Hoist drive and method for driving a double hoist carrying apparatus

In a double hoist carrying apparatus having a hold hoist comprising a rotatable hold hoist drum and hold motor means for providing raising and lowering rotation of the hold hoist drum, a close hoist comprising a rotatable close hoist drum and close motor means for providing raising and lowering rotation of the close hoist drum, and a carrying member suspended from the hold hoist drum and having a frame and a plurality of openable and closeable members attached to the frame, the combination comprising:

• hold rope means connected between the hold hoist drum and the frame of the carrying member for suspending the carrying member from the hold hoist drum and for raising and lowering the carrying member in response respectively to raising and lowering rotation of the hold motor means;
• close rope means connected between the close hoist drum and the openable and closeable plurality of members for closing and opening the plurality of members in response respectively to the application of tension and slack to the close rope means by the close hoist drum;
• hold motor power supply means for supplying a.c. power at a first hold frequency to the hold motor means;
• close motor power supply means for supplying a.c. power at a first close frequency to the close motor means;
• the hold motor means has a frequency to torque first hold curve having a first slope, the hold motor means rotating the hold hoist drum at a raising rotational speed determined by the first hold frequency and the first hold curve slope;
• the close motor means has a frequency to torque first close curve having a first slope, the close motor means rotating the close hoist drum at a raising rotational speed determined by the first close frequency and the first close curve slope;
• and
• the first close frequency and first close curve slope being such that the first close curve lies along higher frequency values than the frequency values along which the first hold curve lies at corresponding torque values on the first close and first hold curves greater than a first predetermined torque value, and the close hoist drum thereby has a faster raising rotational speed than the raising rotational speed of the hold hoist drum at said greater torque values such that the close hoist drum applies tension to the close rope means to close the plurality of openable and closeable members.

In a method of operating a carrying member for raising, carrying and dumping material, the carrying member and any material it may carry comprising a load, the load being suspended by close rope means from a close hoist drum rotated by a close motor and by hold rope means from a hold hoist drum rotated by a hold motor, the steps comprising:

• operating the close motor along a first close speed to torque curve having speed values decreasing with increasing torque output to rotate the close hoist drum and raise the load;
• operating the hold motor along a first hold speed to torque curve having speed values decreasing with increasing torque output a greater amount than the speed values of the close speed to torque curve to rotate the hold hoist drum and raise the load;
• applying the load to the close motor and hold motor to cause the close motor to increase its torque output to assume a portion of the load and consequently decrease in speed as it operates along the first close speed to torque curve and to cause the hold motor to increase its torque output to assume a portion of the load and consequently decrease in speed a greater amount than the speed decrease of the close motor as the hold motor operates along the first hold speed to torque curve;
• in response to the greater speed decrease of the hold motor than the speed decrease of the close motor so that the close motor speed is higher than the speed of the hold motor, increasing the portion of the load assumed by the close motor;
• and
• in response to the increase in the portion of the load assumed by the close motor, increasing the speed of the hold motor to approach the speed of the close motor so that the load assumed by the hold motor also increases and the load assumed by the close motor decreases, whereby the load is shared by the close motor and hold motor at their approaching raising operating speeds.