Hydraulic Power Systems for Forklifts

By Sean D. Ross International Sales Manager BF MICO, Incorporated SF Hydraulic Flow P Control Valve EF As seen in Industrial Vehicle Technology '95 Magazine Materials Handling & Lift Truck Edition andDiesel Progress Engines & Drives Magazine April 1996 Hydraulic Power Brake

Single Hydraulic P Accumulator Charging Valve

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Low Pressure Warning Switch

MICO designs, manufactures and sells hydraulic components Pedal Actuated Single Hydraulic and systems for heavy duty, Power Brake Valve off-road vehicles and equipment. Forklift Hydraulic Power Brake Systems by Sean D. Ross

Hydraulic power brake sys- hydraulic power brake systems pressure drop across the boost tems provide safe, reliable commonly in use: boosted hydraulic piston. The resulting force is trans- braking for forklifts and other brake systems and full power ferred to the master cylinder piston. systems. While Brake line pressure increases until vehicles in the materials han- both systems use a the boost relief valve opens. Once dling industry. Sean D. Ross to provide power for braking, their the boost relief valve opens, brake of MICO Incorporated, USA, designs and applications are quite pressure increases only by direct discusses the configuration different. mechanical input force to the and operation of the two main Boosted Hydraulic master cylinder. A hydraulic power brake actuator reduces the opera- types which are in common Brake System tor input force required to generate use today. A boosted hydraulic brake system brake pressure. The circuit for a is found on forklift trucks from 2.0 boosted hydraulic brake system Hydraulic power brake systems to 10.0 ton capacity (Figure 1). This with a hydraulic power brake provide safe, reliable braking for system uses a hydraulic power actuator, hydraulic flow control forklifts and other vehicles in the brake actuator which consists of a valve, and load sensing steering material handling industry. These hydraulic boost section coupled to is shown in Figure 2. systems are normally found in a master cylinder. The hydraulic forklift trucks above 2.0 ton boost section includes a boost Full Power Hydraulic capacity. Above 2.0 ton capacity, piston, variable flow restriction, and Brake System a regular "automotive-type" master boost relief valve. When the boost Brake systems for forklifts over cylinder cannot provide pressure section is actuated, system flow 10.0 tons normally require more and fluid volume to the passing across the boost piston is volume and higher pressure than within a reasonable pedal force and restricted. This restriction creates a are available through the boosted stroke. There are two types of

FIGURE 1 Boosted Hydraulic Brake System

Figure 1: Layout of a boosted hydraulic brake system system. These vehicles require a full power hydraulic brake system. FIGURE 2 This system uses a hydraulic Boosted (s), accumulator Brake System charge valve, a pedal actuated single hydraulic power brake valve, and a low pressure warning switch. The accumulator charge valve maintains pressure in the accumu- lator(s) within a pre-set range. The accumulator(s) stores hydraulic energy for actuating the brakes. The pedal actuated single hydraulic power brake valve regulates pres- sure to the brakes with a hydraulic feedback feature for positive opera- tor "feel". The low pressure warning switch alerts the operator when accumulator pressure falls below the accumulator charge valves Figure 2: Hydraulic circuit of a boosted system pre-set limits, (ie. in the event of pump failure, etc.). The high and low pressure settings of the accumulator charge valve, and accumulator volume is selected for FIGURE 3 the required number of "power-on" Full Power Hydraulic (accumulator pressure between Brake System charge valve high and low pressure settings) and "power-off" (accumu- lator pressure between charge valve low pressure setting and maximum required brake pressure) brake applications. In full power hydraulic brake systems fluid volume is not limited by a master cylinder. Therefore, with the appro- priate accumulator, a number of brake applications are available at full brake pressure after engine shutdown or pump failure. The cir- cuit for a full power hydraulic brake system with single open center accumulator charge valve, single pedal actuated hydraulic power Figure 3: Hydraulic circuit of a full-power system brake valve, open center steering, and low pressure warning switch is shown in Figure 3. Both boosted hydraulic and full power hydraulic brake systems are normally integrated into the rest of a machine's hydraulic functions (steering, implement, pilot controls, Sean D. Ross is the International etc.). So when deciding whether to Sales manager for MICO, Incorporated use a boosted or full power hydrau- in North Mankato, Minnesota, USA. lic brake system, engineers must He has a B.S. degree in Mechanical evaluate the parameters of the Engineering from the State University vehicle's hydraulic system and the of New York at Stonybrook and has applicable brake standards to been certified by the Fluid Power ensure vehicle safety. n Society as a Fluid Power Specialist. MICO is a registered trademark of MICO, Incorporated. MICO is registered in the U.S. Patent and Trademark Office as well as in Canada, Great Britain, South Korea, and Australia. MICO, Incorporated MICO West Division 1911 Lee Boulevard (Zip Code 56003-2507) 701 East Francis Street (Zip Code 91761-5514) P.O. Box 8118 / North Mankato, MN U.S.A. 56002-8118 P.O. Box 9058 / Ontario, CA U.S.A. 91762-9058 ( 507.625.6426 Facsimile 507.625.3212 ( 909.947.4077 Facsimile 909.947.6054

Form No. 80-950-155 4/96 (d) Printed in U.S.A.