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Full power hydraulic brake system for earthmoving equipment.

Full power hydraulic brake systems have been used on heavy-duty off-highway vehicles for more than 30 years. The first applications for full power hydraulic brake systems were in mobile mining equipment mining loaders, haul trucks, etc. It was in these first applications that full power hydraulic brake systems demonstrated their superiority over common air over hydraulic brake systems.

Since their introduction in mining, full power hydraulic brake systems have become common in a large variety of other industries including earthmoving, material handling, and agriculture. Some examples of machines using full power hydraulic brake systems include wheel loaders find excavators, forklifts, telescopic handlers, logging forwarders, and agricultural harvesters. The full power hydraulic brake systems for these machines may have slightly different features, but their underlying concept remains the same.

A full power hydraulic brake system may use the machine's existing hydraulic system as a power source. An accumulator charge valve directs the machine's hydraulic system flow into one or more hydraulic accumulators to maintain the accumulator pressure between a high and low limit. The hydraulic accumulators store energy for "power-off" (when the machine is off) braking. A low pressure warning switch warns the operator when the accumulator pressure is below the low limit. Hydraulic brake pressure is controlled by a pedal modulating valve.

Let's examine the design and functions of each of these important components.

Accumulator Charge Valve - The accumulator charge valve can be of open center or load sensing design. The open center design is connected in series with other valves (i.e. steering) and is the first in series to provide priority flow to the hydraulic accumulators. The load sensing design is connected in parallel with other valves, and is supplied by either a variable displacement load sensing pump or a fixed pump with load sensing flow control.

The accumulator charge valve controls the charge rate (measured in gallons or liters per minute) of the accumulators in addition to high- and low-pressure limit. A dual accumulator charge valve design includes an inverted shuttle valve for supplying two accumulators for a split brake system (independent brake lines for front and rear brakes).

Accumulators - The hydraulic accumulators may be of diaphragm, bladder, or piston design. The type of accumulator used may be a function of size (diaphragm designs are common when smaller accumulators may be used) and economics.

Low-Pressure Warning Switch - The low-pressure warning switch should be of piston design. This type of design is usually more durable than the more common diaphragm design. A durable design is preferable given the switch's critical function in a full power hydraulic brake system.

Pedal Modulating Valve - The pedal modulating valve controls the brake pressure with a positive feedback for the operator. The feedback is provided by a pilot passage from the brake line to the bottom of the control spool. Different diameter control spools can be used to make feedback more true (i.e. larger diameter spools for lower brake pressure applications, smaller diameter spools for higher brake pressure applications).

Pedal modulating valves are available in single or tandem (for split brake systems) design and width vertical or horizontal mountings. Pilot modulated pedal modulating valve designs are available: for remote operation via a solenoid valve or a lever modulating valve.

The advantages of a full power hydraulic brake system over a common air over hydraulic brake system are many.

* Redundancy - The system eliminates the need for a redundant power source (i.e. air systems).

* Size - Hydraulic accumulators are smaller than air storage vessels. Also, fewer components are required in a hydraulic system.

* Contamination - Full power hydraulic systems are less prone to contamination than air systems.

* Service - Full power hydraulic systems are less complicated than air systems.

* Control - Pedal feedback is more true.

* Response - Brake fill and application is faster.

Figures 1 and 2 show a typical full power hydraulic brake system for a wheel loader. In this system, a dual accumulator charge valve and tandem pedal modulating valve are required for a split brake system. The accumulator charge valve is an open center design with bypass to steering, implement and/or tank.

This system also incorporates a park brake valve with auto apply. This valve will automatically apply the park/emergency brake if the service brake accumulator pressure falls below a preset trigger pressure.

A large number of variations of the above circuit are possible. The final circuit depends on many factors.

The purpose here is to give a brief introduction to the concepts and advantages of full power hydraulic brake systems. Close consultation with the hydraulic brake system manufacturer is recommended to insure all applicable brake standards are met at the most economical cost.

Sean D. Ross is International Sales Manager for MICO, Inc., North Mankato, Minn.
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Title Annotation:Mobile Hydraulics '97
Author:Ross, Sean D.
Publication:Diesel Progress North American Edition
Date:Aug 1, 1997
Words:785
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