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Examining cleaning room requirements, part 3: centrifugal wheels.

Examining Cleaning Room Requirements, Part 3: Centrifugal Wheels

This series is intended to provide information to permit examination of the foundry's shotblast cleaning requirements, this month focusing on centrifugal wheels.

Blast cleaning of castings is a process wherein abrasive particles, usually steel shot or grit, are propelled at high velocity to impact the casting surface, thereby forcefully removing surface contaminants.

The usual methods of imparting high velocity to the abrasive particles are to use centrifugal wheels or compressed air nozzles. Centrifugal wheels are most widely used because of their ability to efficiently propel large volumes of abrasive. For example, a 75 horsepower (hp) centrifugal wheel can accelerate steel shot to 240 feet per second (fps) at 123,000 lb/hr flow.

To do the same with 1/2 in. direct pressure venturi nozzles at 100 lb/minute per nozzle would require cu ft per minute (cfm)/nozzle x 20, or a total of 5200 cfm at 80 psi. Approximately 940 hp at the air compressor would be required to supply this amount of air, which gives a 940:75, or 12.5:1 efficiency advantage for the centrifugal wheel.

Types of wheels include standard, curved vane, canted vane and direct drive wheels.

Standard Wheels--The standard wheels usually are 19 1/2 in. in diameter and are belt driven. They typically are run at 2250 rpm, giving an abrasive velocity of 240 fps and approximately 1700 lb/hr/hp flow at net hp. (Net hp = motor hp - idle hp).

Curved Vane Wheels -- The curved vane wheel was developed in order to reduce the source noise from the centrifugal wheel. It has been shown that a standard 19 1/2 in. diameter wheel has a vane tip velocity of 191 fps to give a resultant abrasive velocity of 240 fps.

An 18 in. diameter curved vane, however, has a tip velocity of 165 fps to obtain the same 240 fps abrasive velocity. This reduction in rpm and tip speed gives approximately 7dB sound power reduction.

Another advantage of the curved vane wheel is that for a given abrasive velocity, idle hp is less due to the fact that the wheel rpm is less (2100 vs. 2250); and the wheel diameter is smaller (18 in. vs. 19.5 in.). For a 4 in. wide vane-18 in. diameter curved vane wheel, idle hp is 2.6, vs. 4.5 idle hp for a 4 in. wide vane-19 1/2 in. diameter straight vane wheel.

Canted Vane Wheels--A canted vane wheel consists of a centrifugal wheel wherein the eight vanes are alternately canted forward and backward approximately 3-6 [degrees]. This is done to alternately project the abrasive at a slight angle from the wheel so that, from a single wheel, two side-by-side patterns are produced at the impact surface.

These wheels were originally developed for higher horsepower steel descaling in order to reduce abrasive interference. However, recent applications to castings resulted in improvement in cleaning efficiency. Figure 1 shows how the wider pattern improves the wheel's ability to clean the sides of cavities in a hanger machine application.

Direct Drive Wheels--A direct drive wheel is one in which the centrifugal wheel is directly mounted on a motor shaft. For the most part, industry experts agree that for high production, rigorous use blast machines, the V-belt driven wheel is the most rugged and reliable centrifugal throwing device. However, there are many applications where the direct drive wheel may be satisfactory.

This is especially true of wheels mounted on an 1800 rpm motor. These wheels are quieter because of less vibration and lower rpm. Since they have 22-24 in. diameters and give sufficient abrasive velocity, the vane life is increased due to accelerating forces being less on long vanes running at lower rpm.

The main disadvantages of the direct drive wheels are: * abrasive impact velocity is "fixed" and cannot be changed without changing the wheel diameter, changing to curved vanes or running the motor from an expensive variable frequency drive; * motor bearings are not nearly as rugged as standard spindle bearings and, consequently, bearing failure can be a problem. [Figure 1 Omitted]
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Author:Briere, Thomas A.
Publication:Modern Casting
Date:Sep 1, 1989
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