Rough Terrain Forklift

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Rough Terrain Forklift


A forklift is a mobile machine that uses two prongs, or forks, to lift and place loads into positions which are ordinarily difficult to reach. Forklifts generally fall into two categories: industrial and rough terrain. Industrial forklifts are commonly used in ware-houses and around truck and train loading docks. They have small tires designed to run on paved surfaces and are usually powered by an internal combustion engine running on gasoline, diesel, or propane fuel. Some smaller industrial forklifts are powered by an electric motor running off an internal battery. Rough terrain forklifts, as the name implies, are designed to run on rough, unpaved surfaces. They are commonly used around construction sites or in military applications. They have large, pneumatic tires and are usually powered by an internal combustion engine running on gasoline, diesel, or propane fuel. Rough terrain forklifts can have a vertical tower, which lifts loads straight up, or a telescoping boom, which lifts loads up and out from the base of the machine.

The rough terrain forklift dates back to about 1946 when a two-pronged lift attachment was placed on a power buggy or tractor chassis. This early machine was used around construction sites and could lift about 1,000 pounds (454 kg) to a height of 30 inches (76 cm). Rapid development of vertical tower forklifts for industrial use was adapted to rough terrain forklifts as well. By the mid-1950s, capacities of 2,500 pounds (1,135 kg) and lift heights of up to 30 feet (9 m) were available.

In 1958, the first four-wheel drive rough terrain forklift was introduced. It had a capacity of 6,000 pounds (2,724 kg) at a lift height of 22.5 feet (7 m), or 3,000 pounds (1,362 kg) at 35 feet (11 m). In 1962, the first telescoping-boom rough terrain forklift came on the market. The telescoping boom allowed loads to be placed out from the base of the machine, both above grade and below grade. This was especially handy in crowded construction areas where open trenches, construction debris, or other construction work prevented a vertical lift fork-lift from operating close to the area where the material was needed.

Developments during the 1970s and 1980s brought improvements in the telescoping boom design and the introduction of features such as automatic hydraulic frame leveling for increased stability. Requirements of the Occupational Safety and Health Act (OSHA) resulted in improved operator cabs and controls during this period.

Today, rough terrain forklifts are a common sight on construction projects. They handle everything from pallets of concrete block to stacks of plywood to roof beams. The larger models use a telescoping boom with lift capacities up to 10,000 pounds (4,540 kg), vertical reaches up to 40 feet (12 m) and forward reaches of 25 feet (7 m) or more. They are usually a low-profile design and can pass through openings as low as 8 feet (2 m) high to gain access to the interior of a structure. Two-wheel steering, four-wheel steering and four-wheel crab steering (all wheels turned in the same direction) configurations are available.

Raw Materials

The frame, cab, boom, and body of a tele-scoping-boom rough terrain forklift are usually fabricated by the forklift manufacturer. Steel is the most common material for these subassemblies. Some steel or aluminum castings or forgings may also be used. Non-metallic materials such as nylon plastic blocks are sometimes used as guides in the boom assembly. The remainder of the parts are usually purchased as finished products and are installed by the forklift manufacturer. Purchased products include the engine, transmission, axles, wheels, tires, brakes, seat, gauges, lights, back-up alarm, hoses, and hydraulic cylinders. The hydraulic fluid, lubricants, and fuel are purchased in bulk quantities and are added as required.


A typical telescoping-boom rough terrain forklift is long and low with a pair of wheels at the extreme front and another pair located towards the rear. The boom is mounted at the rear of the forklift off a pivot that is raised several feet above the level of the frame. The cab is mounted on the left-hand side of the frame structure with the bottom half of the cab low and between the tires. The hydraulic fluid tank and fuel tank are mounted opposite the cab on the right-hand side. The engine and transmission are mounted within the frame along the center-line of the vehicle.

Beyond this basic configuration, various manufacturers have their own unique designs and options. Some forklifts use a single hydraulic cylinder to elevate the boom, while others use two cylinders. Some models have a side-to-side hydraulic frame leveling capability which tilts the frame up to 10 degrees relative to the axles to compensate for extreme axle articulation. This is used, for example, when the tires on one side of the forklift are up on a mound of dirt and the tires on the other side are down in a rut. Other special features include fork attachments that swing up to 45 degrees left and right to allow exact placement of the load.

The Manufacturing

The telescoping-boom rough terrain forklift is generally manufactured in separate, functional group sections: hydraulics, powertrain (engine, transmission, etc.), electrical, chassis, and boom. Individual components are either purchased or created from raw materials, and joined into subassemblies. The subassemblies are then brought together in the final assembly area where the forklift is completed. The actual flow of work varies from one manufacturer to another, but the following is a typical process.

Materials preparation

  • 1 The raw steel materialsheet, plate, bars and tubesare first cut to size and machined. Plate up to 0.75-inch (1.9 cm) thick is cut into shape, or "burned," by oxyacetylene or plasma gas cutting torches controlled by numerical controlled machines. Thinner steel sheet is cut with a shear and bent into shape as required by press brakes. During the cutting and machining, the steel is held in place with large fixtures, or clamping devices, to ensure dimensional accuracy.


  • 2 The parts which will be welded together are first tack welded in place. These would include components of the chassis, cab, and boom, among others. A tack weld is simply a small weld, or fusion of the two pieces of material, to keep the pieces from shifting during the final welding process. The whole assembly is then welded by numerical-controlled (NC) machines which place the welds in exactly the right areas, with the right welding temperatures, and the right feed rate for the welding rod. This is important to obtain a weld which will provide the required strength and meet the standards of the American Welding Society. As with the machining step, a variety of fixtures are used to ensure dimensional accuracy.

Shot blasting

  • 3 At this station, steel parts are placed on a rotating table or a conveyor belt in a large chamber. When the chamber doors close, the parts are blasted with thousands of BB-sized metal pellets that are shot at high speed from dozens of openings in the walls of the chamber. This process cleans away the rough scale that naturally forms on the surface of steel when it comes from the steel mill. It also cleans away the small welding splatter commonly found in the welded areas. This shot blasting is the first step in preparing the parts for painting.


  • 4 All exposed parts, except the boom, are now painted to protect the surfaces. The boom is painted after the telescoping sections have been manually assembled in step 5. In preparation, all parts are thoroughly washed in a detergent bath and then rinsed. A second acid wash and rinse cleans the metal further and also applies a thin phosphorus coating to improve paint adherence. In the paint booth, fine paint particles are sprayed from a spraygun that also imparts an electrostatic charge to each particle. The part being painted is electrically charged to the opposite polarity of the paint. This causes the paint to be drawn to the part and results in an even coat of paint over the entire surface. After painting, the parts are baked in ovens to produce a hard coating.


  • 5 The parts are now sent to several functional group work stations. The boom is built at one station, the cab at another, the chassis at another, and so on. The boom is made of two to four rectangular sections of long, hollow steel tube. The size of each section is smaller than the previous one and the sections slide, or telescope, into each other. Inside each section, a hydraulic cylinder and chain device cause the boom sections to extend or retract when maneuvering loads. Nylon guides prevent the steel sections from rubbing on each other, and stops are installed to prevent the sections from sliding out of each other when the boom is operating below grade level at a downward angle.

    The chassis work group installs electrical wiring and hoses and bolts the engine supports in place. The cab group installs the instrument panel, controls, wiring, and seat. The powertrain group joins the transmission to the engine, mounts the engine accessories and hydraulic pumps, and connects electrical wiring to various sensors on the engine.

Final assembly

  • 6 All of the subassemblies are now brought to the final assembly area. The tires, wheels, hubs, and brakes are installed on the axles, and the axles are installed on the underside of the chassis. The engine and transmission are lowered into the chassis and secured to their mounts. The drive-shaft(s) connecting the transmission and the drive axle(s) are connected. The cab, fuel tank, and hydraulic fluid tank are installed. The boom assembly is lowered onto its pivot point and the hydraulic cylinders that raise and lower the boom are installed. Hose and electrical connections are made between all the subassemblies. Fluids (oil, hydraulic fluid, fuel) are added as required. Instruction and warning decals are applied in the cab and on the boom.

Start-up and testing

  • 7 Each unit is started and run through a series of functional tests with actual loads for up to 1.5 hours. Any final adjustments or settings are made at this time.


  • 8 Finished forklifts are shipped to the customer or distributor by truck or by rail. Two or three forklifts are usually shipped on the same load to minimize the freight charges.

Quality Control

Inspections and tests are essential to the manufacturing process to ensure the product meets all standards and safety requirements. Critical components are placed on a coordinate measuring machine that optically checks dimensions, alignment, and geometry following fabrication. Welders, and even the NC welding machines, must have American Welding Society certification. Other parts are visually inspected during their fabrication and assembly.

In addition to the part-by-part inspection, the entire forklift design is tested for proper function. One of the critical tests is the American Society of Mechanical Engineers (ASME) stability test. This test determines how much weight can be safely handled at various distances, or reaches, from the forklift. For example, a forklift with a 10,000-pound (4,540 kg) lift capacity is limited to a maximum lift height of 20 feet (6 m) and a maximum forward reach of 8 feet (2 m) when lifting a full 10,000-pound load. For a full 25-foot (7.6 m) forward reach, the load capacity of this forklift is reduced to 2,000 pounds (908 kg) without outriggers, or stabilizing legs, and 3,250 pounds (148 kg) with outriggers. Warning labels and charts in the cab caution the operator of these limitations.

The Future

A wide variety of attachments have been developed for rough terrain forklifts to improve their utility. Winches, booms, and rotating fork carriages allow the forklift to place materials more accurately. Articulating booms, or booms with two separate extendible arms, can reach up and over structures to place loads on interior roof slopes or in the center of upper floors. Other attachments and enhancements can be expected in the future.

Additional built-in safety features are also expected. Load-reach management devices can automatically restrict the reach of the forklift based on the load being handled rather than relying on the operator. These devices would determine the weight of the load using pressure sensors and feed this information to a small electronic memory device which had all the load-reach limitations programmed into it. As the load is being maneuvered into position, the memory would compare the angle and extension of the boom with the safety limits. A warning device or a lock-up mechanism would prevent the operator from over-reaching and possibly causing the boom to fail or the forklift to tip over.

Where To Learn More


Petersen, Julie. "Cat's Big Secret: Killer Forklifts." Mother Jones, November-December 1993, p. 13.

Schwind, G.F. "What's New in the Lift Truck Marketplace." Material Handling Engineering, February 1993, pp. 49-56.

Peter Toeg

Chris Cavette