The information does not indicate whether the screw is acting as a feeder or a conveyor. If it is a conveyor then there must be a serious reduction in capacity because the machine should be capable of handling in excess of the normal feed. As a feeder, there may be a problem of blocking in the infeed channel because of lumps. Send details of the arrangement drawing to tech@ajax.co.uk for a more detailed assessment

As a first check, make sure that the screw has not been damaged during commisioing by someone reversing the conveyor with product in the machine. If this is clear, and the drive is rotating at the correct speed, it is possible that there has been a build up of sugar fines on the suface of the flights and in the corner between the screw flights and the centre shaft. Check that the sugar is not damp.

If the reduction in capacity is slight, then it may be practical to run the screw at a higher speed.

There is obviously a simple reason for the reduced rate of handling. It is presumed that the casing is of circular construction, rahter than a 'U' trough shape, otherwise you would be able to inspect the screw and see the problem. Do not reverse the screw to try and clear the machine. On receipt of further details we will try to advise further.

Lyn Bates ■

A screw feeder has a full screw of material and controls the feed rate from the supply hopper, in which case it will immediately show any reduction in capacity if there is some restriction.

A screw conveyor is a transfer device, where the feed is controlled to the machine by some previous equipment. The design for a conveyor has to allow surplus capacity to avoid being unable to carry the load. Normally, the cross section of a screw conveyor is only 45 % full, and is even less full for abrasive or poor flowing materials. Conveyors will therefore only reveal a reduction in capacity when it is unable to transfer the material that is fed to it and then the inlet will fill up and block the supply.

From your query it would appear that the machine is a feeder. Send details please for a more detailed review.

Lyn Bates ■

Dear Settee Zhang,

Information given by Lyn Bates provides enough hints. Please inform:

1. Whether screw conveyor is horizontal or incline. If incline, its inclination degree.

2. Length of screw conveyor

3. Screw diameter, pitch and rpm.

How about the inspection provision? Is it U-trough or tabular trough. If tabular, is it split type? You should open and see the screw when capacity has become less. Inspect, whether sugar powder has solidified in certain corner space etc. You will be in best position to find reason, as the problematic equipment is in your presence.

Regards

Ishwar G Mulani - Author book: Engineering Science and Application Design for Belt Conveyor

Advisor (consultant) for bulk material handling systems

EmailID: parimul@pn2.vsnl.net.in ■

The most common reason for a screw conveyor reducing in its handling capacity is due to the build up of residue on the shaft and central region of the screw flights. This can be caused by the adhesion of fines that compact to form a large radius on the 'trailing' corner and a smaller radius on the working face of the flight. This behviour tends to be more prone to occur with coarse pitch screws. Ribbon type flights can be used to overcome this difficulty and special geometrical forms, such as the 'Lynflow' ribbon flight, have been developed especially to avoid solids build-up problems.

Close pitch screw can 'log' if material sticks betwen the gap of deep pockets of flights that have a small diameter centre tube.

In the case of sugar handling, if any moisture is present, the sugar can build up on the contact surface due to caking and give a rough surface to resist slip and result in a greatly reduced handling efficiency. ■