Abrasive Wear and RPM

July 17, 2003

For starters, it is important to keep in mind how a screw conveyor works. Material being conveyed must slide against the surface of the flights, causing abrasive wear. For example, if a piece of gummy material is placed in a screw conveyor, it sticks to the flight and co-rotates. There is no wear, but the material has no axial movement.

Abrasive wear is very low in Vincent screw presses despite the fact that they run at higher rpm than competitor's presses. This ties to the fact that Vincent presses use interrupted screws, while most competitors use continuous flighting in their presses.

In part, this low wear characteristic is because the screw flights in Vincent presses are much taller than in competitive machines. This means that the solids pass through our press in bigger chunks. Thus less of the solids are touched and rubbed by the flight surface. In a typical 16" screw press, the flights in a Vincent press are about 5" tall. In comparison, they will be only 2" or 3" in a competitive press.

The reason the Vincent press achieves the same dewatering with a 5" flight is explained by the interruptions in the screw flight. These interruptions allows material to build up, compressing and remaining comparatively stationary, inside the last third of the press. It is only when sufficient solids are admitted to the material in the middle third of the press becomes firm enough to push solids from the cake discharge. In the meantime, compressed solids either co-rotate, without friction, in the compression stages, or compressed solids sit stationary, without causing wear, at the resistor teeth.

This is best seen when the flow of solids is shut off to the press, and the press is left in operation. A press with a continuous screw will clear itself, leaving a relatively empty machine inside.

In contrast, in a Vincent press with the interrupted screw, a plug of cake will remain at the discharge. If one then takes off the screens of the press, it is seen that rather solid material has been co-rotating with the compression flights in the last two or three stages of compression. At the same time, it is seen that solid plugs of cake have been sitting stationary at the resistor teeth. The key point is that no abrasive (or, at least, very minimal) wear has been occurring even though the press was running at full speed.

In summary, abrasive wear is lower in interrupted design screw presses because (a) the cake passes through in larger chunks because of the taller flights, and (b) as material is dewatered and compressed in operation, the solids can co-rotate, without wearing the screw.

What about screen wear? Is it not high because of the co-rotating material? The answer is that the Vincent interrupted screw press works with a relative high screw-to-screen clearance. New, this clearance is 1/16". The result is that a mat layer of solids forms against the screen, minimizing abrasive wear.

The proof is in the pudding. Vincent presses have a superb record of low maintenance. Our history in pulp & paper, plastics recycling, and dairy manure applications is truly impressive.

Issue 140