VOL. 12 November ISSUE YEAR 2011
in Vol. 12 - November Issue - Year 2011
Securing The Investment: Maintenance And Upkeep Of Shot Blasting Machines
Drawing: cross section of Magnetic Separator/Windsifter (red and yellow = sand, blue = abrasive)
Magnetic Separator/Windsifter of a shot blasting machine in a foundry
Opened Magnetic Separator/Windsifter: Cylinders and abrasive veil can be seen
Drawing: cross section of Windsifter: 1) Counterweight on the shuttle valve to adjust the width of the abrasive veil, 2) Abrasive veil, 3) Windsifter slot, 4) Regulating valve for windsifter slot
Wear and tear is an important aspect of shot blasting machines. As machine downtime is costly, it must be reduced to the bare minimum. Operators attach great importance to the material of the wear parts. Of course, this has great influence on the part’s service life. However, there are additional ways to save costs. To make an example, shot blasting machines have been chosen.
The topic of impurities and fines in the abrasive is often underestimated. One rule of thumb is: the smaller the particles, the more aggressive they are. Wear in shot blasting machines increases immensely with a growing fraction of fine particles. It is, thus, important to separate out of the abrasive any undersize particle, i.e. too fine abrasive, impurities like for instance scales, sand, burr etc. and, in particular, dust.
In the case of shot blasting machines used in foundries, the coarse parts must be discharged via sieving conveyor and be disposed of. The sieving conveyor is located below the blasting chamber. In order to purify the abrasive, an additional purifying unit, the magnetic separator, is connected upstream of the usual cascade windsifter. The magnetic separator is responsible for removing the aggressive and harmful-if-split foundry sand from the abrasive circulation. This happens in the process described in the following: The abrasive, roughly cleaned by the sieving conveyor, is conveyed to the hopper above the blasting chamber by the bucket elevator. Then it passes one, better still, two magnetic cylinders. The abrasive being magnetic, as opposed to the sand, is removed from the abrasive-sand mixture by the magnetic cylinders. Afterwards, it passes the windsifter and is fed back to the turbines, completing the cycle. The sand is discharged from the machine. In some cases, it can be re-used. In other cases, it is disposed of.
In the case of shot blasting machines not used in foundries, a windsifter will suffice for the purpose of abrasive purification. However, the windsifter is the third purification step (after sieve conveyor and magnetic separator) in machines used in foundries. The basic principle of this unit is the same for almost all manufacturers of shot blasting machines. However, there are great differences in the design. Various adjustment possibilities are important.
The abrasive runs over an edge as continuous, thin and preferably wide abrasive veil. The air draught of the filter fan negative pressure extracts the fines. Various adjusting flaps enable an optimal setting. With some experience, the maintenance person can set the machine to the proper mixture. The wear will be reduced. Thus, providing for a well-adjustable windsifter is a clear advantage when purchasing a shot blasting machine!
Afterwards the abrasive falls onto a perforated plate or sieve, which separates coarse parts still in the abrasive. This perforated plate must be checked and cleared regularly in order to prevent abrasive accumulation.
Various adjustments of the windsifter
AGTOS machines can be variously adjusted in the windsifter section. It is, for instance, possible to control the air speed in the windsifter. Small particles can be extracted with a lesser air stream than larger particles.
An additional sliding plate enables the adjustment of the windsifter to different slipping properties of the abrasive.
Another possibility for the separation of fine and coarse particles is the so-called blade. This device cuts the abrasive veil improving the extraction of undersize particles.
An adjustable counterweight regulates the backpressure of the abrasive before the discharge edge thus producing an even abrasive veil. One may ask how to control and adjust all these possibilities during operation. There is a solution to this question, too: The design provides a control window that may be opened for adjustments for short periods during operation.
There are even further measures to reduce the fines in the abrasive. One basic measure is to always include the bucket elevator head in the suction system because a lot of dust accumulates due to the movement of the abrasive discharged from the buckets to the sliding plate of the purifying unit. They offer their customers another measure as an option not included in the standard scope of supply. This measure is the depression gauge. It enables the control of the negative pressure in the abrasive sifter from the switchboard or the control room.
The multitude of possibilities for optimisation of the abrasive purifying unit shows the importance of this aspect for the quality of the blasting process itself and the effects on the maintenance intervals and costs.
Many of these points can be considered in purchasing. In the case of older shot blasting machines you should check whether all adjustment possibilities are utilised. A good instruction manual should give advice on the matter.