Assembly of Machines
Assembly of Machines
the connection and attachment of parts, subassemblies, and modules in a definite sequence to produce a finished machine. A module is a detachable or nondetachable grouping of parts. The characteristic feature of a module is that it can be assembled apart from the other elements of the final product. A combination of two or more parts in a module is called a subassembly. There are primary, secondary, and higher order subassemblies. The highest order subassembly may be separated only into parts. The base element, whether a part or module, is that element from which assembly is begun. The amount of labor required for assembly in machine building is 25 to 35 percent of the total labor required to produce an item; if many adjustment operations are
required, as in unit production and small-lot production, it reaches 40–45 percent.
Machine building uses both straight-line and modular assembly. A flow chart of a straight-line assembly is shown in Figure 1. Each element of the final product is indicated on the chart by a rectangle, separated into three segments. The designation of the element is given in the upper segment, its index number is in the lower left segment, and the total number of such elements in the finished product is given in the lower right segment. The index numbers of the elements correspond to the identification numbers of parts and modules in blueprints and specifications. Figure 2 shows the flow chart for the modular assembly of the product whose straight-line assembly is shown in Figure 1. Modular assembly permits the parallel assembly of modules and significantly reduces the time required for assembly.
An assembly flow chart shows the structure and sequence of assembly of a product and its modules. In preparing flow charts, the required control and auxiliary operations are established. The charts give an idea of the technological effectiveness of the product’s design from the standpoint of its assembly. The design determines the assembly methods used.
In designing an assembly process, the assembly rate for both straight-line and modular assembly is determined by dividing the annual amount of working time in minutes by the projected annual output of assembled products or modules in units. If the rate significantly exceeds the average duration of similar assembly operations, then the assembly is conducted according to the principles of lot production. In this case, various finished products or modules are assembled at one work station in batches or lots. If the rate is close to or less than the average duration of similar assembly operations, then the assembly is conducted according to the principles of mass production, with a given assembly operation assigned to each work station. In production processes with low assembly rates, the assembly procedure is split into separate operations. If this is difficult or impossible because of production considerations, then the operations are carried out in parallel and the number of work stations is doubled. The assembly operations must be such that a maximally uniform and complete operation is carried out at each station. This ensures better specialization of the assemblers and improved labor efficiency.
The technological processes of assembly may be classified as individual, group, or standard. Individual processes are developed for the assembly of a single particular item. Group processes can be applied to the assembly of a number of items or modules that have common design characteristics. Standard processes are designed for various categories of connections and modules based on advanced assembly methods on an industry-wide scale.
In formulating assembly operations and their sequence, the organizational forms become apparent. The assembly may be progressive or nonprogressive. The transport of the item being assembled from one work station to another in progressive assembly may be accomplished manually (on a workbench, along a roller conveyor, or on carts), by hoisting machines (cranes or telphers), by periodic conveyors, such as apron conveyors or carts chain driven along a closed rail line, and by continuous conveyors.
Progressive assembly for a stationary product is accomplished on fixed stands arranged in a line. Each assembler or assembly team carries out a single operation, progressing consecutively from one stand to the next. Such assembly is suited to lot production with a high assembly rate, particularly in the assembly of heavy machines, where transport is difficult. In progressive assembly the assembly line must receive a steady supply of interchangeable parts and modules at a rate consistent with the assembly rate. Fitting and adjustment operations can be permitted only when they are integrated with the assembly rate. If high precision is required, interconnecting parts may be fitted to each other or fitted separately to the item being assembled. In this case, the interconnecting parts enter the assembly line in pairs. Progressive assembly shortens the production cycle, decreases in-process stock, permits increased specialization of the assemblers, increases chances to mechanize and automate production, and reduces the labor required. The mechanization of assembly operations is designed to replace partially or completely the manual labor of an operator by equipping work stations with electrical, pneumatic, or hydraulic tools and devices. The aim of automation is to transfer control functions to assembly machines and systems. The mechanization and automation of processes may be individual or complex, that is, integrated into systems.
Nonprogressive assembly is used in individual and small-lot production.
In formulating the technological process of assembly, devices, methods, and means of technical inspection are established in order to determine how the precision of the shape and dimensions and the relative position and movement of the elements of a product are related to given specifications. Checks must be made on the mutual positioning of the components in the product, on the quality of the connections made (the strength of and the torque applied to threaded connections and the alignment of contacting surfaces), the correct installation and presence of parts in connections, the total weight of modules and of the final product, and the balancing of rotating parts.
A distinction is made between intermediate and acceptance inspection. Intermediate inspection is carried out after the completion of complex assembly operations and after operations in which defects are most likely to occur. In acceptance inspection all the assembled items and the most sensitive modules are tested. The technological processes of modular and straight-line assembly are established in the flow sheets.
The major trends in improving the efficiency of assembly are mechanization and automation, the elimination of adjustment operations, the reduction of the number of parts and modules used, the standardization of fastenings and other parts in a product, and the reduction in a number of multisection dimensional links.
REFERENCESNovikov, M. P. Osnovy tekhnologii sborki mashin i mekhanizmov, 4th ed. Moscow, 1969.
Korsakov, V. S. Osnovy tekhnologii mashinostroeniia. Moscow, 1974.
Spravochnik tekhnologa-mashinoslroitelia, 3rd ed., vols. 1–2. Moscow, 1972.
V. S. KORSAKOV