Dual cylinder actuated boom arm

A road maintenance vehicle comprising:

• a support housing mounted on the vehicle;
• a mast which is rotatably secured to said housing;
• a frame fixedly secured to said mast;
• a boom member attached at one end to said frame and adapted to hold a road maintenance tool at another end;
• a first hydraulic cylinder including a piston and a piston rod and having a piston rod end and a cylinder end, said piston rod end being pivotably secured to a first side of said mast and said cylinder end being secured to a first side of said support housing;
• a hydraulic control circuit in fluid communication with said first hydraulic cylinder;
• an overload protection circuit in fluid communication with said first hydraulic cylinder;
• and,
• a means for selectively disabling said overload protection circuit, said means for selectively disabling being in communication with said overload protection circuit and comprising said piston and a port located on said first hydraulic cylinder, wherein said port of said means for selectively disabling is so located on said cylinder that said overload protection circuit is only operative when said boom arm is within approximately 20 degrees of a line extending perpendicular to a longitudinal centerline of the vehicle.

A linkage mechanism which allows a substantially 180.degree. pivoting motion of a boom arm around a pivot axis of a mast mounted on a support frame, the boom arm being securable at a first end to the mast and supporting a tool on a second end, the linkage mechanism comprising:

• a first hydraulic cylinder including a piston and a piston rod and having a piston rod end and a cylinder end, one of said piston rod end and cylinder end being pivotably secured to a first side of an associated mast and the other of said piston rod end and cylinder end being secured to an associated support frame;
• an overload protection circuit connected to said first hydraulic cylinder such that the associated mast, and an associated boom arm, will pivot around a pivot axis in the event the associated tool meets an obstruction, wherein the associated mast will pivot away from the obstruction in order to avoid damaging the associated tool, said overload protection circuit comprising a first valve in fluid communication with said first hydraulic cylinder;
• and,
• a means responsive to a movement of said piston for disabling said overload protection circuit, wherein said means for disabling comprises a second valve which is in fluid communication with said first valve and selectively allows a flow of hydraulic fluid from said hydraulic cylinder through said first valve of said overload protection circuit.