KWS Manufacturing

Screw Conveyor Example

Establish Characteristics of Bulk Material Determine Conveyor Size and Speed Based on Capacity Calculate Horesepower Requirements Calculate Torque Requirements Component Series Selection

There are many different factors to consider when designing a screw conveyor. The purpose of this example is to provide a step-by-step process that a KWS engineer would follow when designing a screw conveyor. It is important for the person designing the screw conveyor to understand how each factor affects the final screw conveyor design.

The following information will be used for the example –

  • Bulk Material: Corn Meal
  • Capacity: 25,600 lbs/hr
  • Screw Conveyor Length: 16-feet, 0-inches from centerline of inlet to centerline of discharge
  • Screw Conveyor Degree of Incline: 0° Horizontal

The screw conveyor for the example is control fed at the inlet by another screw conveyor.

Screw Conveyor Example – Step 1:
Establish Characteristics Of Bulk Material

The first step is to look up corn meal in the Bulk Material Table and write down the following information –

Processing Corn Meal with a KWS Screw Conveyor
Bulk Material: Corn Meal
Maximum Particle Size: minus 1/8-inch and smaller
Bulk Density: 32 to 40 lbs/ft3
Trough Loading: 30A (30-percent)
HP Factor: 0.5
Component Series: A1-A2
Abrasiveness: I
Corrosiveness: I
Flowability: III

Note – For the example, the additional characteristics provided in the “Note” column of the bulk material will not be considered in order to simplify the solution. Please refer to the Bulk Material Factors section for more detailed information.

Screw Conveyor Example – Step 2:
Determine Conveyor Size and Speed Based on Capacity

Information Given for Example

  • Bulk Material: Corn Meal
  • Capacity: 25,600 lbs per hour
  • Length: 16’-0” from centerline of inlet to centerline of discharge
  • Conveyor Type: Horizontal

Information Provided from Bulk Material Table:

Bulk Material Maximum Particle Size (in.) Bulk Density (lbs/ft³) %
Loading
Hp Factor Component Series Abrasiveness Corrosiveness Flowability
Corn Meal -1/8 32-40 30A 0.5 A1-A2 I I III

Now that the characteristics of the bulk material to be conveyed have been determined, the next step is to calculate the capacity in cubic feet per hour. Always use the lowest bulk density when calculating capacity in order to get the highest potential volumetric capacity.

The recommended trough loading percentage according to the Bulk Material Table is 30A, or 30-percent.

Fill in the blanks and calculate the Capacity:

Calculate Capacity Calculate Selection Capacity

Nomenclature

SC = Selection Capacity (ft3/hr)

CFH = Required Capacity in Cubic Feet Per Hour (ft3/hr)

CF = Capacity Factor

The Selection Capacity will be used to select the proper screw conveyor diameter and speed from the Capacity Table. Using the Recommended Trough % Loading and calculated Selection Capacity, select the proper conveyor diameter. The Selection Capacity must be less than the maximum capacity given in the Capacity Table. Fill in the blanks below based on your previous calculations:

Fill in the Blanks, Conveyor Diamter, Capacity at Max RPM and Capacity at 1 RPM

The Actual Conveyor Speed is calculated by dividing the Selection Capacity by the Capacity at 1-RPM.

Fill in the blanks below:

Actual Conveyor Speed is Calculated
Capacity Table
Trough Loading Screw Dia. (in.) Max. RPM * Capacity in ft3/hr
At Max. RPM At 1 RPM
Trough Loading 30%
30% A
4 139 57 0.4
6 120 179 1.5
9 100 545 5.5
12 90 1,161 12.9
14 85 1,768 20.8
16 80 2,496 31.2
18 75 3,375 45.0
20 70 4,375 62.5
24 65 7,085 109.0
30 60 12,798 213.3
36 50 18,440 368.8

Screw Conveyor Example – Step 3:
Calculate Horsepower Requirements

Information Given for Example

  • Bulk Material: Corn Meal
  • Capacity: 25,600 lbs per hour
  • Length: 16’-0” from centerline of inlet to centerline of discharge
  • Conveyor Type: Horizontal

Information Provided from Bulk Material Table:

Bulk Material Maximum Particle Size (in.) Bulk Density (lbs/ft³) %
Loading
Hp Factor Component Series Abrasiveness Corrosiveness Flowability
Corn Meal -1/8 32-40 30A 0.5 A1-A2 I I III

Calculating Horsepower

Friction Horsepower

A 12-inch diameter screw conveyor was selected in Step 2 of the example. The Diameter Factor (DF) selected from the Diameter Factor Chart (HP Section) is 55 for a 12-inch diameter screw conveyor.

Diameter Factor Chart (DF)
Dia. Factor Dia. Factor Dia. Factor Dia. Factor
4 12 12 55 18 135 30 377
6 18 14 78 20 165 36 549
9 31 16 106 24 235    

Since the bulk material to be conveyed is corn meal, a wood hanger bearing is selected for the application. The Hanger Bearing Factor (HBF) selected from the Hanger Bearing Factor Chart (HP Section) is 1.7 for a wood hanger bearing.

Hanger Bearing Factor Chart (HBF)
Bearing Type Bearing Factor Bearing* Class
Ball, Roller, or none 1.0 2
Bronze, or Wood 1.7 2
Plastic, Nylon, UHMW, or Teflon 2.0 3
Hard Iron, or Stellite 4.4 4

The screw conveyor speed calculated in Step 2 is 62-rpm.

Fill in the blanks and calculate the Friction Horsepower (FHP):

Calculating Friction Horsepower (FHP)

Nomenclature:

DF = Conveyor Diameter Factor
HBF = Hanger Bearing Factor
L = Conveyor Length (ft)
S = Conveyor Speed

Material Horsepower

The capacity (CP) given in the example is 25,600 lbs. per hour. Please note: Do not use Selection Capacity (SC) to calculate horsepower. The screw conveyor length is 16-feet.

The Material Factor (MF) or HP Factor for corn meal is 0.5 from the Bulk Material Table.

Caclulating Material Horsepower (MHP)

Nomenclature:

MF = Material Factor
CP = Capacity (lbs/hr)

The calculated Material Horsepower (MHP) is 0.21-HP and must be corrected since it is less than 5-HP. Using the Corrected Material HP Chart below, locate 0.21-HP on the horizontal axis of the chart, draw a line vertically until it intersects the curved line, then move horizontally to determine the Corrected Material Horsepower of 0.5-HP.

Corrected Material HP Chart

Corrected Material HP Chart Example

Fill in the blanks and calculate Total Shaft Horsepower (TSHP) by summing the Friction Horsepower (FHP) and the Corrected Material Horsepower (MHP) as follows:

Calculatingtotal Shaft Horsepower (TSHP)

Drive efficiency (e) for a typical screw conveyor drive unit with shaft-mounted reducer and V-belt drive is 88-percent, or 0.88.

Screw Conveyor Drive Unit with Shaft-Mounted Reducer and V-Belt Drive

Total Shaft Horsepower (TSHP) is typically rounded up to the next commonly available motor size. The most commonly available motor size for the example would be 1-HP. The drive unit selected for the example is 1-HP at 60-rpm. The speed of the drive unit is typically rounded to the closest 5-rpm increment.

Screw Conveyor Example – Step 4:
Calculate Torque Requirements

Information Given for Example

  • Bulk Material: Corn Meal
  • Capacity: 25,600 lbs per hour
  • Length: 16’-0” from centerline of inlet to centerline of discharge
  • Conveyor Type: Horizontal

Information Provided from Bulk Material Table:

Bulk Material Maximum Particle Size (in.) Bulk Density (lbs/ft³) %
Loading
Hp Factor Component Series Abrasiveness Corrosiveness Flowability
Corn Meal -1/8 32-40 30A 0.5 A1-A2 I I III

Calculating Full Motor Torque

A 1-HP at 60-rpm drive unit was selected for the example screw conveyor. Full Motor Torque is calculated with the following equation below:

Calculating Full Motor Torque

HP = Nameplate Horsepower of the motor on the screw conveyor
S = Screw Conveyor Speed

The torque rating of the drive shaft, coupling shafts, coupling bolts and conveyor screw must be greater than Full Motor Torque for proper design.

A 12-inch diameter screw conveyor was selected for the example. The minimum standard shaft size for a 12-inch diameter screw conveyor is 2-inch diameter. The corresponding pipe size is 2-1/2-inch schedule 40 pipe with 5/8-inch diameter coupling bolts. The materials of construction of the screw conveyor is carbon steel.

Standard Screw Construction
Standard Screw Construction (by Shaft Size)
Shaft diameter (in.) 1 1-1/2 2 2-7/16 3 3-7/16 3-15/16 4-7/16
nominal pipe Size 1-1/4 2 2-1/2 3 3-1/2 4 5 6
Coupling Bolt dia. (in.) 3/8 1/2 5/8 5/8 3/4 7/8 1-1/8 1-1/4

Maximum torque ratings for each screw conveyor component are shown in the Torque Tables below. Maximum torque ratings are based on a safe stress value for the specific material of construction. The screw conveyor components will have an infinite life under normal operating conditions.

Shaft Dia. Carbon Steel Torque Values
Shaft Coupling Bolts (2-Bolt) Pipe – Schedule 40
C-1045 Grade 5 A-53
Torsion Bolts in Shear Pipe in Shear Pipe in Bearing
Safe Stress Torque Rating Safe Stress Torque Rating Safe Stress Torque Rating Safe Stress Torque Rating
PSI In-Lbs PSI In-Lbs PSI In-Lbs PSI In-Lbs
1 8,750 1,000 15,500 3,400 6,700 3,100 6,700 2,200
1-1/2 8,750 3,800 15,500 9,100 6,700 7,600 6,700 5,600
2 8,750 9,500 15,500 19,000 6,700 14,200 6,700 8,900
2-7/16 8,750 18,700 15,500 23,000 6,700 23,000 6,700 13,200
3 8,750 35,400 15,500 41,000 6,700 31,900 6,700 17.500
3-7/16 8,750 53,000 15,500 64,000 6,700 42,700 6,700 24,700
3-15/16 8,750 76,400 15,500 121,300 6,700 72,600 6,700 58,200
4-7/16 8,750 110,200 15,500 168,800 6,700 112,900 6,700 101,300

The torque rating of the screw conveyor components from the Torque Tables:

Drive and Coupling Shafts = 9,500 inch-lbs.
Coupling Bolts = 19,000 inch-lbs.
Pipe in Shear = 14,200 inch-lbs.
Pipe in Bearing = 8,900 inch-lbs.

The torque rating of each screw conveyor component is much higher than the Full Motor Torque generated by the drive unit. The screw conveyor in the example is designed properly and will function many years with minimal maintenance or downtime.

Screw Conveyor Example – Step 5:
Component Series Selection

Information Given for Example

  • Bulk Material: Corn Meal
  • Capacity: 25,600 lbs per hour
  • Length: 16’-0” from centerline of inlet to centerline of discharge
  • Conveyor Type: Horizontal

Information Provided from Bulk Material Table:

Bulk Material Maximum Particle Size (in.) Bulk Density (lbs/ft³) %
Loading
Hp Factor Component Series Abrasiveness Corrosiveness Flowability
Corn Meal -1/8 32-40 30A 0.5 A1-A2 I I III

The recommended Component Series for corn meal is A as listed in the Bulk Material Table. Corn meal is a light, non-abrasive and free-flowing bulk material.

The 12-inch diameter by 16-feet long screw conveyor for the example is constructed from carbon steel with the following construction as selected from the Component Series Table:

Screw Dia. Shaft Dia. Cover Thickness Series A
Screw Number Thickness (Min.)
Helicoid Sectional
4 1” 14 Ga. 4H206 N/A 14 Ga.
6 1-1/2” 14 Ga. 6H304* 6S309 14 Ga.
9 1-1/2 “
2”
14 Ga. 9H306*
9H406*
9S309
9S409
14 Ga.
12 2”
2-7/16”
3”
14 Ga. 12H408*
12H508*
12H614*
12S409
12S509
12S612
12 Ga.
14 2-7/16”
3”
14 Ga. 14H508*
14H614*
14S509
14S609
12 Ga.
16 3” 14 Ga. 16H610* 16S612 12 Ga.
18 3”
3-7/16”
12 Ga. N/A 18S612
18S712
10 Ga.
20 3”
3-7/16”
12 Ga. N/A 20S612
20S712
3/16”
24 3-7/16” 12 Ga. N/A 24S712 3/16”
30 3-15/16” 10 Ga. N/A 30S816 3/8”
36 4-7/16” 3/16” N/A 36S916 3/8”

Screw: 12H408 helicoid screws with 1/8-inch nominal flight thickness and 2-inch shaft diameter
Trough: TUA1212 angle flange trough with 12-gauge trough thickness
Cover: COV1214UFL flanged cover with 14-gauge cover thickness

The recommended Bearing Material Series is 1-2 as listed in the Bulk Material Table.

The 12-inch diameter by 16-feet long screw conveyor for the example can utilize several different hanger bearing materials as selected from the Component Series Table:

Series Bearing Material
1 Nylatron, Plastech, UHMw, Wood, Ball
2 Plastech, Gatke, Ball
3 Bronze, Hard Iron
4 Hard Iron, Hardsurface, Stellite, Ceramic

Based on a Bearing Material Series of 1-2, the hanger bearings can be Nylatron, Plastech, UHMW, Wood, Gatke or ball bearing. We are selecting wood for the example because wood hanger bearings are very cost-effective and durable.