The advances that have been made in welding and thermal cutting processes have provided the means of shaping and joining large sections of iron and steel. These processes have replaced castings in the production of machine frames, machine bases, and other structures. Correctly designed welded frames are stronger and lighter, but rigidity has not been sacrificed.
Designing for Strength and Rigidity. Machine designs must have sufficient strength so that members will not fail by breaking or yielding when subjected to normal operating loads or reasonable overloads. Strength designs are common in road machinery, motor brackets, farm implements, and like structures.
For weldments in machine tools and other machin- ery, rigidity as well as strength is important, since excessive deflection under load would result in lack of precision in the product. A design based on rigidity requires the use of design formulas for sizing members. Some parts of a weldment serve their design function without being subjected to loading much greater than their own weight (dead load). Some typical parts are dust shields, safety guards, and cover plates for access holes. Only casual attention is
required in sizing such members.
Design Formulas. The design formulas for strength and rigidity always contain terms representing the load, the stress, and the strain or deformation. If two of the three terms are known, the others can be calculated. All problems of design thus resolve into one of
the following:
(1) Finding the internal stress or deformation caused by an external load on a given member.
(2) Finding the external load that may be placed on a given member for any allowable stress or deformation (3) Selecting a member to carry a given load without exceeding the specified stress or deformation.
In designing within allowable limits, the designer should generally select the most efficient material section size and section shape. The properties of the material and those of the section determine the ability of a member to carry a given load.
Sizing of Steel Welds. A weld is sized for its capability to withstand static or cyclic loading. Allowable stresses for welds for various types of loading are normally specified by the construction standards applicable to the job. They are usually based on a percentage of tensile or yield strength of the metal to ensure that a soundly welded joint can support the applied load for the expected service life. Allowable stresses or stress
ranges are specified for various types of welds under static and cyclic loads. The allowable stress ranges for welded joints subjected to cyclic loading specified in
current standards are based on testing of representative full-size welded joints in actual or mockup structures.
The primary requirement of machine design for a machine and some of its members is rigidity. Such members are often thick sections so that the movement under load can be controlled within close tolerances. Whereas low-carbon steel has an allowable stress in tension of 138 MPa (20 ksi), a welded machine base or frame may have a working stress of only 14 to 28 MPa (2 to 4 ksi). In these cases the weld sizes should be designed for rigidity rather than load conditions.
A practical method is to design the weld size to carry one-third to one-half of the load capacity of the thinner member being joined. This means if the base metal is stressed to one-third to one-half of the normal allowable stress, the weld would be strong enough to carry the load. Most rigid designs are stressed below these values.
Welding Conditions. Designers specifying welding procedures for machinery fabrication should specify the following:
(1) Joint type, groove angle, root opening and root face
(2) Electrode type and size to be used
(3) Current type, polarity and current in amperes
(4)Arc length (arc voltage)
(5)Travel speed
(6) Welding position i.e., flat, horizontal, vertical, overhead
(7) Test procedures for weld metal and joints