Helix delta-T Horizontal Curves
Helix delta-T version 6 has a powerful capability to
design conveyors which are curved in the horizontal and vertical plane.
A picture of a
horizontally curve conveyor is shown below - note the idlers are tilted up on
the inside of the curve in order to prevent the belt from straightening and
falling off the conveyor.
<===Positive Belt Drift
Negative Belt Drift ===>
The belt tension
T in a curved belt has a resultant force Ft towards the centre of the curve. The
resultant force Ft is given by
where Ft is motivating force towards centre of curve,
T is belt tension, Is is idler spacing and R is horizontal curve radius.
This motivating force needs to be balanced by tilting
up the idler on the inside of the curve. The weight of the belt and material (if
loaded) creates a balancing force to oppose the motivating force. The trick is
to know how much to tilt the idler and ensure that the conveyor can operate
under all conditions.
Negative Belt Drift - If you tilt the
idlers up too much the belt will drift away from the centre of the curve and
this is called a negative belt drift
Positive Belt Drift - If the idler is
not tilted up enough the forces will not be in balance and the belt will tend to
drift towards the centre of the curve.
The objective is to select a banking angle which will
result in negative belt drift under some operating conditions and positive belt
drift under others and ensure that the belt and material will stay on the
conveyor. If the banking angle required or belt drift is excessive you need to
increase the curve radius or decrease the belt tension. Normally Helix
Technologies aims to limit the banking angle to a maximum of about 8 degrees on
the loaded side of the belt.
Horizontal Curve Calculations
To calculate the
banking angles required and resulting belt drift in Horizontal curves requires
you to first input the conveyor geometry including entering the X, Y, Z
co-ordinates for the points along the conveyor. See the
Entering X,Y.Z co-ordinates help topic. Once you have the conveyor geometry
you can go to the Input, Input Horizontal Curves menu on the
main form. This will
display the following form
As can be seen
from the above image the first sections of the conveyor are straight and the Y
co-ordinates entered are all 0. Then from point 6 onwards the Y co-ordinates are
increasing as the offset increases. The drawing is actually for a single
horizontal curve but in order to improve accuracy of the geometry multiple
points have been added along the curve path.
The radius of
the curve is entered in the Horiz Curve Radius column, in this case it is a
constant radius and it is drawn by the software as a red line. It is often best
to draw the conveyor in a CAD drawing program and obtain the X,Y,Z points from
the CAD drawing
Once you have
drawn the conveyor and entered the curve radii, click the Horizontal
Curve Detail Tab to display the following:
Curves Datacontrol above the graph allows you to scroll through all the
intersection points in the conveyor that are horizontal curves. as you scroll
through the curves, the details of the curve are displayed in the
Horizontal Curve Inputs tabsheet on the right hand side of the form.
The inputs in these tables are extracted from other input data in the program
but the following inputs relate specifically to the Horizontal curves:
Horizontal Curve Radius - this is a very important input, the larger
the radius the less the resultant force towards the centre of the curve. Always
use the maximum radius that can be incorporated in the conveyor.
Mass per m - this is a calculated value from the capacity on the
Spacing - this input affects the motivating force, see formula at top
Angle - this is the angle at which the idler set is tilted up on
the inside of the curve. If you alter this value and press Enter, the program
will re-calculate the Belt Drift for all the operating conditions of the
conveyor and draw them in the main Graph on the form.
Drift Graph - this graph shows the amount the belt will drift for the
particular load and operating case. For instance the Starting Empty belt drift
is usually the highest positive belt drift and is shown by the intersection of
the green graph with the "Starting Empty" vertical line. The Braking Full (or
Coasting Full if no brakes are fitted) graph will usually be the highest
negative belt drift calculated. The belt drift is
Idler Face Widths also affect the calculations considerably, sometimes
it is necessary to use an idler roll with a longer face width than you normally
would for the belt selected. Getting more belt and material load on the centre
roller increase the balancing forces. Three roll idlers are better than 2 roll
idlers because 2/3 of the belt and material are balancing versus half for a two
Belt Friction - some designers allow for a friction force between the
belt and idler to counter the tendency of the belt to drift up the idlers. The
inputs for the us1, usm and us2 are Coulomb friction factors. Helix recommends
that these are set to zero because if the belt is wet the force will be
Conveyor Tensions tab shows the belt tensions at the curve under the
various operating conditions.
Tension Rise - The belt is elongated on the outside of the horizontal
curve due to the curvature and this induces an additional tension in the outside
edge and a reduction in tension in the inside edge. The Tension Rise is
calculated and shown. You should ensure that when this additional tension is
added to the operating belt tensions that it remains within acceptable limits.
The normal allowable tension rise during running is 15% and starting is 150% -
the belt manufacturer must provide the limits for the belt. When this tension
rise is subtracted from the lowest tension it should remain above the allowable
minimum sag tension.
Cross Section Drawing tab shows a drawing of the carry idler tilted at
the banking angle you input for the curve.
You can copy the
Belt Drift Graphs into the Windows clipboard using the Copy
button above the graph and then paste these graphs into a Word document as part
of a report, or you can view and print the report using the button provided at
the top of the form.
In the graph above the banking angle is bit too large
because the negative drift is more than positive drift. This was done purposely
on this conveyor as subsequent points along the horizontal curve have higher
belt tensions resulting in more positive drift and it was decided to keep the
whole curve at 4 degrees banking. you may of course vary the banking angle along
the curve or even at each idler station if you wish, but this is harder to
implement on site.
Adjustable Angle Idlers - it is good
practice to provide a means of adjusting the banking of the idlers on site in
order to allow fine tuning of the banking angle.
Side Guide Rollers - it is common to
install side guide rollers to prevent the belt from slipping off the idler set
completely in the case of excessive belt drift which may be cause by uneven
loading or belt tension variations - refer photo at the top of this help topic
for an example of the side guide rollers.
Belt Troughability - it is important
the belt is flexible enough to trough correctly under the loaded and empty
Photo of Horizontally Curved Conveyor
A photo of a 4400m long 4400tph Horizontally curved
conveyor with 2000kW installed power.
Horizontal Curve Reports can be
generated and printed or saved for each curve.
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