A SPECIAL GRAB FOR EVERY TYPE OF BULK MATERIAL
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For the producon of all goods and energy, raw materials are the basic ingredient. Therefore, raw materials are shipped in large quanes worldwide. All materials have their own properes and for fast and efficient handling, specific grab soluons are required. The most efficient and reliable grabs for handling large amounts of bulk material are rope-operated mechanical grabs. These grabs have a beneficial rao between dead weight and grabbing capacity. Bulk materials have properes from extremely light to very heavy; and from fine and free flowing to coarse and abrasive. There is a wide range of bulk handling grabs, opmised for specific materials. Clamshell grabs can be used for most bulk materials. For certain materials or specific operaons other grabs are a beer soluon. Hereaſter the scope of applicaon and working of the different grab models is described. WORKING OF A CLAMSHELL GRAB In order to get a good filling a clamshell grab must be able to penetrate the bulk material. This is the case for all fine and free flowing materials like coal, agribulk and ferliser. A standard clamshell grab with normal digging force is suitable for this operaon. See opposite the infographic of the working of this kind of grab: For more difficult to penetrate materials, like iron ore, a heavy duty grab with more closing force is required. The higher closing force is generated by applying more rope sheaves in the grab. These grabs have more dead weight by which the digging force will increase as well. See below the infographic of this heavy duty grab for ores. Besides the higher weight and closing force this open model grab has a wide spread for collecng the material from a bigger surface. In general it can be said that larger pieces create more resistance, making it more difficult for the shells to dig into the material. Normally clamshell grabs can be used for material with parcle sizes up to about 50 mm. Larger pieces will have a negave effect on the grab filling. When the material contains many lumps it will have too much ‘digging resistance’, the grab shells can’t enter the material and the grab filling will be poor. WORKING OF A TRIMMING GRAB Trimming grabs are ideal for handling coarse and lumpy material. These materials are praccally impossible to penetrate. Instead of digging into the material, trimming grabs can “scrape” the material together and even with low material levels, the grab shells can sll be filled completely. Trimming grabs are suitable for parcle sizes from 50 to 250 mm. For larger pieces an orange peel grab is a beer soluon. WORKING OF AN ORANGE PEEL GRAB Like a clamshell grab, an orange peel grab has to dig into the material for a good filling of the shells, but where a clamshell grab has two long cung edges for penetrang the material, an orange peel grab has six or eight sharp shell points to penetrate the material. Besides this lower ‘penetraon resistance’, the closing force of the whole grab is concentrated only on these shell points. These two factors, in combinaon with ‘flexible arm and shell joints’, make it possible for an orange peel grab to penetrate difficult materials with small intermediate spaces. The working is made clear in the info graphic below. Orange peel grabs are suitable for stones and rocks from 250 mm up to 2-3 metres. Another material that is commonly handled A SPECIAL GRAB FOR EVERY TYPE OF BULK MATERIAL Peter Visser, Sales Director and Co-owner, Verstegen 34 EDITION 71: SEPTEMBER 2016 WWW.PORTTECHNOLOGY.ORG DRY BULK AND SPECIALIST CARGO HANDLING
Transcript of A SPECIAL GRAB FOR EVERY TYPE OF BULK MATERIAL
For the production of all goods and energy, raw materials are the basic ingredient. Therefore, raw materials are shipped in large quantities worldwide. All materials have their own properties and for fast and efficient handling, specific grab solutions are required. The most efficient and reliable grabs for handling large amounts of bulk material are rope-operated mechanical grabs. These grabs have a beneficial ratio between dead weight and grabbing capacity.
Bulk materials have properties from extremely light to very heavy; and from fine and free flowing to coarse and abrasive. There is a wide range of bulk handling grabs, optimised for specific materials. Clamshell grabs can be used for most bulk materials. For certain materials or specific operations other grabs are a better solution. Hereafter the scope of application and working of the different grab models is described.
WORKING OF A CLAMSHELL GRABIn order to get a good filling a clamshell grab must be able to penetrate the bulk material. This is the case for all fine and free flowing materials like coal, agribulk and fertiliser. A standard clamshell grab with normal digging
force is suitable for this operation. See opposite the infographic of the working of this kind of grab:
For more difficult to penetrate materials, like iron ore, a heavy duty grab with more closing force is required. The higher closing force is generated by applying more rope sheaves in the grab. These grabs have more dead weight by which the digging force will increase as well. See below the infographic of this heavy duty grab for ores. Besides the higher weight and closing force this open model grab has a wide spread for collecting the material from a bigger surface.
In general it can be said that larger pieces create more resistance, making it more difficult for the shells to dig into the material. Normally clamshell grabs can be used for material with particle sizes up to about 50 mm. Larger pieces will have a negative effect on the grab filling. When the material contains many lumps it will have too much ‘digging resistance’, the grab shells can’t enter the material and the grab filling will be poor.
WORKING OF A TRIMMING GRABTrimming grabs are ideal for handling coarse and lumpy material. These materials are
practically impossible to penetrate. Instead of digging into the material, trimming grabs can “scrape” the material together and even with low material levels, the grab shells can still be filled completely.
Trimming grabs are suitable for particle sizes from 50 to 250 mm. For larger pieces an orange peel grab is a better solution.
WORKING OF AN ORANGE PEEL GRABLike a clamshell grab, an orange peel grab has to dig into the material for a good filling of the shells, but where a clamshell grab has two long cutting edges for penetrating the material, an orange peel grab has six or eight sharp shell points to penetrate the material. Besides this lower ‘penetration resistance’, the closing force of the whole grab is concentrated only on these shell points.
These two factors, in combination with ‘flexible arm and shell joints’, make it possible for an orange peel grab to penetrate difficult materials with small intermediate spaces. The working is made clear in the info graphic below.
Orange peel grabs are suitable for stones and rocks from 250 mm up to 2-3 metres. Another material that is commonly handled
A SPECIAL GRAB FOR EVERY TYPE OF BULK MATERIALPeter Visser, Sales Director and Co-owner, Verstegen
34 EDITION 71: SEPTEMBER 2016 WWW.PORTTECHNOLOGY.ORG
DRY BULK AND SPECIALIST CARGO HANDLING
with an orange peel grab is scrap.Because of openings between the shells
(and the flexible joints) smaller pieces can fall out. Therefore orange peel grabs are not suitable for handling sand, gravel and aggregates.
ENVIRONMENTAL FRIENDLY GRABSFor handling fine materials a grabs with high enclosed shells will minimise material loss and environmental pollution. With these shells the influence of the wind on the material inside the grab is minimal.
WORKING OF A SINGLE ROPE GRABThis type of grab is especially designed for cranes with a single drum and is normally used when port cranes are not available and ship board cranes must be used. Most of these ship cranes are single drum cranes that cannot operate 2-4 rope grabs, however, the single rope grab is able to connect to any crane
Most single rope grabs are provided with a mechanical hook system to secure the closing of the grab. To open the grab, this mechanism must be uncoupled. Besides the completely mechanical grabs with a touch-and-go opening system, there is a manual opening system (by pulling a tagline) and a remote control opening system.
WORKING OF A DREDGING GRABFor underwater operations, it is advised to use special grabs for digging operations, which includes clay and hard ground covers, as well as grabs for maintenance dredging (mainly silt and mud). The grabs are available in two and four rope versions. The inner side of the shells are smooth without any abrupt transitions, which minimises resistance during digging into the bottom and also makes it easier to unload sticky materials, such as clay and mud. The dredging grabs have a volume of 16 cubic metres and a deadweight of 25 tonnes.
For hard layers and difficult to penetrate materials, it is recommended to use lips with teeth. There are bottom lips with removable or exchangeable teeth and lips with permanent teeth (cut from the bottom lips). Permanent teeth (cut into the bottom lips) are very suitable for extreme hard layers.
CHOOSING THE RIGHT GRAB The material which has to be handled is very important for choosing the right grab for your operation. This together with the capacity and the type of crane and the specific characteristics of your loading or unloading operation will determine the type of grab best suitable. Choosing the right grab is extremely important for making your logistic operation profitable. A difference of 10% to 20% in capacity can easily occur per cycle. In case of transhipment of large amounts of bulk material this results in a huge capacity loss.
ABOUT THE AUTHOR
Peter Visser is Sales Director and Co-owner of Verstegen. With more than 20 years’ experience in the field of grabs for transshipment of dry bulk materials, he is a highly appreciated partner for many stevedoring companies and crane manufacturers worldwide.
ABOUT THE ORGANISATION
Verstegen is the world's leading manufacturer of rope operated mechanical grabs for the dry bulk industry. Stevedoring companies,
port authorities as well as steel works and power plants are using grabs of Verstegen for handling all kinds of bulk materials. More than 10,000 Verstegen grabs operate in more than 100 countries worldwide.
ENQUIRIES
Verstegen GrabsMarconibaan 203439 MS NieuwegeinThe NetherlandsTel: +31-30-6062222E-mail: [email protected]
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DRY BULK AND SPECIALIST CARGO HANDLING
