1. A worm conveyor comprising: a conduit tube, two conveying helices in said tube, one mounted inside the other with no intervening structure, both helices having a pitch of the same sense, the pitch of the inner helix being greater than that of the outer helix, and means for rotating both helices in the same angular direction, with the inner helix being driven at a speed which is at least as great as the outer helix, the pitch of the inner helix being about 10 percent greater than that of the outer helix, said means for rotating both helices including means for driving the inner helix at a substantially greater rotational speed than the outer helix.

3. A worm conveyor comprising: a conduit tube, two conveying helices in said tube, one mounted inside the other with no intervening structure, both helices having a pitch of the same sense, the pitch of the inner helix being greater than that of the outer helix, and means for rotating both helices in the same angular direction, with the inner helix being driven at a speed which is at least as great as the outer helix, the pitch of the inner helix being about 10 percent greater than that of the outer helix, said means for starting comprising a motor and a direct drive connection between said motor and said inner helix, clutch means between said motor and said outer helix, and means for operating the clutch to connect the outer helix and motor a predetermined time after starting the motor.

4. A worm conveyor comprising: a conduit tube, two conveying helices in said tube, one mounted inside the other, both helices having a pitch of the same sense, the pitch of the inner helix being greater than that of the outer helix, means for rotating both helices in the same angular direction, and start-up means connected to said means for rotating, for starting the drive to said inner helix first and then, after a time interval, starting said outer helix, said start-up means comprising a motor, a direct drive connection between said motor and said inner helix, clutch means between said motor and said outer helix, time-delay means actuated by said motor when it starts, and clutch-actuating means actuated by said time-delay means after a time lapse.

5. A worm conveyor comprising: a conduit tube, two conveying helices in said tube, one mounted inside the other with no intervening structure, both helices having a pitch of the same sense, the pitch of the inner helix being greater than that of the outer helix, means for rotating both helices in the same angular direction, with the inner helix being driven at a speed which is at least as great as the outer helix, the pitch of the inner helix being about 10 percent greater than that of the outer helix, and start-up means connected to said means for rotating for starting the drive to said inner helix first, said starting means then operating after a fixed time interval to automatically start said outer helix.