Patent ID: 8109069

Claim:
A mower for cutting grass growing on a turf surface, which comprises: (a) a traction vehicle supported for rolling over the ground by a plurality of wheels, wherein at least some of the wheels are powered by a traction drive system to propel the traction vehicle over the ground; (b) a grass cutting unit operatively connected to the traction vehicle, wherein the grass cutting unit is supported in a mowing position such that at least some of the weight of the grass cutting unit is directly borne by the ground when the cutting unit is in the mowing position; (c) a hydraulic cylinder coupled to the cutting unit to lift the cutting unit out of engagement with the ground to place the cutting unit into a raised transport position out of engagement with the ground and to lower the cutting unit into engagement with the ground to place the cutting unit into the mowing position; (d) a lift and lower hydraulic circuit for the cylinder with the hydraulic circuit having a fluid supply circuit to the cylinder and a fluid return circuit from the cylinder; (e) a counterbalance system that is effective to shift desired amounts of the weight of the cutting unit between the ground and the traction vehicle, which comprises: (i) a single proportional hydraulic valve connected between the supply circuit and the return circuit to establish a variable backpressure in the return circuit that counterbalances at least a portion of the weight of the cutting unit on the ground by shifting the portion of weight from the ground to the traction vehicle; and (ii) an electronic controller operatively connected to the proportional valve with the controller being responsive to a variable input parameter for substantially continuously and infinitely varying the backpressure being provided in the return circuit in a band lying between a minimum backpressure and a maximum backpressure in concert with variation in the input parameter.