Shot peening improves the fatigue resistance of parts

Controllable gear shot peening

Gears, connecting rods, and crankshafts are prone to bending stress and failure during operation; components such as compression springs, transmission shafts and torsion bars are prone to torsion stress and failure during operation. Shot peening can effectively improve the fatigue resistance of parts.

Improve the bending fatigue strength of parts

Bending fatigue is a common form of fatigue failure. Because the surface of the part is the area where the tensile stress is concentrated.

Shot peening/shot peening is the most effective in overcoming this failure situation. The cantilever beam under bending load, the bending deformation causes its surface to be in a state of tensile stress. Any change in the radius or geometric shape of the surface layer will cause an increase in tensile stress, while the opposite side is in a state of compressive stress. This is a destructive fatigue load. situation. The tensile stress part of the alternating stress leads to the formation and extension of fatigue cracks.

1. gear

Gear strengthening is a very typical and common application in shot peening. Gears of any size and form can be shot blasted/shot peened to improve the bending fatigue properties of the tooth profile roots. The meshing of the gear teeth is similar to that of the cantilever beam, and the load formed by the contact of the tooth surface produces a bending stress at the root position below the contact point.
Gears are usually shot blasted/shot peened after hardening. Increased surface hardness will increase the compressive stress level proportionally. According to the process parameters, the residual compressive stress of the gear after carburizing and shot blasting/shot peening can range from 1 170 to 1 600 MPa. To strengthen carburized gears, usually harder steel shots (55~62HRC) are used. If you want to have a smaller impact on the tooth surface, you can choose a steel shot with a lower hardness (45~52HRC), and the compressive stress level will be about 50% of the high hardness shot.
Introducing a compressive stress in the vicinity of the pitch line of the tooth surface by hammering, grinding, honing and vibrating light decoration is an effective method to enhance the gear's fatigue resistance to pitting. After the gear surface is finished, the contact stress can be further reduced, but care must be taken not to cover more than 10% of the shot peening layer. The smoothing treatment of shot peening small dents can distribute the contact load on a larger surface area, which can further reduce the contact stress.
The fatigue strength of the strengthened gear is increased by 15% to 30%
In some gear processing and manufacturing, after controlled shot blasting/shot peening, the fatigue strength of gears after 1 million cycles can increase by 30% or more. The following organization/specification standards confirm that the use of controlled shot peening can significantly increase the bending load of the tooth surface: Lloyd’s Register, an increase of 20%; Det Norske Veritas, an increase of 20%; ANSI/AGMA 6032-A94 marine gear specifications , An increase of 15%.

2. Connecting rod
The connecting rod is an important part of the engine to transmit power. It mainly bears the alternating load generated by the gas pressure and the reciprocating inertial force. Therefore, when designing the connecting rod, you should first ensure that it has sufficient fatigue strength and structural rigidity. Obviously, in order to increase the strength and rigidity of the connecting rod, the structural size cannot be simply increased, because increasing the weight of the connecting rod will increase the inertial force correspondingly, so An important requirement of connecting rod design is to ensure sufficient strength and rigidity in the lightest possible structure. The stress concentration of the connecting rod, that is, the most common part of failure, is near the center of the big-end hole on both sides of the round corner.

The index based on the fatigue safety factor of the connecting rod, in order to meet the design requirements of strength and rigidity, and the light-weight economic optimization plan is to polish before any boring machining of the connecting rod whether it is forging, casting or powder metallurgy. Pill/shot peening, which can avoid the additional cost of shielding holes. There is no need to do other pre-treatment or post-treatment on the connecting rod before and after strengthening.

The fatigue strength of the strengthened connecting rod is greatly improved

3. Crankshaft
Under normal circumstances, all of the crankshaft journal surface changes and transfer fillets need to be shot peened/shot peened. This includes bearing journals and crank pins. The crank pin bearing fillet is where the stress is concentrated, and the stress occurs at the bottom of the crank pin transition fillet, because when the engine is ignited, the crank pin is located at the top dead center. The stress analysis determined that the crank pin transition fillet is often the initiation part of the crack, and it expands to the adjacent main bearing fillet until it causes the destructive fracture of the entire crankshaft.

A large number of tests have proved that whether it is a forged medium carbon steel crankshaft, a cast steel crankshaft, a ductile iron crankshaft, or austempered ductile iron crankshaft, shot blasting/shot peening is an effective method to improve the fatigue resistance of the workpiece and prolong the service life. During the actual operation of the engine, the load on the crankshaft is mainly bending/torsion composite load. Bending fatigue damage is a common form of crankshaft failure. When the engine output is large and the crankshaft is subjected to a large torque, the torsional fatigue damage occurs. It becomes the main form of failure. A fatigue comparison test was carried out on a crankshaft test piece of a diesel engine. The material was Armco 17~10Ph stainless steel. The test results found that the unstrengthened crankshaft and the shot-peened/shot-peened crankshaft have measured fatigue strengths of 293 MPa and 386 MPa after 1 billion alternating stress loads, respectively. Shot peening improves the fatigue strength of the crankshaft by about 30%.

Improve torsional fatigue strength of parts

Torsional fatigue is also a form of failure that can be effectively overcome by shot blasting/shot peening because the tensile stress is concentrated on the surface of the workpiece. The stress generated by the torsional load can be in the horizontal direction or in the vertical direction, while the tensile stress occurs in the direction of 45° with the horizontal axis of the part.

Low-strength materials are prone to failure and fracture due to torsional fatigue at the vertical shear plane. Because they are less resistant to shear force than tensile force. High-strength materials are most susceptible to failure and fracture at 45° to the horizontal axis of the workpiece, because they are less resistant to tensile forces than to shear forces.

1. Compression spring

Compression springs are subject to high alternating loads and stresses mostly occur on the surface of the spring material, so shot blasting/shot peening strengthening is also an ideal process to improve the fatigue strength of compression springs. The spring material causes tensile stress during rolling, drawing, coiling and compression. In addition to the working conditions of high alternating load after service, the coiling process itself will cause destructive tensile stress on the inner ring of the spring. Shot blasting/shot peening introduces a counteracting compressive stress to transform the surface into a residual pressure layer with a strength of about 1 035MPa. This is 60% of the ultimate tensile strength (UTS) of the spring. As a result, the fatigue life of the spring is extended to 500,000 load cycles without failure.

In addition to compression springs, shot blasting/shot peening is also suitable for strengthening processing of other springs such as tension springs, torsion springs, suspension springs and leaf springs. The fatigue failure point usually occurs in the place where the residual tensile stress and the alternating load stress are concentrated, so the strengthening position of different springs is different.

2. transmission shaft

The transmission shaft is driven by rotation, so a tensile load is generated on the rotating member. Since most of the drive shafts are large load bearing members, they are suitable objects for shot blasting/shot peening. The failure of the drive shaft usually occurs at the splines, cuts, fillets and keyways.
3. Torsion bar
Torsion bars and lateral thrust bars are components used in suspension and other related systems. The torsional deformation of the torsion bar achieves a buffer effect to maintain stability. When used in parts subject to repeated alternating loads such as automobile suspension systems, shot blasting/shot peening can achieve lighter parts and longer service life.

For the purpose of lightweight, the automobile industry uses hollow torsion bars. Shot blasting/shot peening is applied to the outer ring of the torsion bar where the load stress is concentrated. For heavy vehicles (such as heavy-duty trucks, racing cars, etc.), cracks will also occur in the inner ring of the torsion bar, which is also subjected to torsional loads.

Post time: Apr-26-2021

Yancheng Ding Tai Machinery Co., Ltd.
No.9 Huanghai West Road, Dafeng District, Jiangsu Province, China
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