experimental construction and the test program have not found the design inadequate, the work of redesign may be just that of minor revision. If major flaws and shortcomings have been exposed, then the work of redesign may reach major proportions, and entirely new concepts may have to be sought for major components and even for subsystems.
1.9 ROLE OF ALLOWANCE, PROCESS CAPABILITY, AND TOLERANCE IN DETAILED DESIGN AND ASSEMBLY
1.9.1 Allowance ( A
Allowance is the difference of dimension between a female member and a male member of the assembly. A practical example of a male member would be a shaft, and its corresponding female member would be a hole. The concept of allowance is illustrated in Fig. 1.6(a). If the allowance is positive, the type of fit is termed a clearance fit; if the allowance is negative, the type of fit is termed an interference fit; if the allowance has a very small positive value or a very small negative value, it is called a transition fit. The various types of fits are described in Section 1.9.4.
Fig. 1.6(a) Concept of allowance in a shaft-hole combination. 1.9.2 Process Capability ( P)
A production process which is set up to produce a certain dimension ( D) of a part will not be able to produce all parts to the set-up dimension. There will always be a few oversized parts with dimension
Dmax and a few undersized parts with dimension Dmin. The difference between Dmax and Dmin is
termed process capability. A precise production process such as grinding will have a narrower band width (Dmax – Dmin) as compared with a less precise production process such as, say, rough turning on lathe. Referring to Fig. 1.6(b), a frequency distribution curve of machine cut parts follows a bell- shaped normal distribution curve.
1.9.3 Tolerance (T)
The product designer is aware that a process set up at a size D will occasionally produce pieces, which are either oversized or undersized. For this reason, a designer always specifies a part dimension (D + T/2), where T is the tolerance prescribed by the designer. In order to avoid the
production of defective parts, the tolerance T should be matched to the process capability P such that
P is less than T. The usual ratio of T/P is 2. This ratio is termed relative precision index. Figure
1.6(b) indicates the relationship between P and T.
Fig. 1.6(b) Tolerance and process capability. 1.9.4 Types of Fits
Taking the example of the assembly of a shaft and hole, the allowance A is the difference of
dimensions Dh and Ds (see Fig. 1.6(a)). A designer usually starts with an ideal allowance between a shaft and a hole.
(i) Clearance fit. For a bearing, allowance is positive, i.e. (Dh – Dx) = positive. This is called clearance, and this type of fit is called a clearance fit. The designer visualizes the clearance fit for successful operation of a bearing in which the shaft and hole are separated by a film of lubricant. (ii) Interference fit. In the case of a press fit, a designer recommends a negative clearance so that (Dh – Ds) = negative. This type of fit is called interference fit. A practical example of this is a bush fitted in a housing bore.
(iii) Snug fit and transition fit. A third type of fit is called the snug fit which has (Dh – Ds) = zero (small positive value or a small negative value in practice). Such a fit is useful for location of a
component having a bore with a pin. It is termed transition fit. Figure 1.7(a) illustrates the designer’s concept of a clearance fit. This will be discussed in detail now from Fig. 1.7(b), it may be observed that due to the inability of a process to produce an exact dimension, it becomes essential for the
designer to specify certain permissible variation, T which is more than the process capability P. This permissible variation has to be indicated on any realistic detailed drawing. Figure 1.7(b) shows the shaft hole combination of Fig. 1.7(a) modified to take care of tolerance for the production of the shaft and the hole. Figure 1.7(c) illustrates a conventional diagram of fits.
Fig. 1.7 Fundamentals of a limit system. 1.9.5 Selection of Fit
A designer’s concept of fit, manufacturing specifications for a fit, and conventional diagrams of fit are shown in Figs. 1.7(a)–(c), respectively.
A fit can be obtained by a combination of two apparently independent variables: 1. Tolerance grades, 18 in Number: IT-1 to IT-18.
2. Fundamental deviation FD denoted by lower case letters such as c, d, e etc. for shafts and capital letter H for holes.
a few preferred combinations are adopted in practice. One typical example followed by BS4500 is illustrated in Fig. 1.8.
Fig. 1.8 Preferred combinations of fits. 1.9.6 Specific Principles
1. Tolerance need not be assigned to atmospheric surfaces. For such surfaces, finish should be good enough to prevent corrosion and ill effects of pollutants. However, for journal bearing type design a positive clearance is desirable.
2. For a location fit such as pulley on a shaft (where subsequently a key shall be driven in), a location fit or a transition fit is essential.
3. A situation where one component has to be rigidly fitted to another, an interference fit or a press fit is recommended. An example of such a fit is fitting a gear on a shaft for transmission of power.
4. From process engineering point of view, tolerance grades IT-1 to IT-6 are obtained by lapping, honing, microfinishing and precision grinding.
5. Tolerance grades IT-7 and IT-8 represent finished turning and semifinished turning, whereas grade IT-9 is rough turning. All tolerances above grade IT-9 belongs to various primary processes such as rolling, casting, forging etc.
1.10 SUMMARY OF DETAILED DESIGN PHASE
We have seen that the detailed designs, involving large commitments for design work, requires careful preparation of capital budgets and time schedules. This is the first step. Top management, in the light of these estimates, must decide whether to continue with the design project. If the decision is favourable, then a project organization must be developed.
The second and third steps involving the overall designs of subsystems and components are similar in many ways to the preliminary design. In the fourth step, the detailed design of parts is undertaken, and followed in the fifth step by the preparation of assemblies for the components and subsystems.
In the sixth and seventh steps, the prototype is built and tested. The difficulties encountered in both of the operations, constructing and testing, become the subjects of analysis in the eighth step. Also, performance is predicted under conditions of customers operation, and any shortcomings, evident or anticipated, are included in the difficulties.
The final step is the making of revisions. A major problem to be tackled is that the design as a whole should be least affected. Small revisions can start a chain of consequences which could destroy the originality of the earlier design.
Finally, the interative character of design work should be noted. After the revisions have been made in the redesign step, building of new prototypes and subsequent testing may follow, again leading to further revisions. A successful project is, however, highly convergent so that only a few iterations are required to reach a final solution. The high rate of convergence stems from the high confidence levels which are required in critical decisions.