Welding Basics - the Art Behind the Repair

The Basics

The two authoritative bodies to which engineers, manufacturers, and industry at large look for welding guidelines and standards are the American Welding Society (AWS), and the American National Standards Institute (ANSI).  Properly accredited training programs and classes base their technical and background training on the principals set forth by these two groups.

Of the six major welding processes described by the AWS, arc welding is most often used in the production and repair of mobile power equipment. Original Equipment Manufacturers (OEM’S) often utilize Metal Inert Gas (MIG) arc welders primarily because MIG welding offers a degree of automation, higher production rates, and is easy to use.  This may or may not be available in the field, and is not required to complete a satisfactory welded repair.

Of the five types of standard joints (Figure 1), on M-B equipment you will typically see tee joints used for frame members and gussets, lap joints for broom hoods, and corner joints for built up swing and hitch pieces.  To construct these joints, four basic weld types could be used.  These include v-groove, bevel, plug and the very common fillet weld (Figure 2). The fillet weld involves build up of the weld material where the two pieces meet.  No removal (grinding) of the build up is necessary.  The other three types of welds shown require that material be removed from the part to be welded to provide space for the weld or filler material.
 

Considerations in Design

Figure 3 shows two parts that would normally be joined by a fillet weld around the circumference of the hole in part A.  In the design of this part, an engineer would have conducted an analysis to determine the stresses between the parts.  After applying a safety factor, the engineer would have determined what type of weld was appropriate.  When this is determined, manufacturing time, appearance, and the intended environment for the pieces and the weld might also be evaluated to arrive at a final decision on the proper type of weld.  Storage and use of a piece of equipment outdoors, for instance, could give rise to corrosion and weathering, which should be accounted for.
 

Repair Considerations

If part A were to be removed and replaced, a staggered intermittent fillet weld could be considered for the repair to join the parts.  If the parts shown in Figure 3 were part of a broom attachment that was typically stored outdoors, rain could enter through the spaces in the intermittent weld and become trapped.  If the surfaces surrounding the area to be welded were prepped (cleaned) properly before the welding, an opportunity has been created for rust to develop between the parts.  Ongoing rust would certainly reduce the appeal of the equipment and likely contribute to a weakened bond between the welded parts.  Further, given the particular application, an intermittent weld may not be of sufficient strength when all variables are considered.  If the primary reason to attempt an intermittent weld were to save on welding time and material, it could easily prove to be a false savings over the intended life of the equipment.

Summary

In the example shown in Figure 3, proper heat, proper technique, and a fillet weld around the entire circumference of the part would  ensure that the equipment performs well and withstands harsh weather for many years.  Much thought and careful analysis goes into the decision on what type of weld to use on M-B equipment.  In repair work, do not substitute a different weld type without     consulting the OEM.  Beyond that, only   certified welders should attempt repair work on mobile equipment.

Questions regarding the type of weld or the appropriate type of repair are welcome at the M-B Companies Service Department.  Additionally, M-B Service and Engineering personnel can refer you to sources of more complete information than is provided in this article.