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Shot peening of steel belts is a cold working process that strengthens steel belts and reduces operating stresses. It involves striking the surface with small steel pellets called shot (typically made of stainless or carbon steel).
For an ideal indentation to form, the surface of the steel belt must first be put under tension to mimic operating conditions and create a strengthening effect. [1] The compressed shot creates an indentation on the cold-worked metal, introducing high compressive stress. Overlapping indentations can create/cause a continuous layer of residual stress. Because most fatigue and stress corrosion failures originate at the surface, the compressive stress layer reduces surface cracks. The compressive stresses from shot peening can also extend the belt's lifespan. [2] Steel belts typically run at a speed of 15 to 20 ft/min (approximately 5 to 6 m/min) initially but may be increased if the leveling function is sufficient. The faster the belt runs, the less effective the peening process becomes. [3] The process starts with low pressure and increases in steps until a noticeable effect is seen in the belt curve. For a precipitation-hardened stainless-steel belt, the required pressure can be as high as 90 PSI. [4] Peening starts from the center of a section and progresses towards the edge of the belt. Several lighter passes are less likely to over compress the surface and cause distortion compared to a single high-pressure pass. If the shot becomes contaminated with oil, it is less effective as a blasting medium because oil can clog the air blast system. If oil pickup is unavoidable, the peening equipment must be cleaned frequently. Peening is most effective and visibly verifiable when performed on flat, accessible surfaces. [5]
Portable shot blasting units can be used to flatten deformed press belts and prepare them for reuse. These units are designed for field use. The combined weight of the blaster, valve, air hose, and other components is about 25 kg (55 lb), with the blasting machine itself weighing 9 kg (20 lb).[ citation needed ]
Two universal channels, typically 500 mm (20 in) wider than the belt, are welded together to ensure uniform treatment of the belt's surface. It may take several hours to install the blasting unit, including assembling the carriage frame. [6]
An electric shut-off valve is mounted on the inlet air hose to protect the belt from over-blasting, should it suddenly stop during the blasting operation. The valve solenoid must be connected (interlocked) to the press machine's power supply to be effective. An air supply of 4,200 liters (1110 gallons) per minute at a pressure of 87 PSI is typically required. [7] The unit is typically supplied with a flexible air hose that has a minimum bore diameter of about 2.5 cm (1 inch) connecting it to the local air supply. A common blasting medium is tungsten shot with a hardness exceeding 40 on the Rockwell Hardness test. [8]
The machine operates by drawing a quantity of tungsten shot from the bottom of the scroll case into high-velocity nozzles. The shot is blasted onto the surface of the belt, and air is vented through the filter socks. Any shot carried with the air is filtered out and dropped back into the scroll case.[ citation needed ]
Since the 1980s, the standard procedure for addressing the issue of deformed belts has been to turn the belt over, using what was previously the back side to form the new product side. [9] This method flattens the belt by equalizing stresses on both sides. However, the belt typically deforms again, although in the opposite direction, over time. As a result, it often becomes necessary to turn the belt over again after approximately one year, [10] which involves cutting the belt, removing it from the press, turning it, and reinstalling it.
