The Operational Characteristics of Refuse Handling Grabs PETER J. SCOTT and JOHN R. HOLMES City Engineer and Surveyors Department Coventry Corporation Coventry, England ABSTRACT The paper* examines the type of grab generally in use for the handling of refuse in incineration plants in the United Kingdom and the Continent of Europe. The factors affecting the selection of the various types of grab are examined together with their operational characteristics and the effect of the grab upon refuse density in transit and after discharge. Studies are made of four-rope and electro-hydraulic actuation and the impact of grab selection on the cost and size of the crane system into which it is connected. The relationship between grab payload and eight is ex- amined and examples of operational test results are included in the paper. Criteria for the selection of crane and grab systems for refuse handling are suggested. INTRODUCTION Refuse is one of the few raw materials that has the characteristic of a continually variable density, this being dependent upon place of origin, method of collection, depth of storage, time of storage or various combinations of these factors. This being so the selec- * The paper has been written in metric units in accordance with current European practice. Refuse densities have been expressed in tons/cubic meter (tons/m3) and refuse volume in cubic meters (m3). For the purposes of this paper the following conversions will be satisfactory to accord with U.S. practice: U.S. ton/cubic yard = 0.7 tons/m3 cubic yard = 0.77m3 32 tion of suitable handling devices for refuse presents special problems not usually encountered when dealing with more homogeneous materials. This paper sets out to examine the problems associated with the selection of refuse grabs and attempts to rationalize the factors to be considered and the performance to be expected for the various types of refuse grab now in operation in incineration plants in the United Kingdom and the Continent of Europe. The authors do not claim that the characteristic curves and other derived relationships are definitive under all conditions of opeloh. These characteristics and derived relationships are put forward as good indicative guide lines of performance and as a sound basis for selection. As more incineration plants are built, particularly in the larger sizes, it is hoped that much more will become known about the tech- nology of refuse handling and the long term character- istics of some of the newer types of handling grabs now being installed. REFUSE GRA The design of the refuse bunkers of almost all in- cineration plants necessitate the use of Electrieal Over- head Travelling Cranes for the transfer of the refuse from the bunker to the incinerators and for carrying out occasional bunker levelling and grading duties. The arduousness of this duty is recognized in that in every case the cranes are specified as Extra Heavy Duty Cranes in a class equivalent to steel works grabbing cranes. The handling of refuse is performed by grabs which can be divided into groups determined by the method of
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The Operational Characteristics of Refuse Handling Grabs
PETER J. SCOTT and JOHN R. HOLMES City Engineer and Surveyors Department
Coventry Corporation
Coventry, England
ABSTRACT
The paper* examines the type of grab generally in use for the handling of refuse in incineration plants in the United Kingdom and the Continent of Europe. The factors affecting the selection of the various types
of grab are examined together with their operational characteristics and the effect of the grab upon refuse
density in transit and after discharge. Studies are made of four-rope and electro-hydraulic actuation and the impact of grab selection on the cost and size of
the crane system into which it is connected. The relationship between grab payload and 'Neight is examined and examples of operational test results are included in the paper. Criteria for the selection of crane
and grab systems for refuse handling are suggested.
INTRODUCTION
Refuse is one of the few raw materials that has the
characteristic of a continually variable density, this
being dependent upon place of origin, method of
collection, depth of storage, time of storage or various combinations of these factors. This being so the selec-
* The paper has been written in metric units in accordance with current European practice. Refuse densities have been expressed in tons/cubic meter (tons/m3) and refuse volume in cubic meters (m3). For the purposes of this paper the following conversions will be satisfactory to accord with U.S. practice:
U.S. ton/cubic yard = 0.7 tons/m3 cubic yard = 0.77m3
32
tion of suitable handling devices for refuse presents special problems not usually encountered when dealing with more homogeneous materials. This paper sets out
to examine the problems associated with the selection of refuse grabs and attempts to rationalize the factors to be considered and the performance to be expected for the various types of refuse grab now in operation in incineration plants in the United Kingdom and the Continent of Europe. The authors do not claim that the characteristic curves and other derived relationships are definitive under all conditions of openrtloh. These characteristics and derived relationships are put forward as good indicative guide lines of performance and as a
sound basis for selection. As more incineration plants are built, particularly in the larger sizes, it is hoped that much more will become known about the technology of refuse handling and the long term character
istics of some of the newer types of handling grabs
now being installed.
REFUSE GRABS
The design of the refuse bunkers of almost all in
cineration plants necessitate the use of Electrieal Overhead Travelling Cranes for the transfer of the refuse
from the bunker to the incinerators and for carrying out occasional bunker levelling and grading duties. The arduousness of this duty is recognized in that in every case the cranes are specified as Extra Heavy Duty Cranes
in a class equivalent to steel works grabbing cranes. The handling of refuse is performed by grabs which can be divided into groups determined by the method of
actuation employed and the basic shape and form of the device. Grabs are either actuated by the conventional four rope system or by electro-hydraulics. The principal shapes are the two jaw clamshell or bucket grab usually fitted with teeth or tines and the
polyp or cactus grab usually with six or eight claws or tines. Each of the grab shapes can be found wi th four
rope or electro-hydraulic actuation. The grabs examined in this paper are examples of eight different types of grab currently in use in incineration plants in the United Kingdom and the Continent of Europe.
The grabs have been chosen in two equal groups repre
senting electro-hydraulic and four rope actua tion.
Considered within these groups are variations on both the prinCipal shapes of clamshell and polyp construction.
The grabs which will be examined in this paper are:
Electro-Hydraulic Actuation
Grab A
Grab B Grab C
Grab D
Heavyweight Multi-Ram Polyp. Heavyweight Mono-Ram Polyp.
Lightweight Mono-Ram Polyp.
Mediumweight Mono-and Double-Ram
Clamshell.
Four Rope Actuation
Grab E Grab F
Grab G Grab H
Heavyweight Polyp.
Heavyweight Long Tine Clamshell.
Mediumweight Long Tine Clamshell. Mediumweight Short Tine Clamshell.
In mosf cases more than one manufacturer produces each type of grab considered and the characteristic curves and other derived data do not necessarily apply to one or other of these manufacturers but is a close indication, based upon actual operational results, of the
performance to be expected of grabs in each classification. Figs. I -8 indicate typical examples of each of
the grabs considered.
PRINCIPAL CONSTRUCTIONAL FEATURES
Before considering their operational characteristics
it is desirable to describe the principal constructional features of the various types of grab under consideration.
Electro-Hydraulic Actuation
Grab A - Heavyweight Multi-Ram Polyp
This is a heavy weight cactus or radial claw grab
normally with six tines or claws each operated by an individual hydraulic ram. The grab is designed to have a low center of gravity while filling with the ability to
operate at acute angles and with refuse of very mixed
character. The electro-hydraulic system is completely self-contained within the grab and the grab is raised and lowered from a single suspension pin located at the
top of the grab. The electrical connections are made to a heavy duty plug and socket connection and carried
Fig. 1 A and 1 B Grab A - Heavyweight multi-ram electro-hydraulic
polyp on test.
33
by a cable of special flexibility from a self-reeling cable drum mounted on the crane. The grab usually requires 15-20 sec to open and close, and is designed to exert a closing force of approximately 2.5 tons per tine (see Figs. I a and I b).
Grab B - Heavyweight Mono-Ram Polyp
This again is a heavyweight cactus radial-claw grab with six or eight tines depending on size. The grab is of a more open construction than Grab A and possesses a single electro-hydraulic ram with a solidly connected linkage system to its six or eight tines. The method of grab suspension and cable connection system is similar to that of Grab A. In some installations a tandem pulley block is fitted to increase the centers of the hoisting ropes and eliminate the tendency of this type of grab to slew. The electro-hydraulic system is designed to lie within the body of the grab pro-tected by the steel pivot arms of the tines. The combined open and close time is about 20-30 sec depending on size and the grab is designed to exert a closing force of approximately 1.0 - 1.5 tons per tine (see Figs. 2a and 2b).
Grab C - Lightweight Mono Ram Polyp
Features of both preceding types of polyp grab are to be found in this range of lightweight grabs manu factured in Denmark and widely used in incineration plants in that part of Europe. The grab is constructed of light alloy steels and is operated by a single hydraulic ram connected by a linkage system to each of its tines. The tines are not rigidly pivoted but possess a special connection system allowing, the manufacturers claim, a better digging or burrowing action into the refuse. The electro-hydraulic system is totally enclosed behind a steel shield and the grab suspension and cable connection system are similar to those discussed earlier. The grabs open and close times are between 20-25 sec and a thrust of about 0.75 tons is exerted by each tine (see Figs. 3a and 3b).
Grab D - Mediumweight Mono- and Double-Ram
Clamshell Grabs
This type of grab consists of two jaws actua ted by one or two hydraulic rams depending upon the size of the grab and the nature of the material to be handled. In
Fig. 2A and 28 Grab 8 - Heavyweight electro-hydraulic mono
ram polyps in operation.
34
Fig. 3A and 38 Grab C - Lightweight electro-hydraulic mono
ram polyp on completion of manufacture.
Fig. 4A and 48 Grab 0 - Good example of electro-hydraulic
long tine clamshell grabs in operation.
35
some earlier types the jaws were of the enclosed type but, more generally, they are now cut away at the sides and the top to give an open aspect to the grab. Long teeth or tines are fitted to the side and bottom faces of each jaw. The electro-hydraulic system is mounted on top of the jaws and is suitably protected. Grab suspension and electrical connections are affected in a similar manner to the other type of electro-hydraulic grabs discussed. The earlier closed bucket models
of this grab were fitted with short teeth or tines and probably represent the application to the handling of refuse of grabs designed for the handling of other materials. These were considered to be more suitable
for use in plants where the refuse contained a high proportion of ash and clinker. Where this grab is
now called for it is usually of the long tine type. The
opening and closing times are between 15-20 sec
and each bucket is designed to exert a closing force in the range of 3-4 tons (see Figs. 4a and 4b).
Four-Rope Actuation
Grab E - Heavyweight Polyp
This grab has been installed in a significant number of incineration plants in continental Europe. It combines the mechanical action of the polyp with the conventional four-rope operation normally associated with clamshell grabs and grapples. The grab is very similar in appearance to its electro-hydraulic counterpart (especially Grab B), the rope system and sheaving blocks being contained within the body of the grab. To minimize the tendancy of the grab to
spin during hoisting and lowering the closing ropes
are located at fairly wide centers. The holding ropes
are spaced at smaller centers and anchored to an
equalizing bar mounted within the head of the grab.
6.3t
The grab's digging power is usually expressed as a percentage of grab weight and the closing purchase is obtained by the number of falls within the reeving
system. The length of rope withdrawn on closing is
between 10 and 15 m and given a hoist speed of
0.8 - 1.0 m/sec the combined open and closing time
of the grab will be 20 - 35 sec (see Figs. Sa and 5b).
Grab F - Heavyweight Long Tine Clamshell Grab
This type of grab is probably the most popular type of refuse handling device in incineration plants in continental Europe. It consists of two cut away jaws each fitted with long teeth or tines on its sides and bottom faces. The grab presents an open aspect and makes a feature of its heavyweight construction. The grab operates on the conventional four-rope principle with the usual precautions taken to prevent slewing during hoisting and lowering. The length of rope withdrawn on closing and the open and close times are similar to those of Grab E (see Fig. 6).
.�, . ' "-'. - ,t .. Fig. 5A and 58 Grab E - Heavyweight 4-rope operated polyps
in operation.
36
Grab G - Mediumweight Long Tine Clamshell Grab
This is very similar in construction and appearance
to its heavyweight counterpart (Grab F) but with a
lower weight to size rato (see Fig. 7).
Grab H - Mediumweight Short Tine Clamshell Grab
This is a grab consisting of two closed jaws fitted to
short teeth or tines and again operate using the con
ventional four-rope operation. This type of closed
bucket design is not now popular and most earlier
•
, ' " -'
� , . • •
•
grabs of this type were undoubtedly designed for the
handling of coal or other materials and then super
ficially modified to handle refuse. [t is now rare Iy in
stalled and is increasingly being superseded by the open
long-tine clamshell grabs (Type F and G) described
above (see Fig. 8).
GRAB OPEN PLAN AREA
Studies of the dimensions of all the grabs considered
show that the design characteristics of each grab gives a
more or less constant relationship between the closed
Fig.6 Grab F - Heavyweight rope operated long tine clamshell
operating in refuse bunker.
37
Fig. 7 Grab G - Mediumweight rope operated long tine
clamshell feeding a furnace.
38
Fig.8 Grab H - Typical construction of closed bucket short
tine clamshell grab but with electro hydraulic
actuation as for Grab D.
volume of the grab and its open plan area presented to the face of the refuse bed. Marked differences are, however, to be found in the value of this rela tionship between one grab and another, particularly between the polyp and clamshell shapes.
The mean open plan area per m3 of volume for polyp grabs of either actuation system is about 2.8m2 and for clamshell grabs 1.7m2. This information may be seen to have some significance in the operational characteristics discussed later.
DERIVATION OF OPERATIONAL
CHARACTERISTICS
It is well established that the performance and behavior of any refuse handling device is specifically related to the density of the refuse being handled and partially to the other properties of the material. Refuse grabs do not possess any inherent properties related to payload, compaction, and other behavior which can be applied indiscriminately as constants over the whole range of refuse likely to be handled by the grab. The
39
operational evidence seems to show that bunker refuse density has a finite and repeatable relationship to the performance of any grab and it is on this fundamental principle that the derivation of the operational characteristics in this paper have been founded. Against a common base of mean bunker refuse density a series of curves have been drawn for each type of grab which shows mean refuse density in the closed grab, (Figs. 9 and 12) the density of the refuse after discharge from the grab (Figs. 10 and 13) and the compaction effected upon the refuse during transit in the grab. (Figs. 11 and 14). The curves are based on actual operational results in incineration plants over a consider-able number of grabbing cycles and include allowances for random factors such as operator efficiency, bunker design and ergonomic considerations. It is conceivable that under artificial test conditions operational results better than those depicted will occur. The authors' experience is that, even with the best of the grabs considered, long term incineration plant results tended to moderate the more spectacular results obtained under demonstration test conditions.
o
O'
01
OPERATI ONAL CHARA CTERlSTK:S OF EL ECTfO HYDRA ULIC
GRABS.
MEAN REFUSE D EN SITY IN CL OSED GRAB
------
It",3
05
o
02
APPROXIMATE REFUSE DEN SITY IN CHUTE AFTER GRAB DISCHARG E.
FIGURE 10 0] 04 It.3
Figs. 9 and 10
SUMMARY OF GRAB OPERATIONAL
CHARACTERISTICS
Electro Hydraulic Actuation
Grab A - Heavyweight Multi Ram Polyp
Characteristic Curves A (Figs. 9-11) show that this design of grab, particularly due to its multi-ram hydraulic system and secondarily its dead weigh t, obtain the highest closed grab refuse density of all the grabs considered in this paper. Mean Refuse Densities in the Refuse Bunker of 0.2 tons/m3 are compressed to 0.7 tons/m3 in the closed grab and revert to 0.4 tons m3 on discharge in the furnace feed chute, i.e. ra tios of I : 3.5: 2.0 respectively. These characteristics lead to grabs of relatively low and, thereby managable, capacities and, consequently, to cranes with economical safe working loads. A study of all the characteristic curves (Figs. 10 and 13) shows that the refuse after discharge from the grab and into the Furnace Feed Chute retains, in some measure, the compaction
3.0
2·5
2·0
15
T YPICAL C OMPACTI ON FPCT ORS OBTAINED WITH EL EC T RO HYDRAULIC GRAB S.
I ·U"--:--- --.-------.--------. 0' 1 02 03 0·4
----REFUSE DENSITY &JNKER t/11I 3----
KEY:- CURVE 'A� HEAVYWEI01T MUL TI RAM POlyp
CURVE'B'. HEAVYWEIGiT MONO RAM P'OLYP CURVE'C' L IGHTWEIGHT MONO RAM POLYP. CURVE "D'MEDIUMWEI01T MOt-Of,DOUBLE
RAM CL AM SHELL
Fig. 1 1
effected upon it during its transit in the closed grab. Post discharge densities from this grab are quite high and may be felt by some incineration system manufacturers to be a disadvantage, Nevertheless, if it is considered that grabs sized 7-8 m3 represent the limits of the technology, Table 1 shows that an incineration plant of 100 tons/h installed capacity can be fed satisfactorily by a crane of 10 tons safe working load fitted with a grab sized at 6.0 m3 , and that two equally rated cranes will provide a reserve capacity of 100 percent.
The manufacturers claim that their good results derive from the capacity of the electro-hydraulic grab to operate efficiently at acute angles of repose. As its actuation system is self contained, it does not suffer the reduction in efficiency which arises where deadweight and angle of repose are more significant factors in the efficiency of the grab. With electro-hydraulic grabs the power unit can be designed to impose whatever closing force is required regardless of grab weight. With the polyp grabs considered here, the force exerted is about 2.0 - 2.5 tons per tine. As almost all modern incineration plants involve refuse being dis-
07
06
05
tlml
0.3
OPERAT I ONAL CHARA CTERISTI CS OF ROPE OPERATED GRABS
__ �URVEt;'
_-.... URVE 'H'
MEAN REFUSE DENSITY IN GRAB- FULLY CLOSED
02 '---_---.---�---.::-::--�-"---T:_ 01 02 03 04 VIol
05
OA
FIGURE 13 __ ._URVE'E'
tlml
o
__ �URVE'F'
CURVE'c' &
CURVE'H'
A PPIOXIMATE REFUSE DENSITY IN CHUTE AFTER GRAB DI5CHAICE.
O·I+----�----------� 01 02 03 04
____ REFUSE DENSITY IN BUNKERt/,.l __
Figs. 12 and 1 3
40
charged into the bunker from one side only, a significant part of any grab's operational life involves working at acute angles of repose. A capacity to operate at acute angles increases the advan tage of the electro-hydraulic grab.
Grab B - Heavyweight Mono-Ram Polyp
Characteristic curves B (Figs. 9-11) and Table 2 representing the operational characteristic of this grab show a marked reduction in the closed grab refuse densities obtained in comparison with the properties of Grab A. The dead weights of Grab B, over most of the size range, are heavier than Grab A so that its lower handling capacity must be attributed to its single-ram hydraulic system. As an example, Mean Refuse Densities in the Bunker of 0.2 tons/m3 become 0.55 tons/m3 in the closed grab and revert to 0.32 tons/m3 on discharge into the furnace feed chute i.e. ratios of 1 :2.75: 1.6 respectively. Grab weight/payload factors are good and the maximum size of grab required for refuse capacities of 100 tons/h
04
J5
JO
25
1·5
TYPICAL COMPACTION FACTOR S OB TAINED WITH fOPE OPERATED
GR ABS.
IUL----______ ,-____ �----� ________ �
01 02 03 04
REFUSE DENSITY IN BUNKER tlml
CURVE E HEAVYWEIGHT POLyp
CURVE F HEAVYWEIGHT LONG TINE ClAMSHELL. CURVE G MEDIUMWEIGHT LONG TINE
CL AMSHELL. CLRVE H MEDIUMWEIGH T SHORT TINE
ClAMSHELL
Fig. 1.fl
41
is still within the manufacturing capabilities of the industry. The grab sized at 7.5 m3 can be operated well within the capabilities of a crane of 12 tons capacity. The post discharge refuse densities are not as high as with grab A. Grabs of this particular type sized at 6.0 m3 are currently in operation in the United Kingdom.
Grab C - Electro-Hydraulic Lightweight
Mono-Hydraulic System Polyp
The manufacturers of this grab feature its light weight to size ratio and do not subscribe to the conventionally held belief that, irrespective of the system of actuation, grab dead weight is a relevant factor in its refuse handling capability. Characteristic Curves C (Figs. 9-11) and Table 3 indicate the lowest closed grab refuse density compared to the two earlier polyps considered. With Mean Refuse Densities in the Bunker of 0.2 tons/m3 this grab can be expected to compress this to 0.46 tons/m3 within the closed grab reverting to 0.27 tons/m3 on discharge into the furnace feed chute i.e. ratios of 1: 2.35: 1.35 respectively. The grab's light weight gives very good grab weight/payload factors and, in spite of the relatively low closed grab refuse density, economical crane safe working loads are obtained. A 100 tons/h refuse capacity with 25 lifts of 4 tons each can be accommodated with a grab of 9.0m3 fitted to a crane of 10.0 tons safe working load. Grabs of this type sized at 8.0 m3 are giving satisfactory service in Scandanavia. This manufacturer also includes in his range medium and heavyweight polyp grabs with more powerful electro-hydralic systems. These latter types have not to date been put forward for refuse handling but it is probable that their performance would approach that of Grabs A and B.
Grab D - Medium Weight Single and Double Ram
Clamshell Grabs
These grabs, which are successfully installed in a number of U.K. incineration plants, are represented by Characteristic Curves D (Figs. 9-11) and Table 4. With Mean Refuse Densities in the Bunker of 0.2 tons/m3 the closed grab densities become 0.38 tons/m3 and revert to 0.22 tons/m3 on discharge from the grab i.e. ratios of I: 1.96: 1.1 respectively. In common with the other clamshell grabs to be considered la ter, these grabs are distinguished by lower compaction factors and post discharge refuse densities than is to be expected with the other electro hydraulic grabs. The low closed-grab refuse densities do not permit grabs of low cubic capacity but the crane safe working loads are in the same order as that of the cranes fitted to the
other electro-hydraulic grabs. It seems that this type of grab is rapidly being superceded in popularity by the other types of electro-hydraulic grab.
Four Rope Actuation
Grab E - Heavyweight Polyp Grab
Characteristic Curves E (Figs. 12-14) and Table 5 represent the characteristics of this type of grab, second in popularity only to the Heavyweight Long Tine Clamshell (Grab F) in incineration plants in Continental Europe. Grabs of this type, sized at 6m3 , are installed and operating in what is at present the largest incineration plant in Europe at Ivry in Paris.
The characteristic curves indicate closed grab refuse densities of 0.55 tons/m3 with mean refuse bunker densities of 0.2 tons/m3 reverting to 0.32 tons/m3 on discharge from the grab i.e. ratios of 1 : 2.75: 1.6 respectively. With hourly capacities varying from 10 to 100 tons/h the corresponding grab sizes are from 1 to 7 .5m3 , the latter size being well within the limits of refuse grab construction technology. The safe working loads of the cranes are seen to be somewhat higher than the electrohydraulic polyps earlier described but not as high as the clamshell grabs to follow. The grab weight/payload factors are good and this type of grab is clearly designed to satisfy those who wish to take advantage of the grabbing capabilities of the polyp but who have reservations on electro-hydraulics as the most satisfactory method of actuation. Advocates of the polyp grab of either system of actuation also make a point of its very high average filling factor about 90-100 percent as compared with 70-80 percent for other grab shapes.
Grab F - Heavyweight Long Tine Clamshell Grab
Without doubt this is the most popular grabbing device for handling refuse in European incineration plants. This simplicity of design and well tried method of operation are seen as overriding advantages compared to the relatively short rope life and large grab sizes entailed in its selection. Characteristic Curves F (Figs. 12-14) denote that, in well designed heavyweight versions of this grab, closed grab refuse densities of 0.48 tons/m3 occur with mean refuse bunker densities of 0.2 tons/m3 and revert to 0.27 tons/m3 on discharge i.e. ratios of 1: 2.4: 1.35 respectively. From Table 6 grab sizes vary from 1 to 8.5m for refuse handling capabilities of 0.4 to 4.0 tons per lift with crane safe working loads varying from 3 to 13.75 tons, a marked increase, particularly in the bigger sizes
42
over the safe working loads encountered in the polyp and other electro hydralic grabs considered so far.
Grab G - Mediumweight Long Tine Clamshell Grab
The reduced weights of t�s grab are seen to have a marked impact upon its refuse handling capability. As with Grab F it possesses the long tine and open aspect considered most suitable for refuse and again is favoured for its simple construction and well tried method of actuation. Characteristic Curves G (Figs. 12-14) and Table 7 give the principal characteristics of this grab and show that, with mean bunker refuse densities of 0.2 tons/m3 the grab compacts to 0.4 tons/m3 and discharges into the incineration feed chute at a density of 0.22 tons/m3 corresponding respectively to ratios of 1: 2: 1.1. The low compaction capabilities of this grab result in large volume lifting and moderatelyt high crane safe working loads.
Grab H - Mediumweight Short Tine Clamshell
This range of what are, basically, coal grabs superficially modified by the fitting of short tines to the tips of the buckets are described by characteristic Curves H (Figs. 12-14) and Table 8. These grabs possess the lowest closed grab refuse density of all those considered and result, over most of the range, in the largest volume grab sizes. In the example mean bunker densities of 0.2 tons/m3 become 0.35 tons/m3 in the closed grab and revert to 0.2 tons/m3 on discharge into the incineration feed chutes i.e. ratios of 1:1.75:1 respectively. The refuse is deposited into the incineration system in, approximately, its original bunker condition with no residual compaction due to its transit in the grab. Crane safe working loads are higher and the size of grab required at the higher houdy refuse handling capacities render this an inappropriate device for this duty.
GRAB PERFORMANCE AND OPEN PLAN AREA
In the description of the constructional features of the grabs, reference was made to the relationship between the closed volume of the grab and its open plan area. The tables below show the marked differences that exist between the value of the relationship with each type of grab and, in particular, between the polyp and clamshell shapes. Having now considered the relative operational characteristics of the grabs it is interesting to note the empirical relationship that appears to exist between grab payload and the corresponding ratios of open plan area per m3 of closed volume.
Electro-Hydraulic Actuation
Type of Grab
Grab A - Heavyweight
Multi-Ram Polyp
Grab B - Heavyweight
Mono-Ram Polyp
Grab C - Lightweight Mono-Ram Polyp
Grab D - Mediumweight Mono-&-Double
Clamshell
Open Plan Compaction Factor
Area
per m3
of
Closed
Volume
3.6m2
2.8m2
2.2m2
1.4m2
at
O.2t/m3
Bunker Refuse
Density
3.5
2.75
2.30
1.90
Four-Rope Actuation
Type of Grab
Grab E - Heavyweight
Polyp
Grab r - Heavyweight
Long Tine Clamshell
Grab G - Mediumweight
Long Tine Clamshell
Grab H - Mediumweight
Short Tine Clamshell
Open Plan
Area
Per m3 of
Closed
Volume
2.8m2
2.2m2
1.7m2
1.5m2
Compaction Factor
at
O.2t/m3
Bunker Refuse
Density
2.75
2.40
2.0
1.75
It can be seen that apart from actuation system, deadweight, and other random factors, grab design expressed in plan area as a ratio of closed volume has an effect upon the performance and efficiency of the grabs and highlights in particular the properties of the polyp in this respec t.
PRESENTATION OF TABULATED DATA
In setting ou t Tables 1 to 8 direct size for size comparisons between one grab and another have been avoided. The characteristics of grabs vary so widely, as do the crane systems into which they are connected, that the only relevant basis of comparison must be payload. To do this a range of hourly refuse loads from 10 to 100 tons has been chosen and, taking mean cycle times and operation factors, a payload per grabbing cycle has been derived. From this, loads, grab sizes, safe working loads and weight/payload factors have been calculated. As laid out, the tables enable valid comparisons to be made across the whole range of grabs studied.
43
The operational Test Results in Tables 9-16 are based on actual test results obtained by the authors on visits to incineration plants in the United Kingdom, Germany, France, Switzerland, Holland and Denmark. The costs of grabs and crane systems detailed in Table 17-18 are approximate order of costs based on mid 1971 prices in the European Free Trade Area. Equipment originating in other parts of Europe bears an element of import duty on its cost.
GRAB SELECTION
Experience with Refuse Grabbing Cranes in the United Kingdom and Europe has shown that the velocities of all crane motions, and in particular the hoist motion, should not exceed 0.8-1.0 m/sec. These are considered optimum limiting velocities consistent with good operator control and economical sizing of motor drives and associated equipment. Grabbing time cycles are of about 2-2!h min duration and the cranes are rated to deliver their total refuse payload on an operational factor of 50 min in any hour. This leads to 20 to 25 cycles per hour and it follows that the grab will be required to carry on average 4 to 5 percent of its hourly refuse handling requirement per cycle.
Given these guide lines, the selection of an appropriate grab size and type should commence with the calculation of a mean grabbing duty cycle calculated on the basis of the limiting velocities given above. This mean duty cycle should include manipulation allowances, time required for load weighing systems to operate and grab open and close time of about 20 sec. From the calculation of this mean duty cycle the number of cycles to be contained within the hourly operational factor of 50 min in any hour can be derived and, from this, the average payload per cycle to be expected of the grab. Given the mean refuse density to be expected in the bunker, the characteristic curves will give closed grab refuse densities and sizes for each of the grabs considered. Further reference to the tables will give the safe working loads to be required of the crane system into which the grab is to be connected. The final selection of grab will depend upon the method of actuation chosen and the economic implications of the crane system into which the grab is connected. Apart from the main duty of furnace feeding, the grabs and their associated cranes will be required to carry ou t bunker levelling and grading operations. With most multi-stream plants a single refuse grab and crane is rated to feed the installed hourly tonnage of the plant. Since most such plants are over rated in installed capacity to allow for standby, etc. this allow margins usually adequate for carrying out bunker levelling and grading within the 50 min in any hour operational
factor. With single stream plants and others of special construction special calculations will have to be made. In particular, with single stream plants care must be taken not to reduce crane speeds to such a low level that insufficient time remains for the bunker levelling and grading role of the crane. In general, single stream plants should be fed on a 40/60 min operational factor per hour with 2-272 min cycles giving 15-20 cycles for furnace feeding with the grab required to carry 5-672 percent of the installed hourly tonnage per cycle.
The size and plan area of the furnace feed chute may exercise some limiting control over grab selection since, clearly, the open plan area of the grab must be designed to be contained within the plan area of the feed chute. Again, the depth and volume of the feed chute neck into the furnace and the essential operational requirement to maintain a refuse seal, will limit the period between cycles to a particular furnace. Here the requirements of the furnace maker may be in conflict with those of good crane design, for the one will require his furnaces to be fed in frequent small increments and impose serious limitations regarding the rate of feeding, while the crane designed will seek to keep his speeds down and perform his handling duty in larger but fewer increments. Checking, recalculation and modification of the originally set crane speeds will have to be made when these latter requirements conflict with the original sizing of the grab. It is hoped that the graphs showing post discharge refuse densities (Figs. 10 and 13) will be of value.
CAPITAL COSTS
This paper has avoided detailed comparisons in costs between one grab and another as it is unrealistic to consider these in isolation from the crane system into which the grab is connected. Electro-hydraulic grabs, due to their sophistication, are clearly more expensive than their Simpler four-rope operated counterparts. However, crane safe working loads for a given refuse payload vary considerably depending upon the grab chosen and, more fundamentally, the choice between electro-hydraulic and four-rope actuation has considerable bearing upon the equipment located on the crane. These latter factors would tend to have a
much greater impact upon the cost of the complete crane system than the original cost difference between one grab and another. Four-Rope Operated Grabs require cranes with four separate motor-driven motions and their associated gearboxes, winches, brakes and control equipments. Electro-hydraulic grabs necessitate only three separate motions, namely: hoist, traverse and travel. These variations in crane motive equipment must, for a given safe working load, have a significant
44
impact on crane costs. Evidence in the United Kingdom suggested that the cost difference between three and four motor cranes of a given class and safe working load will be of the order of 10-15 percent. On larger crane sizes particularly, this would tend to cancel out the cost disadvantage due to the selection of electrohydraulics and might tend to show a positive cost benefit in four of its selections.
In the ultimate, the limits of development in refuse grab technology must dictate for each type of grab a limiting size and payload. At this point, in the largest plants, the usual two crane bunker arrangement with one duty crane and one standby providing 100 percent reserve capacity will be replaced by a three crane bunker handling system. In that event, two cranes will be required to handle the plants maximum refuse burning capacity with the third crane on standby providing 50 percen t reserve handling capacity. Under these circumstances, the selection of an efficient grab of manageable proportions which eliminated the necessity for a third crane would have an impact upon the costs of the crane system out of all proportion to the original cost of the grab.
MAINTENANCE COSTS
It is well established that the problems of high rope wear in four rope operated refuse grabbing cranes has yet to be satisfactorily solved. Rope-operated grabs, though of simple robust construction, do necessitate particularly in the open and close ropes, a frequency of rope changing much higher than that encountered in the hoist ropes of electro-hydraulic grabbing cranes. Experience in the United Kingdom suggests an average life of about 800 operational hours for the open and close ropes and 3800 operational hours for the hoisting ropes of four-rope grabbing cranes. In some extreme cases, no doubt contributed to by poor crane operation, ropes lives have fallen to about 500 and 2500 operational hours respectively. The frequency of hoist rope changing in electro-hydraulic grabbing cranes follows a pattern similar to the rope life of hoist ropes in four-rope cranes. Set against this, it must be remembered that the electro-hydraulic grab is a sophisticated piece of equipment requiring maintenance skills probably beyond the standard of maintenance manpower in the average incineration plant. Special grab main tenance contracts will be required and the capital costs of component replacements will tend to be high. Electrohydraulic grabs are not without their problems. All manufacturers in the United Kingdom have had difficulties due to hydraulic leaks, electrical control cable fracture and tine breakages. The rope changing necessitated by four-rope grabs, frequent though it
may be, is a simple operation quite within the scope
of normal maintenance personnel and capable of
being completed within 8 h. Undoubtedly, improved
rope design and better sheaving systems will, in the
course of time, substantially reduce the rope life
problem. Electro-hydraulic grabs, with their simpler suspension systems are more easily disconnected on
failure, enabling a replacement grab to be speedily
fitted bringing the crane back into operation with the
minimum of delay.
CON CLUSIONS
The nature of refuse, its diversity of density and com
position, create a special problem for consultants and designers of refuse incineration plants when considering the selection of grabs and their associated crane systems. With more homogeneous materials, the handling prob
lem is greatly simplified as far as the selection of the .
handling device is concerned since the material has a near constant density. The desired rate of transfer being known, the device is easily sized, leaving only choice of actuation and manufacturer to the descretion of the consultant. This paper has attempted to show the wide
range of operational characteristics, payloads and associated crane systems involved in the selection of
grabs for refuse. The authors would put forward the
case that the selection and specification of refuse grabs should be based upon the careful consideration of all
the relevant factors dealt with in this paper. It should
be reserved for the consultant and designer and not left to the en tire descretion of the crane manufacturer.
However reputable the crane manufacturer, the economic
pressures of commercial competitive tendering will lead him to select the cheapest grabbing device securing to himself the short term advantage of capital saving and leaving his clien t the penal ties of higher opera tional
costs and perhaps, an unsuitable handling device. With
one or two notable exceptions, grab manufacturers in
the United Kingdom and the Continent of Europe have
carried out very little operational research into the
behavior of their grabs and the impact of the grab on
the transit and post discharge densities of the refuse. Particularly with electro-hydraulic grabs, the handling capacity of the device is occasionally underestimated and problems arise when crane safe working loads are pitched too low. It is hoped that consultants and designers will not arbitrarily decide upon one grab actuation sys
tem or one particular grab shape without considering the impact that their decision will have on the capital,
running costs and efficiencies of the complete crane system. At the very least, cost benefit exercises should be carried out using two alternative grabs, one from each of the two groups of actuation systems. Finally, the
authors would make the point that this is a field in need
of a considerable amount of practical research and that
all those in positions of influence in the design and op
eration of incineration plants should work to this end by
encouraging the installation of load weighing equipment
on their cranes, maintaining adequate records of grab
loads and cycles and determining the densities and other
characteristics of the refuse being handled.
ACKN OWLEDGEMEN TS
The authors wish to acknowledge the assistance of
the following manufacturers: Maschinenfabrik August Ridinger, Mannheim, West Germany; Sven Maskinfirma,
Holbaek, Denmark; Demag Foedertechnique, Wetter,
West Germany; Von Rolt A.G. Zurich, Switzerland; Clarke Chapman - John Thompson Ltd., (Crane &
Bridge Division), Leeds, U.K.; Butters Cranes, Hillingdon, Essex, U.K.; and Priestman Brothers Ltd., Hull, U.K.
(Continued)
45
Grab Weight Payload Factors k Crane Safe Working Loads.
Ilean Bunker Refuse
3Density
0.2t .... .
liIIean Closed Grab Refuse Density O. 7t,4113.
Mean Feed Chute Refuse
3Density
0.4t/m .
Re4uired Hourly Capaci ty.
10
20
30
40
50
60
70
80
90
100
Refuse Practical Load per Grab Cycle. Siz.e.
0.4 1.0
0.8 1.5
1.a 2.0
1.6 2.5
2.0 3.0
2.4 3.5
2.8 4.0
3.2 4.5
3.6 5.0
4.0 6.0
Grnu WeiKht P�yloou Fnctors L Crone Sufe Working Loaus.
� .
Mean Refuse Bunker Density 0 . 2 tjt113.
Mean Closed Grab Refuse gensity 0.55t/lll •
Mean Feed Chute Refuse Dena1ty 0.32t/111J.
Required Hourly Capac1 ty.
10
20
30
40
50
60
70
80
90
100
Refuse Practical Load per Grab Cycle. Size.
0.4 1.0
0.8 1.5
1.2 2.5
1.6 3.0
2.0 3.5
2.4 4.5
2.8 5.0
3.2 6.0
3.6 7.0
4.0 7.5
Grab Working Weigh t. Load
2.0 2.4
2.5 3.3
3.0 4.2
3.25 4.85
3.5 5.5
3.75 6.15
4.0 6.8
4.5 7.7
4.75 8.35
5.25 9.25
Grab Working Weight. Load.
2.0 2.4
3.0 3.8
3.75 4.95
4.0 5.6
4.25 6.25
4.75 7.15
5.25 8.05
5.75 8.95
6.50 10.1
6.75 10.75
46
�.
Electro Hydraulic Heavyweight Multi Ram.
Design S.W.L.
2.75
3.75
4.75
5.5
6.0
6.75
7.5
8.5
8.25 I
10.25
Grab Wt./ Payload Factor.
0.2
0.32
0.40
0.49
0 . 57
0.64
0.70
0.71
0.72
0.76
i
Polyp. Grab.
Crane Capaci ty Based on 50/60 minutes opera tion factor and cycle time of 2 .1nutes
Electro Hydraulic Heavywoight Mono Ram.
Design S.W.L.
2 . 75
4.25
5.50
6.25
7.0
8.0
9.0
10.0
11.0
13.0
Grab Wt./ Payload Factor.
0. 20
0.27
0.32
0.40
0.47
0.51
0.53
0.56
0.56
0.59
Polyp. Grab.
Crane Capac! ty Based on 50/60 minutes opera tional factor and cycle time of 2 minutes.
Gl'ab. Weight Pay l nad Factors" Crane Safe Working Loads
Grab Lf·
Mean Bunker Refuse3Densi ty 0.2t/1I •
Mean Closed Grab. Refuse �ns lty 0.46t/1I •
Mean Feed Chute Refuse !f11.ii ty O.27t/1I •
Required Hourly Capacity
10
20
30
40
50
60
70
80
90
100
Refuse Practical Load p�r Grab. Cycle Si:l.e
0.4 1.0
0.8 2.0
1.2 2.5
1.6 3.5
2.0 4.5
2.4 5.5
2.8 6.5
3.2 7.0
3.6 8.0
4.0 9.0
Gr�b. Wf'ight Payload Factors" Crane �;af�_ \!?�klng Loads
Mean Refuse Density in 3 Bunker 0.2t/8 •
Mean Closed Grab. Refuse �nsi ty 0.38t/8 •
\!lean Feed Chute Refuse 9':nsity 0.22t/., •
Required Hourl y Capacity
10
20
30
40
50
60
70
80
90
100
Refuse Practical Load per Grau. Cycle Siz.e
0.4 1.0
0.8 2.0
1.2 3.5
1.6 4.5
2.0 5.5
2.4 6.5
2.8 7.5
3.2 8.5
3.6 9.5
4.0 10.5
Grab. Weight
1.55
1.87
2.0
2.75
3.1
3.65
4.25
4.5
5.0
5.4
Grab. �'clght
1.3
:l.0
3.0
3.5
4.0
4.5
4.75
5.2
5.5
5.75
47
TAIlLE 3.
Electro H:"draul.u; J-2Ji!�\(.:lghl \lo�(�.!1.J:<ll}'JI. Grab.
Working Load
1.95
2.67
3.2
4.35
5.1
6.05
7.05
7.7
H.6
9.4
-, Design Grab. Wt.1 S.W.L. (.>ayload
Factor
2.25 1l.26
3.0 0.43
3.5 0.60
4.75 0.58
5.75 0.65
6.75 0.66
7.75 0.66
8.5 0.71
9.:1 O.7:l
10.:> O.H .----.--- .. -
Crane Capacity Based on 50/60 minutes operational factor and cycle time o f 2 minutes
Electro lIyuraulic Mediumwelght Mono" Double Ram Clamshell Grab.
Working Load
1.7
2.8
4.2
5.1
6.0
6.9
7.55
8.4
9.1
9.75
�slgn S,"',L.
2.0
3.0
4.75
5.5
6.5
7.5
8.:.!� _._. --
tI. :.!:-'i
Grab. wt.1 Payload Factor
0.31
0.40
0.40
0.46
0.50
0.53
0.59
O.6� - ----- -
10.0
10.75 -.--- ..
0.65
0.69 . ----------
Crane Capac! ty Based on 50/60 minutes operational factor and cycle time or 2 minutes.
Grab. Weight Payload Factors � Crane Sate Working Loads
Grab. E .
Mean Bunker Rcfus<'JOcnslty O. !t/q .
MSAn Closed Grab. Refuse !rnsity 0 . 55t" •
Mean Feed Chute Refusto' �nslty O . 3Zt/111 •
Required Hourly Capac i t y
10
20
30
40
50
60
70
80
90
100
Refuse Prac t ical Load per Grab. Cyc le Size
0 .4 1 . 0
0 . 8 1 . 5
1 . � 2 . 5
1 . 6 3 . 0
2 . 0 3 . 5
2 . 4 4 . 5
2 . 8 5 . 0
3 . 2 6 . 0
3 . 6 6 . 5
4 . 0 7 . 5
Grab. Weight Payl oad Factors . Crane Safe Working Loads
Grab. P .
Mean Bunker Refus·3Denaity 0 . 2t/WI •
Mean Closed Grab. Refuse3Denai ty
o.48Ve •
Mean reed Chute Refuse �n81ty 0 . 27 t / • •
-
Requir4;td Hourly Capac i t y
1 0
20
30
4 0
50
60
70
80
90
100
Refuse Prac t1cal Load per Grab. Cycle Si ze
0 . 4 1 . 0
0 . 8 2. 0
1 . 2 2 . 5
1 . 8 3 . 5
2 . 0 4 . 5
2 .4 5 . 0
2 . 8 6 . 0
3 . 2 7 . 0
3 • • 7 .S
4 . 0 8 . 5
Grab. Weight
1 . 80
2 . 25
3 . 0
3 . �5
3 . 5
4 . 25
4 . 6
5 . 0
5 . 5
6 . 5
Grab. Weight
2 . 2
2 . 9
3 . 4
4 . 25
5 . 0
5 . 5
6 . 5
7 . 25
7 . 75
8 . 5
48
Worki ng Load
� . 2
3 . 05
4 . 2
4 . 85
5 . 5
6 . 65 I 7 . 4
8 . 2
9 . 1
1 0 . 5
TABLI. 5 .
Rope Oper.ted Ht;tavywelght Po lyp. Grab.
Design S .W . L .
2 . 5
3 . 5
4 . 75
5 . 5
6 . 0
7 . � --
6 . 23 r
9 . 0 j 1 0 . 0
l��_._L
Grab . I H . / Payload Factor
0.22
0. 36
0 . 4 0
0 . 4 9
0 . 57
0 . 56
0 . 6 1
0 . 64 I ': . 65
_ _ _ I : ; . 6 2 I
I
C rane Capac1 ty Based on 50/60 minutes operational rae tor and cycle time of 2 Dl1nutes .
TABLE 6 .
Rope Operated Heavyweiiht Lonk Tine CI�he l l Grab
Workinc Deslt;n Load S . II . L .
2 . 6 3 . 0
3 . 7 4 . 0
4 .6 5 . 0
5 . 85 6 . 5
7 . 0 7 . 75
7 . 9 8 . 7 5
9 . 3 1 0 . 25
10.45 1 1 . S
1 1 . 35 1 2 . 5
1 2 . 5 1 3 . 75
Grab. Wt./ Payload Factor
0 . 11
0 . 28
0. 35
0 . 38
0 . 4 0
0 . 44
0 . 4 3
O.H
0 . 4 6
0.47
Crane Capaci t y Based on 50/80
lIlinutes operational factor and cycle tll18 of 2 minutes .
Grab. We ight Payload Factors . Crane Sate Workins Loads
Grab, G .
lIean Refuse Densi ty in
3 Bunker 0 . 2t"" •
Mean Closed Grab Refuse
30ens i t y
0 . 4 t / • •
Mean Feed Chute Refuse �ns i t y 0 . 2<t/ ll •
Requ ired Hourl y Capac ity
10
20
30
40
50
60
70
80
90
100
Re fuse Practical Load per Grab. Cycle S i ze
0 . 4 1 . 0
0 . 8 2 . 0
1 . 2 3 . 0
1 . 6 4 . 0
2 . 0 5 . 0
2 . 4 6 . 0
2 . 8 7 . 0
3 . 2 8 . 0
3 . 6 9 . 0
4 . 0 1 0 . 0
Grab Weh;;tlt Payload Factors and Crane Safe W,orklnl Loads
Grab. H .
Mean Bunker Refuse
3Den.oity
0. 2tllo •
Mean Closed Grab Refuse !J"ns i ty 0 . 35t/ .. .
Mean Feed Chute Refuse gensi t y O . 22tAu •
Required Hour l y capacity
1 0
20
30
40
50
60
70
80
90
100
Refuse Practical Load per Grab . Cycle S ize
0 . 4 1 . 00
0 . 8 2 . 50
1 . 2 3 . 5 0
1 . 6 4 . �0
2 . 0 5 . 50
2 . 4 7 . 00
2 . 8 8 . 00
3 . 2 9 . 00
3 . 6 1 0 . 00
4 . 0 1 1 . 5 0
. -
Grab. Welght
1 . 3�
1 . 90
• • 7
3 . 4
4 . �
4 . 9
5 . 6
6 . 4
7 . 1
7 . 8
Grab. Wei ght
1 . 33
2 . 6
3 . 31
4 . 1
4 . 7 5
5 .
5 . 9
6 . 25
7 . 2
8 . 25
49
TAUU: 7 .
Rope Opera t<..'d Medium Weight Lont' T iDe C l amshe l l Grab.
Working Design Load S .W . L .
1 . 7 2 2 . 0
2 . 7 3 . 0
3 . 9 4 . 25
5 . 0 5 . 5
6 • • 7 . 0
7 . 3 � . O
8 . 4 9 . 25
9 . 6 1 0 . �
1 0 . 7 1 1 . 7 5
1 1 . � 1 3 . 0
Grau. Wt .1 Payload Factor
o . n
0 . 4 2
0.44
0 . 4 7
0 . 48
U . 4 9
0. 5() 0 . 50
0 . :; 1
0 . 5 1
Crane Capacity Based on 50/60 minutes operational factor and 2 minutes cycle time .
TABLE 8 .
Rope Operated Medium Welght Short TtDa C lamshel l Grab.
Working Load
1 . 7 3
3 . 4
4 . 5 1
5 . 7
6 . 7 5
7 . 8
8 . 7
9 . 4 5
1 0 . 8
1 2 . 25 , I
Design S .W . L.
2 . 0
3 . 75
5 . 0
6 . 25
7 . 5
8 . 5
9 . 5
1 0. 5,
1 � . 0
1 3 . 5
L---.-L . __
Grab. Wt .1 Payload 'actor
0 . 30
0 . 3 1
0 . 38
0 . 39
0 . 4 2
0 . 44
0 . 4 7
0 . 5 1
0 . 5 0
.--0 . 4 9
I
Crane Capacity Based on 50/80 minute. operational factor and 2 minutes cycle t i .... .
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Appro>:1mat" Order o f Costs o f Refus" G rabs . (L Sterl1ng)
� -
Grab. Types
Hcav}-.·�l&ht �lul ti Ram Po l y p . - Grab. A .
HaavY"'"o1t;ht �lono p.a.zJ Polyp. - Grab. D.
LiGht.weight Mono Ram Po l y p . - Grab. C .
Heavywe iKbt Polyp. - Grab. E .
Heavywc icht Lon!; T i "" C l amshel l Grab . F.
Mediumweight Short Ti De C l amshe l l Grab. II.
2
�900 3300
�600 3700
1400 2100
1 200
II l!JOO
900 1300
3500
4500
2500
2600
1600 I
TAll:.E 17 .
EFTA Countrios - Mid 1971
Grab. S i zos m3
4 5 6 8
3700 3900 4 1 00
5900 7400
4900 5 5 1 0 6500 7300 8100
VOO 2900 3 1 00 3JOO 3500
�900 3200 3500 370(j 3900
�OOO �300 2500 2700 2900
Table 18
Afproxlaate Order of Costs oJ G rabbing Cranes for Refuse Handl ing 17m span.
( L Sterl1n,.)
Sa fe Working loads 2 4 6 8 10 12 14
tonnes
4 Motor Crane -
=-t= Rope Opera ted Grabs. 22000 28000 3'<000 38000 48000
3 Motor Crane -
Electro Hydraul ic Grabs . 20000 ?5000 30000 3'<000 37000 1 3�
41000
U,K. Prices Mid 1971 Exc) uding Grabs .
54
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