　　Module system structure and connection method, exchangeability and compatibility
The function of the module in the product is different, and the combination relationship between it and other modules is also different. The number of other modules connected to a module is different, which can be divided into linear connection (single, two-way), plane connection (four-way) and three-dimensional Connection; according to the differences of module relevance, it can be divided into: rigid connection-there is a direct assembly system, and divided into static and dynamic connections; flexible connection-has direct and physical related premises, but no geometric related premises Convergence.