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How Does a Lift Truck Work: A History of Mechanics & Controls



Forklifts are incredible machines that find good use in a variety of work environments such as warehouses, construction sites, shipyards, and recycling facilities.


Forklifts can perform a variety of tasks. Basically, lift trucks are very necessary in the handling of materials. Most often, the products we sell are at some point bundled or packaged on pallets and are too heavy to be moved or loaded by hand.


Despite the various classes of forklifts available, the general working mechanism of all forklifts is the same which is discussed below.



LIFTING MECHANICS


There are two interconnected parts that compose the lifting mechanism - roller chain pulleys and hydraulic cylinders. Each component has two specific purposes - for lifting loads to the required height, and to maintain balance during movement.


Roller Chain Pulley


A roller chain pulley is used to physically move a load up and down along the mast of the forklift. While the mast is being raised into place with the help of hydraulic cylinders, it will raise the chain sheaves, or pulleys, which lift the chains that are attached to the carriage (the forks mount there). The chains are mounted in a way that one end is fixed to the back portion of the mast and the other end is affixed to the carriage. When the lift cylinders are extended it lifts the chains. Because the rear portion is fixed to the mast and cannot move the chains must ride the pulley over the top of the mast. This causes the carriage to raise up 2:1 vs the mast height. These pulleys are essential to the lifting of heavy objects because they allow the lift chains to smoothly roll as the mast is lifted and lowered. This is the basis description of a two-stage mast. Three stage and quad masts operate in the same fashion with the exception that instead of connecting to the carriage on the first mast rail it connects to another mast channel giving you more lift height.


Hydraulic Cylinders


A hydraulic lift cylinder in a forklift is made up of a hollow tube that has been sealed at one end with a butt plate, has a movable piston and rod at the other end that passes through a gland nut. The piston has seals to keep the hydraulic pressure on one side of it, without a seal there would be no pressure, the piston wouldn’t travel, and the oil would just bypass the piston. The piston is lubricated for easy movement inside the tube by the hydraulic oil. The gland nut also has seals inside of it to help maintain the pressures where they should be and, also to keep the hydraulic oil from pouring out. The cylinder strokes, or travels, because the hydraulic pump creates pressure underneath the piston. When the pressure exceeds the weight of the load, forks, carriage, chains, and mast section the mast section begins to raise, and the load goes with it. The hydraulic pump is there to create the flow of oil, the control valve is as it is named, the valve has sections that control the different functions of the mast and attachments by using spools.


FORKLIFT CONTROLS


Each forklift has different controls for the hydraulic functions, but they are typically levers and linkages. There are options for fingertip controls which electrically control the spool valves. There is also a pilot system that is also referred to as hydraulic over hydraulic, but it is just another way to operate the spools in the control valve.


Driving Controls


A lift truck operates very similarly to an automobile. You sit in an operator’s seat, steer with a steering wheel and accelerate and brake just like the configuration of your car. Like other vehicles, a forklift's driving control consists of acceleration, steering wheel, brake pedals, forward gear, and reverse gear. Using these controls, you can easily maneuver the forklift in varying terrains while keeping the load balanced. All counterbalance forklifts come with rear-wheel steering which enhances the ability to rotate and execute precise turns.


Lifting Controls

  • The hydraulic controls are not more complicated than the driving controls. They are typically comprised of two to five levers depending on functions and attachments. This is because the levers are movement sensitive and require thorough practice to be able to operate them effectively.

  • The first lever is the lift circuit. It is responsible for activating the hydraulic cylinders and controlling the upward and downward movement of the mast which carries the payload. The ability to master the use of this lever lies in using slow movements to adjust the level of the forks.

  • The second lever is the tilt function which is responsible for angling the mast while moving the load. This function becomes useful and necessary while handling different loads smoothly. Push this lever forward to tilt the mast in the forward direction and pull the lever back to bring the mast back to the original position.

  • The third lever is most often designated for the side shift function. This allows approximately 12-16 inches of side-to-side adjustment for exact load placement. It comes in handy when trying to keep pallets close together.

  • If there is any additional levers, they are typically for the fork positioner circuit, but could be for a rotator, slip sheet, single double, etc.


Each forklift's working mechanism and controls should be understood and mastered for best use and to ensure the safety of the operator, workers and the load in the work area.


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