The application of shot blasting strengthening technology can effectively improve and enhance the fatigue life and corrosion resistance of key components in automobiles. At present, many well-known automobile manufacturers and component manufacturers around the world have included reinforcement in their standard production processes. At the same time, reinforcement equipment has also formed a complete modern manufacturing assembly line like other manufacturing equipment. With the continuous development of shot peening strengthening technology, its use in the field of automotive manufacturing to improve and enhance the fatigue life of key components has become a focus of attention, and it has been fully considered and valued in the design of cars, trucks, motorcycles, and other vehicles from the beginning. At present, the design of most engine parts includes the application of spray/shot peening strengthening technology and processes, including: crankshaft (de oxidation and strengthening), connecting rod (strengthening), transmission gear and other shaft parts, ring gear, piston, sun gear and planetary gear, as well as plate springs and circular springs. A large number of automotive components, whether they are castings/forgings, die-casting parts, mechanical cutting parts, or welded parts, require surface treatment using different types of spraying/polishing equipment, such as removing oxide scales, deburring, sandblasting, and cleaning other surface impurities.

There is conclusive data to prove that through shot peening strengthening, the fatigue life of leaf springs can be extended by 600%, the fatigue life of transmission gears can be extended by 1500%, and the fatigue life of crankshafts can be extended by 900%. Effectively improving the fatigue resistance and corrosion resistance of components is of great significance for their service life and safety. By relying on shot peening strengthening technology, parts can be designed to be lighter and more compact. Some parts that were previously required to use expensive materials due to process specifications can now be replaced with low-priced materials. Through shot peening strengthening technology, equivalent or even better performance standards can be achieved.

1、 Shot blasting cleaning in crankshaft manufacturing process

As a part of the manufacturing process, the heat-treated crankshaft needs to be shot blasted to remove the surface thermal oxide skin. The crankshaft is placed on a rotating roller, and during rolling, all surfaces of the crankshaft are fully exposed to the shot flow ejected by multiple throwing heads. The multi angle shot impact thoroughly cleans the outer surface of the crankshaft.

The size of the crankshaft determines the type of shot blasting machine selected. For large engines, the crankshaft size may be as large as 762mm and 6096mm in length, and the crankshaft is placed between a set of rollers installed on the trolley. There are several working methods, and customers can choose to fix the throwing head according to the actual situation of their factory workshop. They can either move the trolley under the throwing head or fix the trolley to move the throwing head above. Regardless of the method chosen, the crankshaft placed between the rollers rotates continuously, ensuring that all surfaces receive sufficient shot blasting cleaning.

Smaller crankshafts, such as those with a diameter of 152-203mm and a length of 914mm, are usually cleaned by a rotary shot blasting machine. The crankshaft is suspended on a lifting hook and then sent into a shot blasting chamber with multiple polishing heads for shot blasting cleaning through the rotation of the suspension chain. The hook rotates in the shot blasting chamber, allowing the workpiece to be fully exposed to high-speed shot flow and pass through while rotating. Its cleaning speed can reach 250 pieces/h, and the cleaning effect is very good.

Although the requirements for process control are not as strict as those for strengthening, modern crankshaft shot blasting cleaning equipment also ensures cleaning quality by monitoring process parameters.

2、 Strengthening of the crankshaft

Due to the operation of the crankshaft under alternating stress, there is a high risk of stress fatigue and strain failure at the transition fillet of the journal surface. At present, the use of shot peening to enhance the fatigue resistance of crankshafts has been widely applied and the results are satisfactory.

The drawback of traditional rolling technology is that due to the limitations of crankshaft processing technology, the rounded corners of each journal are difficult to match with the rollers, often resulting in the phenomenon of gnawing and cutting of the rounded corners. Moreover, the crankshaft after rolling has a large deformation and poor effect. The mechanism of shot peening strengthening is to use shot particles with strict diameter control and certain strength to form a flow of pellets under the action of high-speed airflow and continuously spray them onto the metal surface of the crankshaft, just like using countless small hammers to hammer, causing extremely strong plastic deformation on the surface of the crankshaft and forming a cold work hardening layer. In short, due to the influence of various mechanical cutting forces on the crankshaft during processing, the stress distribution on its surface, especially at the transition fillet of the crankshaft cross-section, is extremely uneven. In addition, it is subjected to alternating stress during operation, which easily leads to stress corrosion and reduces the fatigue life of the crankshaft. The shot peening strengthening process is achieved by introducing a pre compression stress to counteract the tensile stress that the part will experience during future work cycles, thereby improving the fatigue resistance and safe service life of the workpiece.

For the shot peening strengthening process, there are two most critical parameters. One is stress intensity, which is usually measured using "Almen test pieces" for strength testing. Multiple test pieces are fixed on different surfaces of the crankshaft, especially at the corner of the crankshaft section where stress is most concentrated, and shot peening is carried out together. The compressive stress generated on the test pieces affects the bending of the test pieces. The expansion and variation of curvature are proportional to the energy of the projectile impact. Another main parameter for determining shot peening quality is coverage, which mainly refers to the ratio of the area occupied by surface craters after strengthening to the total strengthening surface. This parameter is defined by the crankshaft design engineer and is usually required to be between 100% and 200%. Some crankshaft applications may require coverage above 200%.

Based on the hardness of the crankshaft and the ideal imported compressive stress intensity, the hardness of the shot peening used for strengthening the crankshaft is usually between 50-55HRC, with sizes ranging from S280 to S330 (0.7mm to 0.84mm). The strength range generated on the Almen test piece is approximately 0.008 to 0.010C (0.025 on the Ascale). Compared to shot blasting, the monitoring of process parameters for shot peening strengthening is more stringent. For crankshaft strengthening applications, the parameters that need to be monitored include: shot peening speed, shot peening intensity, shot diameter, shot peening distance, strengthening time, and coverage. Any change in these parameters will affect the surface strengthening effect of the crankshaft to varying degrees.

The correct application of controllable shot peening strengthening technology can significantly improve the fatigue strength of crankshafts and other parts working under high load conditions, thereby greatly extending the fatigue life of the parts. Advanced and precise shot peening equipment with computer programming technology can closely monitor the shot peening process to ensure the consistency and repeatability of shot peening quality. At present, many famous automobile manufacturers and component manufacturers around the world have included reinforcement in their standard production processes, and reinforcement equipment has formed a complete modern manufacturing assembly line like other manufacturing equipment.