Machine Frames, Bolted Connections, and Welded Joints Machine ...

Machine Frames, Bolted Connections, and Welded Joints

Chapter 20

Material taken from Mott, 2003, Machine Elements in Mechanical Design

Machine Frames, Bolted Connections, and Welded Joints We have looked at individual machine elements while considering how these elements must work together in a more comprehensive machine. As the design progresses, there comes a time when you must put it all together.

Machine Frames, Bolted Connections, and Welded Joints At this point you must decide, “What do I put it in? How do I hold all of the functional components safely, allowing assembly and service while providing a secure, rigid structure?”

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Machine Frames and Structures The design of machine frames and structures is largely art. Where the designer envisions how the components of the machine will be accommodated. The designer is often restricted in where supports can be placed in order not to interfere with the operation of the machine or in order to provide access for assembly or service.

Machine Frames and Structures Some of the more important design parameters include: Strength Appearance Corrosion resistance Size Vibration limitation

Stiffness Cost to manufacture Weight Noise reduction Life

Machine Frames and Structures Structural design techniques for machine frames can be seen as designing a building, where techniques used in other application apply. Such as; simple beam, truss analysis, indeterminate beams, deflection theory, rigid frames, finite element analysis techniques and, so on. The designer must choose the appropriate design analysis technique for the application in consideration.

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Machine Frames and Structures Because of the virtually infinite possibilities for design details for frames and structures, this section will concentrate on general guidelines. Factors to consider in starting a design project for a frame are: Forces exerted by the components of the machine through mounting points such as bearings, pivots, brackets, and feet of other machine elements.

Machine Frames and Structures Manner of support of the frame itself Precision of the system: allowable deflection of components Environment in which the unit will operate Quantity of production and facilities available Availability of analytical tools such as computerized stress analysis, past experience with similar products, and experimental stress analysis Relationship to other machines, walls, etc

Many of these factors require judgment by the designer.

Materials As with machine elements discussed throughout this book, the material properties of strength and stiffness are of prime importance. In general, steel ranks high in strength compared with competing materials for frames. But it is often better to consider more than just yield strength, ultimate tensile strength, or endurance strength alone.

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Materials con’t Consider the ratio of strength to density, sometimes referred to as the strength-toweight ratio or specific strength. This is one reason for the use of aluminum, titanium, and composite materials in aircraft, aerospace vehicles, and transportation equipment.

Materials con’t Rigidity of a structure or a frame is frequently the determining factor in the design, rather than the strength. In these cases, the stiffness of the material, indicated by its modulus of elasticity, is the most important factor. The ratio of stiffness to density is called specific stiffness.

Recommended Deflection Limits Only intimate knowledge of the application of a machine member or a frame can give an acceptable deflection value. Deflection Due to Bending: General machine part: 0.0005 to 0.003 in/in of beam length Moderate precision: 0.00001 to 0.0005 in/in High precision: 0.000 001 to 0.000 01 in/in

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Recommended Deflection Limits con’t Deflection (Rotation) Due to Torsion: General machine part: 0.001o to 0.01o/in of length Moderate precision: 0.000 02o to 0.0004o/in High precision: 0.000 001o to 0.000 02o/in

Suggestions to Resist Bending A table of deflection formulas for beams in bending yield the following form for the deflection: ∆ = PL3 / KEI Where P = load L = length between supports E = modulus of elasticity of the material in the beam I = moment of inertia of the cross section of the beam K = a factor depending on the manner of loading and support

Suggestions to Resist Bending Some obvious conclusions are that the load and the length should be kept small, and the values of E and I should be large. The next figure shows the comparison of 4 types of beam systems to carry a load, P, at a distance, a, from a rigid support. A beam simply supported at each end is taken as the “basic case.” The data show that a fixed-end beam gives both the lowest bending moment and the lowest deflection, while the cantilever gives the highest values for both.

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Comparison of Methods

Mott, 2003, Machine Elements in Mechanical Design

Suggestions to Resist Bending The following suggestions are made for designing to resist bending:


Suggestions to Resist Bending

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Suggestions to Resist Torsion Torsion can be created in a machine frame member in a variety of ways: A support surface may be uneven; a machine or a motor may transmit a reaction torque to the frame; a load acting to the side of the axis of the beam (or any place away from the flexural center of the beam) would produce twisting.

Suggestions to Resist Torsion In general, the torsional deflection of a member is computed from: θ = TL / GR T = applied torque or twisting moment L = length over which torque acts G = shear modulus of elasticity of the material R = torsional rigidity constant

Suggestions to Resist Torsion The designer must choose the shape of the torsion member carefully to obtain a rigid structure. 1. Use closed sections wherever possible. Examples are solid bars with large cross section, hollow pipe and tubing, closed rectangular or square tubing, and special closed shapes that approximate a tube. 2. Conversely, avoid open sections made from thin materials (as in the next figure).

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Suggestions to Resist Torsion


Suggestions to Resist Torsion 3. For wide frames, brackets, tables, bases, etc, use diagonal braces placed at 45o to the sides of the frame. See next figure. 4. Use rigid connections, such as by welding members together.


Welded Joints The design of welded joints requires consideration of the manner of loading on the joint, the types of materials in the weld and in the members to be joined, and the geometry of the joint itself. The load may be either uniformly distributed over the weld such that all parts of the weld are stressed to the same level, or the load may be eccentrically applied.

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Welded Joints con’t The materials of the weld and the parent members determine the allowable stresses. The allowables listed are for shear on fillet welds. For steel, welded by the electric arc method, the type of electrode is an indication of the tensile strength of the filler metal. For example, the E70 electrode has a minimum tensile strength of 70 ksi (483 MPa).


Types of Joints Joint type refers to the relationship between mating parts,as shown in the next slide. The butt weld allows a joint to be the same nominal thickness as the mating parts and is usually loaded in tension. If the joint is properly made with the appropriate weld metal, the joint will be stronger than the parent metal.

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Types of Weld Joints


Types of Joints con’t In full-penetration butt-welds, no special analysis of the joint is required if the joined members themselves are shown to be safe. Caution is advised when the materials to be joined are adversely affected by the heat of the welding process. For example, heat-treated steels and many aluminum alloys.

Types of Welds The next figure shows several types of welds named for the geometry of the edges of the parts to be joined. Note the special edge preparation required, especially for thick plates, to permit the welding rod to enter the joint and build a continuous weld bead.

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Welds with Edge Preparation


Size of Weld The five types of groove-weld are made as complete penetration welds. Then, the weld is stronger than the parent metals, and no further analysis is required. Fillet welds are typically made as equal-leg right triangles, with the size of the weld indicated by the length of the leg. A fillet weld loaded in shear would tend to fail along the shortest dimension of the weld that is the line from the root of the weld to the theoretical face of the weld and normal to the face.

Size of Weld con’t The length of this line is found from simple trigonometry to be 0.707w, where w is the leg dimension. The objectives of the design of a fillet welded joint are to specify the length of the legs of the fillet; the pattern of the weld; and the length of the weld. Presented here is the method that treats the weld as a line having no thickness. The method involves determining the maximum force per inch of weld leg length. Comparing the actual force with an allowable force allows the calculation of the required leg length.

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Method of Treating Weld as a Line Four different types of loading are considered here: (1) direct tension or compression, (2) direct vertical shear, (3) bending, (4) twisting. The method allows the designer to perform calculations in a manner very similar to that used to design the loadcarrying members themselves.

Method of Treating Weld as a Line con’t In general, the weld is analyzed separately for each type of loading to determine the force per inch of weld size due to each load. The loads are then combined vectorially to determine the maximum force. This maximum force is compared with the allowables from the next table to determine the size of the weld required.


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Method of Treating Weld as a Line con’t The relationships used are summarized next:


Method of Treating Weld as a Line con’t In these formulas, the geometry of the weld is used to evaluate the terms Aw, Sw, and Jw using the relationships shown in Figure 20-8. Note the similarity between these formulas and those used to perform the stress analysis. Because the weld is treated as a line having no thickness, the units for the geometry factors are different from those of the area properties.

Method of Treating Weld as a Line con’t


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General Procedure




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Eccentrically Loaded Bolted Joints The next figure shows an example of an eccentrically loaded bolted joint. The motor on the extended bracket places the bolts in shear because its weight acts directly downward. But there also exists a moment equal to P x a that must be resisted. The moment tends to rotate the bracket and to shear the bolts.


Eccentrically Loaded Bolted Joints con’t The basic approach to the analysis and design of eccentrically loaded joints is to determine the forces that act on each bolt because of all the applied loads. Then, by a process of superposition, the loads are combined vertically to determine which bolt carries the greatest load. That bolt is then sized.

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