GB551159A - Improvements in or relating to apparatus for producing fluid pressures responsive tochanges in a variable - Google Patents

Abstract

551,159. Fluid-pressure servomotors. FARRINGTON, G. H., and BAILEY, F. J. July 1, 1941, No. 8301. [Class 135] A device for producing a variable fluid pressure corresponding to changes in a variable for transmission to an indicating, recording or controlling apparatus' of the kind in which the variable impulse is applied to a control member through a resilient member is arranged so that the deflection imposed on the resilient members is at least ten times the resulting deflection of the controlling flapper or valve and has a bellows or capsule subject to the pressure arranged to act directly or indirectly on the flapper or valve in the direction to move it towards its closed position. In the form shown in Fig. 2 the resilient element is in the form of a torsion tube 40 mounted in pivot recesses 41 and carrying an axially spaced arm 42 and bell-crank lever 43. The arm 42 carries the baffle 20 co-operating with the nozzle 21 while the lever 43 is connected to a bellows 46 acted on by the pressure in the pipe line 25 in the direction to move the baffle towards the nozzle. The arm 43 is also connected through a spring 55 and link 54 to a lever 51 weight-loaded at 53 and coupled by a link 50 to the sensitive element. The bellows is adjustably loaded by a spring 57 acting on the arm 56 of the bell-crank lever 43. The spring is attached to a nut 59 and threaded on a screw 58 so that by rotation of the screw in the nut the effective length of the spring can be varied. The screw and nut may also be moved bodily by a screwed abutment 60. In a modification, Fig. 1 (not shown) the torsion tube carries oppositely directed arms, one carrying the baffle, the other connected to the sensitive element and acted on by a bellows subject to conduit pressure. In this form the supply conduit is fitted with a filter. The baffle may also be subject to forces fixed or variable which are independent of the pressure in the system and may vary in sympathy with a second variable. In a system for controlling humidity the pressure pipe is provided with local and remote humidity indicators, the local one being adjacent the dials of wet and dry bulb thermometers for calibration purposes. The pipe also leads through a pressure relay to a remote control apparatus which in turn controls a humidity control valve. Adjustment of the pressure relay permits compensation for variations in the sensitive element and may include a ratio adjustment so that the pressure range may be made to correspond with the scales of the indicating or recording instruments. In the form shown in Fig. 4 the baffle 20 is actuated by a variable pressure in the bellows 66 proportional to variations of a variable and is connected through a torsion tube 40 and linkage 43, 74 to the spindle of a valve actuated by a diaphragm subject to the variable pressure produced by movements of the baffle. A second bellows 46 subject to the pressure in the control conduit 71 also acts on the baffle. The parts may be so proportioned that the apparatus will be stable or will go into self-oscillation at certain or all pressures within a desired range with a sufficiently high frequency and low amplitude that the indicators are not visably affected while the undesirable effects of static friction 'are avoided.