Shot Peening Equipment Review

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Shot Peening Equipment Review

Manufacturing Processes & Engineering

Shot peening machinery typically utilizes for propelling shot media include air blast systems and centrifugal blast wheels. In the air blast systems, media is introduced by various methods into the path of high pressure air and accelerated through a nozzle directed at the part to be peened. The centrifugal blast wheel consists of a high speed paddle wheel. Shot media is introduced in the center of the spinning wheel and propelled by the centrifugal force by the spinning paddles towards the part by adjusting the media entrance location, effectively timing the release of the media. Other methods include ultrasonic peening, wet peening, and laser peening (which does not use media).

Shot media choices include spherical cast steel shot, ceramic bead, glass bead or conditioned (rounded) cut wire. Cut wire shot is preferred because it maintains its roundness as it is degraded, unlike cast shot which tends to break up into sharp pieces that can damage the workpiece. Cut wire shot can last five times longer than cast shot. Because peening demands well-graded shot of consistent hardness, diameter, and shape, a mechanism for removing shot fragments throughout the process is desirable. Shot peening equipment is available that includes separators to clean and recondition shot and feeders to add new shot automatically to replace the damaged material.

A popular method for sorting damaged/out-of-spec shot media is the use of shot separators. Production sized separators consist of various levels of precision wire mesh, from 1 or more sizes to sort, and is mechanically shaken. Some applications require a maximum and minimum level of shot diameter. To maintain specifications, shot is slowly introduced where the large shot/contamination will be sorted in the first stage, then shot within specifications are sorted in the second level, then degraded shot below specifications is sorted last. The openings on the wire mesh progressively get smaller in this instance. It is possible to attach a production separator to a shot peener for continuous control of shot quality. Testing methods use a similar concept in a much smaller package, where a technician takes a sample of shot and then sorts the various sizes. Further testing of the samples verifies the quality of the shot media.

Wheel blast systems include satellite rotation models, rotary throughfeed components, and various manipulator designs. There are overhead monorail systems as well as reverse-belted models. Workpiece holding equipment includes rotating index tables, loading and unloading robots, and jigs that hold multiple workpieces. For larger workpieces, manipulators to reposition them to expose features to the shot blast stream are available.

Shot Peening Media Coverage:

Factors affecting coverage density include: number of impacts (shot flow), exposure time, shot properties (size, chemistry), and work piece properties. Coverage is monitored by visual examination to determine the percent coverage (0-100%). Coverage beyond 100% cannot be determined. The number of individual impacts is linearly proportional to shot flow, exposure area, and exposure time. Coverage is not linearly proportional because of the random nature of the process . When 100% coverage is achieved, with an exposure time of 1T, locations on the surface have been impacted multiple times. At 150% coverage (1.5T), 5 or more impacts occur at 52% of locations. At 200% coverage (2T), 5 or more impacts occur at 84% of locations.

Coverage is affected by shot geometry and the shot and workpiece chemistry. The size of the shot controls how many impacts there are per pound, where smaller shot produces more impacts per pound therefore requiring less exposure time. Soft shot impacting hard material will take more exposure time to reach acceptable coverage compared to hard shot impacting a soft material (since the harder shot can penetrate deeper, thus creating a larger impression).

Coverage and intensity (measured by Almen strips) can have a profound effect on fatigue life. This can affect a variety of materials typically shot peened. Incomplete or excessive coverage and intensity can result in reduced fatigue life. Overpeening will cause excessive cold working of the surface of the workpiece, which can also cause fatigue cracks.. Be diligent when developing parameters for coverage and intensity, especially when using materials with different properties (ie softer metal to harder metal). Testing fatigue life over a range of parameters would result in a "sweet-spot" where there is near exponential growth to a peak fatigue life (x = peening intensity or media stream energy, y = time-to-crack or fatigue strength) and rapidly decay fatigue life as more intensity or coverage is added. The "sweet-spot" will directly correlate with the kinetic energy transferred and the material properties of the shot media and workpiece.