1. IN COMBINATION, A CASE, INNER AND OUTER CONCENTRIC DRIVE SHAFTS EXTENDING INTO SAID CASE, A SPUR GEAR TRAIN IN SAID CASE HAVING AT LEAST FIVE GEARS AND HAVING ITS TERMINAL GEARS MOUNTED ON SAID DRIVE SHAFTS AND HAVING THREE OF SAID GEARS CONSECUTIVELY INTERMESHING ON A COMMON LINE OF CENTERS, STUB SHAFTS FIXED IN SAID CASE FOR ALL OF THE GEARS IN SAID TRAIN OTHER THAN SAID TERMINAL GEARS AND THE INTERMEDIATE OF SAID THREE GEARS, A MEMBER IN SAID CASE CARRYING A JOURNAL FOR SAID INTERMEDIATE GEAR AND MOVABLE LATERALLY OF SAID LINE OF CENTERS, AND ADJUSTABLE STOP MEANS ENGAGING SAID MEMBER FOR CONTROLLING LATERAL MOVEMENT OF SAID INTERMEDIATE GEAR WHEREBY ONE OF SAID DRIVE SHAFTS CAN BE SELECTIVELY ADVANCED RELATIVE TO THE OTHER.