ASSESS~1

Assessment of the Digging Force and

Optimum Selection of the Mechanical and Operational Parameters of Bucket Wheel Excavators

for Mining of Overburden, Coal and Partings

Author: Dr.-Ing. Viktor Raaz Senior Engineer Research & Development Fon: ++ 201/828-4551 Fax: ++ 201/828-4830 E-Mail: [email protected]

Krupp Frdertechnik GmbH Altendorfer Str. 120

D-45143 Essen/Germany Fon ++ 201/828-04

Fax ++ 201/828-4830 Internet: http://www.thyssenkrupp.com

2 2

Introduction

The mining of harder materials with Bucket Wheel Excavators (BWEs) requires an

optimum adaptation of the mechanical and operational parameters of the BWEs to

the material properties.

The variety of factors influencing the cutting forces and the wear of the cutting

elements, as well as the complex dependencies of these factors, require a systematic

procedure for the investigation of the materials to be excavated. The results of these

investigations are the basis for the optimisation of the equipment parameters. This

optimisation has to be seen as a continuous process throughout the whole

development period of a BWE, i.e. starting from the design phase through the

execution phase until start-up of the commercial operation.

This paper describes the procedure followed by Krupp Frdertechnik for the

assessment of the necessary cutting force respectively cutting energy and the

optimisation of the mechanical and operational parameters of BWEs for the specific

application.

Influencing factors and dependencies Krupps longstanding experience with BWEs digging of hard materials clearly

demonstrate the interrelations between the specific material properties and the

mechanical and operational parameters on the one hand, and the cutting force,

energy requirement and its influence on the wear of the cutting tools, on the other

hand.

Fig. 1 shows the influence and the dependencies of the energy requirement and the

wear of the cutting tools of BWEs during operation, on the basis of the specific

material properties, the shape of the cutting tools, the BWE geometry and the

selected mining method.

3 3

Fig. 1. Factors influencing the energy requirement and wear of the cutting tools

of BWEs

Req

uire

men

t ene

rgy

for l

iftin

gan

d fri

ctio

n lo

sses

, [kW

h/bm

3 ]En

ergy

requ

irem

ent f

or c

uttin

g,[k

Wh/

bm3 ]

Tota

l spe

zific

ene

rgy

requ

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ent

E SPE

Z, [k

Wh/

bm3 ]

Cut

ting

cros

s se

ctio

nSm

ashi

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men

tatio

n zo

nes

Ener

gy re

quire

men

t rel

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toth

e fr

actu

re s

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ce, [

kWh/

m2 ]

Initi

al fr

actu

re s

urfa

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m2 /m

3 ]N

ew fr

actu

re s

urfa

ce, [

m2 /m

3 ]

Gra

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met

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f exc

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Com

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and

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Spez

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4 4

The following gives a detailed summary of the material characteristics, machine

characteristics and process characteristics.

Material characteristics The material characteristics are decisive for the design, construction and the

selection of the parameters of the mining method of a BWE.

The most important material parameters for the assessment of the required cutting

force respectively required energy for the excavation of the material by BWEs are:

compressive strength

tensile strength

cleavage

fracture behaviour and

stickiness.

The abrasiveness is also of essence. It is decisive for the wear and can be

investigated by one of the numerous abrasiveness tests.

The compressive strength is determined in the laboratory [1]. The tested soil sample

must not deviate considerably from the calibrated lump size. For this purpose

undisturbed material samples of cylindrical or cubical shape are pressed between

two plane-parallel plates, applying a preset compression speed. Fig. 2.1.

5 5

P

A

Fig. 2.1 Compression test

Due to the difficulties of measuring the tensile strength of soil samples on an

undisturbed material element by means of the direct method, the so called

Brazilian Test is regularly used.

In the Brazilian Test a cylindrical soil sample is pressed, perpendicular to the cylinder

axis, between two plane-parallel plates [1]. Thus the splitting tensile strength of the

material is determined. Fig. 2.2.

A

P

Fig. 2.2 Brazilian Test

The tough-brittle fracture behaviour of a material can be characterised by the ratio

between the tensile strength and the compressive strength. While considering the

cleavage of the in-situ formation, the angles of cleft inside the material can also be

determinated.

6 6

In case of softer materials, cohesion and angle of inner friction are checked by a

shearing test. Fig. 2.3. It must be pointed out that the moisture, plasticity, and

consistency are also of high importance [1].

T A

T

P

Fig. 2.3 Shearing test

The wedge test- and the Point-Load-Test are in-situ-search methods, which permit a

quick analysis of an undisturbed material element in an open-pit mine. The Point-

Load-Test is a more suitable method for harder materials.

The wedge test is performed on a cylindrical or cubical soil sample with an edge

length of approx. 150mm. The wedge is loaded until the soil sample is cut. Fig. 2.4.

Thus the cutting force relative to the wedge length and the fracture surface is known.

In case of geometrical deviations of the test sample from the calibrated sample size,

a correction factor must be applied.

P

A

Fig. 2.4 Wedge test

7 7

For the Point-Load-Test samples of any shape are pressed between two cones

having rounded tips [1]. Fig. 2.5 and Fig. 3. Similar to the compressive test, tensile

test and wedge test, the strength index is the ratio between the fracture force and the

square of the equivalent fracture surface diameter.

A

P

Fig. 2.5 Point Load Test

The major advantage of the Point-Load-Test in comparison with the other strength

tests is the simple determination of the size correction factor specific of the material.

This factor is in most cases closely related to the initial fracture surface of the in-situ

formation. Fig.4.

This test method has further the advantage to give additional quantitative evidence of

the inhomogeneity of the in-situ material, by plotting the test results in a dispersion

curve.

To check the anisotropy of the in-situ material at all strength test methods, the

stratification or lamination of the tested materials must be considered as well.

A further in-situ test method is the determination of the seismic wave velocity. This

test gives information on an integrated characteristic value, which demonstrates the

interaction of all material properties in the open-pit mine.

8 8

Fig. 3. Point-Load-Testing device

1.000

10.000

100.000

100 1.000 10.000Cut surface

Forc

e

P ( Value Points)P=10*CA+CB*lg(A)Confidence limits

Fig. 4. Results of the Point-Load-Test

9 9

Traditionally, the cutting force is assessed on the basis of material characteristics via

the specific cutting-length of the cutting tools and/or cutting cross sections of all

buckets in the cut.

Empirical investigations tried to derive the dependencies of the cutting force related

to the specific cutting-length of the cutting tools or to the cutting cross sections of all

buckets in the cut from the material characteristics only [2, 3, 4, 5]. In view of the fact

that in this connection no parameters of the excavator design and no parameters of

the mining methods were considered, the findings obtained are of limited value only.

Alternatively to the above investigations, the effect of the material properties on the

assessment of the cutting force can be determined by the fracture surface-related

energy requirement, the fracturing behaviour of the material and the initial fracture

surface of the in-situ-formation. The effects of the relevant properties of the material

to be excavated, the effects of the excavator design and the effects of the selected

mining methods on the necessary energy and consequential wear are obvious.

BWE Mechanical Parameters The required output of the BWE, the shape of the mine and the mining bench

configuration are the key parameters for the geometrical layout of the BWE.

The most important geometrical parameters are the bucket wheel diameter, the

outreach of the bucket wheel, the pivot point coordinates of the bucket wheel boom

and finally the inclined position of the bucket wheel. Fig. 5.

For better discharge conditions of the buckets and/or improved free cutting properties

in the working block, the bucket wheel has a vertically and/or horizontally inclined

position relative to the longitudinal and vertical axes of the bucket wheel boom [3, 4].

These machine characteristics determine the geometry of the mining block to be cut

and influence substantially the effective output of the BWE [6].

10 10

Horizontal Inclination

Vertical Inclination

Cutting Circle

Boom Outreach (Length)

Slew

ing

Axi

s

Pivot Point (PP)

Pivot Axis

Slewing Point

PP-Distance

BW-Di

ameter

PP-H

eigh

t

Side View

Top ViewView A

A Axis of the Boom

Ground Level

Ahead Positionof the Bucket

Boom

Fig. 5. Geometrical parameters of the BWE

The cutting force is considerably influenced by the bucket design. Influencing factors

are, among others, the number and shape of the buckets, the arrangement of ripping

and cutting teeth, their shape, their arrangement on the cutting blades of the buckets

and the expected wear. Fig. 6.

Fig. 6. Simplified illustration of teeth shape and tooth arrangement on the cutting

blade of a bucket of a BWE

Side view

Top view

Rear view

3D-view

11 11

BWE Operating Parameters

A BWE can only be operated economically if the operating parameters are in

accordance with the technical date of the BWE and the characteristics of the material

to be excavated [6].

The selection of the mining method is given by the material properties, the design of

the BWE and the characteristics of the material to be excavated.

A block is removed by terrace cut, dropping cut or a combination of both. Very often,

the mining method is also a function of the preferred lump size of the excavated

material and/or its stickiness.

The mining geometry of the block is given by the block width and height, the terrace

height and the slope angles of the front and side faces. Fig. 7 and 8.

12 12

Side view

Top view

Rear view

3D-view

Fig. 7. The mining block geometry of a BWE in a terrace cut.

Fig. 8. The mining block geometry of a BWE in a combination. Dropping cut after

the first terrace cut.

Side view

Top view

Rear view

3D-view

13 13

By selecting the slice depths and the basic slewing speeds for each slice respectively

cut, the whole operating process of a BWE in the block can be illustrated. Fig. 9. The

maximum slewing angles for each terrace and slice of the selected block are

accordingly preset.

Fig. 9. Operating method of a BWE in a combined terrace and dropping cut.

The slewing operation during excavation is decisive and preset by the slewing

angles, the basic and maximum slewing speed, the slewing acceleration and the

slewing speed control.

Block excavation time

Advance Selective digging height Slewing speed Output

TimeTerrace cut Dropping cut

Qmax

14 14

In order to have a uniform output, the slewing speed of the bucket wheel boom will

be controlled according to a cosine function. Fig. 10.

Fig.10 Slewing speed and output during a slewing motion with corrected and

uncorrected speed control.

In order to attain a constant output during a slewing cycle within a slice especially in

the dropping cut a corresponding correction factor has to be considered in the

slewing speed control. Of course, this correction factor differs for the terrace cut and

dropping cut and depends especially on the selective cutting height of the bucket

wheel.

The reduced output at the start of the speed control range, as shown in Fig. 10, is

due to the fact that the bucket wheel cuts into the slope.

Cutting force and energy requirement

The width and the depth of each cutting cross section of any bucket depends on the

selected cutting depth, on the slewing angle of the bucket wheel boom and on the

Slewing time

Control range

Acceleration- and braking ranges

Output with correction of speed

Slewing speed with correctionOutput without correction of speed

Slewing speed without correction

15 15

actual cutting position of the bucket on the cutting circle. Of course, the cutting cross

section changes during slewing according to the selected slewing speed.

Fig. 11 shows the projection of a slice with the cutting circle and the possible cutting

position of a bucket at the front slope for a terrace cut.

Fig. 11. Projection of a slice on the front slope for a terrace cut.

The deviation of the cutting circle projection from the vertical line in the front position

of the bucket wheel boom depends on the horizontal and vertical inclination of the

bucket wheel.

According to the slewing speed, the cutting circle moves in each slice from the bench

side to the side slope and vice versa. This process continues until the material in

each terrace of the block is excavated. The actual cutting position of one bucket

moves according to the cutting speed of the bucket wheel from bottom to top.

Due to the vertical and/or horizontal inclinations of the bucket wheel, there are

differing cutting cross sections for slewing to the right and left side.

For any cutting position of the bucket on the cutting circle and any slewing position of

the bucket wheel boom the cutting contour and the cross section of the cut can be

shown. Fig. 12.

SliceFront positionof cutting circle

Actual cutting positionof one bucket

Cutting circle atside slope

Cutting circleat bench side

Terr

ace

/ Slic

e he

ight

Terrace / Slice width

16 16

Fig. 12. Cut geometry in the cutting position of the bucket for simplified bucket

profile.

The sum of the cutting contours and cross sections of all buckets, which are

simultaneously in the cut during each slewing motion, leads to the course of the

cutting force. Fig. 13.

Fig. 13. The required bucket wheel drive power as a function of time for right-

and left-hand slewing.

Slewing time

Driv

e po

wer

Slewing to the left

Slewing to the right

Penetration contours of buckets

Cut cross section byslewing to the right

Cut cross section byslewing to the left

Cut depth

Cut width

Material

17 17

The variations in the cutting forces during a slewing cycle influence the drive power of

the bucket wheel and that of the slewing mechanism.

In order to consider the fracture behaviour of the in-situ-formation, a digitalisation of

the bucket shape with the arrangement and geometrical line-up of the teeth

respecting their shape as well is required.

Considering the bucket shape with the arrangement and geometrical line-up of the

teeth as well as the fracturing behaviour of the material, the smashing/fragmentation

zones for each bucket and thus the size distribution of the excavated material can

also be derived. Fig. 14 and 15.

Fig. 14. The cut cross sections in the cutting position of the bucket and the

clearance of the buckets during slewing to the right

Overlapping of penetration contours of buckets when slewing to the right

Slope edge

Cleft edgesCutting edges

Cut cross section

Penetration contours ofsuccessive bucket blades with teeth

Free cutting contourof bucket

18 18

Fig. 15. The cut cross sections in the cutting position of the bucket and the

clearance of the buckets during slewing to the left

Those areas in which the material is reduced to fines out of the cutting zones of the

teeth and edges - are called smashing/fragmentation zones.

An unfavourable teeth arrangement and excessive wear of the teeth increase the

portion of the smashed material and thus increase the percentage of fines in the

excavated material.

Overlapping of penetration contours of buckets when slewing to the left

Slope edge

Cleft edges

Free cutting contour of bucket

Cut cross section

Penetration contours ofsuccessive bucket blades with teeth

Cutting edges

19 19

A high percentage of fines in the excavated material leads to a bigger new fracture

surface and thus higher energy consumption, which results in an uneconomical

excavation process. A further very important aspect for the correct assessment of the

energy consumption is the influence of the already existing cleavage in the in-situ-

formation. For the determination of the new fracture surface this cleavage has to be

deducted from the total fracture surface of the excavated material.

For the design of new BWEs engineers frequently apply the specific energy

requirement per bank cubic meter of the excavated material. This characteristic can

be derived from the cutting force and the actual output, while considering the lifting

and friction losses. In view of the fact that the material characteristics of the

excavated material, the geometry of the new BWE and the mining method are

neglected, the derived drive power of the bucket wheel can considerably deviate from

the required drive power.

Wear of cutting tools The wear of the cutting tools mainly depends on the material to be excavated,

however, it is also influenced by the mining method within the block and the BWE -

parameters.

The number of contacts of each point of the surface of the cutting tools is the result of

the excavation analysis of all cut cross sections in any position of the bucket wheel in

the block.

The fracturing behaviour of the material depends on the toughness and cleavage of

the undisturbed material and can be considered as angle of cleft of the material in the

cross section along the cutting circle.

20 20

An analysis of the number of contacts of the tooth surfaces with the material during

excavation of a total block allows a prediction on the distribution and intensity of wear

on the tooth surfaces and the cutting edges, considering the abrasiveness of the

material. Fig. 16.

Wear

highlow

Fig. 16. Expected wear distribution on the tooth surface as a function of material

characteristics, BWE data and operational parameters.

In practical operation, the specific wear of the cutting tools is often indicated as ratio

between mass loss of the cutting tools and the excavated material volume.

Similar to the specific energy requirement, the expected specific wear of the cutting

tools can only be determined in connection with the material characteristics, BWE

data and operational parameters.

21 21

Optimisation Reliable material characteristics are the basis for the optimisation of the mechanical

and operational parameters of a BWE.

The optimisation of BWEs can be structured into several successive steps:

1. Detailed analysis of the in-situ-formation, selecting and execution of suitable test

methods for assess of the cutting force, estimation of the fracturing behaviour of

the material, and wear of the cutting tools.

2. During the development phase the most important geometrical BWE parameters

and operational characteristics are determined on the basis of the material

properties, the required output, and the excavation parameters. The goal is to

develop an optimised BWE for the specific application.

3. In the execution phase the shape and arrangement of the cutting tools are

optimised. The goal of the optimisation is the cost-efficient excavation of the

material, which in view of the wear of the cutting tools ensures the required output

in the excavating block.

4. During the commissioning and operating phase, the operational parameters have

to be adapted to the changing material properties to obtain an optimum output.

22 22

Conclusion The economical design of BWEs in hard materials depends on numerous material

characteristics, mechanical and operational parameters.

The material characteristics are decisive for the design of the BWE and the selection

of the mining method. On the basis of numerous material tests, the KRUPP improved

Point-Load-Test-method furnishes extensive characterisation of the in-situ-formation,

statistically proven by KRUPP databases.

A cutting force relative to the length of the cutting contours or cut cross section in

relation to the bucket blades or the bucket itself is not sufficient for an optimum

design of a BWE, especially for hard materials. This design does not consider the

BWE geometry and the required mining method.

The method which should be given preference for the design of a BWE is the method

using a fracture surface-related energy requirement, which considers the BWE

geometry and mining method as well as the fracturing behaviour and the natural

cleavage of the in-situ formation.

The wear of the cutting tools can be predicted by the wear intensity and wear

dispersion on the surfaces of these bodies as a function of the material

characteristics, mechanical and operational parameters.

A cost effective design of a BWE for a specific application is only possible on the

basis of material characteristics determined by means of professional test methods

and statistically proven databases of experienced manufacturers. A maximum

efficiency of the BWE in operation is only guaranteed, if the selected BWE geometry

and the mining method are in agreement with the material characteristics.

23 23

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Felsmechanik, des Erd-, Grund- und Tunnelbaus sowie der Abfalldeponien.

Ferdinand Enke Verlag, Stuttgart, 1997.

2. Himmel W.: Der spezifische Grabwiderstand in Abhngigkeit von der Spanflche

und der Spanform bei verschiedenen Bodenarten. Freiberger Forschungshefte

A265, S.5-37, 1963.

3. Rasper L.: The Bucket Wheel Excavator. Development-Design-Application. Trans

Tech Publications, Vol. 1, Clausthal, Germany, 1975.

4. Durst W., Vogt V.: Bucket Wheel Excavator. Trans Tech Publications, Clausthal-

Zellerfeld, Vol. 7, FRG, 1988

5. Rodenberg J.F.:Contribution to the Assessment of the Specific Cutting Force for

Bucket Wheel Excavators. Continuous Surface Mining. Trans Tech Publications,

Vol. 1, No.1-3/87, Clausthal, Germany, 1987.

6. Lu Zhonglin: Beitrag zur Festlegung der Auslegungs- und Betriebsparameter von

Schaufelradbaggern durch Untersuchung ihrer Einflsse auf das effektive

Frdervolumen und den Energieverbrauch sowie durch Untersuchung des

Entlehrungsvorganges des Frdergutes., Dissertation, TH Aachen, 1983.