Gearbox conveyor differential speed

The back-drive system is a means of controlling the speed of the gearbox pinion shaft (and thereby the conveyor differential speed) using, for instance, a motor or a brake. This could be offset from the gearbox shaft, in the same manner as the main drive, and connected by a belt. This belt would be a timing belt because of the accurate control required. Normally the back-drive is connected directly and in line with the gearbox pinion. 

The difference in rotational speed between the bowl and the conveyor is commonly referred to as the conveyor differential speed, N. Conveyor differential speed is calculated from a knowledge of the rotational bowl speed, S, the gearbox pinion speed, Sp, and the gearbox ratio, Rob ... 

The basic decanter is completed with a drive motor, usually electrical, and a gearbox, which controls the differential speed of the conveyor. 

The construction of a basic centrifuge with all its main components is shown in Figure 2.1. The heart of the decanter is the rotating assembly, which comprises a mainly cylindrical bowl housing an Archimedian screw conveyor, with a small clearance between it and the bowl. One end of the bowl is conical in shape, providing the means whereby solids can be removed from it. Affixed to one end of the bow l is usually a gearbox to effect a small differential speed between the conveyor and bowl. 

Sometimes the frame is mounted on a sub-frame together with the drive motor, and where necessary a back-drive system, to control the gearbox pinion shaft, which will in turn control the conveyor-to-bowl differential speed. The back-drive system will be described later, but for the present it suffices to say that it is essentially a braking motor or similar device coupled to the gearbox pinion shaft. The main motor is offset from the bowl and drives the bowl by means of a set of V-belts. The back-drive can also be offset, in which case it would be connected with a timing belt. The timing belt is to facilitate more accurate speed control. However the back-drive system can also be mounted direct in line with the pinion of the gearbox. 

The gearbox is a major component of the rotating assembly, which creates the differential speed between the bowl and the conveyor. 

To calculate conveyor differential, the same relationships as described for the epicyclic gearbox are used, except, as already stated, the Cyclo differential is faster than the bowl speed whereas the epicyclic is slower. 

Some of the early models of decanter were equipped only to indicate gearbox pinion speed, rather than specifically conveyor differential. This meant that differential speed had to be calculated, and calculated using the nominal bowl speed without a bowl speed measurement, or with just a one-off measurement. With this calculation it has to be borne in mind that bowl speed can vary by 100-200 rpm with change of load, and belt slip, ifany. This could lead to a major percentage error on differential calculation if the differential is low. 

Figure 1.14 Schematic diagram of a horizontal axis scroll decanter centrifuge. The motor drive for the bowl and the gearbox required to produce the differential rotation speed between the bowl and screw conveyor are omitted for clarity.

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