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.
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 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.
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.
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.
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.
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