FORK LIFTS & LIFT TRUCKS

Fork lifts and lift trucks serve the same purpose in that they transport goods, materials, etc., from one place to another and stack them ready for storage, or load them onto trucks, box cars, etc. On the other hand, a lift truck is designed to operate in rough terrain and has as its power source a conventional wheel tractor. Fork lifts are also self‑propelled machines, but they have smaller wheels which are about 10 to 14 in [25.4 to 35.56cm] in diameter and are of a solid rubber design. The lift capacity and lift height of lift trucks is greater than that of the fork lift. Fork lifts are pow­ered by air‑cooled gasoline or petroleum gas engines or some are battery‑powered, while the larger lift trucks use diesel engines exclusively. However, when the unit has to operate inside buildings they employ petroleum gas engines or battery power to drive the wheel motor and the hydraulic pump. Fork lifts are commonly driven hydrostatically or by electric wheel motors, whereas lift trucks use standard transmissions, power-shift transmissions, or are hydrostatically driven. Four‑speed forward and reverse ranges are used in the standard or power-shift transmissions.

The lift trucks are classified by their lift capacity and lift height. Their lift capacity range is between 1400 and 120,000 lb [65.6 and 54,480 kg]. The lift height range is between 9.6 and 42 ft [2‑9 and 12.81 m]. Depending on the lift capacity, either a conventional drive axle or a drive axle having planetary wheel hubs is used. The fork lift uses band brakes which are hydraulically applied, whereas lift trucks use drum brakes which may be applied hydraulically or by oil or air.

The lift of a fork lift and lift truck is similar in design and in turn, in operating principles. The major components are

• The lift frame

• The mast, consisting of the inner and outer mast

• The single‑acting lift cylinders

• The double‑acting tilt cylinders

• The carriage with the forks

The lift frame is mounted to the tractor, and the outer mast is pivot‑fastened at the bottom to the lift frame. The two double‑acting tilt cylinders are fastened to the lift frame or tractor and the rod ends of the pistons are pivot‑fastened to the outer mast. The inner mast is guided by rollers in the channels of the outer mast. Carriage load rollers and carriage thrust rollers are fastened to the carriage frame to support the carriage and guide it through the inner mast. The lift cylinder is fastened to the lower mast cross member, and the piston rod end is pivot‑fastened to the inner mast cross member (crosshead shaft). The ends of the two lift chains are fastened to the sides of the lift cylinders and placed over guide rollers which are bearing‑supported by the crosshead shaft. The other ends of the lift chains are fastened to the bottom of the carrier frame. The two forks are positioned on the shaft and supported in the bores of the carrier frame, and the lower ends of the forks rest against the lower cross frames.

LIFT OPERATION

Before driving the forks under the pallet or positioning them over the load, the operator must first operate the directional control valve to direct oil to the tilt cylinders, either into the piston ends or into the rod ends of the cylinders, to tilt the mast to a suitable fork position. At the same time the lift cylinder directional control valve must be operated to direct oil into or from the lift cylinders to raise or lower the forks. When raising the load, the piston is forced upward, raising the inner mast. Since the lift chains are fastened to the upper end of the lift cylinder piston, and on the bottom to the carriage frame, the extension of the inner mast raises the carriage frame by this extended distance because the chain is shortened. To lower the load, the directional control valve position is reversed, allowing the oil to flow from the lift cylinder to the directional control valve and back to the reservoir. This removes the force from the piston and the weight, lowering the piston, the inner mast, and the carriage.