28/03/2008 - Efficient and trouble-free performance depends upon the operating and maintenance personnel for what effectively can be a self- destructing machine.
Knowledge of the basic components of the airless shot blast cleaning machine will help you understand and appreciate the importance of planned maintenance programs to maintain machine efficiency and performance.
Our airless blast cleaning equipment has six basic elements:
1. The bladed wheel which propels the abrasive by centrifugal force onto the work pieces.
2. A work handling system to present all facets of the component to be cleaned to the blast stream of the wheel or wheels.
3. The cabinet or booth to retain the abrasive within a confined area and allow ease of collection for recycling of the spent abrasive.
4. Abrasive recovery system to collect, transport, elevate and store cleaned abrasive ready for re-use.
5. Separator. Incorporated within the recovery system a separator cleans the abrasive of all contaminants.
6. A dust collector to remove dust and abrasive fines from the blast machine and air- wash, keeping the machine and separator under negative pressure, ensuring dust does not escape to the environment.
Each edition of the Granowski Gazette will focus on one element; this edition will examine the airless abrasive impeller wheel.
1. The Airless Centrifugal Blast Wheel
The wheel is the propulsion means for throwing the abrasive onto the work pieces. The effectiveness of the blast will be determined by the size and type of abrasive used, the quantity of abrasive thrown, the direction in which it is thrown, and the force at which it is thrown. This force is directly proportional to the speed of the wheel, and over the years, many designs of airless impeller wheels have been developed.
The operating mix of abrasive chosen for the particular application will depend on the function required. The efficiency of the blast cleaning will depend on two other variables; the quantity of abrasive and the direction in which it is thrown.
We determine the quantity of abrasive thrown by the use of an ammeter. As the amount of abrasive being fed through to the wheel increases, the ammeter reading goes up. If the ammeter reads anything less than the motor's rated amperage, the operation is below maximum efficiency, and corrections should be made.
There are two fundamental causes of a low amperage reading.
1. A starved wheel.
2. A flooded or choked wheel.
Starved Wheel. When the wheel does not pull full amperage because of lack of abrasive, check the abrasive conveying system to be sure that it is delivering the abrasive as it should. Check the rotary screens, feed spout and feed funnels for any obstructions. A small piece of wire, nut or paper can impede normal abrasive flow to the wheel, and seriously reduce its working efficiency. Also, excessive or sharp bends in the feed pipe to the wheel can reduce its carrying capacity, and therefore its ability to deliver the necessary volume of abrasive to the wheel.
Flooded or Choked Wheel. This condition will arise when too much abrasive is fed to the wheel, choking the feed spout and not allowing an even flow of abrasive into the wheel. To correct this condition, the dipper valve on the abrasive control to the wheel must be adjusted to limit the amount of abrasive released to the wheel, and this is done by adjustment of a set screw.
A worn impeller or a foreign body in the impeller can reduce its capacity to feed the wheel with abrasive, and cause the same condition. Inspect the impeller periodically to be sure that it is clean and in good condition.
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