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Shot peening can be used to recondition distorted steel conveyor belts.
The shot peening process
Shot peening is a conservation process for flattening a deformed steel belt in which the surface of the belt is impacted by small stainless steel or carbon steel balls, called peening shot. Each ball hitting the belt functions as a peening hammer, forming a small indentation, or dimple, on the steel belt surface.
For the indentation to be formed, the steel belt surface layer must yield in tension. The compressed grains help to restore the surface to its original shape by producing a hemisphere of cold-worked metal, highly stressed in compression. Overlapping indentations create a continuous layer of residual compressive stress. It is well known that cracks will not lead up, nor propagate in a compressive stressed zone. Since most fatigue and stress corrosion failures originate at the surface, the compressive stresses from shot peening significantly enhance the belt’s lifespan. Note that:
- Although it is possible to peen while the belt is in production, care must be taken to ensure that there is no loss of shot which would contaminate the finished product or the press system.
- The belt is run at a speed of 15–20 ft/min to start with, but this may be increased if the levelling function is satisfactory. The faster the belt runs, the less effective the peening process becomes.
- The process starts with a low pressure (20 PSI), and work up in steps of 10 PSI until a noticeable effect is seen in the belt curve. For a precipitation-hardened type of stainless steel belt, the required pressure could be as high as 90 PSI.
- If the shot becomes contaminated with oil from the belt, it becomes less effective as a blasting medium, and the oil also clogs the air blast system. If oil pickup is unavoidable, then frequent cleaning of the equipment and washing of the shot will be required.
- Peening has to be done over a flat surface.
- Peening starts from the center of a section and progresses towards the edge. Several light passes across the belt are less likely to over-compress the surface than one heavy pass which could distort the belt in the opposite direction.
- Peening must be applied to the concave side of a curve to stretch the metal on the "short" side.
Portable shot blasting unit
Press belts may become deformed and worn over time. The portable shot blasting unit is primarily used to flatten deformed press belts and prepare the belt material for reuse. Compact and lightweight, the unit is designed for field use and can be carried and operated efficiently. All necessary equipment (excluding the carriage frame and air compressor) fits into a box with dimensions of approximately 350 × 350 × 320 mm. The combined weight of the blaster, valve, air hose, and other components is about 25 kg, with the blasting machine itself weighing 9 kg.
A pair of universal channels (38 mm × 76 mm), typically 500 mm longer than the belt width, must be provided on-site. These channels are welded together to allow the blaster to move smoothly across the belt’s surface. The total installation time, including assembling the carriage frame, generally takes only a few hours, after which the peening process can begin.
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. For best blasting results, an air supply of 4,200 liters per minute is required at a pressure of 6 bar. A flexible air hose is supplied with the unit, connecting the blasting unit to the local air supply. All local supply pipes should have a minimum bore diameter of 1 inch. The recommended shot blasting medium is tungsten shot (beads) with a diameter ranging from 0.2 to 0.4 mm and a hardness exceeding 40 HRC. The machine operates by drawing a quantity of tungsten shot from the bottom of the scroll case into the high-velocity nozzles. The shot is blasted onto the surface of the belt, and most of the shot bounces back into the scroll case. The air is vented through the filter socks, and any shot carried with the air is filtered out and dropped back into the scroll case.
Flattening out deformed belts
Since the 1980s, the standard procedure used to solve the problem of deformed belts was to turn the belt over, i.e. what was previously the back of the belt was used to form the new product side. The belt became flatter when turned due to the equalization of stresses on both sides. However, over time, the belt typically reverted to its original shape, albeit in the opposite direction. As a result, it often became necessary to turn the belt again after approximately one year. This process was extremely time-consuming and costly, as it required cutting the belt, dismantling it from the press, turning it and then reinstalling it. The reinstallation involved belt joining operations such as welding and grinding of the joint, as well as running-in procedures. Additionally, this process demanded specialized equipment for handling the belt, welding jigs, and skilled personnel for joint welding. Compounding the issue, production had to be halted during these operations, with stoppages lasting up to a week not uncommon. The steel belt shot peening process was introduced as a solution to address the belt cross-curvature problem.
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