Screw Conveyors 270

CHAPTER 10
SCREW CONVEYORS
10.1Introduction

The modern screw conveyor is essentially a development of the well-known

Archimedean screw which was conceived some 2000 years ago as a means of raising
water for irrigation. Applications of this device were naturally very limited until
relatively recent times, and its evolution has consequently been slow. A fundamental
feature or the original pattern of Archimedean screw which distinguishes it from other
types of screw conveyor is that the helical screw (or 'flight') is attached to the inner
surface of the cylindrical casing and rotates with it.

In the late 18th century the need for efficient mechanical handling of grain to feed the
expanding world population provided the impetus for the improvement of screw
conveyors for the transport of grain and other dry bulk materials.

In the earliest recorded examples of screw conveyors the flights consisted simply of a
series of wooden ploughs set in a helical arrangement around a wooden shaft. Later
versions used steel flights cut from nut sheet as circular rings, split on one side and
with the two edges then pulled apart to form one helical section of the screw, Any
number of these sections could be riveted together to make a continuous helix of the
required length which would then be fitted to a steel or iron core. Around 900 the
smooth 'helicoid' flight was introduced, formed by rolling a continuous strip of steel
into a helix.

The screw conveyor, one of the oldest and simplest methods used for the movement
of bulk materials, consists of a long-pitch plate-steel helix mounted on a shaft or
spindle carried in bearings within a U-shaped trough. As the element rotates, the
material fed to it is moved forward by the thrust of the lower part of the helix and is
discharged through openings in the trough bottom or at the end.

Screw Conveyors are employed for transportation of pulverized, granular and, less
frequently, small-size lumpy bulk materials to relatively short distances (usually up to
4.0 m horizontally or to heights up to 30 m) in the chemical, flour industries, and in
the manufacture of building materials.
 Screw Conveyors 271

Fig.10.1 Horizontal Screw Conveyor

The movement of the materials is forced through the trough by a rotating screw
(Fig.10.1), which is formed by a helical blade attached to the drive shaft 8 which is
coupled to a drive 1and supported by end bearings 2,6 and by inner bearings 4. The
trough 7 of the round-bottom shape is topped by a cover plate 3 with an opening 5 for
loading the conveyor. A similar unloading opening 9 is provided in the bottom of the
trough. The loading and unloading points can be located anywhere along the trough.

Among the advantages of screw conveyers are simple design, easy maintenance,
small dimensions, convenient intermediate unloading, and tightness which are of
especially high value when conveying dusty, hot or odorous loads. Their drawbacks
are high unit power consumption. substantial rubbing and crushing of the conveyed
material, high wear of the screw and trough, and a high sensitivity to overloads which
can lead to load accumulation in the trough (especially at the bearings).

10.2 Conveyor Components

10.2.1 The Conveyor Screw

This is the rotating part of the screw conveyor which imparts smooth and positive
motion to the bulk material being conveyed. It consists of spiral flying mounted on a
pipe and is made either right or left hand to suit the screw rotation and the desired
direction of material travel.
 Screw Conveyors 272

Fig.10.2 Typical Screw Configurations: (a) Helical Spiral (b) Ribbon Spiral
(c) Cut Flights

10.2.2 The Drive Shaft, End Shaft and Coupling

The drive shaft supports the conveyor screw section and keeps it in alignment. The
end shaft is located at the end opposite the drive shaft. Couplings are used to connect
successive conveyors screw section when more than one section is necessary to make
up the total length of conveyor. The shaft and coupling are secured in the conveyor
screw by coupling bolts as shown in Fig.12.3.

Fig.10.3 Installing Screw Section in Trough and Fitting it onto Coupling

 Screw Conveyors 273

10.2.3 The End Seals

The plate seal (Fig.10.4a) is an economical, effective sealing device, designed for
exterior mounting between the end bearing and the trough end. Standard units employ
lip type seals to contact the shaft but other types of commercial seal cartridges also may
be used. The seal plate and the end bearing are bolted to the trough end by one set of
bolts. Split gland seals (Fig.10.4b) are designed for interior or exterior mounting. They
provide a seal which is effective for many applications. The universal type of seal
(fig.10.4c) is arranged for use with waste packing or with cartridge type lip or felt seals.
An opening at the top of the seal housing facilitates waste repacking, and exposes the
waste for oiling. The packing seal housing is mounted outside the trough and between it
and the end bearing.

Packing gland seals (Fig.10.4d) are effective means for sealing the conveyor both
internally and externally. This seal also is sometimes suitable for pressure or vacuum
service. The packing pressure is adjusted by the gland bolts. Air purge shaft seals
(Fig.10.4e) are arranged for attaching to standard or special trough ends. A constant air
pressure is maintained to prevent material from escaping from the trough along the shaft.
The air purge seal is desirable for sealing highly abrasive materials.

(a) (b) (c)

(d) (e)

Fig.10.4 Various End Seals

10.2.4 The Conveyor Complete with the Trough and the Drive

The trough is the enclosure in which the material is confined and guided in its
movement. A shaft mounted speed reducer makes a simple and compact drive
combination.
 Screw Conveyors 274

10.2.5 Typical Drive Arrangements

Screw conveyers exist in three main types: horizontal, vertical and inclined screw
conveyors. The three types have certain features in common, but differ in other
features and especially in the arrangement.
Some of the drive arrangements are presented in Fig.10.5 below.

a. Horizontal Screw Conveyers

A horizontal (or slightly inclined) screw conveyer (Fig. 10.1) has a screw arranged on
a bearing-supported shaft, a trough with a semi cylindrical bottom arranged coaxially
with the screw, and a drive (electric motor and reducer gear) to rotate the shaft with
screw. A bulk material is fed to the trough through one or more openings in its cover;
as the screw rotates, the material is propelled along the shaft, as a nut would be moved
on a threaded bar if retained from rotation together with the bar. Rotation of the
material together with the screw is prevented by its gravity force and friction on the
trough walls. The material is unloaded from the e conveyer through one or more
openings in the bottom, which are provided with gates.

The conveyer screw may be right or left-handed, with one, two or three leads. The
screw surface may be continuous or discontinuous: band-like or in the form of
separate blades. Continuous screws are used mainly for conveying dry fine-granular
and pulverized materials not liable to slumping; discontinuous screws are effective
with slumping materials. Besides, blade- and band-like screws are used in cases when
thorough intermixing of the material during conveying is essential.

b. Vertical Screw Conveyers

Vertical screw conveyers relate to special types (Fig. 10.5a, and b). The conveyer has
a shaft with a continuous helix, which is suspended on a thrust bearing and rotates in a
cylindrical casing (tube), and a short horizontal screw feeder which rotates in a short
pipe connection. The two screws may be driven from a common drive (Fig. 10.5a) or
separately (Fig. 10.5b). The conveyer is unloaded through a pipe connection at the top
of the casing. The bottom portion of the vertical screw, which receives the fed
material, is made with a varying (decreasing upward) diameter or with a smaller
screw lead. With a large height of conveyer, intermediate bearings are sometimes
provided to prevent radial run-out of the shaft.

The upward motion of bulk material along the vertical screw occurs as follows. As the
material is fed by a horizontal feeder screw to the vertical screw, it is rotated by the
latter. The material is pressed by centrifugal forces to the cylindrical walls of the
conveyer casing. Due to the forces of gravity and friction it lags somewhat behind the
motion of the helix, i.e. rotates at a smaller angular speed than that of the screw;
therefore, it is moved upwards along the screw axis, similar to a nut moving on a
 Screw Conveyors 275

thread, along a helical path, but with a lower axial speed than the speed of a retained
nut which would move along the screw.

In order to develop a sufficiently high centrifugal force, the screw should rotate with a
sufficient frequency, i.e. in contrast to horizontal screws whose rotational speed is
limited by the highest value, the speed of vertical screws should be not less than an
allowable (critical) lowest value which is required to move the material upwards
along the screw.

c. Inclined Conveyors

Screw conveyors can be operated with the path inclined upward, but the capacity
decreases rapidly as the inclination increases. A standard pitch screw inclined at 15°
with the horizontal has 70 per cent of its horizontal capacity; if it is inclined 25° the
capacity is reduced to 45 per cent; if it is inclined 45° the material will move along the
floor of the trough but at a greatly reduced rate. For steep inclines the helix may have
a short pitch and the trough may be made tubular; then the capacity loss is less. Thus
with a jam feed and 45° incline such a conveyor has a capacity about 50 per cent of its
capacity when horizontal.

The bearing hangers of inclined conveyors should be of T design to reduce the

interference with the flow of the material. It may be desirable to use a few turns of
double helix each side of the bearing hangers. With suitable forced feed a screw
conveyor in a vertical position will lift material if the rotating speed is high and there
are no intermediate bearings. Such a conveyor is not self-clearing and should not be
used for material which will sour or spoil.
 Screw Conveyors 276

Fig.10.5 Typical Drive Arrangements: (a) Vertical (b) Horizontal and Vertical
(c) Steep Slop

10.3 Typical Applications

Screw conveyors serve wide variety of purpose in many industries. Some of the
application areas are:

1. When the materials are extremely hot, cast screws and troughs may be used or
the screws and troughs may be made of high temperature alloy metals.
2. If the materials are sticky or viscous, ribbon flight screws may be the choice.
Furthermore, special coatings applied to the screw and troughs may also aid
the flow of the material.
3. When extremely abrasive materials are to be conveyed they may require
screws and troughs made of abrasion resistant metals or the screws may be
provided with hard surface flights.
4. When the materials are corrosive it may be desirable to make the conveyor
screws and troughs of stainless steel, Monel metal, nickel, aluminium, etc.
5. When the materials are to be mixed or aerated a conveyor screw of ribbon
flights or cut flights or one of these combined with paddles may be used to
obtain the desired results.
 Screw Conveyors 277

6. If materials are to be heated or cooled, which conveying they may require

jacketed troughs arranged for circulating heating or cooling media.
7. When contaminable materials are handled they may require self lubricated
bearings, screw and trough construction which will eliminate pockets, creels,
etc.

10.4 Design Considerations

The two essential parameters to be established in the design or selection of a screw
conveyor for a given application are the screw size, and its rotational speed. The
choice of a suitable screw size involves consideration of the overall diameter of the
screw, the diameter of the shaft, the radial clearance between the shaft and the
containing trough and the type and pitch of the helical flight. However, reliable
trouble-free operation of screw feeders and conveyors requires careful consideration
of many factors during design. Some of the important design criteria are:
 Properties of material to be conveyed
 Flight type
 Conveyor screw size
 Maximum lump size
 Required duty of construction
 Properties of material to be conveyed
Knowledge of material characteristics is extremely important when designing screw
conveyors. Of particular importance is the lump size, abrasiveness, corrosiveness,
bulk density and flowability.

Correct interpretation of this data will ensure that the correct flight type is used and
the materials of construction, screw size, flight clearances and the rotational speed are
appropriate for the application.

The flowability of the material will assist with power calculations. Material which
flows easily and has a low angle of repose will generally convey with less power than
cohesive materials with higher angles of repose.

Particular care should be exercised when handling products that aerate easily (e.g. fly
ash, cement, talc) or when handling hydroscopic or damp materials.

10.4.1 Flight type

Selection of the most appropriate flight type can be done after a review of the material
properties. Standard screw flights can be manufactured in varying thicknesses and
materials. Conveying abrasive materials may require the use of bisalloy or other hard
wearing metals depending on screw speeds.
 Screw Conveyors 278

In addition to standard flights, which can be produced in varying pitches, special

flight designs are also available for many applications.
Right Hand flights are most commonly used on screws, although it can be designed to
convey in either direction, and special bi-directional designs are also available. (Fig
10.6).

Fig. 10.6 Right hand and left hand flight

10.4.2 Screw conveyor size

The conveyor screw size is directly related to the material bulk density and the
required throughput but several other factors must be considered. To maximise wear
life, screw speed should be kept to a minimum. If hanger bearings are fitted, the screw
filling level should be kept below the bearing to minimise bearing contamination.
Maximum lump size and the percentage of lumps in the product must be considered.
Maximum lump size

The first and overriding consideration in the determination of a suitable screw

diameter is the amount and size of lumps (greater than 15 mm across the largest
dimension) present in the product to be conveyed. The presence of large hard lumps
may necessitate the use of a screw of significantly greater diameter than would be
indicated by the mass throughput required. As a guide, the radial clearance between
the shaft and the casing should be 1.75 to 3 times the size of the largest lump in the
conveyed product, and up to 4.5 times this dimension if the proportion of lumps is
very high (greater than about 90%). Fig 11.12 allows the selection of screw conveyor
size for products of various lump sizes. It should be noted, however, that the nature of
the lumps may be relevant; so that, for example, if the lumps are soft and readily
degradable they should impose no limitation on the size of the screw. When a screw
conveyor / feeder is required to handle material containing a large percentage of
 Screw Conveyors 279

lumps that do not easily crush, casing clearances, screw diameters and power
calculations must be reviewed accordingly.
The trough is commonly fabricated from flat sheet from 2 and 8mm thick. The screw
pitch t = (0.5 to 1.0) D, where D is the screw diameter. The screw diameter D is
governed by the lump size, being at least twelve times that for loads of uniform lump
size and at least four times the maximum lump size in case of unsized bulk materials.

The screw speed is influenced by the nature of load and screw diameter, increasing
inversely with the bulk weight, screw diameter and the intensity of abrasive action of the
material. Conveyors handling heavy materials operate at around 50rpm and those
designed to convey light loads, at up to 150rpm.

The cross-sectional loading of a screw conveyor is given by

 d2
A = (10.1)
4

where  = the capacity factor which take account of the accumulation of

load at inner bearing (Table 12.2)
D = screw diameter (see table 10.1)

As per Ethiopian Standards ES1050:1975, the recommended diameter is given in

table10.1

Table 10.1 Nominal diameter of screw conveyors (mm) ES1050:1975

D 100 125 160 200 250 315 400 500 630 800 1000 1250

Table 12.2 Values Capacity Factor

Material 
Heavy-weight abrasive loads 0.125
Heavy-weight mildly abrasive 0.250
Light-weight mildly abrasive 0.320
Light-weight non abrasive 0.400

The hourly capacity can be calculated by

Q = 3.6 A v k tons/hr (10.2)

where k is a factor introduced in designing inclined

conveyors(Table12.3)
v = the speed of the conveyor [m/s]
 = specific weight of the material [kg/m3]
 Screw Conveyors 280

Table 10.3 Values of k corresponding the Inclination 

 (degrees) 0 5 10 15 20
k 1.0 0.9 0.8 0.7 0.75

The speed of the conveyor

tn
v= (10.3)
60

where t = pitch of the screw (lead) [m]( Table 12.4)

n = rpm of the screw
As per Ethiopian Standard 1050:1975, the recommended pitch is given in table10.4

Table 10.4 values of pitch of the screw (lead) in mm

t 80 100 125 160 200 250 315 355 400 450 500 560 630 800 1000

The capacity formula can be rewritten,

 D2 tn
Q = 3600  k
4 60

 47  t  n  D 2  k   (10.4)

From practical experience,

n = 60/ D for light non abrasiveload

n = 45/ D for heavy non abrasiveload
n = 30/ D for heavy abrasiveload

The power requirement in kW at the drive shaft for horizontal conveyors,

QL
N h  C0 (10.5)
367

Where N h = power requirement for horizontal conveyor

Co = friction factor (Table 10.5)
L = conveyor length [m]
 Screw Conveyors 281

and for sloping installation

QH QL
Ns   C0 (10.6)
367 367

where N s = power requirement for inclined conveyor

H = level difference

Load per meter [kg/m]

Q
q= (10.7)
3.6 ν

Axial force, P [kg]

Mo
P (10.8)
r tan   

102N o  60
Mo 
2n

N0
 975 kgm (10.9)
n

Where M 0 = the resisting moment

N 0 = N h for horizontal conveyor
= N s for inclined conveyor
r = radius at which the force P is applied r  0.7 to 0.8  
D
2
 = reduced friction angle, tan  = 
 = screw helix angle

The friction factor Co is adopted based on experimental data.

Table 10.5 Friction Factor Co

Material Co
Flour, cereal, saw dust 1.2
Peat, Soda ash, pulverised coal, finely ground chalk 1.6
Coal (lump anthracite and bituminous, air dry brown), rock salt 2.5
Gypsum, dry clay, sand, cement, ash, lime, moulding sand 4.0
 Screw Conveyors 282

Example 10.1

Calculation of a Screw Conveyor

Design a horizontal screw conveyor to meet the following conditions.

- material to be conveyed: moulding sand

- required capacity: Q =35 tons/hr
- the conveying run length: L = 20m
- bulk weight of the material:  = tons/m3

Solution:

1. Main parameters of the conveyor. Considering that the material is heavy and
abrasive, we take the trough loading efficiency  = 0.125 and the screw pitch
t  0.8 D . Hence, shaft speed will also be low, i.e. n  37.5 rpm. A reducer with two
spur gear pairs will suit the purpose.

Substituting t  0.8 D in equation 12.4, we obtain:

4Q 4  35
D3 3
60  0.8  n     k 60  3.14  0.8  37.5  0.125  1.65  1

 0.495m  0.5m.

2. Required power. The resistance to motion factor for moulding sand being Co  4 ,
we determine the power required on the screw shaft from equation 12.5:

QL 35  20
N h  C0  4  7.7 kW
367 367

Taking the efficiency of the reducer with two spur gears   0.9 , the required motor
power will be

Nh 7 .7
N   8.5kW
 0 .9

The torque transmitted by the electric motor to the screw shaft is obtained from
equation 10.9:
 Screw Conveyors 283

N0 7 .7
M o  975  975  200kgm
n 37.5

3. Load propulsion rate ( t  0.8D  0.8  0.5  0.4m ), and from equation 10.3

tn 0.4  37.5
v   0.25m / sec.
60 60

4. The load per meter of conveyor length is obtained from equation 10.7:

Q 35
q=   38.8 kg / m.
3.6 ν 3.6  0.25

5. Axial force along the screw: this can be obtained from equation 10.8 as

D 0 .5
r  0.8   0.8   0.2 m.
2 2

  380 40'  f  0.8  ,   17 0 40'

hence,
Mo 200
P   660 kg .
r tan    0.2  1.5

The shaft is subjected to a complex load made up of

1. the torque with moment M 0 ;

2. tension (compression) by force P;
3. the bending effort between bearings due to the own weight;
4. the transverse component acting on the screw surface and screw edge, and
5. the longitudinal force on this section exerted eccentrically on the turns.

Since not all of these forces can be determined precisely, the calculation for the strength
of the components is usually simplified by estimating according to M 0 and P.
 Screw Conveyors 284

10.5 Review Questions

1. Describe a screw conveyor and its basic components
2. State the typical applications of a screw conveyor.
3. Discuss the different drive arrangement in screw conveyors.
4. What considerations are taken in designing a screw conveyor?
5. Design a screw conveyor to meet the following conditions.
- material to be conveyed; coal
- required capacity; 10 tons/hr
- length of the conveyor; 12 m
- arrangement; inclined (150)
 Screw Conveyors 285

Bibliography

1. Link-Belt Catalogue, 1958. Material Handling and Processing Equipment,

U.S.A.

2. Frederic V. Hetzl, Russell K.Albright, 1941. Belt Conveyors and Belt

Elevators. 3rd edn. New York: John Wiley and Sons, Inc.; London: Chapman
and Hall, Limited.

3. Conveyor Equipment Manufacturers Association, 2002. Belt Conveyors for

Bulk Materials. 5th edn, USA.

4. Patric M., McGuire, P.E, 2010, Conveyors: Application, Selection and

Integration, Taylor and Francis Group, London.

5. Siddhartha Ray, 2008. Introduction to Materials Handling, New Age

International Limited Publishers, New Delhi.

6. A. Spivakovisky and V. Dyachkov, “Conveyors and Related Equipment”,

Peace Publishers, Moscow.

7. Wilbur. G. Hudson. 1954. “Conveyors and Related Equipment”, John Wiley

and Sons, New York,
8. Monte, Armando. 2009. Elementi di Impianti Industrial. Torino: Librenia
Contina.
 Screw Conveyors 286

CHAPTER TWELVE .......................................................................................................................... 270

SCREW CONVEYORS ....................................................................................................................... 270
12.1 Introduction .................................................................................................................................. 270
12.2 Conveyor Components ................................................................................................................. 271
12.3 Typical Applications .................................................................................................................... 276
12.4 Design Considerations.................................................................................................................. 277
12.5 Review Questions ......................................................................................................................... 284
Bibliography......................................................................................................................................... 285

Index
Air purge shaft seals, 273 Ribbon Spiral, 272
capacity, 279 Right hand and left hand flight, 278
Conveyor Components, 271 Screw conveyor size, 278
Couplings, 272 SCREW CONVEYORS, 270
Cut Flights, 272 Split gland seals, 273
Design Considerations, 277 The Conveyor Screw, 271
Flight type, 277 The Drive Shaft, End Shaft and Coupling,
Friction Factor, 281 272
Helical Spiral, 272 The End Seals, 273
Horizontal Screw Conveyers, 274 Typical Applications, 276
Horizontal Screw Conveyor, 271 Typical Drive Arrangements, 274
Inclined Conveyors, 275 universal type of seal, 273
Nominal diameter of screw conveyors, 279 Values Capacity Factor, 279
Packing gland seals, 273 values of pitch of the screw, 280
plate seal, 273 Vertical Screw Conveyers, 274