Forklift Trucks
Forklift Trucks
Forklift trucks are material handling equipments. They are the most commonly used industrial lift trucks. They are the workhorses of material handling because of their flexibility which include (i) indoor / outdoor operation over a variety of different surfaces, (ii) availability with a variety of load capacities, and (iii) availability with a variety of attachments.
A forklift truck is a type of powered industrial truck. Like other powered industrial trucks, its purpose is to move, carry, push, pull, and lift a material load and then stack it or place it in a storage rack (tier). Forklift trucks come in many sizes and capacities. They can be powered by batteries, or internal combustion engines working with diesel or gaseous fuel. Some are designed to be used in a hazardous location or environment where an ordinary forklift can cause a fire or explosion.
Forklift truck is a powerful tool which allows one person to precisely lift and place large heavy loads with little effort. It is defined as a mobile, power-driven vehicle used to carry, push, pull, lift, stack, or tier material. It is also being called as powered industrial lift truck, forklift, fork hoist, and fork truck. It is widely used throughout steel industry and other industries for moving materials and goods.
A forklift truck is a powered industrial lift truck equipped with lifting media made up of a mast and elevating load carriage. It is a vehicle drawn or propelled by mechanical, or electrical power, designed incorporating a powered lift principally to lift, carry or stack goods by means of (i) a fork consisting of one or more arms which support the load, (ii) a platform, or (iii) any attachment or other mechanism. Forklift truck consists of variety of components (Fig 1) needed to handle the task effectively. These components ranges from truck frame, power source to counterweight and are essential for the forklift to function efficiently. Some of these components are given below.
Truck frame – It is the most important component which forms the base of the machine. All of the key components of the forklift including wheels, counterweight and mast are attached to the truck frame.
Counterweight – The counterweight is a cast iron weight attached to the rear part of the forklift. The aim of the counterweight is to counter balance the load that is being lifted. On electric forklift, counterweight is fixed to the lead-acid battery.
Power source – The forklift’s power source of the forklift consists of an internal combustion engine. The engine can be fueled by diesel, or gas (such as LPG, CNG, or natural gas). Electric forklifts derive power from fuel cells or lead acid batteries.
Carriage – The carriage serves as the base to the forklift. The carriage is fixed on mast rails so that it can be easily moved upward and downward.
Mast – The mast is a vertical part that lifts up and pushes down the loads. The mast component consists of interlocking rails which offers horizontal control. Like carriage, mast may be equipped with rollers.
Hydraulic components – These are hydraulic reservoir (tank), gear pump (creates flow / pressure), control valves (To direct flow), control valve levers (direct manual), electronic controls (joystick, fingertip controls), connectors (steel tubing / hosing / fittings, and cylinders (single acting / double acting). Different cylinders are lift cylinders, tilt cylinders, steer cylinders and side shift cylinders
Fig 1 Main components of forklift trucks
Without limiting the definition of the forklift truck, it includes forklifts, order pickers, side loaders, reach trucks, pallet trucks, platform trucks, straddle trucks and lateral stacking trucks. Design and construction of the forklift truck is to be in compliance with the national standards and is to meet various safety norms for lifting equipment and moving vehicles.
Forklifts work on the principle of a see-saw. There is a pivot point (drive axle centre) and counterweight at one end to offset the load weight. In order for see-saw principle to work for a lift truck, the load of the forks is to be balanced by the weight of the lift truck. Too high weight causes it to tip over. Also if the weight is moved further out from the pivot or fulcrum point it also causes tip over. A properly loaded lift truck does not exceed the rated capacity of the truck (as listed on the truck’s data plate.
Stability of the forklift truck is the result of many factors which include wheelbase, overall width at the front axle, weight distribution, and lifting height. Stability refers to longitudinal and lateral stability, the stability triangle, and dynamic versus static stability. In the three points (A,B,C) of the stability triangle, point A is the centre pivot point of the rear steer axle, while points B and C forms a straight line between centre line of each drive axle.
Forklift trucks belong to the family of the powered industrial vehicles / trucks and are the most versatile, useful and widely used equipment. These are self loading, counterbalanced, powered, wheeled vehicles, with the operator seating on the vehicle, designed to raise, move and lower load on forks or other attachments fastened to a mast which is fixed in front of the vehicle to allow lifting and stacking of loads. They are used for lifting, lowering, stacking, unstacking, loading and unloading and maneuvering of medium to large weight, uniform shaped unit loads, intermittently.
Forklift trucks are classified into seven types based on their characteristics. The class 1 forklift trucks are with electric motor, rider, and counter-balanced trucks having solid or pneumatic tyres. The class 2 forklift trucks are with electric motor and they are narrow aisle trucks with solid tyres. The class 3 forklift trucks are electric motor hand trucks or hand / rider trucks with solid tyres. The class 4 forklift trucks are internal combustion engine trucks with solid tyres. The class 5 forklift trucks are internal combustion engine trucks with pneumatic tyres. The class 6 forklift trucks are with electric or internal combustion engine. There are no forklifts in this class. They are tractors with solid or pneumatic tyres. The class 7 forklift trucks are rough terrain forklift trucks with pneumatic tyres.
The important characteristics of forklift trucks are (i) normally needs pallet /skid / container, (ii) requires skilled operator, (iii) equipment needs maintenance facility, (iv) capacity of these equipment vary from 0.5 ton upto around 60 tons, (v) slow travel speed ranging from 10 kilometers per hour (kmph) to 15 kmph, and (vi) suitable for short hauls normally hundreds of meters. Other features of a forklift truck are given below.
- The source of power is diesel or low pressure gas engine or a battery driven motor
- The mast can be tilted forward or backward within a range, for better stability during movement with load and also to facilitate loading and unloading. In a particular design the mast can be moved outboard and inboard on tracks which are laid over the chassis of the truck.
- The mast can be a single mast or can be telescoping in design which allows high lifting of the load for forklift trucks. These trucks are required to run through limited head room areas.
- In certain designs of the forklift trucks, the forks are independently retractable outboard and inboard through pantograph mechanism. Loads are picked up and placed while forks are outboard but are moved inboard for greater stability during movement.
- The operation of the mast and movement of the forks (or any other attachment) are through a hydraulic power pack.
- The body of the forklift truck is purposely built heavy which act as counter load while lifting loads on forks/attachments.
- Forklift trucks are normally provided with solid rubber tyres so as to facilitate operation in different floor conditions. The rear two wheels are steered for manipulation of the forks / attachment fixed in front of the truck.
The weight of the vehicle (and operator) behind the front wheels of the truck counterbalances the weight of the load which is the weight of the vehicle beyond the front wheels. The front wheels of the truck act as a fulcrum or pivot point.
Wheel loadings / weight distribution can be very important in applications where there are floor loading limitations such as elevators / multiple floor buildings / trailers. Empty forklifts have most of their weight in the rear of the truck when unloaded. A forklift undergoes the greatest change in weight distribution when a load is placed on the forks. As much as 90 % of the weight is shifted to the drive axle when the truck is fully loaded.
A forklift truck can have as many as five degrees of freedom (DOF) in its movement (Fig 2). These are (i) horizontal translation (drive wheels), horizontal rotation (turning wheels), (iii) vertical lift (forks along mast), (iv) mast tilt (forward tilt for loading/unloading and backward tilt for travel), and (v) fork translation (to handle different size loads).
Fig 2 Degree of freedom in forklift truck
Rated load capacity – Forklift trucks are available in capacities ranging from 0.5 tons upto 60 tons. The rated load capacity of the forklift truck is specified assuming a rated load centre of 500 mm from the face of the forks of the truck and a lift height of upto 4 metres. Attachments and loads with a centre of gravity higher than 500 mm reduce the load capacity of the truck.
Load capacity – The load capacity of the forklift truck is a measure to indicate the maximum weight load which can be handled as a ‘load’ on the forks at a given ‘load centre’ with the mast held in a vertical position. The load capacity rating is expressed in kilograms at the load centre in millimeters.
The approximate allowable load capacity W of a forklift truck given by w=G x B / (F + L) = M / (F + L) = Wo (F + Lo) / (F + L). Here G is the weight of the empty forklift truck, B is the front axle to truck load centre distance, F is front axle to fork face distance, L is actual load centre which the distance from fork face to load centre of gravity, Lo is the rated load centre, M is the load moment, and Wo is the rated load capacity. L = ULD/2 if load is of constant density and of rectangular shape. Here ULD is the unit load depth. Although it seems preferable (in light of the above equation) to specify a load moment (M) of the forklift truck, it is Wo which is normally specified instead because it is easier to relate to the maximum allowable load weight of the forklift truck.
Two different adjustments can be made with respect to the load capacity of the truck. These are (i) derating (W is less than Wo) which determines the maximum permissible load weight given that the centre of gravity of the load, L, exceeds the rated load centre, Lo, (ii) rerating (new Wo and L = Lo) which modifies the rated load capacity of the forklift truck due to changes to the truck such as adding extra counterbalance weight, a fork attachment, or an extended mast.
Power source – Power source determines where truck can operate (inside / outside) and the gradability of the truck (i.e., how steep an incline the truck can climb). Power source can be internal combustion engine or battery. Internal combustion engine powered forklift truck has gradability of 15 %. The engine fuel can be diesel or gas. The battery powered forklift truck has electric motor and gradability of 10 %. The truck is used for indoor operation and requires battery recharge facilities.
Tyres – Tyres determine in what type of terrain the truck can operate over. Pneumatic tyres are used for outdoor service (rough terrain). Cushion tyres are used both for indoor and outdoor service (smooth terrain) while solid tyres are used for indoor service.
Attachments – Attachments determine the functionality of the forklift truck. Forks of a forklift truck are one of the most common attachments. A pair of forks is used for working with skids, pallets, containers and box shaped loads resting on legs / packers. However, a wide variety of devices called attachments have been designed for attaching to forklift trucks to make them useful for many different tasks.
There are two common attachments used. The first is the mask which is the attachment to the mast of the forklift truck. The mast has shelf guides for example, masking tape on the mast to mark the height of shelves. The second is the carriage which is the attachment to the carriage which moves on the mast. The carriage consists of (i) side shifter which shifts the forks without repositioning the truck, (ii) fork positioner (spreader) by which each fork can move independently (aids load stabilization when a variety of pallet sizes are used), (iii) rotators which enable dumping of a load when used with barrel clamps, and (iv) push / pull which grips / pushes / pulls slip sheet (used together with blades).
The attachment to the forks can be either an enhancement or replacement for the forks. Boom is the attachment which is fixed with respect to the fork carrier. At the end of the boom, a chain pulley block is provided for lifting loads using the hook and slings. The common types of enhancement attachments include (i) blades (platens) which are used together with push / pull for slip sheet handling, (ii) length which can be shorten, lengthen, or extendable. The replacement attachment are (i) blades for slip sheets which can replace forks for slip sheet handling, (ii) clamps used for carton, bale, roll, or barrel, (iii) ram used for coils, (iv) shovel, (v) block forks, more than two forks are used for handling non-palletized loads (e.g., bricks), (vi) trailer hitch , (vii) radio / radio frequency data modem used for communication with the truck operator, (viii) extra counter balance used to increase the load capacity of the truck, and (ix) TV which is mounted between the forks.
Specifications of forklift truck
There are different operating parameters or specifications based on which suitability of a forklift truck for a particular application is determined. The major specifications from operational point of view include (i) rated capacity at specified load centre, (ii) power sources (diesel, gas, and battery etc.) (iii), turning radius, (iv) physical dimensions (length, width, and height), (v) mast height, (vi) lift height, (vii) mast specification (single or telescoping, tilting or non-tilting, and retractable or not), (viii) travel speed, (ix) lifting speed., (x) floor clearance, (xi) free lift (movement of fork without mast movement), (xii) retractable fork or not, (xiii) fork size (length, width, maximum gap between forks etc.), and (xiv) attachments provided. Other important technical specifications are (i) motive power (kilowatt rating), (ii) power transmission system (disc clutch, fluid coupling etc.), (iii) tyre specifications, and (iv) battery and charger specification etc.
The specification of mast includes (i) overall lowered height, which is the distance measured from the floor to the top of the mast when it is fully lowered and vertical and it is critical for entering low clearance areas and doorways, (ii) maximum fork height, which is the distance measured from the floor to the top of the forks in the full raised position and is critical to achieve the desired height to put-away a load, (iii) free lift, which is the distance the forks can be raised without the overall lowered height of the mast increasing and is critical when working inside of a trailer or other low clearance areas, and (iv) ratio of overall lowered height to maximum fork height which is normally the same, but can vary based upon mast overlap / retention.
Capacity rating of the forklift truck
Forklift trucks are specified for a rated capacity at a specified load centre. Load centre is the distance from the heel (vertical face) of the forks to the assumed centre of gravity of the load. However, if the actual centre of gravity of the load goes beyond the specified load centre, the loading capacity of the forklift truck is to be reduced accordingly, so that the moment of the load about the front wheel does not exceed that of the counter-loaded body of truck, and the rear wheels do not loose contact with ground. The stability of a fork lift truck at the rated capacity is of paramount importance from the point of view of safe operation.
Turning radius and aisle width
A forklift truck can move freely through an aisle having its width at least 300 mm more than the maximum width of the load or the forklift truck, whichever is higher. However, if the forklift truck has to work across the length of an aisle, the minimum aisle width requirement can be determined from the equation A = TR + L + X + C where A is the width of the aisle, TR is the minimum turning radius, L is the length of weight resting on fork, X is the distance between centre line of drive (front) wheel to face of fork, and C is the clearance (can be 150 mm).
Clearances
The under clearances of the forklift truck frame, mast, drive, and steering axles can be significant if there are surface irregularities or obstacles. Not all under clearances are indicated on specification sheets. The mast clearance is normally the lowest point and, hence, most often listed. In backing up over bumps or obstacles, the rear (steer) axle clearance is important. Pneumatic tyre trucks have higher under clearances since they are intended for poor surface conditions.
The operator is required to know the forklift truck’s grade angle capability for operating on ramps or over dock boards and dock plates. The incline (in terms of percent) on which the centre of the frame under clearance just touches when the truck is going down from the level to the ramp or when the forklift truck is going down from the level to the ramp or when the forklift truck is going up from the ramp to the level is important. And the so called departure angle (from the wheel arc to the truck edge) of the counterweight is also essential.
Batteries for forklift truck
Engine driven trucks are comparatively cheaper than battery operated truck. Moreover, engine driven trucks can be used almost without the need of electric power. Despite all these, the number of battery forklift trucks continues to increase, particularly for capacities upto 3 tons due to factors like overall lower maintenance cost and lack of smoke, fume and noise. Battery operated forklift trucks are particularly suitable for stores and operations in confined areas. However, pre-requisite to using battery-operated forklift trucks is availability of electric power and battery charging facility.
Types of batteries which can be normally used are either of lead acid or nickel alkaline type of sufficient ampere-hour capacity to ensure at least one full day’s operation. Advantages of lead-acid battery are lower cost, greater energy (kWh) capacity in a given space, and lower internal resistance. Benefits of nickel-alkaline battery are longer life, better mechanical strength, non-corrosive electrolyte (KOH) which does not freeze, shorter recharge period (around 7 hours) and non-critical control of the charging current. The battery voltage has largely been standardized by forklift truck manufacturers.
It is necessary that sufficient battery capacity is provided to allow uninterrupted operation of the forklift truck during normal operation period. Capacity of a battery is rated in ampere-hour for a six hours discharge period. This rating, divided by six, gives the current draw in amperes, which, if continued six hours, completely discharges the battery. The average voltage multiplied by the ampere hours rating gives the total energy capacity of the battery in watt-hour. The battery sizes for forklift trucks of various capacities have been standardized by the manufacturers. The selection of the battery is based on energy rating for a proposed duty cycle of the forklift truck within a given period between two battery changes.
Charging of used up batteries is an essential facility for battery operated forklift trucks. Industrial batteries used in forklift trucks are intended to be recharged around 300 times per year or once in 24 hours on an average. More frequent recharges normally reduce the overall life of the batteries.
Safety aspects
Stability represents a key variable in the design of safe counterbalance forklift truck logistics. Forklift speed represents a major occupational hazard. Relevant principles of traffic safety and information ergonomics are obviously applied to the areas of logistics and transport management design. The insights and developments within the new field of Intelligent Transport Systems (ITS) and its possible applications into industrial environments can improve forklift truck management. Electronic proximity control, with designated vehicles, speed zoning, alerts on blind corners and recognition of pedestrian workers, reduce hazards and avoid collisions on industrial sites, but also generate further benefits from improved logistics, i.e. control of vehicle use, vehicle users and cargo movement.
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