# Shot peening of steel belts

Shot peening is a surface treatment process used to restore deformed or damaged drive belts and improve the durability and mechanical properties of materials. This is done by striking them with small spherical particles called "shot". It can be used to recondition distorted steel conveyor belts, restoring their structural integrity and extending their lifespan.

## The shot peening process

Shot peening is a restoration process for flattening deformed steel belts, in which the surface of the belt is impacted by small balls known as peening shots, made of either stainless steel or carbon steel. Each ball that strikes the belt acts like a peening hammer, creating a small indentation, or dimple, on the surface.
For an indentation to form, the surface of the steel belt must be under tension. The compressed shot helps restore the surface to its original shape by producing an indentation of cold-worked metal, which is subject to high compressive stress. Overlapping indentations create a continuous layer of residual compressive stress. Cracks do not propagate in a compressively stressed zone. Because most fatigue and stress corrosion failures originate at the surface, the compressive stresses from shot peening can extend 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 the shots do not contaminate the finished product or the press system.
* Belts are typically run at a speed of 15 to 20 ft/min initially, but this may be increased if the leveling function is satisfactory. The faster the belt runs, the less effective the peening process becomes.
* The process starts with low pressure, e.g. 20 PSI, and increases in steps of e.g. 10 PSI 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.
* 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 must be done over a flat surface for results to be visible.
* 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 should be applied to the concave side of a curve to stretch the metal on the "short" side.

## Portable shot blasting unit

Portable shot blasting units can be used to flatten deformed press belts and prepare them for reuse. These units are designed for field use and are therefore portable. 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, typically 500 mm longer than the belt width, are welded together, to allow the blaster to treat the belt's surface evenly. The total installation time, including assembling the carriage frame, may take a few hours.
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. The unit is supplied with a flexible air hose that connects it 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 on the Hardness Rockwell C Scale or 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. 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 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. This method flattens the belt by equalizing stresses on both sides. However, over time, the belt typically reverts to its original shape, albeit in the opposite direction. As a result, it often becomes necessary to turn the belt again after approximately one year.
This process is time-consuming and expensive, requiring cutting the belt, dismantling it from the press, turning it, and then reinstalling it. The reinstallation involves various belt joining operations such as welding and grinding of the joint, as well as running-in procedures. Additionally, this process demands specialized equipment for handling the belt, welding jigs, and skilled personnel for joint welding. Production must be halted during these operations, with stoppages lasting up to a week.