Hydraulic carriage drive system

A hydraulic drive system for reciprocally driving a load along a fixed path comprising, in combination,

• pump means operable to deliver hydraulic fluid under pressure,
• an averaging accumulator connected to said pump means for receiving and storing pressure fluid therefrom,
• double acting hydraulic cylinder means for driving the load along said path, said cylinder means including an elongated cylinder having first and second fluid port means located one in each end portion, a piston mounted in the cylinder for reciprocal movement between said first and second port means, and a rod extending through one end of said cylinder and having one end connected to said piston and its other end adapted to be connected to the load for movement therewith,
• first conduit means connected to said first port means and second conduit means connected to said second port means,
• control valve means connected to said first and second conduit means for controlling the flow of pressure fluid to said cylinder selectively through one of said first and second conduit means and simultaneously permitting the flow of fluid from the cylinder through the other of said conduit means to drive said piston,
• third conduit means connecting said control valve means to said averaging accumulator to supply driving fluid through said control valve means and one of said first and second conduit means to selectively drive said piston,
• high pressure braking accumulator means having inlet means and outlet means, said inlet means being connected in fluid communication with said first and said second port means to permit the flow of pressure fluid from said elongated cylinder into said braking accumulator means,
• means connecting said braking accumulator outlet means to said control valve means, and means isolating the high pressure fluid in said braking accumulator means from said pump means and said averaging accumulator,
• said control valve means being operable to block the flow of hydraulic fluid therethrough to brake movement of the load whereby continued movement of said piston and rod and the load connected thereto will force fluid from one end portion of said cylinder into said braking accumulator for storage at a pressure greater than the pressure in said averaging accumulator and to initially direct the stored high pressure fluid to said cylinder to rapidly accelerate said load upon actuation of said control valve means to move the piston in the desired direction and to direct pressure fluid from said braking accumulator and said averaging accumulator to said cylinder to continue moving said piston when the Pressure in said braking accumulator and said averaging accumulator are substantially equal.

In a method of operating a sawmill in which a carriage is reciprocally driven along a fixed path by a hydraulic drive system including a double acting cylinder having its piston connected to the carriage and having its opposed end portions connected through a control valve to an averaging accumulator containing fluid under pressure supplied by a pump with the control being operable to selectively direct fluid under pressure from the averaging accumulator and pump to either end of the cylinder to drive the carriage in the desired direction and operable to connect the other end of the cylinder to the averaging accumulator to brake the carriage and recover energy in the form of pressure fluid returned to the accumulator during braking, the improvement comprising,

• providing a second accumulator having an inlet and an outlet,
• connecting the inlet of the second accumulator to the two end portions of the cylinder and connecting the outlet to the control valve to thereby permit fluid under pressure to flow into the second accumulator to maintain a minimum pressure therein equal to the pressure in the averaging accumulator,
• directing the flow of pressure fluid from the cylinder to the second accumulator during braking while preventing the flow of such fluid from the cylinder to the averaging accumulator to thereby store fluid in the second accumulator at pressure substantially in excess of the pressure in the averaging accumulator during braking, and
• using the high pressure fluid stored in the second accumulator to accelerate the carriage in the opposite direction after each braking action while maintaining such high pressure fluid isolated from the fluid in the averaging accumulator and from the pump until sufficient fluid from the second accumulator has been used to reduce the pressure therein to a level substantially equal to the pressure in the averaging accumulator, and
• using pressure fluid from the averaging accumulator and the braking accumulator simultaneously to continue to drive the carriage for the desired distance.