desired position the bearing housing 118 includes a front side 120 with a hole 138 through which roller chain 110 enters the bearing housing 118 a rear side 122 internal bearing supports 130 132 with bearings 126 128 respectively on opposite sides of sprocket 114 for supporting shaft 116 an end plate 134 and an end bearing 136 for supporting the end of shaft 116 on which the sheave 140 is mounted the coil chain 142 is endless and is of sufficient length to hang near the floor 12 of the hangar h so that it can be conveniently reached and grasped by a person standing on the floor chain guides 144 145 extend from the bearing housing 118 into radially spaced relation to the sheave 140 on either side of the coil chain 142 to prevent the coil chain 142 from disengaging or jumping off of sheave 140 the counterweight 112 which is suspended from lower chain 110 as described above moves from a position in proximity to the bearing housing 118 as illustrated in fig6 when the door is closed to a position near the floor 112 as indicated in phantom lines 112 39 when the door is fully opened as indicated in phantom lines 10 39 a cage 146 is provided to guide the counterweight 112 in a vertical line between its upper and lower positions and to provide a safety barrier to prevent persons from inadvertantly walking or standing beneath the counterweight when it is descending as the door is being opened a power assist mechanism is also provided as an alternative to applying manual force to raise or open the door 10 the power assist mechanism essentially includes an electric motor 148 with.