Morning Shane..

I have a 4km horizontally curved 3000t/h conveyor, with a booster (tripper) drive about half way round, i.e. in the middle of the curve.

It has 3 x 260kW drives at the head, plus 1 x 260kW on the booster, all fitted with TSS fluid couplings.

When I ventured out of my sheltered existence and commissioned it, it was 2km long with 2 x identical head drives.

When I extended it to 4km I put on another drive on the head and one on the booster half way round the curve.

Since there was a 3 year gap twixt initial and additional drives, the motors were not identical, as the motor speeds varied between 1475rpm (old) and 1490rpm (new).

This 1% motor speed difference was quite enough to stuff up the power sharing completely, and varying the oil fills only helped a bit.

The booster always took the most load and occasionally tripped, but we never had any problems with the tension distribution around the conveyor, but then I lead such a sheltered life I probably would not have noticed it anyway..

Regards Shane

LSL Tekpro ■

Shane,

You can write a book on a reply to this question. I am certain Larry will give you the low down on all the dynamic effects but in very blunt terms, if it is out of sync it is a major accident waiting to happen. I have seen belts snapped as a result, tail assemblies pulled out. Pick your weakest point. It all gets down to how bad, when and why before you can give a definite answer.

Col Benjamin ■

Dear Shane,

This is a warning that it is not a book. However, note that we are easily carried away.

Belt vibration or elastic wave propogations ( dynamic, uncontrolled high and low belt tension oscillations traveling between drives) is the critical event that can cause drive slip, excessive oscillating tensile and compression shock waves, unacceptable terminal forces at pulleys, high belt sag and spillage and a host of other vibration induced ills (See our website references).

Dynamic elastic oscillation can be activated by poor control, either through poor instrumentation, drive controls, logic or all of the above. An example of such an event was published from the commissioning of the ZISCO 16 km overland in 1996.

CDI was the mechanical designer and control systems designer (PLC based) except for the inverter drive controls.

ZISCO (Zimbabwe Iron & Steel Co.) is a single flight curved overland driven by its head and tail. Two pulleys and three inverter driven motors are at the head and a single inverter drive is at the tail. The tail accelerates the belt via a 500 second speed ramp. The head drives are controlled by belt tension through a load cell and other logic. Therefore, this classifies as a 16 km booster driven belt.

The control was first given to a inverter/motor drive manufacturer/specialist. Work was performed by the specialist to our specs at their motor/inverter shop. However, upon implementation, the logic did not work. Significant belt vibration occurred during the acceleration cycle. After repeated attempts, through differing logic, they could not stablize the drive torque oscillation between head and tail within the inverter logic and its data highway.

CDI was then contracted to implement a PLC based control which did stabilized the oscillations. THe behavior was published at the South Africa BELTCON 9 conference in 1997 entitled:

"ZISCO Installs World's Longest Troughed Belt 15.6 km Horizontally Curved Overland Conveyor" by the undersigned.

As Colin said, you can write a book on all the variants of why elastic shock waves happen and how to control them. I hope the noted paper's field measurements gives some idea of the effects.

An over simplified answer to Shane's question on why:

Repeating the above: any time a drive pulls (varies torque) upon the belt a shock or elastic (stretch) wave is created that simutaneously eminates from the drive as tensile (high) and compression (low) force differences which resonates along the belt's axis in opposite directions.

When one drive applies varying torque with respect to time, and the booster applies a varying torque that is not belt elastically compatible or amplifies the initial drive elastic wave, a vibration pertubation is created which does not seem to be controllable by the applied logic, we say there is drive instability. ■