Shot peening of steel belts

For other uses, see Peen.

Shot peening can be used to recondition distorted steel conveyor belts. The process is often considered quick and cost-effective compared to other methods and can be performed without interrupting daily production. A deformed steel belt has the following disadvantages:

• Exerts uneven pressure on the product.
• Can cause steel belt tracking problems in the press.
• The steel belt is exposed to unnecessarily high stress.
• Increases the risk of accidental damage to the edge of the belt.
• Reduces the lifespan of the belt.

The shot peening process

Shot peening is a maintenance 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 small 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. Because nearly all fatigue and stress corrosion failures originate at the surface of a part, compressive stresses generated by shot peening provide a significant increase in the lifespan of the belt. Note that:

• Although it is possible to peen while the belt is in production, great 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 leveling function is satisfactory. The faster the belt runs, the less effective the peening process becomes.
• Start 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.
• Always peen against a solid 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

At times, the press belts become deformed and worn out. The primary use for the portable shot blasting unit is to take a deformed press belt, flatten it out while simultaneously conditioning the belt material to be used again. Small in size and weight, the unit is flexible, easy to carry, and easy to operate in field situations. All the equipment needed (excluding the carriage frame and the air compressor) can be packed into a box with dimensions of about 350mm L × 350mm W × 320 mm. The total weight of the blaster, valve, air hose, and miscellaneous components is around 25 kg, and the blasting machine weighs only 9 kg. One pair of universal channels (38 mm * 76 mm) must be provided on-site – usually 500 mm longer than the belt width. The channels are welded together so that the blaster can quickly run through the frame across the face of the belt. The total installation time, including the manufacture of the carriage frame, is limited to just a few hours, after which the peening process can begin.

On the inlet air hose, an electric shut-off valve is mounted to protect the belt from overblasting should it suddenly stop during the blasting operation. The valve solenoid must be connected/interlocked to the press machine's power supply (240 V) to be effective. For best blasting results, an air supply of 4,200 litres per minute is required at a pressure of 6 bar. A flexible air hose is supplied with the unit, connecting the blasting unit and 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

Historically (per the 1980s), the standard method previously used to solve the problem of deformed belts was to turn the belt over, i.e. what was previously the back of the belt is then used to form the new product side. The belt became flatter when turned because of equalising the stresses on the two sides. However, the belt usually changes its shape to eventually acquire the same shape as before being turned but in the opposite direction. Because of this, turning the belt again after approximately one year was often necessary. This method is very time-consuming and thus very costly since it involves cutting the belt, dismantling it from the press, turning the belt, and re-installing it in the press, followed by the belt joining operations (welding and grinding of the joint) and running-in procedures. All these operations also require equipment for handling the belt, special welding jigs, and skilled personnel for joint welding. Added to this is the loss of production during the operation – a stoppage in production of one week is not uncommon. The steel belt shot peening process was developed as a superior solution to the belt cross curvature problem.