A conveyor belt is a system that carries material from one point to another. A conveyor system or material handling system consists of two key components; a loop of material known as the belt and two two pulleys fixed at either end. These pulleys work in a continuous circular motion to pick up material at one end of the belt and drop off material at the other end of the belt.
Since introduced in the early 20th century, conveyors have become a vital part of mine operations. Conveyor systems can be used to transport material in underground and open pit operations, and are especially common in coal mines (Betz, 1998). The simplicity and increased reliability of conveyor systems make it an attractive alternative to conventional truck haulage. While the capital cost associated with installing conveyor systems are similar to those of haul trucks, significant savings can be achieved from lower operating costs (SME, 2011).
Brief History on Conveyor Systems
Conveyor systems were first invented in the late 1800’s (Product Handling Concepts, 2014). These systems were constructed using wood and leather. In 1905, the first mining related material handling systems was implemented in a coal mine. Revolutionizing the industry, costs were cut drastically by quickly and efficiently moving coal from shaft to truck (Product Handling Concepts, 2014). As conveyors systems gained popularity, technological advancements occurred accordingly. As such, material costs associated with conveyor systems dropped. By 1908, ball bearing and steel belts were introduced, smoothing transportation of material.
The most famous application of conveyor systems was by Henny Ford in 1913. He invented the first assembly line for his Model T vehicle. Parts and tools were transported to workers using the assembly. This system drastically reduced the time and cost of producing a vehicle.
Today, conveyor systems are common in many industries including; automotive, agricultural, computer, electronic, food, process, automotive, aerospace, pharmaceutical, chemical, bottling, canning, print finishing, mining and packaging industries (Kaboli, 2016). Many industries use a combination of different types of conveyor systems in order to mass manufacture their products. As mentioned earlier, modern day conveyor systems are an integral part of ore and waste transportation in the majority of mining operations. New applications include transport of material along mine shafts, handling material up steep walls of open pits, moving material to stockpiles kilometers away, and loading material onto transport ships. Technology now allows conveyors to easily span lengths of 20 km – 30 km, with the longest system reaching 100 km (SME, 2011) .
Types of Conveyor System
Conventional Mining Conveyor
The conventional conveyor is the most common conveyor system, uses two or more pulley systems with a belt that rotates about them, carrying medium. The two pulleys can be powered for certain requirements in the conveyor system. The conventional conveyor belt is ‘v’ shaped to better hold the material. Currently, most long conveyors use steel-cord belts. Sorter conveyors often use polyester/nylon fabric known as EP fabric (SME, 2011). Different variations of the belt conveyor have been conceived to deal with challenges unique to certain environments. These variations common to the mining industry include the gravity conveyor, cable belt, pipe conveyor, and sandwich conveyor/apron conveyor.
Similar to the conventional mining conveyor, the gravity conveyor is a two pulley system. However, gravity conveyors travel on a decline benefiting from potential energy. This belt turns a motor within a generator storing electricity as ore is transported. Depending on the height and steepness of the belt slope, the value of the energy produced is often greater than the operating cost of the conveyor. This allows the gravity conveyor system to operate at a profit (SME, 2011).
The cable belt is a conveyor system where the belt is supported between two steel cables on either side. In this variation, the cables absorbs the driving forces of the belt. These are not as common as the conventional steel-cord belts, as capital and operating costs are higher (SME, 2011). These conveyor systems can be covered to reduce outside contamination as shown in the picture to the left (Overland Conveyor Company, 2016).
In the pipe conveyor, the belt is converted into a cylindrical shape in order to fully enclose the material being transported. Benefits of this are the following (SME, 2011):
- Completely eliminates spillage and is more environmentally friendly
- Allows for sharper vertical and horizontal bends because there is no risk of spillage
- Can take a more direct path when navigating irregular terrain reducing civil works
- Can climb slopes 50% greater than trough belts
- The belt can be looped and the underside can be set up to transport material in the opposite direction
The sandwich conveyor is useful when navigating steep slopes. A second belt rides on top of the material in steep sections to hold material in place. This is often assisted by what’s called inverted pressing idlers (SME, 2011).
Components in a Conveyor System
Head and Tail Pulley
There are several components that makeup a conveyor system. The head pulley, tail pulley and belt are the essential components of a conveyor system. The head pulley, located at the discharge point, powers the rotation of the belt and is usually fixed. The tail pulley, located at the feed chute is movable. However, the tail pulley is usually held in place for a long period of time.
The tail pulley is fixed by a counter weight and pulley system, or in situations where space is limited (underground operations) by a piston or winch. Using this variable system, the position of the tail pulley is adjusted to ensure tension and slack of the belt compensate for changing conditions.
The conveyor belt carries material between the head pulley and tail pulley. The conveyor belt is composed of two layers of material; the under layer and the over layer. The under layer provides linear strength and shape, it is composed of woven fabrics that have warp and weft such as polyester, nylon or cotton. The over layer, often referred to as the 'cover', is composed of a material such as rubber or silicone (lower frictional surfaces) and heat/gum rubber (higher frictional surface).
Idlers support the conveyor belt. Additionally, idlers control the slack in the belt. In Lehman’s terms, idlers are rollers that spin using ball bearings. They are strategically spaced based on the forces that are expected to occur at different points in the belt. Spacing of idlers will vary depending on the forces impacted on the belt at a given place. Areas of varying spacing include the carrying zones, impact zones and spacing for return idlers. Furthermore, different types idlers of idlers are used depending on the required belt trough angles.
Belt cleaners (otherwise known as belt streamers) remove any carry back material. Carry back material, muck that sticks to the belt at the discharge point, is returned to the head pulley through the underside of the belt. As such, belt cleaners are typically located underneath the head pulley. Carry back is detrimental for a variety of reasons (ASGCO Complete Coneyor Solutions, 2016):
- It causes excessive buildup on the belt and pulleys and can cause blockages
- Causes belt misalignment due to an artificial crown created by the carry back
- Accumulation of material under the conveyor
- Overall reduced operating efficiency and profitability from higher maintenance costs and lost material
The discharge chute creates a smooth transition of material from either one conveyor system to a stockpile or one conveyor system to another conveyor system. It is located under the head pulley at the discharge point. The discharge chute is most important in conveyor to conveyor transport as it reduces impact on the second conveyor system (increasing wear on the belt and reducing maintenance cost). Furthermore, poor transfer design can lead to premature belt wear, poor ore tracking, material degradation, and dust generation (SME, 2011).
The feed chute, located near the tail pulley, brings material onto the conveyor system. Much like the discharge chute, it is a very important component of the conveyor system. As transfer design is critical to the system, installing a feed chute is imperative to reducing reliability costs (SME, 2011).
Safety systems are implemented to minimize hazards around conveyor systems. Some innovations include emergency stop cords and hazardous zone barricades (or screens). At the beginning of some conveyor systems, a magnetic sensor is used to detect if any rock bolts or other metallic materials are on the belt. Since these materials can rip the belt, if detected, they are immediately expelled through a secondary chute.
Design of a Conveyor System
The Goodman Conveyor Company, an industry expert, has outlined the steps in designing a conveyor system (Goodman Conveyor, 1999). A simplified version has been summarized below, along with contributions from others sourced below.
- Determine the desired conveyor capacity
- Identify the material and its characteristics
- Choose a troughing angle
- Determine belt width
- Select belt speed
- Determine the idler spacing
The first step in designing the conveyor’s capacity is to determine the desired capacity required to transport the desired amount of material. This capacity, in t/h, should be the peak surge volume that is expected. Conveyor capacities can range anywhere up to 40, 000 tonnes per hour for a 3200 mm wide and 45 mm thick belt (E D Yardley, 2008). By finding conveyor capacity, sizing of other conveyor parameters can be calculated. Material Characteristics
An important design consideration for a conveyor system is to characterize the material handled. Furthermore, parameters such as the over cover belt material, the maximum incline a conveyor can scale, and trough design can be identified from these material characteristics. For example, a abrasive material such as ore from an in pit conveyor system will have much different characteristics than coal. Characteristics of the material that must be considered include:
- Angle of repose: static angle between the free formed pile of the material and the horizontal
- Angle of surcharge: dynamic angle of repose (usually 5 – 10 degrees less than angle of repose)
The trough angle is the angle of incline on either side of the belt (as mentioned earlier conventional belts forms a ‘V’ shape). Trough angles between 35 and 45 degrees generate a higher carrying capacity than a flat belt with equal widths (Goodman Conveyor, 1999). However, belts with higher trough angles require belt material that can provide a higher flexibility. An optimal trough angle is calculated based on material characteristics and limitations of the material chosen for the belt (Goodman Conveyor, 1999).
Belt width is a function of the largest lump size (largest product particle size), operating speed, and the desired belt capacity (SME, 2011). The largest belt belt width of all conveyor system is 3.2 metres (SME, 2011). A general rule of thumb for calculating belt width is that the belt should between three to five times the height of the maximum lump. If there are more particles of larger sizes in the mine operation, a higher width should be used (SME, 2011).
Typical speeds for conveyors can vary from one meter per second (small conveyors) to 8.5 metres per second (overland conveyors). Spreading conveyors, can reach speeds of up to 15 metres per second with a handling capacity of up to 40,000 t/h (SME, 2011).
Belt speed is determined by several different factors and benefits. Increasing belt speed, increases belt capacity, which decreases belt width and tension. Moreover, this increases the importance of the design of transfer points, and generally reduces the life of all conveyor components (Goodman Conveyor, 1999). As such it is important to find a balance between, capacity required and reliability costs.
While spacing can vary depending on the load requirements, many belt systems will use four feet spacing for carrying zones, one foot for impact zones (such as loading) and eight to ten feet for return idlers on the underside of the belt (Precision Pulley and Idler, 2009). Furthermore, idler spacing is determined based on belt weight, material weight, idler rating, belt sag, idler life, belt rating and belt tension. Moreover, a general rule of thumb is to limit belt sag to 2% of idler spacing (Goodman Conveyor, 1999).
Technical Design of a Conveyor System
In order to ensure safe operation and retain the highest reliability ratings in the system, it is imperative that conveyors are designed to handle all types of loads and forces. In the following section, equations are introduced to determine; forces on the conveyor belt, the minimum required drum diameter, drum power required, and motor sizing.
Any time an object is in tension, it is vital to find the forces acting on the item and and the energy the item possesses. The forces felt by the conveyor belt are a function of the skid plat, the friction of the idler rollers, the mass of the belt, the mass of the rotating drum roller, and the mass of the materials being conveyed over the belt (Siegling Belting - Transilion). All of this is used to determine the effective pull in the equation below:
FU = uT * g(m + mB / 2) + uR * g(mB / 2 + mR)
- uT is the coefficient of friction of the skid plat
- uR is the coefficient of friction of the support rollers
- mB is the mass of belt
- mR is the mass of all rotating drum/rollers not including the drive drum
- m is the mass of the material transported by the belt at a given time
- Fu is the effective pull
This is then used to calculate the maximum belt force using the equation:
F1 = FU * C1
Where C1 is a calculation factor provided by the belt manufacturer.
Next, a second calculation factor, C2, is calculated and compared to C1 using the following equation:
C2 = F1 / bo
Where bo is the width of the belt.
If C2 is below the proposed belt constant, then the proposed belt can withstand the stresses felt in the conveyor system.
Horizontal bend conveyors are used to bypass obstacles and reduce the number of broken routes in the conveyor system. However, horizontal bend conveyors depend on a number of factors which include; belt stretching forces (elastic properties of the belt), horizontal curve radius and the distance between support rollers (M. Grujić, 2011). All these factors are transitively characterized in the equation below. As horizontal bend conveyors are expensive, it is necessary to justify the value of implementation.
- T is the actual distance of belt at the distance x.N
- Lc is curve length (in metres)
- Rc is the curve length (in metres)
- x is the distance in metres of the observed point from the beginning of the curve (in metres)
- Fx is the value of the force acting in some point in the curve
It is important to note that for horizontal bends the maximum belt force should be higher then a straight conveyor system. This should be verified by finding both the C1 and C2 values for this special circumstance and comparing the values.
A correctly sized drum diameter is used to calculate the minimum power required for the system to operate. The drum diameter is a function of belt width, the effective pull of the belt, and the arc of contact on the drum (Siegling Belting - Transilion). The drum diameter is determined using the following equation:
dA = (FU * C3 * 180) / (bo * b)
Where: C3 is a calculation factor provided by the manufacturer b is the arc of contact on the drum
Drum Drive Power and Motor Size
Parameters used to determine the drum drive power include the drum diameter calculated in the above section, and the belt speed (Siegling Belting - Transilion) It is calculated using the following formula:
PA = (FU * v) / 1000
Where v is the belt speed.
The drum drive power is used to find size the motor required. Assuming an efficiency of 80%, the motor size is calculated using (Siegling Belting - Transilion) (Betz, 1998):
PM = PA / 0.8
It is imperative to create a conservative motor design. When calculating motor size, one should always round to the next largest motor. A design calculation can be seen to the right. Although the design is crude, it is a good estimation for compromise between excessive capital expenditure and system reliability. Under designing a conveyor motor size can result in inadequate belt speeds and failure. While over design can result in voltage dip problems (Betz, 1998).
Maintenance – Step by Step Guide
Maintenance on conveyor belts can be characterized into three broad steps.
The first step, the most complicated one, involves shutting down the conveyor system and emptying the medium on the belt. After the belt is shut down; bearings, universal joints and pulleys are checked and lubricated (odesie - Technology Transfer Services, 2016)
Next, the tension, wear and lubrication in the belt is checked. Moreover, sprocket alignment, wear and screw set is then checked. If a v-belt is within the system; the tension, wear and sheave alignment is checked. Electrical connections to the conveyor are checked. The gearbox is checked and oil is added to the proper level. Lastly, the general condition of the system is checked (odesie - Technology Transfer Services, 2016).
The third step is starting the system and ensuring it runs properly; if this is not the case then the system needs to be turned off and checked for any irregularities (i.e. back to step one). The last step involves adding medium and continuing with production (odesie - Technology Transfer Services, 2016).
Types of Maintenance
There are 4 different types of maintenance, each summarized below (Lodewijks, 2002):
- Preventative maintenance
- Opportunity based maintenance
- Corrective maintenance
- Condition based maintenance
Preventative maintenance is calendar based maintenance. Part replacements occur after certain run times. This type of maintenance relies solely on reliability data provided by the manufacturer. The manufacturer’s preventative maintenance run times are generally conservative. This means many parts are being replaced well before they are at a serious risk of failing. While some parts of the conveyor system can be replaced while the conveyor system is operational, preventative maintenance often requires shutting down the system. This results in a costly loss of production. However, when compared to run until failure, part replacement intervals are lower and overall failure time is lower.
Opportunity Based Maintenance
Opportunity based maintenance refers to taking advantage of overall system down time to replace parts. An example of this would be if mining operations halted, the conveyor system would be shut down and thus replacements could be made to the conveyor system. Similar to preventative maintenance, the replacement of any part is not strictly based on run time. As such, parts may not need to be replaced. Furthermore, this will result in greater up time, but runs a higher risk of costs associated with part failure.
Corrective maintenance (otherwise known as run to failure maintenance) is when the system is run until components within the system undergo failure. When failure occurs, downtime is generally much longer and more expensive than the previously discussed maintenance types. If failure is not detected immediately, catastrophic damage can be inflected on the system which can amplify conveyor downtime and costs.
Condition Based Maintenance
Condition based maintenance involves monitoring different components of a system to predict when failure might occur. When failure can be predicted, the appropriate corrective actions can be taken to minimize the effect on production. Recent studies show that up to 90% of mechanical failure can be predicted (Schools, 2015). In many cases, maintenance can be carried out without interrupting production. Condition based monitoring uses heat and vibration sensors to predict when a component is at risk of failure. Vibration signatures received from the sensors in an area of the conveyor will be much different when the conveyor is running smoothly compared to when it requires maintenance. This would give accurate close to failure readings of major components such as major drives, idlers and pulleys (Schools, 2015). Heat sensors are used, as parts will often fail when they reach a certain temperature. By monitoring the temperature of vital parts, maintenance can be carried out before parts reach a critical temperature. Condition based maintenance interrupts production for maintenance only when it is necessary.
Factors Effecting Reliability
Although, conveyor systems provide reduced downtime when compared to haul trucks, it is imperative to limit downtime on these systems. Reliable and available equipment is essential in conveyor systems because they deliver small amounts of material over long periods of time (Overland Conveyor). Furthermore, choosing a suitable type of maintenance for the mining operation with reduce operational costs while maximizing uptime. Unlike haul trucks, production halts when one conveyor fails. This is why there is a great focus on reliability when it comes to conveyor systems.
The type of maintenance scheduling has a great effect on the overall reliability and availability of the conveyor system. Corrective maintenance results in the worst reliability and availability in a mining conveyor system. As failure is unexpected, maintenance crews are not as prepared when compared to preventative maintenance. Moreover, there is also a significantly higher diagnostic time associated with run to failure maintenance. Lastly, corrective maintenance increases the risk of a domino effect of secondary failures caused by the original failure.
Condition based monitoring, in theory, provides the greatest reliability and availability. Being able to accurately predict when a component is reaching failure based on sensors is objectively better than estimating when it will fail based on past experience. In practice, the accuracy of the predictions is a function of the accuracy of the sensors used. In the past, trouble collecting quality and meaningful sensor data has made it difficult to justify the additional expense and effort associated with condition based maintenance. However as sensor technology has improved significantly, condition based maintenance now provides the greatest reliability and is the most economical maintenance option (Schools, 2015). According to the Conveyor Equipment Manufacturers Association (CMEA), well maintained conveyor systems can operate reliably at 90% availability.
Number of Conveyors in Parallel
In a system made up of numerous elements, the reliability of the system depends on the number of elements in the system and the reliability of each element. Assuming each conveyor has the same reliability, a conveyor in a parallel system can be assigned a coefficient of readiness, ri. The reliability of a number of conveyors in parallel is then modeled with:
Therefore, it can be seen that the reliability of the system decreases with each additional conveyor added. To maximize reliability, it is advantageous to make each conveyor as long as practically possible to minimize the number of components in the system (A.A. Balkema, 2000).
Common Electrical and Mechanical Issues
There are several issues associated with interplay between electrical and mechanical parts contingent towards system reliability. The most common issues are listed in the table below.
|Electrical Issues||Mechanical Issues|
|Voltage Dip from High Currents||Gearbox variations|
|Stalling due to externally imposed voltage fluctuations||Sprocket variations|
|Difference in voltage supplied by two power pulley systems||Fluid coupling variations|
|Variations in motors characteristics||Wet clutch couplings|
|Large transient pulsation in the systems|
Of the electrical and mechanical issues listed, the bolded problems are the most common. The voltage dip from high currents is caused by severe starting problems from conventionally designed conveyor systems. Gearbox variations is caused by starting problems or undue stress on the dominant motor and drive train. The slippage of wet clutch coupling generate new difficulties in such operations (odesie - Technology Transfer Services, 2016). All these issues have an effect on system reliability. The underlying issue is caused by having multiple components in the conveyor system, thus triggering electrical and mechanical trips.
Variable Speed Drives
A variable speed drive is a piece of equipment that controls the speed of machinery. These controllers have a combination of computer and power electronic technology that have advanced control algorithms. In the past, they were known to have poor reliability. However, with recent technological advances, they are known to reduce downtime in conveyor systems and actually improve overall reliability (odesie - Technology Transfer Services, 2016). Overall, the advantages of variable speed drives surpass the disadvantages making it more economical to use variable speed drives.
Reliability of Horizontal Conveyor System
With increasing remote mining locations and increasing depths of underground mines, processing plants are increasing in depths from mining operations. This results in material to travel long distances for processing. Horizontal conveyors allow conveyor system to take a more direct route and navigate terrain more efficiently. Thus, this reduces the number of components required in the system, increasing reliability of the overall conveyor system.
Belt Conveyor vs Haul Trucks
Efficiency and costs between conveyor systems and haul trucks are still disputed between professionals in the industry. The disadvantage of conveyors is that they are not as flexible, and require a greater capital cost. In surface operations, installation of in pit crushing and conveying systems may cost more initially than the capital cost of trucks. However, in pit crushing systems operate more efficiently and at a reduced cost. Trucks are less efficient for the following reasons (E. D. Yardley, 2008):
- Only 40% of the energy consumed by trucks is expended during hauling payload. The other 60% is spent hauling the truck body
- Trucks tend to be empty on return
- 80% of energy consumed by conveyors is used by delivering payloads
- Energy costs for trucks are 3 times greater on flat land and 8 times greater on pit slopes than energy costs for conveyors
The cost benefit of conveyors is generally seen as tonnages increase and the haul distance to the plant increases. The McIntosh Engineering Hard Rock Miners Handbook provides several rules of thumb for when it is advantageous to use conveyors over haul trucks (J, 2000):
- When underground daily mine production exceeds 5000 tonnes
- When conveying distance exceeds 2 km
- Beyond 1 km, the cost of transporting a tonne of material by conveyor is 1/10th the cost of transporting it by truck (Conveying Equipment Manufacturers Association, 2005)
Conveyor System Costs
Capital costs of increased conveyor system increase as tonnages increase. Costs are summarized in the table below of an In-Pit conveyor system with fixed conveyors using a steel idler system and a base length of 610 metres (InfoMine USA, 2014).
|Belt Width (cm)||Capacity (mtph)||Motor Size (HP)||Capital Cost ($)||Additional Cost ($/m)|
Different types of In-Pit conveyor system will have varying costs, a shiftable In-Pit conveyor systems will have different costs than a fixed In-Pit conveyor system. Costs are summarized in the table below of an In-Pit conveyor system with shiftable conveyors using a steel idler system and a base length of 610 metres (InfoMine USA, 2014).
|Belt Width (cm)||Capacity (mtph)||Motor Size (HP)||Capital Cost ($)||Additional Cost ($/m)|
It is important to note that varying the material used to construct the conveyor system will also vary the capital cost of the conveyor system.
Similar to capital costs of associated with In-Pit conveyor system, overland conveyor system increase in capital costs as tonnages increase. Below are two tables comparing the capital costs associated with overland conveyors, both with a base length of 1,615 metres, and materials weighing 801 kg/cu m and 1602 kg/cu m, respectively (InfoMine USA, 2014) (HIC Conveyor Products Factory India, 2003).
|Belt Width (cm)||Capacity (mtph)||Motor Size (HP)||Capital Cost ($)||Additional Cost ($/m)|
|Belt Width (cm)||Capacity (mtph)||Motor Size (HP)||Capital Cost ($)||Additional Cost ($/m)|
When comparing the two tables above, it is important to note, that when material weights double there is an increase in capital costs as well as additional costs per metre for implementing a conveyor system.
The Conveyor Equipment Manufacturers Association (CMEA) estimates annual operating maintenance costs at 2% of the purchase cost and 5% of the belt cost (Conveying Equipment Manufacturers Association, 2005).
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