The negative dobby with single jack is known as Keighley dobby. The negative dobby with double jacks is known as climax Dobby. It consists of pattern cylinder, pattern chain, straight and bent lever, top and bottom hook, needle, baulk lever, clink, harness for Healds and spring for heald under motion.
Working:
Depending on the presence of peg straight the bent lever is lowered and thus bottom hook is falling on knife if bent lever is lowered and top hook falls on knife if straight lever is lowered.
Due to the movement of T-lever the 2 knifers will be describing to and fro motion. During the forward movement of knife, it drags the hook, baulk lever and thus the heald shaft is lifted through outer and inner jacks.
When peg is not present, the hook will be clear from the knife and the springs bring the heald shaft, lower line of shed.
Disadvantages of negative dobby:
During heald shaft lifting, negative dobbies can consume more energy to overcome heald shaft weight, warp tension and reversing motion pull.
This results in increased irregularity in the operation of the weaving machine.
The uncertain bottom position of the heald shaft results to form irregular shed.