PURPOSE: To mount a drive motor assembly of different type on a same adapter by selectively bringing first and second surfaces of the adapter into contact with a support plate to provide the drive engagement between a drive member and the adapter of the drive motor assembly of first and second types.

CONSTITUTION: A first surface 26 of an adapter 23 is brought into contact with a support plate 16, center openings 24, 21 of the adapter 23 and the support plate 16 provided the drive engagement between a drive member 18b of a drive motor assembly of second type and a second surface 27 of the adapter 23, and the drive engagement between the assembly and a back up pad 10 is provided. To provide the drive engagement between the drive motor assembly of the first type and the back up pad 10, the second surface 27 of the adapter 23 is brought into contact with the support plate 16, and the center openings 24, 21 of the adapter 23 and the support plate 16 provide the drive engagement between a drive part 18a of the drive motor assembly of the first type and a bar 28 of the adapter 23.