State of the Art Hydraulic Systems for Movable Bridges

Page 3HYDRAULIC CYLINDERS:The application of hydraulic cylinders to operate movablebridges has been the most widely acceptable to date. Be itlift, swing type, or bascule design, cylinders can easily beinstalled on both old and new bridges. Two cylinders perspan are normally required; however, any number is possible.There are basically two types of cylinders available formovable bridges, welded and tie rod design. There is a widevariety of mounting styles. Most common are sphericalbearing clevis, both ends and intermediate trunnion. Thetie rod designs are inexpensive and are used in manydifferent industry applications because, regardless ofmanufacturer, they are all dimensionally interchangeable.The welded type hydraulic cylinder is so constructed that itcan be serviced in place even after years of exposure, andgenerally, stainless steel piston rods are standard whereasthe tie rod design most often requires that you order stainlesspiston rods speciEically as most manufacturers do notoffer stainless steel rods as standard. For civil engineeringprojects stainless steel, chrome plated piston rods arethe optimum.'$1.PROPORTIONAL VALVES:Controlled acceleration and deceleration are extremelycritical to the safety requirements in bridge operationsince oil flow equals speed. This is accomplished byincreasing or decreasing oil flow to the hydraulic cylinder(~).This is accomplished by using todays proportionaldirectional control valves. These are extremely rugged andversatile valves that operate from a proportional forcecoil. The force controlled servo mechanism, an int.egra1part of the directional control itself, provides an adjustableforce output proportional to the amount oE DC currentbeing applied. Typical power supply is 0 - 9 volts DC/800miliamps.The adjustable force delivered from the servo is used tovary the travel of the valves main spool. This spool is sodesigned to offer excellent metering of the hydraulic flu~d,both to and from the cylinder(s). Thus the cylinder(s)speed becomes infinitely variable.