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78ANALYSIS OF DESIGN ERRORS IN MECHANICAL ENGINEERING
Alisher Mamadjanovich Mamadjanov
Tashkent State Technical University
Shokhrukh Sadirov
Fergana Polytechnic Institute
ABSTRACT
This article examines an analysis of design errors in mechanical engineering that would be made in any stage of making design documentation by creating more opportunity to understand why making fewer errors in design in mechanical engineering is important.
Keywords: design errors, dimensions, drawings, technical documentation, manufacturing process, manufacturing process, strength calculations, stiffness calculations, kinematic calculations, dimensional chains, tolerance.
INTRODUCTION
Drawings are a carrier of information about the product, its design, dimensions, materials, special processing and, indirectly, about the manufacturing technology. The drawing provides a specific and unambiguous execution of the part, since the information contained in the drawings is mandatory for the contractor [1-5]. Only an error-free execution of the drawing ensures the production of a suitable part. According to the statistical analysis of machine failures, 60-90 % of these problems are related to design and manufacturing errors. Most of the errors are detected during the manufacturing process and the first test of products. Some errors are detected only in the course of operation after a long time, reducing the inter-repair period of the product or the resource of its operation as a whole.
METHODOLOGY
The reasons for errors are inherent in the essence of the design process. The creative process of designing is the ideal process in the designer's imagination. Based on the data of the technical specification, the conducted research, information materials and practical experience, the designer creates a mental image of the product, which is reflected in the drawings [6-11]. But there is an error between the designer's idea and its actual implementation, even with the most subtle insight into the problem. In the design process, the designer has to take into account a number of requirements and restrictions. These factors are often contradictory and do not allow you to create the sample that the designer wanted. Any design can be considered as imperfect, lagging behind the imaginary ideal design-the standard. The standard embodies all the best that scientific and technical achievements give. The removal of the real quality of the product from the standard serves as a criterion for the perfection of the design. If the removal is greater
than the average engineering level of a given time, then the design can be considered erroneous.
An error is a deviation of the design result from the accepted norms laid down in advance in the technical conditions and restrictions, a deviation from the standard or objective law that exists in nature. There are obvious (obvious) and hidden errors.
Obvious (obvious) errors are easily detected when comparing the design with the standard or when checking it according to the objective laws of mathematics, physics, mechanics and other laws that are known to an ordinary engineer. Obvious errors include errors in dimensional circuits, strength, deviations of parameters (force, speed, pressure, etc.). Obvious errors are detected when checking technical documentation by analytical or graphical methods known to an ordinary engineer. Hidden errors are not detected by verification and appear, as a rule, in new developments where a working principle that has not been tested in practice is applied or there is not enough information to implement an already known principle [12-17]. In such structures, ordinary methods of control and analysis do not give an answer or give an incorrect, distorted answer to the question of the operability and suitability of the design.
Hidden errors are identified after performing special calculations or developing expert opinions of major specialists. In such cases, it is advantageous to build an experimental model, when testing it, most of the hidden errors will be revealed.
DISCUSSION
The reasons for errors in technical documentation can be very diverse: ignorance, erroneous judgment, inability to cover all the issues of the problem, negligence, indifference, etc. Errors in design documentation are classified into the following groups: Group I-design errors; II - errors in calculations; III - errors in dimensions. The following errors belong to group I.
1. Errors caused by the wrong direction of development. These errors are already embedded in the technical specification for the development and arise from a wrong understanding of the work that the product should perform, or the processes for which it is created. Such errors should be disclosed already in the initial stages of development: in the technical proposal, draft design. The developer is given the right to critically analyze the terms of reference and identify all inaccuracies and errors in it [18-21]. A significant role in this process is played by the heads of groups, bureaus, and chief project engineers, who are responsible for the correct direction of design developments. Errors in the wrong direction of development are hidden errors and are not always detected when checking the design documentation and checking its compliance with the requirements of the technical specification.
2. Errors in the application function of the designed product. New products must meet their functions, be efficient and reliable.
3. Errors in the compliance of the designed product with the physiological requirements of the service personnel. The shape, size and control devices should ensure convenient and reliable operation.
4. Errors in the choice of material, when the properties of the material and its processing do not ensure the normal and reliable operation of all components and mechanisms.
5. Errors in the choice of the shape of the parts. The shape of the parts contributes to their production from the material specified in the drawing, the most effective technological methods.
6. Errors in the use of the material. The material can be used irrationally: with excessive thickness of the walls, ribs, etc.
7. Errors in the assessment of the psychological and social aspects of the new product. The design must meet the new requirements of operation, take into account the wishes of the person, the requirements of fashion, compliance with the environment, etc.
8. Aesthetic errors and product non-compliance with safety requirements. The appearance of the product should be pleasant and correspond to its functional application. Temperature, noise, vibration of the product must be within the normal range.
The following errors belong to group II.
1. Errors in strength calculations. As a result of these errors, the dimensions of the dangerous cross-sections may be unreasonably small or large. If the size of the dangerous cross-section is underestimated, the product will fail prematurely or break down. If the dangerous cross-section is increased, the product weight and material consumption unnecessarily increase. These errors are based on an insufficient or erroneous assessment of the actual forces acting in the product, the adoption of an incorrect calculation scheme, calculation methods, or the assumption of errors in the calculations.
2. Errors in the stiffness calculations. These errors lead to vibrations that exceed the permissible limits. As a result of vibrations, the product cannot perform its functions.
3. Errors in kinematic calculations. As a result, the product will not meet the parameters for which it is designed.
The largest part of errors belongs to group III.
1. Errors in the calculation of dimensional chains. They occur when the dimensions and permissible deviations are incorrectly calculated, including when the movement of the mechanism is incorrectly determined.
2. Errors in determining the size of the mechanism bottleneck. As a result, there is a case when the product cannot be assembled. The reason for the error: an inaccurate
calculation or a calculation in which the place for assembly work was not taken into account.
3. Errors due to developer's negligence. Errors can be made when calculating the size or when recording a correctly calculated size and the tolerance to it.
Errors in this group are detected when checking the drawings are shown as a discrepancy between the specified size and the actual location of the structural element in scale.
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Fig.l. Design error: the length of the toothed rail is insufficient, since the movement of the rail x1 is greater than the length of the support x2
RESULTS
The correct dimensioning and tolerance of the drawings is an important process that shows the quality of the technical documentation. Dimensions and permissible deviations in the drawings determine: the accuracy of the assembly process; interchangeability of components and products; the use of rational technological processes in the manufacture of parts.
The developer's good knowledge of manufacturing and assembly technology (basing, installation, clamping, tools, operations, transitions) allows you to correctly and accurately set the dimensions in the drawings. Rationally chosen dimensions and extreme deviations can reduce the complexity of manufacturing a part by 15-20 %, without changing its design.
The mistakes made by the developer in the design documentation depend on the focus of his attention and mental state during the development period.
They are often associated with haste and carelessness. All mistakes made must be identified and corrected in a timely manner before the design documentation is put into production [22-25]. A reliable error detection system creates favorable conditions for avoiding errors at all.
The appearance of errors in the design documentation is usually due to certain motives. According to the signs of error occurrence, there may be motivated or unmotivated errors.
Motivated errors have a certain basis of occurrence. They seem to have a logical justification for their occurrence, associated with the ignorance or absent-mindedness of the developer. Motivated errors can also be related to the scale of the drawing. Most
often, the dimensions are set according to the actual size of the drawing, although the image is made in an enlarged or reduced scale. Sometimes the dimensions and permissible deviations of the holes are set on the shafts, and the dimensions and permissible deviations of the shafts are set on the holes [26-30]. The holes and the shaft can have different nominal sizes, etc. Sometimes incorrect dimensions are added due to an erroneous image, section, or cross-section. The developer's absentmindedness can cause the size to be set on a different dimension line, which will definitely lead to an error. Sometimes the length of the movement of the mechanism, the place for assembly, etc.are not taken into account. Unmotivated errors are called random errors that cannot be explained in any way.
CONCLUSION
When assessing the impact of errors, it is necessary to consider the design in its inseparable connection with its intended purpose and application. Here, such factors as the serial production of the product, the reliability of the design, etc. are important. Error analysis shows that errors are relative, depending not only on objective factors, but also on the experience and qualifications of the expert who determines the error. Products designed to be manufactured in a single production will be erroneous for mass production and vice versa. It is very difficult to assess errors of an economic nature, and errors of a social nature are detected only after a certain period of operation.
Fig.2. Design error: no space is provided for the relative mutual movement of the gear rails; it is necessary to cut off the shaded areas
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