UNIT ASSEMBLY IN AIRCRAFT PRODUCTION

Assembled units of an aircraft (helicopter) with small amount of parts which have specified design and technological requirements (each unit generally has separate assembly drawing) are the objects of Unit Assembly.

Technological units of an aircraft (helicopter) are:

1. Flat frame units which form the outline of an aircraft (spars, ribs, frames, beams).

2. Spatial frame units ("spiders" of a wing, skeletons of the equipment).

3. Panels of an airframe, and parts (framework), joined to the skin (or reinforced primary structure).

4.Constructions (assembly units) such as doors, covers, hatches, dashboards, fairings.

5. Units and panels of inborn systems and airborne equipment (control panels, collectors, seats, gears). These assemblies are usually referred as mechanical assembly.

The constituent components (sub-assemblies) which are assembled separately and joined to the unit are called Sub-Units. They usually do not have any separate assembly drawings.

Assembly is a combination of technological operations for installation of parts and units in its own position assigned by assembly drawing. Joining of these sections in assembly units are conducted by methods specified in the process documentation.

These two large stages of assembly should be precisely marked and optimized during development and realization

Let's consider regulations (rules, principles), which should be used at development and realization of these stages of the master schedule assembly.

It is known, that for positioning of work pieces, parts or articles in selected coordinate system surfaces, lines, points are used. These surfaces are called Bases.

According to their purpose, Bases are subdivided into constructional (designer), technological and measuring. The technological Bases, in their turn, are divided in positioning and assembly bases.

Positioning bases are the surfaces, lines, points which are used for positioning of work pieces, parts or articles with regard to selected coordinate system. Such definition is usable for all operations of processing and assembly.

At the first stages of development of master schedules, all possible versions of positioning bases for an article on the basic operations are considered. Then the scheme of basing is layed out. This is a sketch where the adopted sets of positioning bases are marked

The combination of scheme of basing, methods and facilities of parts installation and positioning is called method of assembly. There are three groups of assembly methods applied in aircraft production. At first the mutual positioning of parts is considered by matching of positioning bases of parts to the joint. Secondly, positioning bases of parts, match with positioning bases of assembly equipment (presence of mechanical contacts). Thirdly, positioning bases of parts, match positioning bases are created by optical-mechanical-coordinate systems by no mechanical contact.

The methods of assembly with positioning bases on parts are (without use of an assembly device) distinguished by different kind of positioning bases, which are as follows:

a) Assembly by mating surfaces of interfaced parts or assembly by installing on a base part, i.e. assembly by drawing;

b) Assembly by marks on some (base) parts;

c) Assembly by technological (assembly) holes in parts of an assembled packet.

The methods of assembly with assembly device are as follows:

a) Assembly on bases - surfaces, which includes:

- any surfaces (which do not form the outlines)

- external surface of a skin which form the outlines

- internal surface of a skin;

- outer surfaces of a skeleton ("from a skeleton");

b) Assembly on bases - holes, which includes:

- structural holes under attachment bolts (HAB);

- technological locating-base holes (LBH);

- technological coordinate-fixing holes (CFH).

For unit assembly, the optical-mechanical coordinate system method is not applied practically.

Assembly Bases are the surfaces, lines, points of mating parts which directly form a joint. In other words these are the surfaces prepared by special way and coordinated according to the kind of joint.

The assembly base and units in a joint are restricted to some degrees of freedom. Therefore they may be considered as supplementary or positioning bases.

Combination of assembly bases of parts in a unit is evident from the assembly drawing. The character of real assembly process depends on coordination of assembly bases (if it is required).

Measuring Bases are the surfaces used for determination (definition) of relative position of article refer to means of measurement.

The selection of bases at article design, part and manufacture affects the accuracy of assembly unit and efficiency indexes of an assembly (technological cost price, labor-intensiveness, duration) process.

It is necessary to follow on three principles of basing:

1.       Principle of identity (unity) of bases: the technological bases should coincide with design or bases and materialized in it.

Such method corresponds to more common principle of machine-building technique. There should be a shortest path of transferring the sizes from the drawing up to an article. Rejection of special (arbitrary) technological bases eliminates intermediate calculations so-called base sizes. Thus errors can be reduced.

As example of keeping this principle (see fig.) is arrangement of placing holes (PH) of a rib wall on the basic constructional horizontal axis of a wing. By this, the coordination of positioning bases of a part and assembly device becomes simpler.

2.       Principle of combination (coincidence) of bases: positioning bases it is necessary to select those surfaces of parts, which are assembly bases at the given stage of assembly or it could be considered in future assembly bases.

Following this rules allows to increase the accuracy of assembly and to reduce volume of adjustment.

As you can see in a fig 2.1, this principle is provided for bands – its outline are positioning and assembly bases simultaneously. Positioning and assembly bases of fitting attachment does not coincide. The violation of this principle of combination (coincidence) of bases results to attachment fitting with a band adjustment by removal of allowance compensation of a gap by the gasket (on a place).

3.       Principle of permanency of bases: once selected positioning bases should be saved at all stages of assembly and manufacture of parts. The positioning bases at first assign for assembly units, then for sections, panels, units and, at last, for parts.

For a unit (see fig. 2.1) such base should be an attachment hole because it used for a rib installation by next stage of assembly of the assembly unit, and also securing a part at its machining, for example, at milling a foot of attachment fitting.

The principles of basing should be taken into account when selecting the sequence of the installation of parts in an assembly position. Parts, which assembly bases can serve as positioning supplied on assembly first (principle of combination (coincidence) of bases). Then, in order of geometrical deposition on assembly supplied parts and units participating in formation of outlines or ensuring rigidity.