TT Drives have, in principle, been abandon. Many underground mines in North America applied the concept with poor results. Germany also had many problems such as slip instability.

Drive traction becomes unstable when the interface driving friction surface between TT belt and main belt is contaminated with water, sand, and clay like slimmy materials. When drive friction is lost on one intermediate drive, it distributes into tension and compression shock waves that exaccerbate the adjoining drives load sharing either side of the disturbed unit. One drive will then see more demand power and one drive less power that may cause its instability and so on. The results manifest themselves in failed take-up systems, failed tail and intermediate drive pulleys and failed splices and ultimately failed belts.

Often, the belt tensions became so severe that the belt width was reduced. In one case the fabric belt width reduction dropped from 36 inches to 32 inches over time. The belt fabric became so damaged it had to be replaced.

To overcome this instability, I invented the Power Strip in 1992. The Power Strip, unlike the TT drive is nearly full length and only sits on the center idler roll. This concept was successfully installed many years ago (~2001) by ESKOM in South Africa. Various authors have referred to this concept as either Power Strip or Power Band. I did not apply for the patent on this concept.

The Power Strip has commercial advantage in long overlands of about 15-20% cost savings over conventional trough belts due to belt, power and civil/structural cost savings.

Recently, our technical partner, Laing O'Rourke, showed a very significant cost savings by using the Power Strip to connect a number of existing conveyor systems transporting product in a series of belts. The belts had various fabric and steel cord constructions. The lot was married into one horizontally curved conveyor. Advantages included:

1) removal of many transfer stations

2) usage of existing belts with many differing belt constructions that were spliced together to carry material but eliminated the tension carrying member reliability and duty

3) reduced power demand or used new reserve power to increase production

4) increase belt production capacity using the existing belt components

5) better belt tracking with horizontal curves

6) increase overall reliability

7) reduced maintenance demands ■

TT- Drives

Our experiences with TT-Drives are much better than those of Mr. Nordell.

In 1973 the first TT-Drive in Germany was used in the open-cast mine Fortuna. The total drive power was 9 x 430 kW, there from 3 x 430 kW on the TT drive. All in all this seven month test run brought positive results. This technology was not followed furtheron, because it was not needed in lignite open-cast mining (existing sufficient belt strength, hauling ways controllable without intermediate drives).

Since the beginning of the 80s the Deutsche Steinkohle AG uses TT drives as intermediate drives in underground mining conveyors to avoid transfer stations and to realize larger axle distances by existing drive components and conveyor belts.

Inbetween some hundred TT drives have been installed and many of them are still running. In spite of the difficult environmental influences underground, which the TT drives were exposed to, no problems occured by the loss of friction. The installed drive power was transferred totally to the driving belts.

There are no problems with moisture ,the friction even increases.

When correctly designed, no slipping actions are to be expected.

Regarding this there are no negative experiences.

The main reasons for the use of TT drives in the german hard coal mining were:

•avoidance of relatively expensive steel cord conveyor belts

•usage of standard equipment (drives, pulleys, gearboxes, couplings, electrical motors etc.)

•avoidance of transfer stations and hence considerable reduction of costs, wear and dust

Last but not least it can be said, that for 30 years the TT drive technology is a reliable and well working drive technology in the german hard coal mining . ■