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Eliminating the Discharge Snub Pulley

On high-tension conveyors, not only are snub pulleys expensive to buy and install, but they are an undesirable maintenance item. Any snub pulley maintenance causes a major shut down, while maintaining their idler substitutes is a quick fix.

The substituting idlers must be placed to exactly share the load. This leads to some calculation problems easily solved by Circles Geometry.

The following sample procedure precisely places approach idlers in a curve to the discharge pulley to eliminate the need for a snub pulley:

Assume the conveyor is a straight slope and the idler spacing has been determined and idlers located using that spacing all the way to the discharge end.

Determine the required transition distance and height (to equalize edge and center belt tension) using your favorite formula or the chart below.

From: Belt Conveyors for Bulk Materials © Conveyor Equipment Manufacturers Association

Define the return belt line (slope and a point). Note; if you are using v-returns and are still at high tension on the return side you should use a return side transition also.

Calculate the pulley center elevation. (The horizontal pulley location, X is known from the specifications of the conveyor.) The pulley surface will be set tangent to the top side of the return belt line or return transition line. Your calculation of this can be checked by using Circles Geometry, “Circle tangent to two lines calculator” as shown. (Input the second line as tangent and vertical at the forward pulley surface). We have, then, eliminated the snub pulley.

Calculate the transition radius from the belt line to the lifted discharge pulley: R = pulley radius – transition height

Define the trough belt line. A point (on the belt line) and a slope (the belt slope) define a line..

Calculate the concave curve: Enter the above pulley transition circle (radius and x and y at the pulley center) and the belt line (point and slope) into Circles Geometry. Enter a fitted circle radius equivalent to the required concave curve radius for the conveyor at the discharge tension.

Circles Geometry will provide the location of the tangent points (point of curvature and theoretical belt line point at the pulley) and the fitted circle center.

Locate the transition starting point. (2801.0682, 306.0636 in the example) on your incomplete conveyor profile. Find the location of the next idler beyond this point using the established spacing. For each idler in the curve, calculate the center of the roller.

: (The idlers should be supported so that the center rolls are located as calculated.) Print out of idler locations follows with idler center roll X and Y for each idler in the curve:

  Circles Geometry Set 9
Small Circle Outside Large Circle
Large Circle Center X  2742.2334
Large Circle Center Y  600.2379
Large Circle Radius  300
Small Circle Radius  0.5
Point Slope Line
Line X  2802
Line Y  306.1236
Line Slope  -5
Number of Small Circles   15
Parallel Lines: Equal spacing
"Equal spacing, parallel lines"  3
,
,
1 X Circle center = 2802.0735
1 Y Circle center = 305.7563
1 X Tangent = 2801.9739
1 Y Tangent = 306.2463
,
Line to Line = 3
2 X Circle center = 2805.0105
2 Y Circle center = 306.3684
2 X Tangent = 2804.906
2 Y Tangent = 306.8574

Line to Line = 3
3 X Circle center = 2807.9416
3 Y Circle center = 307.0099
3 X Tangent = 2807.8322
3 Y Tangent = 307.4978

Line to Line = 3
4 X Circle center = 2810.8668
4 Y Circle center = 307.6807
4 X Tangent = 2810.7526
4 Y Tangent = 308.1675

Line to Line = 3
5 X Circle center = 2813.7862
5 Y Circle center = 308.381
5 X Tangent = 2813.6671
5 Y Tangent = 308.8666
,
Line to Line = 3
6 X Circle center = 2816.6996
6 Y Circle center = 309.1107
6 X Tangent = 2816.5757
6 Y Tangent = 309.5951

Line to Line = 3
7 X Circle center = 2819.6072
7 Y Circle center = 309.87
7 X Tangent = 2819.4785
7 Y Tangent = 310.3531

Line to Line = 3
8 X Circle center  2822.5089
8 Y Circle center = 310.6587
8 X Tangent = 2822.3753
8 Y Tangent = 311.1405

Line to Line = 3
9 X Circle center = 2825.4046
9 Y Circle center = 311.4771
9 X Tangent = 2825.2662
9 Y Tangent = 311.9576

Line to Line = 3
10 X Circle center = 2828.2944
10 Y Circle center = 312.3251
10 X Tangent = 2828.1512
10 Y Tangent = 312.8042

Line to Line = 3
11 X Circle center = 2831.1782
11 Y Circle center = 313.2029
11 X Tangent = 2831.0303
11 Y Tangent = 313.6805

Line to Line = 3
12 X Circle center = 2834.0561
12 Y Circle center = 314.1106
12 X Tangent = 2833.9033
12 Y Tangent = 314.5867

Line to Line = 3
13 X Circle center = 2836.928
13 Y Circle center = 315.0482
13 X Tangent = 2836.7705
13 Y Tangent = 315.5227

Line to Line = 3
14 X Circle center = 2839.7939
14 Y Circle center = 316.0158
14 X Tangent = 2839.6316
14 Y Tangent = 316.4887

Line to Line = 3
15 X Circle center = 2842.6538
15 Y Circle center = 317.0136
15 X Tangent = 2842.4867
15 Y Tangent = 317.4849

Notice that the data output file can be opened in your spreadsheet. Ready for you to calculate structural details for idler supports, thus:

09/17/2011

Top side concave curve idlers, as in this example, are not heavily loaded if they are located properly.

The selected 300 foot radius is such that the belt will not lift off the idlers under any running condition. Those calculations are not shown here.

The full set of numbers for 15 idlers.