The product life cycle consists of three main stages: manufacture, use, and salvage. At each stage, the product participates in the corresponding process: in the first, the production process; in the second, the operating process; in the third, the disposal process.

Accordingly, each stage imposes specific requirements on the product. We now consider the requirements on the product at each stage of the life cycle.

Since the product design is based primarily on its intended operation, with secondary attention to its manufacture and disposal, we begin by considering the requirements on operation.

The product requirements on the product design include requirements on the characteristics of its components and requirements on the design as a whole.

The requirements on the components are noted on the corresponding working drawings, while the requirements on the design as a whole are noted on the assembly drawing and in the explanatory caption.

The characteristics of the components at the operational stage include the hardness of the material, strength, rigidity, dimensional precision, shape of the surface, relative position of the surfaces, surface quality, and wear resistance.

The characteristics of the design as a whole at the operational stage include the strength, rigidity, vibration resistance, disequilibrium of the rotary components, and precision of the links in the dimensional chains.

The requirements on these characteristics are determined by the characteristics of the operational process and the corresponding operating conditions.

At the manufacturing stage, the characteristics on which requirements are imposed may be divided into three groups: (1) some of the operational characteristics, which are directly determined by the manufacturing stage; (2) characteristics ensuring certain operational characteristics; (3) characteristics ensuring satisfactory behavior of the product components.

For the product components, the characteristics of the first group include the hardness of the material, strength, rigidity, dimensional precision, shape of the surface, relative position of the surfaces, and surface quality.

The characteristics of the design as a whole at the manufacturing stage include the strength, rigidity, vibration resistance, disequilibrium of the rotary components, and precision of the links in the dimensional chains.

For the product components, the characteristics of the second group include, for example, the precision of alignment of the geometric axis with the component’s axis of rotation. The tolerance on alignment of the axes is determined by the balance requirements.

For the product components, the characteristics of the third group include, for example, the machinability of the materials and the characteristics of the technological bases.

At the disposal stage, the characteristics of the product design on which requirements are imposed may be divided into two groups: (1) some of the operational characteristics; (2) characteristics ensuring satisfactory behavior of the product components.

The first group includes the strength and hardness of the materials; the second includes the characteristics of the technological bases, for example.

Table 1 itemizes the characteristics on which requirements are imposed.

Table 1

Product characteristic

Stage of product life cycle

operation

production

disposal

1. Strength

+

+

+

2. Rigidity

+

+

+

3. Vibrational stability

+

−

−

4. Hardness

+

+

+

5. Wear resistance

+

−

−

6. Dimensional precision, shape, and relative position of surfaces

+

+

−

7. Precision of links in dimensional chains

+

+

−

8. Surface quality

+

+

−

9. Disequilibrium of rotating parts

+

−

−

Analysis of Table 1 indicates some characteristics common to all stages of the life cycle.

In addition, the requirements imposed at different stages of the life cycle are inconsistent.

For example, high strength is required at the operational stage, but low strength is desirable at the manufacturing stage.

As a result, in specifying the requirements on the common characteristics, we must resolve the conflict between the requirements at different stages of the life cycle.

To that end, the first step is to consider the selection of the criteria.

The criterion selected is minimum total expenditure on the product at all stages of the life cycle: for example, minimum total cost of ensuring the required product quality at all stages of the life cycle.

Since the product is designed to meet operational goals, we first specify the requirements on the product characteristics on the basis of the operational requirements.

Hence, the conflict between the requirements at different stages of the life cycle is reconciled by lowering the requirements at the operational stage. For example, the strength of the part at the operational stage must be decreased in order to facilitate its manufacture and disposal.

However, limits on the decrease in strength are imposed either by engineering requirements or by economic concerns, when the cost of the loss of operational efficiency due to the lower strength exceeds the cost savings in manufacture and disposal.

Thus, to reconcile the requirements on the characteristics at different stages of the life cycle, we lower the values of the characteristics in the operational stage.

However, this is not possible in all cases: for example, in relation to the characteristics associated with the product’s operational safety.

The values of the common characteristics in the operational stage can only be decreased within certain limits. For example, excess decrease in strength will result in product failure.

Thus, in reconciliation of the requirements on the characteristics at different stages, we must focus on a certain permissible range at the operational stage.

To resolve the conflicts in the requirements, we need to know the dependence C = f(xi), where C is the manufacturing (or disposal) cost, while xi is the ith characteristic of the product. This dependence may take different forms.

If there is no extremum in C = f(xi), the cost will increase with increase in the product characteristic at the manufacturing (or disposal) stage.

In the presence of an extremum, however, the cost will first decrease with increase in the characteristic, and then begin to increase.

For example, with low rigidity of the part, its manufacturing cost will be high on account of the time required to install and strengthen the workpiece.

With increase in rigidity, these time expenditures will decrease. At some rigidity value, the manufacturing cost will begin to increase.

This must be taken into account in reconciling the requirements on the characteristics at different stages of the life cycle.

In the general case, two forms of C = f(xi) are possible (Fig. 1).

Fig. 1.
figure 1

Graph of the product’s cost C as a function of the value of characteristic xi in the absence (1) and presence (2) of an extremum.

Full size image

Thus, reconciliation of the requirements on the product characteristics at different stages of the life cycle must include the identification of permissible ranges for each characteristic at the operational stage and subsequent decrease in the value of each characteristic so as to minimize the total product cost in all stages of the life cycle.

CONCLUSIONS

(1) The requirements on the product quality at different stages of the life cycle may be reconciled by lowering the requirements at the operational stage.

(2) In resolving the conflicts, the criterion is minimum total product cost in all stages of the life cycle.

(3) In reconciling the requirements on a characteristic of the product at different stages of the life cycle, a lower limit is set by the value in the operational stage below which the product cannot function.

(4) In resolving the conflicts, we need to know how the manufacturing (or disposal) cost depends on the value of specific product characteristics.