Clive Hibbert, director of engineering – Joy UK, outlined the research behind Joy’s approach to fluid couplings in Joy’s recent Mining Perspectives magazine.

Fluid couplings were established as being suitable for longwall application by the early 1990s, but on AFCs rated up to 700 hp. Higher power with external cooling had been developed in other applications and as power was not a limitation, development work focused on developing a device to suit the bigger size requirements. Joy worked with Voith to develop the Turbo Transmission Technology (TTT) now used on Joy’s AFCs.

Joy testing of a high torque clutch indicated several disadvantages Hibbert said, including: “complex mechanical equipment (including a large volume pump); extremely reliable monitoring and control devices for normal operation; and limitations due to heat dissipation needs with frequent starting at high loads.”

One of the advantages Joy identified was the ability of fluid couplings to cool down much faster than a clutch system. This is crucial because the electric motors powering an AFC produce optimum torque when brought up to speed under no load. This generates heavy slip which dissipates as heat. When an AFC is started often or is overloaded heat buildup can cause a system shutdown.

“During the early work on the clutch system, Joy recognised that a 1000 hp gearbox/clutch combination with its limited heat capacity would take up to two hours to cool down if the oil over heated,” Hibbert said.

“However, a fill-controlled water coupling with an ability to purge the hot water could be refilled with cold water in less than five minutes.”

Another advantage of fluid couplings is the characteristic of torque limitation with provides protection for transmission and chain in the event of over voltage.

Water utilisation was limited in the design by changing from ‘open loop’ to ‘closed loop’ water circuits, using temperature monitoring and solenoid valves to retain water within the TTT circuits until pre-set temperatures required additional cold water to purge the coupling.

“A fundamental characteristic of the TTT system is the simplicity of construction. The coupling housing is directly mounted between the gearbox and electric motor in a similar manner to the other fluid couplings on AFC transmission,” Hibbert said.

Control of the TTT is achieved by two solenoid valves that control the flow of water. Hibbert said the TTT system has been designed to be extremely fault tolerant and only excessively high water temperature will shut down the AFC.

In addition, routine maintenance of the TTT system is simple and, unlike clutch systems, does not need oil and filter changes.

The need to start an AFC in abnormal conditions is paramount to the success of a mine when mining conditions are unfavourable, Hibbert said.