chain 26 normally carrying the weight of the upper panel 11 as well as the lower panel 12 tension in the chain 26 allows each panel 11 12 to balance the weight of the other panel the chain 26 is attached to the upper panel 11 with a chain rod 71 assembled through and anchored to a bracket 72 bolted to the upper panel 11 tension in the chain 26 due to the weight of the door panels 11 12 ordinarily prevents counterclockwise rotation of the bell crank lever 54 as viewed in fig3 the length of the cable 46 is arranged to control and keep the roller cam 34 in the cylindrical portion of the cavity when the chain 26 maintains the bell crank 54 in the position illustrated in fig2 and 3 inspection of fig2 reveals that the caliper housing or block 33 rigidly fixed to the door panel 11 is ordinarily arranged to slide freely along the door guide rail flange 21 in the event that the chain 26 supporting the door panel 11 breaks or otherwise suffers a loss of tension the bell crank lever 54 is released the bell crank 54 is thereby enabled to pivot counter clockwise under a bias force developed by the compression spring 36 and transmitted by tension in the cable 46 tension in the cable 46 is released when the bell crank 54 is freed by loss of tension in the chain 26 to pivot counter clockwise and in turn the cable releases the compression spring 36 from the compressed condition of.