Design Considerations of Welded Assemblies | Machine Design

Welding is the most important method of joining the parts into a complex assembly. The design considerations of welded assemblies play an important role at the time of fabrication. In this topic, we are going to discuss design considerations of welded assemblies.

Assembly welding

Table of Contents

The general principles in the design of welded assemblies

Select the material with high weldability
Use of a minimum number of welds
Do not shape the parts based on casting or forging
Use standard components
Avoid straps, laps, and stiffeners
Select the proper location of the weld
Prescribe the correct sequence of the welding

Select the material with high weldability

In general, low carbon steel is more easily welded than high carbon steel. Higher carbon content tends to harden the welded joint as a result of which the weld is susceptible to cracks.

For ease in welding, maximum carbon content is usually limited to 0.22 per centimeter square.

Use of a minimum number of welds

Distortion is a serious problem in welded assemblies. It creates difficulties in maintaining the correct shape, dimensions, and tolerances of finished assemblies.

A metallic plate or component does not distort when it is heated or cooled as a total unit uniformly and it has the freedom to expand or contract in all directions.

In welding, however, only the adjoining area of the joint is heated up, which has no freedom to expand or contract.

Uneven expansion and contraction in this adjoining area and parent metal results in distortion. When distortion is prevented by clamping fixtures, residual stresses are built up in the parts, and annealing is required to relieve these stresses.

Since distortion always occurs in welding, the design should involve the minimum number of welds and avoid over-welding. It will not only reduce the distortion but also the cost.

Do not shape the parts based on casting or forging

In designing a welded assembly to replace a casting, it is incorrect to duplicate its shape or appearance by providing protrusions, brackets, and housing.

The designer should appreciate that welded assemblies are different from castings, having appearances of their own. A correctly designed welded assembly is much lighter than the corresponding casting. It should reflect its lightweight characteristics, flexibility, and economy of the material.

Use standard components

The designer should specify standard sizes for plates, bars, and rolled sections. The nonstandard section involves flame cutting of plates and additional welding.

Standard tubular sections should be used for torsional resistance. As far as possible, the designer should select the plates of equal thickness for a butt joint.

Avoid straps, laps, and stiffeners

The stiffness of the plate can be increased by making bends, indentations in the form of rib, or corrugations by press working.

If at all the stiffener is required to provide rigidity to the plate, it should be designed properly with minimum weight. The use of a separate stiffener involves additional welding increasing distortion and cost.

Select the proper location of the weld

There are two aspects of selecting the correct location for the welded joint.

The welded joint should be located in an area where stresses and deflection are not critical. Also, it should be located at such places that the welder and welding machine has unobstructed access to that location.

It should be possible to carry out pre-weld machining, post-weld heat treatment, and finally weld inspection at the location of the weld.

Prescribe the correct sequence of welding

The designer should consider the sequence in which the parts should be welded together for minimum distortion.

This is particularly important for a complex job involving a number of welds. An incorrect sequence of welding causes distortion and sometimes cracks in the weld’s metal due to stress concentration at some point in fabrication.

A correct welding sequence distributes and balances the forces and stresses induced by weld contraction.

For example, to prevent angular distortion in the double V butt joint, a sequence is recommended to lay welding runs alternatively on opposite sides of the joint.

An example of incorrect and correct ways of welded design is illustrated in the below figures.

In figure 1 (a), it is necessary to prepare bevel edges for the components prior to the welding operation. This preparatory work can be totally eliminated by making a slight change in the arrangement of the components shown in fig. 1 (b)

Fig. 1 Saving of preparatory bevelling (a) Incorrect (b) Correct

Many times fabrication is carried out by cutting steel plates followed by welding. The aim of the designer is to minimize the scrap in such a process. This is shown in figure 2.

Fig. 2 Reduction of scrap

The circular top plate and annular bottom plate are cut from two separate plates resulting in excess scrap as shown in fig 3.

Fig. 3 Reduction of Scrap

Making a slight change in design, the top plate and annular bottom plate can be cut from one plate reducing crap and material cost, shown in the figure. Accumulation of welded joints results in shrinkage stresses. A method to reduce this accumulation is shown in figure 4.