A rail-mounted stacker reclaimer is one of the main machines used in the coal stockyard of a thermal power plant. It performs two important functions: stacking coal into stockpiles and reclaiming coal from stockpiles.
The machine generally operates on rails installed along the stockyard and can serve two parallel stockpiles, with one stockpile on either side of the rail track. Depending on the stockyard arrangement, the machine works with a yard conveyor for receiving and transferring coal during stacking and reclaiming.
How Does a Bucket Wheel Stacker Reclaimer Machine Work?
A stacker reclaimer combines stacking and reclaiming functions in a single machine.
During stacking, coal coming through the yard conveyor is transferred through the tripper and intermediate conveyor to the boom conveyor. The boom conveyor carries the coal to the required location and discharges it onto the stockpile.
During reclaiming, the bucket wheel cuts into the stockpile and collects coal. The material collected by the buckets is transferred through the bucket wheel chute and boom conveyor to the central discharge arrangement and finally onto the yard conveyor.
The machine can move along the stockyard through its long-travel mechanism, while the boom can be raised or lowered through luffing and rotated horizontally through slewing.
Major Movements of Stacker Reclaimer
The main movements of a stacker reclaimer are:
1. Long Travel
Moves the complete machine along the rail track from one position to another.
2. Slewing
Rotates the revolving superstructure horizontally so that the boom can be positioned across the required stockyard area.
3. Luffing
Raises or lowers the boom to control its operating elevation.
4. Bucket Wheel Rotation
The bucket wheel rotates during reclaiming and penetrates the stockpile to extract coal.
5. Conveyor Operation
The boom and intermediate conveyors transport coal between the stockpile and yard conveyor.
The coordination of these movements allows the machine to build and reclaim the stockpile efficiently.
Modes of Operation of Bucket Wheel Stacker Reclaimer Machine
A stacker reclaimer mainly operates in two modes:
- Stacking
- Reclaiming
1. Stacking Operation
During stacking, coal is transported by the yard conveyor towards the stacker reclaimer. The tripper raises the yard conveyor belt and directs the material through a discharge guide chute onto the intermediate conveyor. The intermediate conveyor transfers the material to the boom conveyor. The boom conveyor carries the coal towards its discharge end, from where the coal falls onto the stockpile.
The stacking operation generally starts at one end of the stockpile with the boom in a relatively low position. The machine travels along the stockpile while coal is discharged from the boom conveyor. When the stacker reaches the opposite end, the travel direction is reversed. This reciprocating movement continues to build the stockpile.
As the stockpile height increases, the boom is raised through the luffing mechanism, and the stacking cycle is repeated. The exact stacking pattern depends on the stockpile design, required capacity and control philosophy of the particular machine.
2. Reclaiming Operation
During reclaiming, the bucket wheel removes coal from the stockpile in layers. The bucket wheel rotates and its buckets penetrate the stockpile, picking up coal. As the buckets rotate upward, the coal is retained and guided by the bucket wheel chute.When the bucket reaches the appropriate discharge position, the coal falls into the bucket wheel discharge chute and is transferred to the boom conveyor.
The boom conveyor transports the reclaimed coal towards the central discharge system. The material then passes through the central and portal chutes onto the yard conveyor.The entire stockpile can be reclaimed using different methods, including bench-type and modular reclaiming.
Bench-Type Reclaiming
In the bench method, the reclaimer starts at one end of the stockpile and works on the upper bench or layer. The machine reclaims the complete length of that bench. After reaching the other end of the stockpile, the machine returns to the opposite end. The boom is then lowered to the next bench and reclaiming starts again. This process continues progressively from the upper bench towards the lower benches.
Modular Reclaiming
In the modular method, reclaiming starts from the top of the stockpile at one end. The reclaimer takes a series of cuts, approximately one bench deep, and then returns to the other end. The boom is lowered to the next bench and another series of cuts is taken. The process is repeated from one bench to the next until the required reclaiming module is completed.
Components of Bucket Wheel Stacker Reclaimer Machine
1. Bucket Wheel and Drive
The bucket wheel is the main component responsible for reclaiming coal from the stockpile. Depending on the machine design, the bucket wheel drive may consist of a hydraulic drive or an electric motor with fluid coupling, brake and gearbox. The drive is mounted on a suitable drive frame supported by the machine structure.
In some designs, the gearbox output shaft is connected to the bucket wheel shaft through a keyless shrink-disc arrangement.
The bucket wheel must develop sufficient torque to penetrate the stockpile and extract coal under varying material conditions.
2. Tripper Conveyor
The tripper arrangement raises the yard conveyor belt and guides the coal through the discharge chute. During stacking, material from the yard conveyor is discharged through the guide chute onto the intermediate conveyor. After passing over the discharge pulley, the yard conveyor belt returns to its normal level through the belt-return arrangement. The tripper therefore provides the required material-transfer path between the yard conveyor and the stacker system.
3. Boom Conveyor
The boom conveyor transports coal between the material-transfer point and the stockpile during stacking, and between the bucket wheel and discharge system during reclaiming. The reversible boom conveyor drive is mounted on a drive frame and supported by the revolving structure. The boom conveyor includes belt tensioning and adjustment arrangements at the boom head. Important conveyor protection devices include:
Zero Speed Switch: A zero speed switch monitors pulley rotation or belt movement. If the belt speed falls below the preset operating limit, the switch can initiate a conveyor trip.
Belt Sway Switch: Belt sway switches are installed on both sides of the conveyor. If the belt moves excessively away from its normal alignment and operates the switch, the conveyor can be tripped to prevent belt and structural damage.
Pull Cord Switch: Pull cord switches are installed along the boom conveyor walkway. They allow the conveyor to be stopped manually during an emergency.
4. Intermediate Conveyor
The intermediate conveyor receives material from the tripper arrangement during stacking and transfers it towards the boom conveyor. Depending on the machine design, the material may pass through a vibratory feeder or belt feeder arrangement before entering the intermediate conveyor.
The intermediate conveyor forms an important part of the material-transfer path during stacking.
5. Boom Luffing Arrangement
The luffing arrangement raises and lowers the boom according to the required stacking or reclaiming position. In the arrangement described here, boom movement is achieved using a hydraulic system with two hydraulic cylinders. The operator cabin can also be maintained in a level position through a hydraulic levelling arrangement. A hydraulic power pack provides the required hydraulic power for luffing and other hydraulic functions such as chute operation, depending on the equipment design.
6. Slewing Arrangement
The revolving superstructure of the stacker reclaimer is supported on a large-diameter slewing bearing. An external ring gear attached to the revolving structure is driven through pinions connected to the slew drive units. The slew drive may consist of an electric motor, slip clutch, brake and planetary/bevel-helical gearbox arrangement, depending on the machine design. The slewing system allows the boom to move horizontally across the required operating range.
Slew Protection
Slip Clutch:T he slip clutch limits the torque transmitted to the slew mechanism and helps protect the drive and structural components from excessive loading.
Rotary Cam Limit Switch: The rotary cam limit switch controls the allowable slew range and interlocks the boom position with the yard conveyor. It can prevent the boom from slewing over the yard conveyor unless the boom has reached the required safe elevation. Protection settings should be maintained according to the manufacturer’s specified limits.
7. Long Travel Mechanism
The long-travel mechanism moves the stacker reclaimer along its rail track. It allows the machine to travel from one position to another along the stockpile. It also consists of :
Travel Limit Switch
Travel limit switches are operated by track-mounted strikers and prevent the machine from travelling beyond its permitted operating position. They can initiate stopping and allow the appropriate reversal sequence.
Automatic Rail Clamp
Automatic rail clamps help prevent unintended movement of the machine, particularly during strong wind or storm conditions. The clamps are released through the hydraulic system when travel operation is initiated and the required permissive conditions are satisfied.
Dead Stock: Why Material Remains After Reclaiming
A bucket wheel reclaimer machine cannot always recover 100% of the material stored in a stockpile. Some coal may remain in areas that are outside the effective digging envelope of the bucket wheel. This residual material is commonly known as dead stock or inaccessible coal.
Where Does Dead Stock Occur?
Dead stock can mainly occur due to:
- Bucket-wheel geometry – the bucket wheel has a defined cutting and digging envelope.
- Boom reach – material beyond the effective reach of the boom cannot be directly recovered.
- Stockpile cross-section – the shape and slope of the stockpile influence bucket-wheel accessibility.
- Machine rail position – the location of the rail track determines the reclaimer’s working geometry.
- Structural clearance – the machine cannot be operated beyond its designed mechanical clearances.
- Stockpile base geometry – material near the base or lower portions of the stockpile may remain outside the bucket-wheel cutting path.
Therefore, even when the main reclaiming operation is completed, a certain quantity of coal may remain in inaccessible zones.
How Is Dead Stock Recovered?
Where recovery of the remaining coal is required, additional equipment or a different operating method may be used, such as:
- Bulldozer – pushes residual coal into an accessible reclaiming area.
- Wheel loader – collects coal from areas that cannot be reached by the bucket wheel.
- Auxiliary reclaiming equipment – may be used depending on stockyard design.
- Stockpile geometry optimization – proper stacking and stockpile shaping can reduce inaccessible areas.
The amount of dead stock is not a fixed percentage. It depends on the Stacker Reclaimer design, rail position, boom geometry, stockpile dimensions and operating method.
Safety Precautions for Stacker Reclaimer Operation During High-Wind Conditions
A Stacker Reclaimer has a large exposed structure and a long boom, making wind an important operating consideration. High winds can create additional forces on the machine and may cause unintended movement if the machine is not properly secured.
Key Safety Precautions
1. Follow Defined Wind Limits: Always follow the wind limits specified by the OEM and site operating procedure. Wind limits should not be generalized across different machines.
2. Monitor Wind Conditions: Continuously monitor wind conditions where a wind monitoring system is provided. Take action according to the approved operating procedure when the specified limit is approached.
3. Move to the Designated Safe Position: When high winds approach the permitted operating limit, position the machine and boom as specified by the OEM/site procedure.
4. Secure the Machine: Use the designated rail clamps or anti-drift system where provided to prevent unintended machine movement during strong winds.
5. Restrict Travel and Machine Movements: Avoid unnecessary travel, slewing or luffing during severe wind conditions. Travel limit switches provide an additional layer of protection against excessive travel.
6. Restrict Personnel Access: Keep personnel away from exposed areas of the machine, travel path and areas below elevated structures during severe weather.
7. Inspect Before Restarting: After a high-wind event, check the rails, wheels, travel system, rail clamps, boom, structural components and safety devices before returning the machine to operation.
8. Never Bypass Safety Devices: Wind-related alarms, interlocks, limit switches and other protective systems should never be bypassed to continue operation.
Key Point
High-wind operation requires control of wind exposure, machine position, boom configuration, travel movement and securing systems. The exact wind limits and storm-position requirements must always be based on the specific machine’s design and operating procedure.
Preventive Maintenance Of Stacker Reclaimer Involves Following Works:
1.Checking all hydraulic hoses for any leakages.
2. Checking luffing hydraulic cylinders and tripper hydraulic cylinders for any leakage.
3. Checking all power pack and its hydraulic components for any leakages.
4. Checking for cleanliness of all components of stacker reclaimer including the hydraulic power pack.
5. Checking hydraulic oil tank for sealing and leakages.
6. checking all oil filters (Suction line, pressure line and return line) and if required replace.
7. Checking oil cooler.
8. Checking the luffing and slew hydraulic oil level in power pack tank.
9. Checking all hydraulic valves cleanness.
10. Checking the hydraulic oil condition by visualize.
11. Checking Slew and bucket drive hydraulic motor and its cleanness.
12. Checking rail clamp and mounting fasteners.
13. Checking auto lubrication system and its working condition for slew bearings and long travel wheel bearings.
14. Checking all boxes of stacker reclaimer for oil level, cleanness of breather.
15. Checking all couplings for its smooth working.
16. Checking conveyor belt joints condition and its top and bottom rubber condition.
17. Checking all idlers, pulleys, skirt rubber, scrapers, pulley bearing and coffin box condition.
Frequently Ask Questions (FAQs) on Bucket Wheel Stacker Reclaimer Machine
Stacking builds stockpiles. Reclaiming extracts material and feeds it back to conveyors.
They automate coal storage and recovery, ensuring steady boiler fuel supply.
It reduces equipment needs, saves yard space, and improves coal blending.
The major components include the conveyor system (boom conveyor, intermediate conveyor, belt feeder), luff mechanism, slew mechanism, travel drive, and bucket wheel.
In modular type reclaiming, coal is removed in small repeated blocks from top to bottom from one end. In bench type reclaiming, coal is taken layer by layer from the pile.
The slew angle is the range through which the boom can rotate left and right. In most designs, it is around ±110°, but the exact value may change as per machine design and yard layout.
The luffing angle is the range through which the boom can move up and down. In most designs, it is approximately between ±6° to ±12°, though the exact value may change as per machine design.
To prevent derailment in high wind, stacker reclaimers use storm brakes, rail clamps, and anchoring devices. These keep the machine fixed on rails and stable against wind pressure.
There is no single global value, as limits depend on machine design and local standards. As per Indian Standard IS 14416/17, an alarm is given at 20 m/s (≈72 km/h) and all motions must stop at 25 m/s (≈90 km/h). Other regions follow their own wind codes, but most machines are designed for safe operation below 20 m/s and must be secured beyond this limit.
Stacker reclaimers use pile collision sensors, boom limit switches, ultrasonic or laser detectors, and anti-collision systems. These devices stop or slow the boom when it comes too close to the heap, preventing impact damage.
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