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Staged modulation improves forklift brake control

Pedal Movement
Pedal movement provides low-pressure braking for maneuverability, or high-pressure braking for fast, safe stops.
Taylor Model TE-155S 80-hp diesel-powered forklift has more than 71/2-ton lift capacity and almost 15-mph travel speed. Pedal-operated staged-modulation of hydraulic brake system helps operators safely maximize performance.
Iron Rugged
Pictured below are two versions of rugged ductile-iron 1100 Series modulating brake valves: suspended pedal, at left, and treadle, at right. Simplified schematic diagram at immediate left shows typical arrangement when connected for tandem braking circuit.

When a forklift operator makes an approach to pick up or stack a load. cont trof the movement of the forks is obviously critical to speed and accuracy. Lessobvious, but equally as important, is control of the vehicle’s braking system Forklift builder Taylor Machine Works, Inc., Louisville, Miss., installs staged-modulation hydraulic brake Ives from Carlisle Braking System in their equipment to help operators maximize performance.

Staged modulation in a hydraulic brake System provides a non linear pressure rise in response to pedal motion. As the vehicle operator first depresses the brake pedal, brake pressure rises gradually in response to pedal position, making it very easy to control When high brake pressure is requlred, depressing the pedal farther brings a rapid rise in brake pressure. Typlcally, the vehicle operator brakes with low pressure most of the time Full-pressure stops are made only rarely generally under panic conditions. In either case, the valve spring-returns.

Carlisle Braking Systems, Bloomington, Ind accomplishes staging by putting dual-rate pressure-regulating springs in their modulating control valves The rate of pressure rise is predetermined in both the lower and upper portions of the plot shown above by sring selection at the factory. The transition point between the upper and lower plots may be field-adjusted by turning an external screw located under the pedal. These changes can be made without disassembling the umt, and the screw is locked in place once the transition point is set. Carlisle has offered the staging option in the past only in their high-end 2100 Series modulating control valves used on large wheel loaders and haulage trucks. By adding the staging option to the more economical 1100 Series, Carlisle brings smooth modulation to valves suitable for forklift truck and small wheel loaders. The highflow 1100 Series may be used in open-center, closed-center, and load-sensing hydraulic systems, and models are offered for single and tandem circuits. Its dampedspool design provides noise-free modulation across the range of all output pressures.

For a typical tandem braking circuit, as dlagrammed at right, the pressure source is a dedicated pump or an accumulator. With the modulating valve at rest, pressure to both circuits is blocked and return lines from the brakes are connected to tank. When the operator depresses the pedal, the valve shifts quickly through the all-ports-blocked center position and then begins to pass flow to the brake circuits. As brake pressure increases, the pilot forces developed in opposition to the pedal force eventually take command of the spool and shift it to its center position, locking that level of pressure in the brake circuits. When the operator releases the pedal, spring and pilot forces combine to return both spools to their original positions.

Series 1100 valves are designed with larger internal passages and contours that limit flow forces. The resulting high flow capability reduces brake reaction time and shortens stopping distance— particularly when used for wet brakes with large displacements (an industry trend today)

Options for the modulating valves include: treadle pedal actuation, automotive- style suspended pedal for firewall- mounting, horizontally mounted valves with treadle pedal for constrained space, hydraulic or pneumatic pilot supply, and adjustable proximity switches for stop-light actuation.

Phil Redenbarger. section manager product development, Carlisle Off- Highway Braking Systems. describrd the operation of this component.