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The Important Role of Shot Blasting Machine for Material Strengthening

Mar. 02, 2020

Shot blasting strengthening process of shot blasting cleaning machine can effectively improve the service life of gears. This process uses steel shots to hit the curved parts of gear tooth roots at high speed, which generates compressive stress on the surface, destroys the tensile stress existing inside the workpiece, improves the bending fatigue resistance of the gear tooth root, and increases its service life. The principle and effect of shot peening are high residual compressive stress. Because the metal medium (steel shot or shredded shot) impacts the surface of the part at high speed, it will cause plastic deformation on the surface. This deformation will extend to the surface of the material and create a compressive stress under the surface to offset the part produced during the manufacturing Bad tensile stress. The residual compressive stress delays the formation of fatigue fracture of the part, thereby extending the safe service life of the part. For the most critical gear parts in automotive transmission parts, after the heat treatment I sequence, the surface is subjected to continuous shot impact by shot peening, and the residual austenite in the crystal phase structure of its tooth surface material can also be converted into horse Austenite, thereby increasing tooth surface hardness. Coverage can be observed visually, while intensity needs to be measured using Al [J test strips. The part proofing tool (PVT) is designed to place the test specimens in specific locations. In these test specimens, the areas where the parts have strength requirements and must be tested for strength are determined. According to different applications, the gear strengthening is generally required. The intensity range is 0.015 ~ 0.03 (on the grade "A"), and the coverage is 100% ~ 200%. For the strength measurement alone, for gears, the maximum shear stress occurs at the corner radius of the tooth root and the transition area of the gear, while the two tooth surfaces of the gear, the driving surface and the driven surface, are constantly subjected to work. Increased load, so the focus of gear strengthening is to strengthen these parts: the driving surface, the driven surface, the root of the tooth. From the perspective of strengthening requirements, mainly including two parameters of strength and coverage, of course, different gears have different requirements, which must be determined according to their final application environment.

Three types of shot peening strengthening methods include the following three methods: centrifugal shot peening, compressed air shot peening, and hybrid (nozzle and head) From the perspective of output, in mass production, centrifugal shot peening is better than the other two methods. In the centrifugal force shot blasting strengthening process, the steel shot overflows from the hole on the impeller shaft to the blade under the action of centrifugal force, and then is thrown out by the high-speed rotating blade at a fixed angle.In a unit time, the area covered by the shot blast flow is larger than the nozzle spray. Shot, but the application of shot peening is limited by the size of the gear to be processed. If it is a pinion with a small tooth root area, multiple shot heads need to be fixed at different angles to achieve the desired strengthening effect. Compressed air type shot peening must use multiple nozzles to align specific parts of the tooth surface and root, so that the shot flow is more concentrated and the target is more determined, which can achieve the effect of precise strengthening.

Effect of process parameters on strengthening effect:

Regardless of the strengthening technology, the goal is to obtain a constant and repeatable strengthening strength. Therefore, it is necessary to understand which key variables in the process will affect the final strengthening result. For centrifugal force shot peening, the head, the angle of the shot (including the position of the head, the movement of the directional sleeve, etc.), the speed of the head, and the speed of the shot will affect the final strengthening effect. For direct pressure shot peening, variables such as nozzle, nozzle size, spray pressure, and spray angle (involving nozzle movement, multiple nozzles) will affect the final strengthening effect.

1. In centrifugal shot peening, the impact of the process variables on the strengthening effect. The head diameter determines the speed at which the steel shot medium is ejected from a fixed angle. At the same rotating speed of the head, the strengthening strength produced by the larger diameter head is higher. The throwing head is directly driven by a variable frequency motor. The speed of the throwing head can be changed by changing the frequency of the motor, thereby changing the initial speed of the steel shot. The power of the tossing head determines the number of shots shot in a unit of time. The head is usually permanently fixed at a specific position in the shot blasting room, but the direction of the shot can be changed by adjusting the position of the directional sleeve. The position of the directional sleeve ultimately determines the angle at which the steel shot is ejected by the head. 

2. The influence of process variables on strengthening effect in direct pressure shot peening. The shot pressure in shot peening is similar to the head speed in shot peening. The greater the pressure, the greater the intensity. For complex gear strengthening, usually in a closed loop, the nozzle pressure is monitored by a proportional regulator. If there is any deviation from the preset pressure value, the system will issue an alarm to instruct the machine to shut down. The nozzle mounted on a multi-axis robot can be moved freely.The robot handles the movement of the nozzle with the assistance of a servo drive system and a dynamic controller, allowing the nozzle to move along the outer edge and internal route of the part, such as along planetary gears and gear rings The contour can be sprayed to achieve a uniform, consistent and highly repeatable strengthening effect. The size of the nozzle determines the amount of shot media that is sprayed onto the surface of the part. For the shot peening process, the most important factor is to obtain precise directionality, the target is accurate, and the steel shot is hit without error to a specified point on the surface of the part to achieve the required strength. When strengthening a group of planetary gears and ring gears at the same time, special attention must be paid to the fact that their tooth root angles are different. Therefore, the setting of each nozzle position for shot blasting is very critical. Reasonable settings can achieve the ideal gear Ring gear strengthening effect. Other process parameters that affect the results of shot peening include: flow rate of steel shot, size of steel shot, consistency of size, etc. The material flow is controlled by a dedicated flow control valve. In the shot peening equipment, the control valve is installed in the feed area of the shot head.By adjusting the opening of the flow control valve, the flow of steel shots entering the shot head through the valve is adjusted. The control valve is installed in the pressure tank outlet area. The size of the steel shot (or shred) will directly affect the coverage and strengthening time. The resulting residual stress is lower, but the strengthening layer is deeper; of course, the diameter of the steel shot must be smaller than the fillet radius of the gear transition zone. The size of the steel shot needs to have a high consistency. In the strengthening equipment, a vibrating screen is used to filter the shot to ensure that the size of the steel shot is consistent, thereby achieving a stable strengthening effect.

Industry blasting machine manufacturer believes that the shot peening process is developing rapidly and the scope of application will become wider and wider. Now, by adopting a precise automatic control system to implement synchronous monitoring of process parameters, the flexibility of the shot peening system has been greatly enhanced, so that a more ideal part strengthening effect can be obtained. With the development of technology, automotive engineers will no longer be limited to single shot peening or shot peening. The new hybrid system can combine the advantages of both technologies to meet the specific production requirements of more users.

Shot Blasting Cleaning Machine

Shot Blasting Cleaning Machine

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