Helix Technologies, a Perth-based engineering software developer, has recently added a new pipe conveyor design feature into its Helix DeltaT6 Conveyor Design Software.
In a pipe conveyor, the belt is formed into a circular tube that fully encloses the conveyed material. The conveyor belt is an open trough under the loading chute and it is trained and formed into an enclosed tube for the length of the conveyor until it is once again opened to a troughed shape at the discharge pulley.
The DeltaT6 Program has always had the capacity to design troughed belt conveyors using conventional methods such as ISO 5028 (DIN22101) and CEMA. In 2005 Helix added the VISCO method which uses the Viscoelastic properties of the belt rubber to estimate the friction factor and demand power (resistance) of the conveyor and this software has been widely used in Australia and many other countries to design troughed conveyors, including long overland horizontally curved conveyors.
Conveyor dynamic analysis
The Helix deltaT6 software also has an advanced Dynamic Analysis module for calculating the transient belt tensions in the flexible conveyor belt during starting and stopping the conveyor. The magnitude of these transient belt tensions is required to ensure the conveyor can be started safely and that during stopping it does not develop excessive belt sag which can cause material spillage and belt and structural damage to the conveyor.
This Dynamic Analysis feature has made the Helix delta-T6 program a software tool used in more than 25 countries around the world.
The addition of the pipe conveyor design capability adds a new dimension to the DeltaT6 software, and although pipe conveyors generally have a higher capital and operating cost than troughed conveyors, they have the advantage of transporting the material in an enclosed environment and they can also navigate tighter (smaller radius) curves than normal troughed conveyors, which means they can be used in situations where the troughed conveyor would not be possible.
Helix pipe conveyor calculation method
The resistance of a pipe conveyor may be broken down into four main categories, namely:
- Belt to idler indentation resistance
- Material and belt flexure losses
- Idler rotation (rim drag) resistances
- Belt to idler scuffing losses
Belt to idler indentation resistance
In a pipe conveyor, the folded belt adds additional load on the idler rolls imparted by the stiffness of the belt. There are also more idler rollers (normally six for pipe conveyor versus three for a troughed conveyor) and more idler face length is in contact with the belt.
The gravitational force resulting from the mass of the material and belt is taken on the three lower idlers, as it is in a conventional troughed belt. The upper two wing rollers and the top centre roller also have indentation losses due to the folding of the stiff belt into a tubular shape.
There is also a resultant force on the lower three rollers due to the belt tension in a convex vertical curve. In a concave vertical curve, there is a resultant force applied to the three upper rollers due the belt tension. In addition, the wing rollers (the two on each side of tube) must also take the resultant force due to the tensioned belt being curved around horizontal curves.
The belt to idler indentation forces in a pipe conveyor may be summarised as follows:
- Belt folding force – on the four side rollers and top roller
- Gravitational forces due to belt and material mass – on bottom centre and lower wing rollers
- Convex curve belt deviation load – on bottom centre and lower wing rollers
- Concave curve belt deviation load – on top centre and upper wing rollers
- Horizontal curve belt deviation load – on lower wing and upper wing rollers on inside of curve
In the Helix delta-T6 program, when you perform a pipe conveyor calculation, all of these individual indentation losses are calculated for each section of the conveyor and added to arrive an equivalent friction factor ‘f’ for indentation resistance.
The user can see the resulting proportion of conveyor resistance attributed to indentation, flexure, rolling resistance and belt scuffing in the viscoelastic friction factor report.
Material and belt flexure resistance
In a pipe conveyor, as well as a troughed conveyor, the belt will tend to sag down to some extent between supporting idlers under gravitational forces induced by the material and belt mass.
The pipe tube will also tend to bulge slightly between idler stations and there is a resulting resistance loss due the flexure of the material and belt as it deforms in travelling from one idler station to the next.
The total material and belt flexure loss is a function of the belt tension, the amount of belt sag, the resistance of the material moving/shearing (internal co-efficient of friction of the material) and amount of belt flexure resistance due to its stiffness.
Adjusting the material flexure
To adjust the amount of resistance due to belt and material flexure you need to adjust the material flexure adjustment factor input value on the viscoelastic belt properties input form. The default input value is set to 1.0 and this is the setting required for iron ore. You need to adjust this input value to reflect the relative internal co-efficient of friction of the material being transported. For example, if it is say dry wheat, use a factor of 0.8 or even 0.7, and if the material is very hard, sharp angular ore or rock, use a value of say 1.1 or 1.2. The amount of flexure also depends on the amount of belt sag and also the troughing angle of the idlers, the sag is calculated automatically and adjusted for each section.
Idler rotation resistance
In a pipe conveyor, as well as a troughed conveyor, the idler rollers have a resistance to rotation. The amount resistance depends on the manufacture of the idler, bearing and seal type. The actual value of the resistance can vary considerably from idler to idler and for a pipe conveyor, due to the higher number of idler rollers, this resistance can have a considerable effect on the total pipe conveyor resistance.
Idler skew and tilt resistance
If the idler rolls are not aligned perpendicular to the belt travel direction, a scuffing resistance is generated. The magnitude of this scuffing resistance depends on the amount of misalignment as well as the co-efficient of friction between the belt and idler roll. The co-efficient of friction will in turn depend on whether the belt surface is dry, wet or moist.
Example of pipe conveyor friction factor
The conveyor resistances for each section of the conveyor are calculated using the methods shown in the viscoelastic calculation method as described above. The four main resistance components (indentation, flexure, idler rotation and skew and tilt resistance) are then added to give a total resistance ‘R’ for each section of conveyor. This total section resistance in Newtons is then used to back calculate the Friction factor μ because the masses and idler loads m are known.
Pipe conveyor friction factor report
The report pictured shows the values of each component of the conveyor resistances.
It can be seen from the report that in the carry sections of the pipe conveyor, the total friction factor varies in sections with no horizontal curves and increases in the curved sections (increases from 0.0247 in section four with no horizontal curve to 0.0323 in section five with 300 m radius horizontal curve).
On the return belt sections from 26 onwards, the friction factor is higher than carry sections at about 0.045 to 0.056. However, this does not mean the section resistance is higher in the return run because the mass is much lower as there is no material being transported.
Proportions of indentation, flexure, idler drag and idler skew (scuffing) losses
The report shows the proportions of the resistances as a percentage of the total for each section:
Carry side (Section 11):
- Indentation loss is about 54.2 per cent
- Flexure loss is about 2.6 per cent
- Idler Drag loss is about 37.9 per cent
- Idler Skew loss is about 5.3 per cent
- Total μ = 0.0347
Corresponding Return side (Section 29):
- Indentation loss is about 35.1 per cent
- Flexure loss is about 1.2 per cent
- Idler Drag loss is about 60.1 per cent
- Idler Skew loss is about 3.6 per cent
- Total μ = 0.0514
There is lower flexure loss on the return run (no material) and the additional idlers make the idler drag losses proportionally higher on the return side than on the carry side.
Indentation losses are lower on the return run than the carry side due to no material mass.
The proportions of each resistance component can vary widely depending on the belt rubber properties, belt speed, idler spacing and idler rimdrag. The designer should explore different settings to get an optimal design.
This example calculation demonstrates some of the design inputs and considerations required for a pipe conveyor design; further details and sample calculations can be obtained from the Helix Technologies websites at www.helixtech.com.au and www.helixweb.com.au.
A demonstration version of the software can be downloaded from the websites.