SubscribeThe construction of grabs has a long history that dates back to approximately the end of the nineteenth century.
During a sailing trip along the Danish coast, the author discovered a grab with the year 1892 imprinted on it. It is probably a Kampnagel or a H.&O. Arnesen grab. These grabs were later registered for patent in the USA by the company Linkbelt and then manufactured under license by Krupp in Germany.
A large number of different grab types have been produced over the course of the years and a certain degree of standardisation has been achieved, which is however mostly specific to a company. The following is applicable for nearly all the different types of grabs:
- Single-rope clamshell bucket
- Two-rope clamshell bucket
- Four-rope clamshell bucket
- Electrohydraulic motor grab
- Remote control, diesel driven grab
Naturally the same is true for grapples (orange peel buckets) be they operated by ropes, electro-hydraulically or diesel hydraulically and logging tongs in the same drive category as well as other special grab types.
The construction of grabs is machine construction, referring to calculation and design.It is however often underestimated that a great deal of experience and feeling is required for the details, which frequently cannot be expressed in numbers and formulas.
All of the aforementioned grab types (we by no means claim that the list is complete) are developed and conceived in accordance to the corresponding inner mechanics of the bulk goods that are to be loaded or unloaded with due consideration for the piled weight of the individual goods and the conditions where they are to be put into operation.
Many of today’s manufacturers of grabs entered into grab construction via slight detours. Be it as a “by-product”, because for example cranes were already constructed or simply by coincidence, because a customer had at some time made an enquiry as to whether he could also buy a grab. This is the reason why many of the grab types have company specific characteristics and their own design. This itself is due to the fact that nearly every manufacturer approached the construction of the grab in a different way, with a different way of looking at things and differing requirements.
Or a constructor simply had a good idea that he could put into operation in a grab. Like a manufacturer of grabs in the Netherlands who are well-known for their scissor grabs. They are extremely successful with this grab because nobody has been able to manufacture a grab to the same quality level or one, which lasts as long as this one does.Furthermore, there are a relatively large number of grab manufacturers in this field who have more or less developed their own concepts and their own designs, however not as successfully.
The supposedly simple technology of grabs means that competitors often copy existing models when they introduce a new model or type to the marketplace.Naturally, based on the mechanics, the functioning of a grab can easily be copied. The small fine points are however decisive for the quality of operation and the long-life of the grab.
The one and two joint trimming grabs had their heyday at the time when tweendeck ships were employed more and more as bulk goods transporters. The coarser and heavier the bulk material is, the more difficult it becomes to cut through it, as is often the case for heavy ores and similar. The grabs that are used for this should have a wide opening shell because the distance that the jaws of the grab travel during cutting must be as far as possible in order to ensure a good filling of the shell. This is exactly the case with a trimming grab.
Thanks to the long distance the shells have to travel, the trimming grab sinks into the bulk good more slowly and the relationship between the closing pressure of the jaws and the achievable filling level is good, especially when heavy ores are concerned. The scissor grab also has similar advantages in this respect.
What we have just been talking about relates to ores. The same grab and pick-up procedure is applicable to a whole series of other bulk goods. Each bulk good to be loaded or unloaded has its own specific piled weight and apart from very few exceptions, very different inner mechanics. The inner mechanics are for example to be taken into consideration for the filling of the shell: how does the bulk good distribute itself? For this reason, in order to be able to achieve maximum loading performance, grabs, which are specific to the bulk material, have to be used. This is however, in most cases not possible due to the cost considerations involved.
Probably well known to many of the readers, it is exactly these cost considerations that lead to frequent arguments between technicians and businessmen in the companies when a grab is purchased.
A certain number of compromises have to be made in the concept of the grab that can lead to differences in opinion, not only between technicians and the management in the companies themselves, but also sometimes between the suppliers of the grabs and the customers.
Look at this example:
A 12 m3 motor grab is requested that is to be used as a universal machine for just about all different kinds of bulk goods. In the case of this motor grab you have to reduce the bucket volume from 12m3 to for example 6m3 for bulk goods to be loaded with a higher piled weight like ore, even so it will never achieve the same loading performance as the same power unit with 6.0 m3 ore buckets.
Basically this can be said about all grab types. Similar conditions apply for two and four-rope columnar grabs but additional important components have to be taken into consideration. Traditionally speaking, most grab types are built for a specific purpose and in this regard are specialised. If the grabs are then used for a job, which goes beyond the conceived area of employment, then certain losses in efficiency have to be expected.
The above is really just a small superficial look at the grab sector.
For two and four-rope grapples or the same as electrohydralic models there are once again a whole series of other influencing factors that have to be taken into consideration. More shells = more mechanics / hydraulics. The distribution of power in a grapple is different to that of a clasmshell bucket.
An additional field is mechanical two and four-rope grabs for the loading of wood bundles and long-cut timber.
Whilst automatic mechanical single-rope grabs as well as electro and diesel hydraulic, remote controlled motor grabs are used for the loading of wood bundles, it has to be remembered in all cases that wood is a natural product and that a tree when it grows does not adapt its form to the grabs that it will one day be transported by.
Finally the grabs that are produced uniquely should be mentioned. Like for example electro-mechanical grabs and mechanical single-rope grapples due to their special “inner workings” and the idea that is behind them. The EMG from ZGS is a good example of an idea taken from the field of machine construction into the construction of grabs. Originally this variation of power transmission comes from the construction of planes. Two constructors who formally built the undercarriage for planes quite simply modified something for the grabs
Or the grabs that stand out simply due to their size or their weight like for example the four-rope clamshell bucket and grapples with 50 or more tonnes dead weight. The four-rope models of salvage grabs get even bigger and heavier having in excess of 100 t dead weight. These are mainly used on floating cranes for the salvage of wrecks.
When grabs of this size are concerned it has to be expected that the customer may carry out other experiments in order to try something out or simply because a new requirement suddenly exists. That means that the grabs sometimes have to do jobs that the customer did not originally think about.
A short story from Russia (then still the USSR): there the crew of a floating crane that was only meant for dredging mud from the harbour basin and the shipping channel started to fish boulders out of the water that weighed between 60 and 70 t, with the grabs that we had delivered.
This was not planned because one did not know that such boulders were there. But because they were there they simply had to be removed.
The Suez Canal Association lifted all of the mooring dolphins from the canal bed that dated back to the “single lane period” with the salvage grabs. Originally shipwrecks should be salvaged with this salvage grab.
And last but not least, in order to show that not only large grabs can be used for other purposes, another customer frequently bent a very light, closed grab for foodstuffs- something which we first of all had no explanation for. Until the day that someone told us that this extremely small grab was used to clamp over the bitts of the ship and to warp the ship to the pier.
A motor grab was also used to demolish a building.
The areas of employment of grabs are as you can see not as limited as many people think.
(Sigvard F. Orts (born 1939) Managing Director of ORTS GmbH, has been building nearly all kind of grabs for 30 years.Contact: sigvard.orts@orts.gmbh.de)