Forklift Structure and Working Principle
This topic will discuss the working principle of forklifts. Based on the "Forklift Safety Manual" published by Taylor, a heavy-duty forklift manufacturer in the United States:
Most forklifts are counterbalanced forklifts, like a seesaw, so the center of gravity of the load must first be found. This center of gravity is called the load center, and its length is half the length of the pallet. For example, if the pallet dimensions are length (d) 1000mm and width (w) 1200mm, the load center is 500mm.
The load center of a forklift is often 500mm or 600mm, so to know the standard load center of a forklift, you must find it in the forklift's specification sheet or forklift drawing. Forklift tonnage refers to the maximum load value of the forklift for loading, unloading, and handling goods, designed based on the structural strength of each part, the pressure of the hydraulic system, and stability. The stability of a counterbalanced forklift is simply based on the lever principle (seesaw). Under critical conditions, when a small force is applied to the load side, the forklift will rotate forward. In vehicle design, a safety factor of 1.4 is typically set to ensure safe operation of forklifts. Load curves show the relationship between allowable load and load center; operating within the range of values on the load curve can prevent accidents. The load center directly affects the load weight. A load center shorter than the standard load center does not affect the load weight, but a load center longer than the standard load center reduces the load weight.
For example, if the load center of a load is 400mm, but the standard load center is 500mm, the load weight will not be affected. However, if the load center is 600mm and the standard load center is 500mm, the load weight will decrease. Engine-powered forklifts, also known as internal combustion forklifts, use diesel, gasoline, or liquefied petroleum gas as fuel and are powered by an engine. Their load capacity ranges from 0.5 tons to 45 tons. They are generally classified as counterbalanced internal combustion forklifts, container forklifts (front-loading), and side-loading forklifts. Forklifts are generally classified into three types: counterbalance forklifts, container forklifts (front-loading), and side-loading forklifts. These use diesel engines as power sources and have a load capacity of 3.0 to 6.0 tons. The forks are mounted on the sides of the forklift, allowing for direct side-loading of goods, making them primarily used for lifting long, thin items such as wooden sticks and steel bars.
While there are many types of forklifts, they are all basically composed of four main parts: the power unit, the chassis, the working parts, and the electrical system. Differences in the structure and installation location of these four parts result in different types of forklifts.
Counterbalance forklifts are the most common type of forklift. We will now discuss the composition of each part using this type of forklift as an example.
01 Power Unit
The power unit of a forklift supplies power to the forklift's working parts for loading and unloading goods and for the operation of the wheeled chassis. It is generally installed at the rear of the forklift and also serves as a counterweight.
Electric forklifts use batteries and DC series motors. Their drive characteristics are closest to the requirements of constant power soft characteristics, and their traction performance is superior to that of internal combustion engines. In addition, it operates smoothly and quietly, emits no exhaust fumes, is easy to maintain, and is simple to operate; operating costs are low, and the overall service life of the vehicle is long. Disadvantages include: the need for charging equipment, high initial investment, long charging time (generally 7-8 hours, fast charging 2-3 hours), short continuous working time after a single charge, and the battery's susceptibility to shock and vibration, requiring specific road surfaces. Due to battery capacity limitations, the electric motor power is low, resulting in lower vehicle speed and climbing ability. Therefore, battery-powered forklifts are mainly used in warehouses and workshops with narrow aisles, short transport distances, good road surfaces, small lifting capacities, and where high speed is not required. Only battery-powered forklifts can be used in flammable goods warehouses or places requiring clean air. Battery-powered forklifts are also used in cold storage warehouses where internal combustion engines are difficult to start.
The mechanical characteristics of internal combustion engines do not meet the requirements for the constant power soft characteristics of forklift prime movers; their output power increases with increasing speed. Therefore, internal combustion engines must be equipped with mechanical transmissions, torque converters, or hydraulic transmission devices to increase output torque before they can be used. Internal combustion forklifts, unlike battery-powered forklifts, have the following main advantages: no need for charging equipment, longer operating time, higher power, stronger climbing ability, lower road surface requirements, and lower initial investment. With a suitable transmission method, they can achieve ideal traction performance. Disadvantages include: noise and vibration during operation, exhaust fumes, more frequent maintenance, higher operating costs, and a shorter overall lifespan. Therefore, internal combustion forklifts are generally superior. For forklifts with a lifting capacity of medium to high, internal combustion forklifts are preferred.
Among internal combustion forklifts, diesel engines are the most common, with almost all forklifts with a lifting capacity of 3 tons or more using diesel engines. This is because diesel engines consume less fuel. However, diesel engines are relatively heavy, noisy, and vibrate more. Forklifts with smaller lifting capacities can use gasoline engines, which are smaller and lighter, but consume more fuel; gasoline is expensive, and its exhaust contains more harmful components and is flammable. In some countries, there are also forklifts using liquefied petroleum gas (LPG) engines, which have lower fuel prices and produce less exhaust fumes.
In recent years, the use of liquefied petroleum gas (LPG) engines as power units in internal combustion forklifts has been increasing both domestically and internationally. Many are dual-fuel forklifts, capable of using gasoline, diesel, or LPG as fuel. Germany has seen a 160% annual growth rate in LPG forklift usage, and the number is also rising in the US and Japan. Currently, there is a growing outcry against vehicle exhaust pollution. Therefore, the use of LPG engines is becoming more widespread in industrial vehicles driven by internal combustion engines, including forklifts. This is because using LPG engines not only avoids air pollution and reduces environmental hazards but also reduces engine wear and extends engine life. It also lowers fuel costs.
02 Chassis
The chassis is the direct working mechanism that bears the entire weight of the goods and performs tasks such as picking up, lifting, and stacking. It consists of the working device that directly performs loading and unloading operations and the hydraulic transmission system that operates the working device. Based on design, manufacturing, and varying working conditions, it comes in various structural forms. The forks are the fork-shaped components that directly bear the goods. They are mounted on the fork carriage via hooks, and the distance between the two forks can be adjusted according to operational needs and locked by a positioning device.
The fork carriage is a structural component welded from steel plates, featuring a roller assembly. The inner mast has vertically oriented grooved tracks on its inner side. The fork carriage is connected to the inner mast in the same way, and can only move vertically along the tracks of the outer mast.
The inner mast is a frame structure welded together from two grooved sections serving as columns and beams. Its lower part connects to the forklift's drive axle (front axle). With the aid of a tilting hydraulic cylinder, the mast can tilt at a certain angle in the forward and backward directions. Forward tilting facilitates loading and unloading, while backward tilting prevents the goods on the forks from slipping off when the forklift is moving.
The lower end of the lifting hydraulic cylinder is on the outer mast crossbeam, and the upper end connects to the inner mast crossbeam and the sprocket. One end of the lifting chain is connected to the lower part of the outer mast, and the other end passes over the sprocket and connects to the fork carriage. When pressurized oil is supplied to the hydraulic cylinder, the piston rod moves upward at a speed v, driving the sprocket and inner mast to rise at the same speed v. Due to the principle of the movable pulley, the chain pulls the fork carriage to rise at a speed of 2v. When the hydraulic cylinder reaches the end of its full stroke, the inner mast is in the extreme position above the outer mast, and the fork carriage is in the extreme position above the inner mast. When the oil pressure is released, the goods or forks and other components descend under their own weight.
The steering system is used to make the forklift move in the direction determined by the driver. Forklift steering systems can be divided into two types according to the energy required for steering: mechanical steering systems and power steering systems. The former uses the driver's physical energy as the steering energy and consists of three parts: the steering gear, the steering transmission mechanism, and the operating mechanism. The latter is a steering device that uses both the driver's physical energy and engine power as steering energy. Under normal circumstances, only a small portion of the energy required for forklift steering is provided by the driver; most of it is provided by the engine through the steering assist device. However, even if the steering assist device fails, the driver should generally still be able to independently handle the steering task. During forklift operation, steering and travel are frequently alternating; to reduce the driver's workload, internal combustion forklifts often employ power steering systems. Commonly used power steering systems include integral power steering, semi-integral power steering, and steering assist devices. The braking system is the system that slows down or stops the forklift. It consists of brakes and a brake transmission mechanism.
Braking systems can be classified into three types according to the braking energy source: manual braking systems, power braking systems, and servo braking systems. The former uses the driver's physical energy as the braking energy; power braking systems rely entirely on the potential energy in the form of air pressure or hydraulic pressure converted from engine power; the latter is a combination of the former two.
The composition, function, and working principle of the forklift chassis and other parts are very similar to those of automobiles. Therefore, due to space limitations, content identical to that of automobiles will not be elaborated upon, while content different from that of automobiles will be introduced.
On a counterbalance forklift, a counterweight is located at the rear to balance the weight of the goods at the front. The forklift's power unit (internal combustion engine) or battery is generally located at the rear to provide partial balance.
03 Working Section
The working section of the forklift directly bears the entire weight of the goods and performs tasks such as picking, lifting, and stacking. It consists of the working devices that directly perform loading and unloading operations and a hydraulic transmission system that operates the working devices. Due to design, manufacturing, and different working conditions, it has various structural forms.
The forks are the fork-shaped components that directly bear the goods. They are mounted on the fork carriage via hooks. The distance between the two forks can be adjusted according to operational needs and is locked by a positioning device.
The fork carriage is a structural component welded from steel plates, featuring a roller assembly. The inner mast has vertically oriented grooved tracks on its inner side. The fork carriage is connected to the inner mast in the same way and can only move vertically along the tracks of the outer mast.
The inner mast is a frame structure welded together from two grooved sections serving as columns and beams. Its lower part connects to the forklift's drive axle (front axle). With the help of a tilting hydraulic cylinder, the mast can tilt at a certain angle in both forward and backward directions. Tilting the mast forward facilitates loading and unloading, while tilting it backward prevents goods on the forks from slipping off when the forklift is moving.
The lower end of the lifting hydraulic cylinder is on the outer mast crossbeam, and the upper end is connected to the inner mast crossbeam and the sprocket. One end of the lifting chain is connected to the lower part of the outer mast, and the other end passes over the sprocket and connects to the fork carriage. When pressurized oil is supplied to the hydraulic cylinder, the piston rod moves upward at a speed v, driving the sprocket and inner mast to rise at the same speed v. Due to the principle of movable pulleys, the chain pulls the fork carriage to rise at a speed of 2v. When the hydraulic cylinder reaches its full stroke, the inner mast is at the upper extreme position above the outer mast, and the fork carriage is at the upper extreme position above the inner mast. When the oil pressure is released, the goods or forks and other components descend under their own weight.
