Quoted from Steve_B:There is also a small spring that goes over one of the shafts.
To elaborate a little on @steve_b's note, some motors have springs and some don't.
These open frame motors are made with two cast plates that are riveted together to hold the gear shafts in place. Some motors have extra spacers between the two plates (maybe 1/2" tall?). Those spacers allow the rotor to slide back and forth along its shaft. When power is applied the rotor is pulled into the stator (or laminated plates) and starts turning, and at the same time the small gear on the rotor shaft engages with the rest of the gear train. When power is cut the rotor slides out of the stator and disengages the gear at the same time. This allows the rotor to spin down without driving the gear train. It's a simple clutch mechanism that allows the motor to stop suddenly which is important for Score Motors that need to stop in their index positions.
Motors that are mounted with the rotor on a horizontal shaft (e.g. Williams Score Motors) use a spring to push the rotor out of the stator when power is cut. Motors that are mounted with the rotor on a vertical shaft (e.g. Gottlieb Score Motors) rely on gravity to pull the rotor out of the stator so they don't require springs.
Also a couple of Gottlieb motors I checked have their fiber washers on the bottom of the frame. So when power is cut and the rotor drops it falls onto the washer as it spins down.
/Mark
