June cover feature article

Water compatible power transmission

Gearboxes lift weight in water

Power transmissions which work immersed in water help the disabled, provide toys for the rich, and offer new opportunities in marine and offshore engineering. Tom Shelley reports

New gearboxes, which work immersed in water, use combinations of plastic and steel gears and rely on water for both cooling and lubrication.

Their first use in the UK is in a swimming pool which converts at the touch of a button into a ballroom. It has been constructed for a rich VIP who wishes to remain anonymous.

In Sweden, they are being increasingly used to raise and lower the floors of pools for treating the injured and disabled, and offer many opportunities for engineering in the marine and offshore industries.

Anders Stigsson of Stigsson Konstruktion in Sweden says that he designs each of his water immersion gearboxes for each individual application.

His first application in the UK is for a swimming pool being built on the third floor of No.1, Cambridge Gate, Regents Park, London. The pool installation has been designed by Rainbow Pools and is being installed as part of major rebuilding work being undertaken by Balfour Beatty, under the direction of architect Daniel Shabetai of The Conservation Practice.

The idea of building swimming pools with floors which can be raised and lowered seems to come from Sweden, where it has become quite popular. A common reason for building them is to be able to quickly convert to different uses, such as to reduce depth to teach small children swimming, or to increase depth for diving. In hospitals, swimming pools with lifting floors can be used to allow wheelchair access for the disabled undergoing therapy, or to cater for the needs of patients with different kinds of conditions. Floors are usually raised and lowered by hydraulic cylinders, acting on cables passed over pulleys. Complexity arises because of the need to closely co-ordinate the motions of the cylinders, in order that the floor lifts equally at each corner. There is also a need to design systems so that oil leaks cannot contaminate the water. One solution is to place the cylinders beneath the pool, and another is to use water hydraulics. Both systems take up a significant amount of space.

In the construction at Cambridge Gate, a listed Edwardian building, space is very limited. Swedish pool maker Invarmex Steel has therefore provided a system in which a 3kW 900 rpm AC induction motor drives a pair of 20mm diameter stainless steel shafts running alongside one end of the swimming bath. These drive other shafts along the sides through right angled gearboxes. The longitudinal shafts engage two pairs of worm drives on each side. The worms then act on spur gears forming the first stages of gear trains which engage on lead screw jacks in recesses inside the swimming area.

The worm in each case is made of stainless steel, acting on a glass-reinforced plastic gear. The plastic used is one specially developed for the task. Mr Stigsson is reluctant to divulge exactly what it is formulated from, except to say that it is significantly stronger than nylon.

The rest of the gear train also consists of alternating stainless steel and plastic spur gears, culminating in a large plastic gear attached to the top of the stainless steel lead screw. The lead screw runs in a plastic nut attached to the side of the floor.

Bearings are woven cotton-reinforced plastic and design life is in excess of 10 years. No installed water immersed drive trains from Stiggson are yet old enough to have reached this figure.

The floor itself is made of 600 x 600mm acid etched glass panels. The panels are each made up from two 12mm thick sheets of glass, laminated together by a 1mm thick layer of transparent polymer adhesive. The whole construction is supported on 100 x 100mm square, hollow, stainless steel box sections. The air volume is sufficient to make the whole construction buoyant when it is in the water. The floor therefore exerts no weight until raised clear of the water level to form the floor of the ballroom. Since it is 8 x 3.5m in total area, it then weighs about two tonnes. Raising it fully from the lowest to the highest position will take about 40 minutes.

When the floor is raised, water overflows through ducts around the sides of the pool to re-enter the area underneath the floor. A grating round the outside, which would allow a higher water flow rate is ruled out by the need to use the raised pool floor as a dance floor. The reverse water flow process occurs during lowering. It is the need to accomplish the water transfers process which requires the slow rate of lifting or dropping of the floor. Pools made for medical and municipal customers usually run off water from above a rising floor into a holding tank. The water is then pumped back underneath.

The pool lining is also made of 316 stainless steel panels, apparently made in Sheffield. The outer shell is concrete.

While Stigsson is mainly interested in making more gearboxes for swimming pools for hospitals and for the disabled, the technology clearly lends itself to other applications.

Many marine and offshore drive systems are complicated by the need to lubricate them with oil or grease. In inland lake and river environments, there is now seen to be a need to at least use biodegradable oils. Even on dry land, water is available almost anywhere. There is much to be said for machinery which could be lubricated from any tap, well or river.

Prior to its being taken over by the client, the house is to be used as a venue for the British Interior Design Exhibition from June 4-22.

Stigsson Konstruktion

Rainbow Pools

Invarmex Steel

Interior Design House

Design Pointers

* A combination of alternating glass-reinforced plastic and stainless steel gears allows and requires a gearbox to be lubricated and cooled by water

* Both spur gear and worm drives can be made in this way

* Use of fibre-reinforced plain bearings enables such gearboxes to have design lives in excess of 10 years

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