The M15 fire control quadrant, used on the M109 series howitzers, measures the angle of elevation of the gun
(2) A circular projection, centered lengthwise on the back side of the bracket, is machined flat for a mounting pad. A
horizontal keyway across the pad center provides for quadrant alinement. The mounting pad is drilled for four attaching cap
screws of corrosion-resistant steel.
(3) The cast housing pivots on two shafts held in the ends of the bracket and forming an axis parallel to the bracket
mounting pad keyway. A gunner's quadrant seat bar is permanently mounted on top of the housing.
(4) The cross-level knob operates a wormshaft meshed with a gear sector fixed on the right bracket shaft in the
housing to adjust the quadrant to a vertical position, correcting for any cant in the gun carriage. This position is established by
centering the bubble in the cross-level fire control level incorporated in the level assembly.
(5) The elevation knob operates a shaft on which are mounted worm and spur gears, The worm gear is meshed with a
gear sector pivoted on a fixed, transverse shaft, The level assembly is mounted on the hub of the gear sector. The spur gear
drives the elevation counter through a differential assembly. Turning the elevation knob clockwise causes the gear sector and
level assembly to rotate clockwise and increases the number of mils indicated on the elevation counter. Counterclockwise
knob rotation does the opposite. At zero gun elevation, the bracket mounting pad keyway is horizontal, the elevation fire
control level bubble is centered, and the elevation counter reads 0000 mils.