Completed flow ensures efficient changeover

A novel approach offers efficiency and reliability benefits in hydraulic valve sequencing and the generation of reciprocating flows. Tom Shelley reports

Flow sensing sequence valves and a new reciprocating valve respond to completion of flows and allow work at full pressure in all stages, ensuring maximum efficiency. A typical application has been to improve reliability and performance of changeover operations in large agricultural ploughs, while the reciprocating valve is expected to open up news ways of efficiently operating continuously cycling cylinders. The basic principle is to use a valve which is continually trying to switch to its next position but is prevented from doing so by a differential pressure generated by the return oil flowing from the cylinder. When the return flow stops because the cylinder has reached the end of its travel, the valve is able to switch, initiating the next operation in the sequence. Full pressure is available at all times and there are no user adjustments required, the valve being set up to cope with all general operation variations encountered when changing tractors or ground conditions. The ploughs are made by Dowdeswell Engineering based in Stockton, Warwickshire. Modern ploughs have two sets of ploughshares, so that when the plough reaches the side of the field, and is turned round and pulled back in the opposite direction, the second set of ploughshares, which is handed in the opposite sense to the first set, pushes the earth aside in the same direction. With large ploughs, which can weigh in excess of two tonnes, it is necessary to fold the plough before turning to reduce inertia and ground clearance. In order to complete the cycle of folding and turning, four separate cylinder movements are required. It is normal to control the whole operation by one actuation of a control valve. Tradtionally, sequence valves are operated by pressure. In this case, pressure operation is impractical because the valve would need three successive pressure increases, impossible to achieve if full pressure is required mid way through the cycle. To make matters worse, the whole system is vulnerable to spurious pressure peaks generated by the tractor driving over the rough field. By sensing flow, however, all these problems are overcome. Applied to a single valve and cylinder, the same method has been used to produce high efficiency reciprocation in excess of 120 cycles per minute. A commercial product with a wide range of potential applications in the agricultural and construction industries is in development. Hydraulic Actuators and Controls Jeremy Fuller at HAC Pointers * Improved sequence valves change over when no more return flow is sensed from a cylinder * This is more reliable than sensing rises in pressure and allows full hydraulic pressure to be used in all stages * A similar principle has been demonstrated in a new, high efficiency reciprocating valve