Patent ID: 7228782

Claim:
A boom-holding control device for use in heavy construction equipments, comprising: a boom-holding valve ( 15 ) provided on a fluid pressure line ( 14 ) interconnecting a boom control spool ( 11 ) and a piston-side chamber ( 13 a ) of a boom cylinder ( 13 ) for preventing boom deadweight-caused drainage of a hydraulic flow from the piston-side chamber ( 13 a ) to thereby keep a boom in a holding condition; a boom release valve ( 19 ) for releasing the boom from the holding condition in response to at least one of a boom-down pilot signal pressure (P dn ) supplied from a boom cylinder remote control valve ( 20 ) and a travel signal pressure fed from a travel control operator-interface device; a solenoid-actuated changeover valve ( 25 ) provided on a travel signal line ( 24 ) interconnecting the travel control device and a pressure receiving part of the boom release valve ( 19 ) for selectively opening and blocking off the travel signal line ( 24 ); a travel selection switch ( 26 ) for issuing electric travel signals (I t ) when activated to assume a travel position (TR); and a boom release switch ( 28 ) for generating electric boom release signals (I hd ) when activated to assume a boom release position (R); wherein the solenoid-actuated changeover valve ( 25 ) is shifted to an opening position ( 25 b ) to release the boom from the holding condition at the time of simultaneous activation of the travel selection switch ( 26 ) and the boom release switch ( 28 ).