15.3.1 Overview
When designing a threaded fastener joint, it is important that the designer considers all production variations that might influence the joint’s performance. These variations come under the following broad categories:
- • Process Variations
- • Material Variations
- • Tolerancing
It is important that the designer specifies the acceptable variation of the above parameters in the engineering drawings, also citing the relevant company, national or international standards.
Production testing and inspection is an important part of Quality Control and it provides feedback on achievement and limits variations. It is important that the designer therefore considers the necessary testing and inspection and facilitate this (as far as possible) within the design.
15.3.2 Process Variations
The performance of the threaded fastener joints is affected by the following:
- a. Dimensional tolerances, e.g. fastener/hole fits, hole positional tolerances.
- b. Surface finish on the mating part of the design.
- c. Preload variations.
- d. Thread profile and dimensional tolerances.
- e. Lubrication conditions.
The above variations are limited by the designer specifying an acceptable level on the drawings or by invoking standards.
15.3.3 Material Variations
Performance of the joint can be affected by the following material variations:
- a. Ultimate strength (compressive and tensile)
- b. Proof strength (compressive and tensile)
- c. Moduli of the material
- d. Poisson Ratio
- e. Friction characteristics
- f. Corrosion characteristics
15.3.4 Tolerancing
Tolerances can affect the joint characteristics such as its load carrying ability, stiffness and backlash. It is likely that the larger the tolerances, the greater the joint strength variation.
When considering the effect of manufacturing tolerances it is necessary to assume some statistical distribution of the variations. If all tolerances were assumed to be at their worst possible value, then the design is far too pessimistic. However, there can be cases where this has been designed in deliberately and so it is important that it is not overlooked completely.
When using statistical analysis it is essential to recognise the distribution of the variation. Manufacturing variations can generally be classified as Gaussian. However, Skewed-Gaussian or Rectangular distributions can be more suitable for certain manufacturing methods. Inspection and testing can affect the distribution by truncating the ends of the distribution.
It is important that the designer obtains information on the variation of important joint characteristics and examine test results in order that he can have confidence in his statistical analysis. It is important that the tolerances specified take account of the variation in the controlled characteristic. In order to optimise production yield and minimise costs, it is important that the tolerances are increased to the maximum consistent with the performance to be achieved.