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Digger science
 Hydraulic excavators
Drag-line excavators
Pros and cons |
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| The Barley Pickers use three hydraulic cylinders |
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| An old car axle supplies rotational movement |
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Hydraulic excavators
The Barley Pickers opt to build a hydraulic machine quite similar to the excavators found on most building sites.
The idea behind hydraulic systems is very simple: force is applied at one point and transmitted to another by means of an incompressible fluid.
A pump draws this fluid (usually hydraulic oil) from a reservoir or tank and pumps it into the rest of the system at pressure. The reservoir ensures that the whole system is constantly full of oil. A drop in the volume of oil and the resulting pocket of air would cause a loss of pressure, efficiency and power.
The movement of fluid in the system is directed by a spool valve. This valve has two connection points that lead to a cylinder (one port at each end, so that movement is possible in both directions) and a low-pressure return connection back to the reservoir. The cylinder contains a ram or piston.
With the machine in neutral, the spool valve directs fluid directly back to the reservoir. When the machine is engaged, the valve directs fluid under pressure to one end of the cylinder, pushing the ram so that it extends. At the other end of the cylinder, fluid is released back to the reservoir. With the valve directing fluid in the opposite direction, the ram retracts.
The spool valve also has a pressure-release valve, which has a hose that connects straight back to the reservoir. When the ram reaches the end of its stroke, or if the load is too great, the pressure-relief valve will open and allow fluid to flow back to the reservoir. The pressure-release valve is set so that it cuts in if the rest of the system is being overloaded, to prevent damage.
Most hydraulic systems have more than one cylinder. The Barley Pickers' has three: the first moves the upper arm or 'boom', the second moves the forearm or 'dipper', and the third moves the bucket so that it cuts into the sand and scoops it up. The whole motion is similar to extending your arm downwards with your hand cupped and then scooping backwards. But unlike the joints of the human arm, hydraulic joints don't have rotational movement. They either extend or retract. Rotational movement in the Barley Pickers' machine comes from swivelling the whole 'arm' from where it's connected to the Land Rover.
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| An electric winch drags the Hammerlocks' bucket along |
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Drag-line excavators
The Hammerlocks build an old-fashioned drag-line excavator. This involves a winched bucket suspended from an A-frame on the back of a vehicle. The machine reverses into position, drops its bucket and then drives forwards a little. An electric winch then drags the bucket along, so that it digs into the sand aided by a cutting edge with teeth on the open side. The bucket is lifted with a hand-winch.
Drag-line excavators are used for fairly shallow digging, for example for removing the top soil on open-cast mining sites. They are capable of moving very large quantities of earth.
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| Hydraulic systems are very efficient when they work, but there's a lot to go wrong |
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| The Hammerlocks make do with a manual winch to lift their bucket. Operating the winch takes time. |
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Pros and cons
Barley Pickers
A hydraulic machine with a slewing (rotating) action is capable of digging deep holes.
The Barley Pickers replicate a design that is used on building sites throughout the world.
On the other hand ...
Hydraulic systems are relatively complex, so there's quite a lot to go wrong.
The Land Rover the Barley Pickers find has a rotten chassis, and the weight of the excavating arm places a huge strain on the vehicle.
Excavators have relatively small buckets and are not designed for transporting large amounts of earth.
Hammerlocks
The Hammerlocks' machine is much simpler.
It has a much bigger bucket.
On the other hand ...
A drag-line machine cannot dig so deeply into the sand.
The Hammerlock scavengers are only able to find one electric winch, which they use to drag their bucket. They have to make do with a manual winch to lift the bucket. Operating the winches takes time.
Though the Hammerlocks build a good machine, it takes them most of the test to figure out how to work it properly.
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