Materials conveyors: A guide for specifying belt conveyor systemsTemitayo Oketola | January 26, 2021
Conveyor systems are essential mechanisms used for raw materials transportation and inspection in many industries. These material handling systems come in numerous designs, with the incorporation of rollers, chains, and more commonly, belts. Nevertheless, many managers and engineers are still plagued with the challenge of designing efficient conveyor systems.
This article presents the basic steps for designing a belt conveyor system. It will take into account the belt sizing, speed, capacity and motor requirements. However, before proceeding to the design of a conveyor belt, some key terms must be understood.
Understanding key terminologies
- Capacity (C) describes the rate at which the bulk material is being conveyed by the belt. It is measured in ton per hour (TPH) or kg/hr.
- Lump size describes the maximum dimensions of the bulk material being conveyed. For instance, the lump size of a steel grit is its diameter.
- Troughing angle describes the angle the belt forms on the carrying side while running over idler rollers. Belts are troughed to allow the conveyor load and to transport materials. Common troughing angles include 15°, 20°, 25°, 30°, 35° and 40°.
- Surcharge angle describes the angle that the surface of the material makes with the horizontal when it is at rest on a moving conveyor belt.
The design process
Before proceeding to design a conveyor belt system, engineers must set a design specification. Common design specifications set during the early design stages include the volume flow rate of bulk material, conveyor capacity, transporting distance and the material lump size. These will serve as a guide for the design of the conveyor belt to be made.
Conveyor belt dimension, capacity and speed
The first step in the design of a belt conveyor with a specified conveyor capacity is to determine the speed and width of the belt. The magnitude of the belt speed can be determined using:
- C = Conveyor capacity
- r = Material density (kg/m3)
- V = Belt speed (m/s)
- A = Conveyor’s belt cross-sectional area
Belt speed should be specified such that it does not cause spillage of the raw material being conveyed. Suitable conveyor belt speed typically ranges between 2.5 m/s and 3.5 m/s. Higher belt speed usually translates to a higher horsepower requirement. Additionally, the service life of a belt conveyor running at an extremely high speed may be shortened considerably due to excessive vibration.
The specification of belt width will depend upon the material lump size. As a rule, the specified belt width must be wide enough to handle the material lump size.
Roller diameter and RPM
Rollers are metal cylinders that support the conveyor belt and facilitate the belt’s rotation in all directions. They ensure that the belt tension is maintained. A wrongly sized roller will often seize up during operation, resulting in sharp edges that cause the conveyor belt to slip.
The relationship between roller diameter and its number of revolutions per minute is given as:
- n = Number of revolutions per minute
- V = Belt speed (m/s)
- D = Roller diameter (mm)
So, for a belt speed of 1.3 m/s and roller diameter of 100 mm, the number of revolutions per minute would be obtained as 248 rpm.
Belt power, tensions and motor specification
The power required at the drive pulley drum can be determined using:
- F = Total tangential force at the drive pulley (N)
- Pp = Power required at the drive pulley drum (kW)
- V = Belt speed (m/s)
Similarly, the power required to produce lift can be determined using:
- C = Conveyor capacity (kg/s)
- L = Lift required (m)
- PL = Power required for the conveyor to produce lift
So, let’s say there is a need for a conveyor system with a capacity of 400 TPH (or 111.1 kg/s) to produce a lift of 15 m. The power required would be 6.23 kW.
The minimum motor power requirement can then be obtained by substituting the Pp (or PL) values in the equations given below.
- Pmin = Minimum motor power (kW)
- h = Efficiency of the reduction gear
The spacing of idlers along the conveyor belt plays a crucial role in the overall economy of the conveyor system. They affect the cost and life of the belt, drive power requirement and tension rating of the belt.
According to the Conveyor Equipment Manufacturers Association (CEMA), the best spacing for idlers depends on:
- The sag on the belt between idlers
- The weight of the belt and the live load it carries
As a rule, the idler spacing should not exceed the values shown in Table 2, and the load on the idler should not exceed load ratings. CEMA also recommends that the sag should be limited to 3% when the belt is operating under normal load and 4.5% when the loaded belt is standing still.
The design procedure for the conveyor belt system is broad, tedious and iterative. There exist many published design guides or national standards that designers can use. While this article presents useful information about conveyor belt design, designers are advised to also obtain copies of the CEMA and ISO guides for reference.