What Is Part Exchange?

Interchangeability means that in a batch of parts (or parts) of uniform specifications, any one of them can be installed on the machine to meet the specified functional requirements without selection, repair or adjustment.

There are countless examples of interchangeability in daily life and industrial production. People's bicycles are usually manufactured according to interchangeability. If a certain part of the bicycle is broken, you can buy the same specifications in the hardware store and replace it to restore the bicycle's function. These bicycle parts can be used interchangeably within the same specification, and they are all interchangeable parts.

In the machinery and instrument manufacturing industry, the interchangeability of parts and components means that in the batch of parts of the same specification, one of them can be installed on the machine without any selection or repair, and can reach the specified requirements. Use required characteristics.

Interchangeability in the machinery and instrument manufacturing industry usually includes interchangeability of geometric parameters and interchangeability of performance parameters. The geometric parameters generally include the size, geometry (macro, micro) and mutual positional relationship. The properties of mechanical products include hardness, strength, stiffness, heat transfer, and other physical and chemical parameters. ^[1]

The principle of interchangeability begins with the manufacture of weapons. In China, weapons produced as early as the Warring States Period (476-222 BC) could meet the interchangeability requirements. Tomb of the First Emperor Qin of Xi'an

Modern mechanical parts are interchangeable, so it is possible to carry out efficient, decentralized and specialized production of thousands of parts and components in one machine, and then centralize them for assembly. Therefore, the application of the interchangeability principle has become one of the powerful means to improve production levels and promote technological progress, and its main functions are as follows:

From a design perspective, the components are interchangeable, which can maximize the use of standard parts, universal parts and standard parts, greatly simplifying the drawing and calculation work, shortening the design cycle, and conducive to computer-aided design and product variety. diversification.

From a manufacturing perspective, interchangeability is conducive to mutual cooperation. A large number of standard parts can also be produced separately in specialized workshops or factories. Due to the single product, large number, and detailed division of labor, high-efficiency special equipment can be used, and computer-aided Processing creates the necessary conditions for the specialization of production, which will inevitably increase output and quality, and significantly reduce production costs. During assembly, because the components are interchangeable, no auxiliary processing is required, so that the assembly process can be carried out continuously and smoothly, so it can reduce the workload of assembly work and shorten the assembly cycle, so that it can be used in assembly lines and even automated assembly , Promoted the development of production automation, and significantly improved efficiency.

From the point of view of use and maintenance, if the parts are interchangeable, after the parts are worn or damaged or lost, they can be replaced with another new spare part (such as the pistons of cars, tractors, piston pins, piston rings, etc.) Such spare parts) not only facilitates maintenance, but also significantly reduces the maintenance time and cost of the machine or instrument, ensuring the durability and continuity of mechanical product work, thereby extending the service life of the product and significantly increasing the value of the product.

In short, interchangeability is of great significance in improving product quality and reliability, and improving economic benefits. It has become a universally observed principle in the modern machinery manufacturing industry and plays an important role in China's modernization. However, it should be noted that the principle of interchangeability does not apply in all cases, and parts cannot be interchanged when the economic principle is only met by a single formulation. ^[2]

Interchangeability is divided into internal interchange and external interchange according to the use occasion, and is divided into full interchangeability, incomplete interchangeability, and non-interchangeability according to the degree of interchange. According to the purpose of interchange, it is divided into assembly interchange and function interchange.

Interchangeability according to use occasion

Internal interchange: The interchangeability of various parts within a standard component is called internal interchange;

External interchange: The interchangeability between standard parts and their related parts is called external interchange;

For example, for a rolling bearing, the fit between the outer diameter of the outer ring and the base hole, the inner diameter of the inner ring and the journal is interchangeable;

Interchangeability

Full swap: Also known as absolute swap. Parts do not need auxiliary processing and repair, nor need selection, to achieve a technical state achieved by interchange. The use of full interchange makes large-scale, assembly work in mass production simple, saves labor and time, reduces costs, stabilizes quality, and facilitates replacement parts during use and maintenance. But sometimes the tolerances of the parts will be very small, and the machining will be difficult or even impossible. ^[3]

Incomplete interchangeability: Incomplete interchangeability is required when the components need to be matched (but cannot be further processed) to be assembled into a machine with specified functions during assembly. An incomplete interchange is proposed in order to reduce the manufacturing cost of parts. In mechanical assembly, when the accuracy of the machine assembly is very high, if the full interchange is used, the tolerances of the parts will be too small, causing processing difficulties and high costs. At this time, incomplete interchange should be adopted to enlarge the manufacturing tolerances of the parts, and the selective fitting method should be used to divide the matching parts into several groups according to size, and then match according to the group, that is, large holes and large shafts, small holes and small The shafts match. Each part in the same group can be completely interchanged, but it cannot be exchanged between groups. In order to facilitate manufacturing and reduce costs, internally interchangeable parts should be incompletely interchanged. But for the convenience of use, the external interchangeable parts should be completely interchanged.

Not interchangeable: when the parts are reassigned, the parts that require processing to assemble to complete the specified functions become non-interchangeable. Generally, high-precision parts need two parts that cooperate with each other, or can be researched to complete their functions.

For standard parts, the interchangeability between standard parts and their components is called external interchange; the interchangeability between internal parts of standard parts is called internal interchange. For example, for a rolling bearing, the fit between the outer diameter of the outer ring and the base hole, the inner diameter of the inner ring and the journal is external interchange; the fit between the outer ring, the inner ring raceway diameter and the rolling body is internal interchange. Interchangeability can be further divided into complete interchange and incomplete (or limited) interchange according to the degree of interchange.

Interchangeability

Assembly interchangeability: The interchange where the specified geometric parameter tolerance meets the assembly requirements is called assembly interchange;

Functional interchangeability: The interchange that specifies both the geometric parameter tolerance and the mechanical physical performance parameter tolerance that meets the requirements for use is called a functional interchange.

The above-mentioned external interchange, internal interchange, complete interchange and incomplete interchange are all assembly interchanges. The purpose of assembly interchange is to ensure product accuracy, and the purpose of functional interchange is to ensure product quality.