Minimalist motor plumbs hidden depths

Citation

(1998), "Minimalist motor plumbs hidden depths", Industrial Lubrication and Tribology, Vol. 50 No. 3. https://doi.org/10.1108/ilt.1998.01850cad.007

Minimalist motor plumbs hidden depths

Minimalist motor plumbs hidden depths

Keywords: Motors, Seawater

It is never a great time for your motor to give up on you. But if that motor is hundreds of metres below sea level on a multi-million dollar pipeline project, failure is likely to fray a few nerves.

Therefore reliability was top priority for Seafloor Dynamex when designing Hydro-Motor ­ the new seawater driven hydraulic motor at the heart of its Hydro-Digger seabed excavation system. Operating conditions for such a motor will always be tough, so designers decided to minimise the chance of problems by paring the number of components down to a minimum and eliminating seals and bearings.

The result is a horizontally mounted, high- torque, medium-speed, orbital-piston motor run on seawater. Power is developed by four variably aligned pistons mounted within a ring-shaped rotor. Pistons rotate between two sets of wheels.

Rotation is effected by a hollow channel within the 1.2m diameter circular rotor being fed by seawater at around 22bar. Water enters the rotor via a hollow steel tripod that is connected to the inlet hose. Once fluid reaches the rotor it forces through this circumferential channel at speeds approaching 20m/s, causing motor rotation at around 200rpm.

Water is held at pressure in the rotor's hollow by three aluminium bronze doors, spaced at one third intervals. There are also three evenly spaced water outlets in the rotor, each immediately preceded by a tapered section of trackway. These tapered sections gradually constrict the cross-sectional aperture presented to the open door, causing it to smoothly close and water to escape through the following outlet.

Hasler expands: "Each door opens and shuts six times a second. As a door is closing, there is some water backflow which has the effect of feathering this process and prevents mechanical wear."

At the end of the tapered trackway section, the door opens once more and that section of hollow rotor is immediately replenished with high pressure water from the tripod chamber.

Finally, the area within the rotor accommodates a four-section 1.2m span propeller, which aids overall motor rotation.

The circular film of water in the rotor provides the motor with lubrication as well as actuation. Sills says: "It is a liquid bearing system. The motor effectively floats on a film of seawater."

The Hydro-Motor has provided Seafloor Dynamex with a 1997 Manufacturing Industry Achievement Award. Winner of the Engineering Team section, the judges said about the company's motor: "We were impressed by the simplicity of the design, use of a minimum number of moving parts and its complete insensitivity to external pressure."

Hasler suggests innovation is necessary through force of circumstance. "As well as being the HQ of Europe's oil industry, Aberdeen is the world's leading technical centre. Oil is more difficult to extract here than anywhere else, so the levels of engineering required are also higher."