Design Considerations Of Castings 4r3424

DESIGN CONSIDERATIONS OF CASTINGS: Complex parts, which are otherwise difficult to machine, are made by casting processes using sand mould. The cast components are stable, rigid and strong compared with respect to some other part or structure with machined or forged parts. Eg:- machine tool bed and structure, cylinder block of internal combustion engine, pump and gear box housing. The general principles for the design of casting are as follows: 1. Always keep the stressed areas of the part in compression. Cast iron has more compressive strength than its tensile strength. 2. Round all external corners. It has two advantages:

It increases the endurance limit of the component.

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It reduces the formation of brittle chilled edges.

3. Wherever possible, the section thickness throughout should be held as uniform as compatible with overall design considerations. Abrupt changes in the cross – section result in high stress concentration. If the thickness is to be varied at all, the change should be gradual. 4. Avoid thickness of the metals at the junctions. Even after the metal solidifies, the central portion still remains in the molten stage, with the result that a shrinkage cavity or blowhole may appear at the centre The concentration of metal can be avoided in two ways:

One is to provide cored opening in webs and ribs.

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Stagger the ribs and webs.

5. Avoid very thin sections. The minimum section thickness depends upon the process of casting, such as sand casting, permanent mould casting or die casting. The minimum thickness for grey cast iron component is about 7 mm for parts upto 500 mm long, which gradually increases to 20 mm for large and heavy castings. 6. Shot blast the parts wherever possible. The shot blasting improves the endurance limit of the component, particularly in case of thin sections.

DESIGN CONSIDERATIONS OF FORGINGS: Forged components are widely used in automotive and aircraft industries. They are usually made of steels and non – ferrous metals. They can be as small as the gudgeon pin and as large as crankshaft. Forged components are used under following circumstances:

Moving components requiring light weight to reduce inertia forces. Eg:- connecting rod of I.C. engine.

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Components subjected to excessive stresses. Eg:- aircraft structure.

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Small components that must be ed by other structure or part. Eg:- hand tools and handle.

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Components requiring pressure tightness where the part must be free from internal cracks. Eg:- valve bodies.

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Components whose failure would cause injury and extensive damage are forged for safety.

In order to obtain maximum benefit from forged components, following principle should be adopted:1. While deg a forging, advantage should be taken of the direction of fibre lines. In case of forged parts, the fibre lines are arranged in a favourable way to withstand stresses due to external load. 2. The forged component should be provided with an adequate draft. 3. A parting line divides the forging into two halves which ensures the minimum depth to which the steel must flow to fill the die impressions. 4. The forging should be provided with adequate fillet and corner radii. Small radius results in folds on the inner surface and cracks on the outer surface. Large radius is undesirable. Sharp corners result in increasing difficulties n filling the material, excessive forging forces and poor die life. 5. Thin sections and ribs should be avoided in forged components. A thin section cools at a faster rate in the die cavity and requires excessive force for plastic deformation. It reduces the die life and the removal of the component from the die cavities becomes difficult. A properly designed forging is not only sound with regard to strength but it also helps reduce the forging forces, improves die life and simplifies die – design.

DESIGN CONSIDERATIONS OF MACHINED PARTS: Machined components are widely used in industrial products. They are usually made from ferrous and non – ferrous metals. The general principles for the design of machine parts are as follows:1. Avoid machining. Machining operations increase the cost of the product. 2. Specify liberal tolerances. Depending on the functional requirement of the component, the designer should specify most liberal dimensional and geometric dimensions. 3. Avoid sharp corners. The sharp corners result in stress concentration. 4. Use stock dimensions. Using stock dimensions eliminates machining operations. 5. Design rigid parts. Any machining operations such as turning or shaping induce cutting forces on the components. The component should be rigid enough to withstand such forces. 6. Avoid shoulders and undercuts. They usually involve separate operations and separate tools, which increase the cost of machining. 7. Avoid hard materials. They should be avoided unless such properties are essential for the functional requirement of the product.

DESIGN CONSIDERATIONS OF WELDED ASSEMBLIES: Welding is the most important method of ing the parts into a complex assembly. The general principles in design of welded parts are as follows:

Select the material with high weldability. A low carbon content steel is more easily welded than a high carbon steel.

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Use minimum number of welds.

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Do not shape the parts based on casting or forging.

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Use standard components.

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Avoid straps, laps and stiffeners.

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Select proper location for weld.

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Prescribe correct sequence of welding.

TOLERANCES: Tolerance is defined as permissible variation in the dimensions of the component. The basic dimension is called the normal or basic size, while the difference between the two limits is called permissible tolerance. The two limits are sometimes called the upper and lower deviations. There are two systems of specifications for tolerances, namely unilateral and bilateral. 

In unilateral system, one tolerance is zero while the other takes care of all permissible variation in basic size. They are used for shafts and holes.

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In case of bilateral tolerances, the variation are given in both directions from normal size.

TYPES OF FITS: When two parts are to be assembled, the relationship resulting from the difference between their sizes before assembly is called a fit. Depending upon the limits of the shaft and the hole, fits are broadly classified into three groups:1. Clearance fit It is a fit which always provides a positive clearance between the hole and the shaft ver the entire range of tolerances. In this case, the tolerance zone of the hole is entirely above that of the shaft. Hole Zero line Shaft

Fig.:- diagram of clearance fit

2. Interference fit It is a fit, which always provides a negative clearance over the whole range of tolerances. In this case, the tolerance zone of the hole is completely below that of the shaft. Shaft

Hole

Fig.:- diagram of interference fit 3. Transition fit It is a fit, which may provide either a clearance or interference, depending upon the actual values of the individual tolerances of the mating components. In this case, the tolerance zones of the hole and the shaft overlap.

Shaft

Shaft

Shaft

Hole

Fig.:- diagram of transition fit.

There are two basic systems for giving tolerances to the shaft and the hole:1. Hole basis system (H) The different clearances and interferences are obtained by associating various shafts with a single hole, whose lower deviation is zero. The size of the hole is the basic size. This system has an advantage over the shaft basis system, because holes are machined by standard drills or reamers having fixed dimensions, while the shafts can be turned or ground to any given dimension. Due to this reason, the hole basis system is widely used. 2. Shaft basis system (h) The different clearances or interferences are obtained by associating various holes with a single shaft, whose upper deviation is zero. The size of the shaft is the basic size, while the clearance or interference is applied to the dimensions of the hole.

B.I.S SYSTEM OF FITS AND TOLERANCES: According to a system recommended by the Bureau of Indian Standards, tolerance is specified by an alphabet, capital or small, followed by a number eg.:- H7 or g6. The description of tolerance consists of two parts:

Fundamental deviation. The fundamental deviation gives the location of the tolerance zone with respect to the zero line. It is indicated by an alphabet – capital letters for holes and small letters for shafts.

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Magnitude of tolerance. The magnitude of tolerance is designated by a number, called the grade. The grade of tolerance is defined as a group of tolerances, which are considered to have the same level of accuracy for all basic sizes. There are eighteen grades of tolerances with designations – IT1, IT2 ... IT18. IT – International tolerance. Eg.:- the tolerance for a shaft of 50 mm diameter as the basic size, with the fundamental deviation denoted by the alphabet ‘g’ and the tolerance of grade 7 is written as 50g7.

The fit is indicated by the basic size common to both components followed by symbols for tolerance of each component. Eg.:- 50H8/g7 or 50H8 – g7.

SELECTIVE ASSEMBLY: The selective assembly is a process of sorting the manufactured components into different groups, according to their sizes and then assembling the components of one group to the corresponding components of a matching group. 

It results in closer clearance or interference without involving costly machining methods.

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It results in finer clearance or interference even with large manufacturing tolerances.

The main drawbacks of this method are:

100% inspection is required for components and there is additional operation of sorting the components.

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Interchangeability is affected and servicing or replacement of worn out components becomes difficult.

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The method can be used only when large number of components are manufactured, otherwise some groups may not contain sufficient number of components.

It is particularly useful in the case of interference fits.

TERMINOLOGY RELATED TO FITS AND TOLERANCES: Certain terminology connected with limits and tolerances is given below:1. Deviation It is the algebraic difference between a size ( actual, maximum etc) and the corresponding basic size. 2. Actual deviation It is the algebraic difference between an actual size and the corresponding basic size.

3. Upper deviation It is the algebraic difference between the maximum limit of size and the corresponding basic size. 4. Lower deviation It is the algebraic difference between the minimum limit of size and the corresponding basic size. 5. Zero line It is a straight line to which the deviations are referred in a graphical representation of limits and fits. 6. Fundamental deviation It is one of the two deviations that is conventionally chosen to define the position of the tolerance zone in relation to the zero line. 7. Tolerance zone It is the zone bounded by two limits of size of a part in the graphical representation of tolerance. It is defined by its magnitude and by its position in relation to the zero line.