Usage:
Installing Reinforcing Plates
Machine components fail primarily because of a localized overstressed condition. This overstressing is caused either by overloading, which stretches, compresses or bends the member, or by repeated flexing which cracks the member without visible bending. The former may occur, for example, in a track roller frame, the latter in a scraper gooseneck. In either case, the stresses can be reduced by properly applying reinforcements to the weak section. Improper application may have the opposite effect. High stresses usually occur at definite, predictable locations in machine members, although the locations are not always apparent until a failure occurs.
The understanding and application of several basic rules will help in the successful design and installation of reinforcing plates:
Rule 1 - Avoid sudden changes in cross-sectional area. Notches, sharp corners, and abrupt changes in area give rise to points of stress concentration. Reinforcements should be designed to enable loads to be transmitted smoothly through the reinforcement and from the reinforcement to the machine component (Figure 54). Notches and cracks should be filled with weld metal; corners should be curved with a generous radius; changes in width or thickness of plates should be gradual; ends should be tapered.
Figure 54 - Changes in cross sectional area must be made smoothly and gradually.
Several examples of well designed reinforcement plates are shown in Figure 55. Note the tapered ends and rounded corners. In most cases, tapered plates add the same strength to beams subjected to bending stresses as rectangular plates do (Figure 56). The tapered plates weigh about half as much as rectangular plates and require no more time to install.
Figure 55 - Properly designed reinforcements have tapered ends and rounded corners.
Figure 56 - Rectangular reinforcing plates add no more strength than tapered plates.
Rule 2 - Place reinforcements as far as possible from the neutral axis for bending loads. The neutral axis of a beam is located on a line of zero stress, which is usually at or near the center of the beam. Reinforcing plates of a given thickness will be most effective when placed as far as possible from the neutral axis (Figure 57). Note that plates added to the top and bottom of a beam parallel to the neutral axis add considerable more strength than plates of the same thickness added to the sides of the beam.
Figure 57 - Bending Load Reinforcements
Rule 3 - Use a circular or box section for torsional loads. Reinforcements for shafts subjected to torsional stresses should be continuous around the shaft and should run the full length that is in torsion. (Figure 58). Square sections should be reinforced by adding plates on all sides. Rectangular sections are strengthened by adding plates to the opposite sides that are closest together. I-beams, channels and angles should be reinforced in such a manner that the final section is boxed. In all cases, the circular or boxed section is much stronger in torsion and is preferred for torsional loads.
Figure 58 - Torsional Load Reinforcements
Rule 4 - Consider the effect on the entire beam before adding a reinforcement. Relatively small reinforcing plates added to a beam may reduce the stresses directly under the plate, but they have a tendency to increase stresses at each end of the beam (Figure 59). A long tapered plate is much better because it spreads the load uniformly through the beam instead of concentrating it in two spots.
Figure 59 - Long reinforcing plates distribute the load evenly along the beam.
Rule 5 - Avoid making transverse welds at the ends of reinforcements. Transverse welds create high stress areas that usually lead to cracking. Once a crack has formed, it will progress across the beam until it eventually causes a failure. The end of the reinforcing plate should be left open (Figure 60). Again, the ends are tapered to spread the load over a greater area of the beam.
Figure 60 - Transverse welds at the end of reinforcing plates should be avoided.
Rule 6 - Use gusset reinforcements to reduce stresses in sharp corners and other small areas. Gussets should be designed and installed so the forces are distributed and transmitted uniformly and are not concentrated in one spot. Gussets can be triangular or they can be designed with a radius (Figure 61). They should not be welded in the sharp corners created by the machine components and the gusset. Welding in these areas creates localized overstressed conditions that originate cracks. The thickness of the gusset should be about the same as or slightly less than the thickness of the plates it is reinforcing. If possible, the gusset should be welded on both the inside and outside.
Figure 61 - Gussets reduce stresses in sharp corners and other small areas.
Controlling Heat Distortion
Figure 62 - Members can be pulled into alignment by shrinking of the weld.
Distortion due to thermal expansion and contraction is a problem with all types of welds. There are three basic methods of minimizing distortion: peening, jigs and fixtures, and special welding techniques. With the first method, the weld metal is stretched immediately after it is deposited by a series of hammer blows. Peening has limited application in service welding, however, because of the lack of precision possible with hand peening and the possibility of strain hardening the weld metal. When the work is clamped in a special jig or fixture, the clamping force overcomes the force of thermal contraction of the weld deposit, and the deposit itself stretches. This method, too, has limited application in service welding because of the expense of special jigs and the difficulty of clamping large components.
Special techniques in the positioning of welding members and special pass sequence is usually the simplest method for controlling distortion in service welding. Members can be positioned initially somewhat out of alignment, and the shrinking of the weld deposit can be relied upon to pull the member into the proper position (Figure 62). A pass sequence that builds the deposit up equally on each side of the joint will allow contractive forces to balance each other and prevent distortion (Figure 63). Sufficient tack welding will greatly reduce distortion when the final weld is made. In each case, no attempt is made to eliminate the expansion and contraction; rather, the forces are utilized to give the desired final shape to the joint.
Figure 63 - A proper pass sequence allows contractive forces to balance each other.
The controlled expansion and contraction of metal has another application in service maintenance and reconditioning work - heat straightening. Large components such as track roller frames that are slightly bent can be straightened easily by controlled heating and cooling. The principle involved is based on upsetting the metal or making it shorter and thicker in the area where heat is applied. (Figure 64). If the outer or convex side of the bend is heated, the heated area expands. If the inner or concave side of the bend is kept relatively cool, however, the heated area cannot expand lengthwise as much as it wants to because of the rigidity of the cooler portion. The heated area, therefore, expands outward and becomes thicker. When the heated area cools, it contracts and effectively shortens the convex side of the bent beam. This shortening counteracts the bend and straightens the beam. If one heating and cooling operation does not bend the beam enough, the process can be repeated on either side of the original area.
Figure 64 - If metal is forced to expand unevenly when heated, it becomes "upset".
Selecting the Proper Reinforcing Steel
Many types of steels are available for use in reinforcing machine operated under sever conditions. High-strength structural steel, heat treated steel and mild steel are recommended for various field reinforcements and reconditioning procedures. High-strength structural steel is a weldable steel with a minimum yield strength of 45,000 psi and a minimum tensile strength of 70,000 psi. This type of steel is not heat treated. E7018 electrodes are recommended for welding high strength structural steel. Figure 65 gives the names and producers of some commercially available steels that fall into this category as well as some heat treated steels that are considered weldable. E11018 electrodes are recommended for the latter. A mild grade of hot rolled structural steel can also be used as reinforcing material. E60 Series electrodes are recommended for welding mild steel. SAE 1017, SAE 1018, SAE 1020 and SAE 1021 grades fall in this category and are produced by nearly all steel companies.
Figure 65 - High Strength Structural Steels and Weldable Heat Treated Steels
Rebuilding By Welding
Components of track roller frames on track-type machines (such as links, rollers and sprockets) that are exposed to considerable wear in the normal course of operation are often rebuilt by welding. In many cases automatic welding equipment is best used for these operations, and specific instructions for this type of rebuilding work is beyond the scope of this book. Refer to the Caterpillar Parts Rebuilding literature listed in the Reference Books section (Page 67) for additional information on these subjects.
Other components such as teeth and cutting edges, which are exposed to the abrasive effects of rock, sand and soil, experience a marked amount of wear. It is usually more economical to replace these components as they become worn, but in some instances it may be desirable to rebuild these parts by welding. In addition, surfaces adjacent to cutting edges and members such as ripper shanks that run in the soil will wear away from the abrasive action of the soil. These surfaces can be restored to their original dimensions by arc welding. The built-up weld deposit on restored surfaces must be strong enough to support a hard overlay deposit and hard enough to resist abrasion. Use E7018 electrodes on modern earthmoving machines where high strength steels are used freely; E7018 electrodes give the best weld buildup material. The material can be deposited in any position on any of the low and medium carbon steels.
Hardsurfacing
The service life of parts that are subjected to constant abrasion can be extended greatly by hardsurfacing the affected areas with beads of weld. Hardsurfacing is most commonly applied to rebuilt parts, but new components can be treated in the same manner. On parts such as bucket teeth that are exposed to severe wear, the entire area can be covered with a layer of hardsurfacing material. Since the material is hard and brittle, the layer should not be more than 1/8 inch thick. Otherwise, it will have a tendency to crack and break off. The leading edge of a cutting tool may break off if it is extended more than two beads away from the base metal (Figure 66).
Figure 66 - Service life of parts can be extended by hardsurfacing.
For most applications, a pattern of stinger beads of hardsurfacing material has been found to be more economical. In applications where heavy rocks are being handled, the beads should be placed parallel with the flow of the abrasive material (Figure 67). The parallel beads will support the rocks and protect the base metal while offering the least resistance to flow. Beads may be laid perpendicular to flow in areas that are covered completely. Surfaces exposed to the abrasive action of sand, soil or small stones should be protected with stringer beads that are perpendicular to the flow (Figure 67). The spaces between the beads will fill up with soil and the soft base will be completely protected. Another pattern used frequently is the diamond shaped pattern (Figure 67). This pattern is designed to prevent dirt from packing between the stringer beads and is used where self-cleaning action is desired.
Figure 67 - Different bead patterns are required for various applications.