SubscribePurchasing a grab can be more important for the calculation of a vessel or a contract than it seems for the first sight.
It happen many years ago that a shipping company got a contract, which was calculated with four vessels to handle the given bulk- volume (cement- clinker) within the given time- schedule. After a while it appeared that the grabs on this four vessels were working so efficient and fast, that the time for charge and discharge was much shorter than calculated before.
The shipping company calculated new and found out that they are able to fulfil the time- schedule and the necessary bulk- volume also only with three vessels, because of the unexpected short charge / discharge times in the harbours. So they took out one vessel from this contract, but earned the money for the calculated four vessels. This story, of course, is a special case, but it shows the impact which grabs can have.
Grabs are the link between the vessel and shore. It depends on the grabs (and on the crane- drivers) how long the vessel will stay in harbour ( if there is not limit set from shore- site with the possibilities of the “take-away”). But how to know which kind of grab fits the best to the requirements?
What kind of bulk cargo has to be handled?
What kind of equipment is already at the place?
The dead weight and the volume of the grabs has to correspond with the cranes, where they are working on. For this the SWL (Safe Working Load) of the crane is important and what kind of grab the crane is able to operate.
For the “open- close” of a grab are different technical solutions possible:
- Fully mechanic (“mechanical rope grab”: single-rope, two- rope or four-rope type)
- Electro- hydraulic grab
- Diesel- hydraulic grab
The advantages of a single- rope grab are, that he can be operated on every kind of crane and they are quiet robust. So they fit for areas or circumstances where is only low maintenance possible. Disadvantages are that this grabs are the “slowest” solution. Additional can accrue problems with the working high, because of the length of the closing rope. This means in harbours with tide it can happen that during the time when the water is on the lowest point the grabs can not reach the top of the hoppers. Or at least the necessary time per cycle rises up, because the jib has to be lifted much more to reach the hopper on the shore- site.
Some manufacturers offer their mechanical single- rope grabs also with a remote- control for opening. But this is not a big advantage and is not really increasing the discharge rate. On the other hand the grab has then some electrical and hydraulic parts, which are increasing the possibility of a malfunction. The advantage of the single- rope grab, that he is fully mechanic and easy to maintenance, is partly lost.
The two- rope and four- rope grabs need a crane with 2 (4) winches. One rope is the holding- rope, the other one is for closing and opening the grab (=2 rope grab). A four- rope grab is working with the same mechanism, but he has 2 holding- ropes and two closing- ropes. The positions of the 4 ropes in the top of the grab has, not exactly but nearly, to correspond with the position of the 4 winches in the jib- head.
The closing force of all mechanical rope grab depends on their dead weight and how often the rope is folded inside the closing mechanism. The more often the rope is folded (same Principe like “block and tackle”) the slower the grab is closing, but the more power he has.
Electro- hydraulic grabs working with hydraulic- cylinders. They are faster than mechanical- rope grabs and they need only one hook, not 2 or 4 winches. But to run an electro- hydraulic grab additional equipment is necessary, which has to be installed on the crane. An electro- hydraulic grab needs electrical power- supply from the crane to run the electro- motor, which is connected to the hydraulic- pump.
The necessary crane equipment are e.g. cable-drum, rope-drum, guide- rollers, turn-around pulley and the electrical main board, which controls the functions of the grab. All this has to be installed on the crane and this means that electro- hydraulic grabs cannot be operated on a crane without this equipment.
There are some intermediate solutions e.g. mobile cable-drum with integrated main board, but this is a quiet complicated solution. Diesel- hydraulic grabs have an Diesel- engine installed instead of the electro- motor. So they can operate independent from any external power- supply. They are controlled over a remote- control.
The advantage of this grabs are that they can operate on every kind of crane without any further equipment like single- rope grabs, but they are also fast like electro- hydraulic grabs. So this grabs are kind of mixture of the advantages of the mechanical single- rope grabs and electro- hydraulic grabs.
What kind of bulk- cargo will be handled with the grab? For heavier bulk, e.g. iron ore, pig- iron, sand, clinker, a heavier and powerful grab is necessary. The mechanic- physical behaviour of the bulk is also different. The angle of repose is different, e.g. coal has approx. 40° and clinker- stone 30°. Different is also how the grab is able to go through the bulk with his knifes.
In some bulk it can happen, if the design of the clamshells is not optimised, that the grab is pressing himself out from the bulk during closing. The formula for the calculation if the grab and bulk fits to the SWL of the crane is as follows: (bulk density x grab volume) + grab dead weight < SWL of the crane.
After all this technical points the interesting point for the purchaser are: the price and the maintenance costs, as well costs for spare parts.
A grab with the low costs is a grab without malfunctions. And a grab with the lowest costs is a grab without malfunctions, good grade of filling of the clamshells and a high number of cycles per hour. The purchase price of a grab is only in the second row of the costs during the life time of a grab.
The costs, which are caused by the grab during the years, are more important and most important are the costs which are NOT caused by the grab, because he had no malfunctions and no breakdowns during operations in the harbours. A fast, efficient and reliable grab can save and earn a lot of money over the years.
Grabs look all the same from outside, but the inside “features” are the invisible points which decides about the quality, liability and effectively of a grab. The design of the clamshells.. How is the kinematics, the “flow of the force” inside the construction and the bearings of the grab? Balanced and optimised kinematics means that the grab is using his force at the optimum point and it ensures a long lifetime of the bearings and the construction of the grab itself.
After checking the possibilities of the cranes, it is up to the wishes of the customer what kind of priorities he wants to set for his decision. The more information the grab manufacturer gets from the customer about the crane, the intend of use (stationary or switching from crane to crane) and the bulk, which will be handled mainly with this grab, the better for both sides.
