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Poka Yoke Station
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Hozelock Ltd
The market leader in garden watering equipment, Hozelock is also one of the UK’s flagship companies, exporting its products to many parts of the world.
Although its products are generally of low unit cost, they are produced in very large numbers. The Company has thus adopted the Poka Yoke approach to manufacturing - where key stages in the assembly process incorporate their own quality checks. In June 1996 add2 was chosen to design and build three such Poka Yoke units for use on a new micro-pump production line.
The new micro-pump is mains driven, fully submersible and of an elegantly simple design - all but three of the eleven component parts being made of injection moulded plastics material. In the first assembly operation, six components are installed in the main body casing to complete the rotor assembly and pump chamber. Once the cover of the chamber is fitted, however, how can one tell that the other items have been installed correctly - if at all?
The test station that add2 designed is a small ‘mechatronic’ instrument which back-drives the pump rotor - first one way, then the other. Since a correctly assembled rotor is physically prevented from turning in one direction, measuring the current drawn by the back-drive motor enables the unit to decide whether or not the assembly is complete. This technique also checks the ‘freeness’ of the rotor bearings and shows up any other impediment to a smooth action.
A relay output contact interlocks the unit with the next operation down-stream, so that only a product that passes the first assembly check can go on to the next stage. In practice, the use of these units adds no more than three seconds to the overall process cycle time.
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