Shaft seals are a necessity to stop leakage and to prevent the grease from being sucked out of the bearing. They are disk shaped rubber seals with a spring that provides a tight seal around the shaft. They go on the high pressure side of the bearing. Also a rotary coupler needs at least two bearings to prevent deflection between inner and outer races. The JT rotary coupler meets these requirements. From their description, I would guess that the Craft Supplies unit doesn't ... their description says bearing which sounds like one bearing to me.