SCREW CONVEYOR BASIC DESIGN CALCULATION CEMA (Conveyor Equipment Manufacturer Association) Approach

HISTORY & APPLICATION

HISTORY:

The first conveyor as such as designed by

ARCHIMEDES (287 – 212 BC) for

removing water from the ship and to

raise water from river to farm land

The next technological advancement of

importance in screw conveyor design was

patented on march 29, 1898 by Frank C.

Caldwell.

This construction is now known as the

helicoid flight.

APPLICATION:

Screw conveyor s are bulk material

transporting devices capable of handling

a great variety of materials which have

relatively good flowability.

Flowability is defined in the CEMA

material classification standard and

denotes: “the degree of freedom of

individual material particles to move past

each other”

This characteristic is important in a screw

conveyor operation as the screw helix.

The many advantages of screw conveyor

is feasibility of nomerous feed and

discharge of the material to be conveyed.

Another used is the blending of several

ingredients to make a finished product or

to make a pre-mix for some product

Screw conveyors may be operated

horizontally, on an incline or vertically.

MATERIAL CODE & BULK MATERIAL CHARACTERISTICS

MATERIAL CODE

A study has been made to

define the characteristic of

bulk materials in terms which

are readily recognized.

Table 2-1

Material Classification Code Chart

Major Class Material Characteristics Included Code Designation

Density Bulk Density, loose Actual, lb/ft3

Size Very Fine No. 200 sieve (0.0029") and under A200

No. 100 sieve (0.0059") and under A100

No. 40 sieve (0.016") and under A40

Fine No. 6 sieve (0.132") and under B6

Granular 1/2" and under C1/2

3" and under D3

7" and under D7

Lumpy over 16" to be spesified D16

X = actual maximum size Dx

Irregular Stringy, fibrous, cylindrical, slabs,etc E

Flowability Very free flowing flow function > 10 1

free flowing 4 < flow function < 10 2

average flowability 2 < flow function < 4 3

sluggish flow function < 2 4

Abrasiveness Midly abrasive index 1 - 17 5

Moderately abrasive index 18 - 67 6

Extremely abrasive index 68 - 416 7

MATERIAL CODE & BULK MATERIAL CHARACTERISTICS

Table 2-2

Material Characteristic

Material Weight

Material Code Component Series Mat'. Factor Vertical lb/ft3

Ash, black ground 105 105B635 1A-1B-1C 2.0 -

Ashes, coal, dry __ 1.2" 35 - 45 40C1/246TY 3D 3.0 V

Ashes, coal, dry __3" 35 - 40 38D346T 3D 2.5 -

Cement, Clinker 75 - 95 85D336 3D 1.8 -

Cement, Portland 94 94A10026M 2D 1.4 V

Cement, Aerated (Portland) 60 - 75 68A10026M 2D 1.4 V

Coal, Anthracite, sized__1/2" 49 - 61 55C1/225 2A-2B 1.0 -

Coal, Bitumious, mined 40 - 60 50D335LNXY 1A-1B 0.9 -

Coal, Bitumious, slack 43 - 50 47C1/245T 2A-2B 0.9 -

Dolomite, crushed 80 - 100 90C1/236 2D 2.0 -

Dolomite, lumpy 90 - 100 95Dx36 2D 2.0 -

Fly Ash 30 - 45 38A4036M 3D 2.0 -

Gypsum, raw__1" 70 - 80 75D325 2D 2.0

Limestone, crushed 85 - 90 88Dx36 2D 2.0

Limestone, dust 55 - 95 75A4046MY 2D 1.6 - 2.0

BULK MATERIAL

CHARACTERISTIC:

A study has been made to

define the characteristic of

bulk materials in terms which

are readily recognized.

The table shows in the first

column the range of density

that is usually experienced in

handling material

The material table is a guide

only. The material code and

the material factor (Fm) are

based on experience of several

conveyor manufacturers.

SELECTION OF CONVEYOR SIZE AND SPEED – 1

MAXIMUM LUMP SIZE

The Allowable size of a lump in a screw

conveyor is a function of the radial

clearance between the outside diameter

of the central pipe and the radius of the

inside of the screw trough.

Table Maximum Lump Size

Class 1 Class 2 Class 2

10% lumps 25% lumps 95% lumps Ratio

Ratio R=1.75 Ratio R=2.5 R=4.5

Max. Lump (inch.) Max. Lump (inch.) Max. Lump (inch.)

6 2 3/8 2 5/16 1 1/4 3/4 1/2

9 2 3/8 3 13/16 2 1/4 1 1/2 3/4

9 2 7/8 3 9/16 2 1/4 1 1/2 3/4

12 2 7/8 5 1/16 2 3/4 2 1

12 3 1/2 4 3/4 2 3/4 2 1

12 4 4 1/2 2 3/4 2 1

14 3 1/2 5 3/4 3 1/4 2 1/2 1 1/4

14 4 5 1/2 3 1/4 2 1/2 1 1/4

16 4 6 1/2 3 3/4 2 3/4 1 1/2

16 4 1/2 6 1/4 3 3/4 2 3/4 1 1/2

18 4 7 1/2 4 1/4 3 1 3/4

18 4 1/2 7 1/4 4 1/4 3 1 3/4

20 4 8 1/2 4 3/4 3 1/2 2

20 4 1/2 8 1/4 4 3/4 3 1/2 2

24 4 1/2 10 1/4 6 3 3/4 2 1/2

Radial

Clearance

(inches)

Pipe OD

(inches)

Screw

dia.

(inches)

SELECTION OF CONVEYOR SIZE AND SPEED – 1

CONVEYOR SIZE & SPEED

In order to determine the size and speed of a screw conveor, it is necessary first to establish the material code number. This Code number controls the cross-sectional loading that should be used.

The capacity table (below) gives the capacities in ft3/hour at one revolution per minutes for various sized and various cross-sectional loading.

The basis for the Capacity Table is as follows

察 = 宋.挿��� 拶�匝 − 拶�匝 皿 皐 掃宋層挿匝�

Where:

C : Capacity [ft3/hour.rpm]

Ds : Diameter of screw [inches]

Dp : Diameter of pipe [inches]

P : pitch of screw, [inches]

K : percent trough loading

SELECTION OF CONVEYOR SIZE AND SPEED – 2

In order to determine the conveyor speed can

be calculated by the formula:

� = 晒.察�宋.察�層 .察�匝 .察�惣察層

Where:

N : Conveyor speed [rpm],

ふ≤ recoﾏﾏeﾐded ﾏax. Rpﾏぶ Q : Required capacity [ft3/h]

C1 : Conv. Capacity at one rpm (see table below)

CFo : Overload capacity factor [110% - 120%]

CF1 : Conveyor pitch factor

CF2 : Type of flight factor

CF3 : Mixing paddle factor

Pitch CF1

Standard Pitch = diameter of screw 1

Short Pitch = 2/3 diameter of screw 1.5

Half Pitch = 1/2 diameter of screw 2

Long Pitch = 1 1/2 diameter of screw 0.67

Conveyor Loading

15% 30% 45%

Cut Flight 1.95 1.57 1.43

Cut & Folded Flight NR 3.75 2.54

Ribbon Flight 1.04 1.37 1.62

None 1 2 3 4

1 1.08 1.16 1.24 1.32

Compiled by Masda Ehsan

Special Conveyor Mixing Paddle Capacity Factor, CF3

Factor CF3

Standard paddles per pitch set at 45o reverse pitch

Special Conveyor Pitch Capacity Factor, CF1

Special Conveyor Flight Capacity Factor, CF2

Type of Flight

Description

SELECTION OF CONVEYOR SIZE AND SPEED – 3

SCREW CONVEYOR CAPACITIES

at max. RPM at one RPM

A-15 45% 6 165 368 2.23

A-25 9 155 1,270 8.20

B-15 12 145 2,820 19.40

B-25 14 140 4,370 31.20

C-15 16 130 6,060 46.70

C-25 18 120 8,120 67.60

20 110 10,300 93.70

24 100 16,400 164.00

at max. RPM at one RPM

A-16 D-16 30% 6 60 90 1.49

A-26 D-26 9 55 300 5.45

A--36 D-36 12 50 645 12.90

A-46 D-46 14 50 1,040 20.80

B-16 E-16 16 45 1,400 31.20

B-26 E-26 18 45 2,025 45.00

B-36 E-36 20 40 2,500 62.50

B-46 E-46 24 40 4,360 109.00

C-16

C-26

C-36

C-46

Material Class

Code

Degree of

Trough Loading

Screw Dia.

(inches)

Max.

RPM

Capacity (ft3/hour)

Capacity (ft3/hour)Degree of

Trough Loading

Screw Dia.

(inches)

Max.

RPM

Material Class

Code

SCREW CONVEYOR CAPACITIES

at max. RPM at one RPM

A-35 E-15 30% 6 120 180 1.49

A-45 E-25 9 100 545 5.45

B-35 E-35 12 90 1,160 12.90

B-45 E-45 14 85 1,770 20.80

C-35 16 80 2,500 31.20

C-45 18 75 3,380 45.00

D-15 20 70 4,370 62.50

D-25 24 65 7,100 109.00

D-35

D-45

at max. RPM at one RPM

A-17 D-17 15% 6 60 45 0.75

A-27 D-27 9 55 150 2.72

A-37 D-37 12 50 325 6.46

A-47 D-47 14 50 520 10.40

B-17 E-17 16 45 700 15.60

B-27 E-27 18 45 1,010 22.50

B-37 E-37 20 40 1,250 31.20

B-47 E-47 24 40 2,180 54.60

C-17

C-27

C-37

C-47

Material Class

Code

Degree of

Trough Loading

Screw Dia.

(inches)

Max.

RPM

Capacity (ft3/hour)

Material Class

Code

Degree of

Trough Loading

Screw Dia.

(inches)

Max.

RPM

Capacity (ft3/hour)

COMPONENT GROUP NORMAL SERVICE HEAVY SERVICE

Trough Cover Trough Cover

6 1 1/2 16 ga. 16 ga. 6 1 1/2 14 ga. 16 ga.

9 1 1/2 14 ga. 14 ga. 9 1 1/2 10 ga. 14 ga.

9 2 14 ga. 14 ga. 9 2 10 ga. 14 ga.

12 2 12 ga. 14 ga. 12 2 3/16 14 ga.

12 2 7/16 12 ga. 14 ga. 12 2 7/16 3/16 14 ga.

14 2 7/16 12 ga. 14 ga. 14 3 3/16 14 ga.

16 3 12 ga. 14 ga. 16 3 3/16 14 ga.

18 3 10 ga. 12 ga. 18 3 3/16 12 ga.

20 3 10 ga. 12 ga. 20 3 3/16 12 ga.

24 3 7/16 10 ga. 12 ga. 24 3 7/16 3/16 12 ga.

Component Group 2A, 2B, 2C and 2D

Regular flights and heavy trough

Screw Dia.

(inches)

Coupling Dia.

(inches)

Thickness, US standard

gauge or inches

Thickness, US standard

gauge or inchesCoupling Dia.

(inches)

Screw Dia.

(inches)

Component Group 1A, 1B and 1C

Regular flights and regular trough

EXTRA HEAVY SERVICE

Trough Cover

6 1 1/2 14 ga. 16 ga.

9 1 1/2 10 ga. 14 ga.

9 2 10 ga. 14 ga.

12 2 3/16 14 ga.

12 2 7/16 3/16 14 ga.

14 3 3/16 14 ga.

16 3 3/16 14 ga.

18 3 3/16 12 ga.

20 3 3/16 12 ga.

24 3 7/16 3/16 12 ga.

Component Group 3A, 3B and 3D

Regular flights and heavy trough

Screw Dia.

(inches)

Coupling Dia.

(inches)

Thickness, US standard

gauge or inches

HORSEPOWER REQUIREMENT (HORIZONTAL SCREW CONV.)

The horsepower required to operate a horizontal screw conveyor is based on proper

installation, uniform and regular feed rate.

The horse power requirement is the total of the horsepower to overcome conveyor friction

(HPf) and the horsepower to transport the material at specified rate (HPm)

�皿讃 = 鯖.�.�纂 .��層宋宋宋宋宋宋

�皿� = 察. 鯖.�.�讃 .�� .�� 層宋宋宋宋宋宋

�皿 = �皿讃 + �皿� ��蚕

Where:

C : Equivalent design capacity [ft3/h], e : Drive efficiency

L : Total length of conveyor [ft] N : operating speed [rpm]

Fb : Hanger bearing factor Fm : Material factor

Ff : Flight Factor Fo : Power overload factor

Fd : Conveyor diameter factor

HORSEPOWER REQUIREMENT (HORIZONTAL SCREW CONV.)

Hanger Bearing Factor, Fb Flight Factor, Ff

Component Group Bearing Type Fb

Group A Ball 1.0 15% 30% 45% 95%

Group B Babbit 1.7 Standard 1.0 1.0 1.0 1.0

Bronze Cut Flight 1.10 1.15 1.20 1.30

Wood Cut & Folded Flight NR 1.50 1.70 2.20

Group C Plastic 2.0 Ribbon Flight 1.05 1.14 1.20 -

Nylon

Teflon Paddle Factor, Fp

Group D Chilled hard iron 4.4

Hardened alloy sleeve None 1 2 3 4

Fp 1 1.29 1.58 1.87 2.16

Screw Diameter Factor, Fd

Screw Dia. (inch) Fd Mechanical Efficiencies of Speed Reduction Mechanisms

4 12

6 18

9 31 V-belts and sheaves 0.94

10 37 Precision roller chain on cut tooth sprockets, open guard 0.93

12 55 Precision roller chain on cut tooth sprockets, oil tights casing 0.94

14 78 Single Reduction helical or herringbone eclosed gear reducer or gearmotor 0.95

16 106 Double Reduction helical or herringbone eclosed gear reducer or gearmotor 0.94

18 135 Triple Reduction helical or herringbone eclosed gear reducer or gearmotor 0.93

20 165 Low ratio (upto 20:1 range) enclosed worm gear speed reducers 0.9

24 235 Medium ratio (20:1 to 60:1 ratio range) enclosed worm gear speed reducers 0.7

Compiled by Masda Ehsan Gigh ratio (60:1 to 100:1 ratio range) enclosed worm gear speed reducers 0.5

Note : Contact Gear Reducer Manufacturer for the exact Mechanical Efficiencies

Type of Speed Reduction MechanismApproximate

Efficiencies

Type of FlightConveyor Loading

Standard paddles per pitch set at 45o reverse pitch

HORSEPOWER REQUIREMENT (HORIZONTAL SCREW CONV.)

HORSEPOWER REQUIREMENT (HORIZONTAL SCREW CONV.)

HORSEPOWER REQUIREMENT (INCLINED SCREW CONV.)

ADVANTAGES: Inclined screw conveyor is often very desirable as it may solve a conveying

problem with a minimum of equipment and occupy a minimum space

DISADVANTAGES:

The capacity, or the maximum available capacity of a given screw conveyor decreases with

increase of incline

The horse power per-unit capacity increases.

ADDITIONAL POWER to lift the material, the formula to calculate horsepower to lift the

material as follow:

�皿� = 察 .� .�惣惣宋宋宋 . (掃宋)

�皿 = �皿讃 + �皿� + �皿� ��蚕

Where:

HPh: Horsepower to lift the material [HP]

C : Equivalent design capacity [ft3/h]

h : actual height of lift [ft]

e : Drive efficiency

PROBLEMS ASSOCIATED WITH INCLINED SCREW CONVEYOR

Several things can be done to overcome many of the problems associated with inclined screw

conveyor:

Limit the use of standard screw components to inclines of less than 25o, preferably not

over 15o

Use close clearance between trough and screw

Increase the speed over that applicable for a horizontal screw conveyor of the same size

Use short pitch screws, 2/3 or ½ pitch