The challenge: to build a machine capable of demolishing parts of the outbuildings of a disused power station. While the Chaos Crew opted for a hydraulic brick crusher, the Beach Boys tried a medieval-style battering ram.

Battering ram

Pros

• Quicker to build.
• Less to go wrong.

Cons

• Danger of exhaustion
• Can't be deployed in a multidirectional way – limiting access. If the rammer is not heavy enough, it won't work.
• If the rammer is too heavy, the vehicle will collapse.
• Low accuracy.

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Hydraulic crusher

Pros

• Manoeuvrability.
• Accuracy.
• Engine power.

Cons

• Long building time.
• Hydraulics can be slow moving – especially if pump isn't big enough.
• Many elements involved – more chance of failing.

Hydraulic power - an alternative to brute force?
A fluid, usually water or oil, is used to transmit power from a pump to a hydraulic motor or a hydraulic ram (piston). Because liquids are nearly incompressible, large pressures can be built up to exert very great force within a hydraulic ram.

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How has hydraulic power developed?
In 1795, the British engineer Joseph Bramah invented the hydraulic press using the principle of hydraulic power, and then he came up with hydraulic power transmission, using a pump and motor. During the 19th century, hydraulic power was used for cranes, dock gates, swing-bridges and lifts. Perhaps the most famous example is the hydraulically operated Tower Bridge in London.

Today hydraulic power is used in farm tractors to control ploughs and in earth-moving equipment, presses, winches and aircraft main control surfaces, landing gear and brakes. Most car braking systems are hydraulic, too.

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What are hydraulic rams and pistons?
A hydraulic ram consists of a cylinder with a piston inside. One end of the piston is attached to a rod that moves – pushing, lifting, pressing. Inside the ram, the end of the piston to which the rod is attached is called the 'rod side' and the opposite side is called the 'piston side'.

The surface area of the 'piston side' is always greater than the 'rod side' because the rod itself takes up some space. It is therefore the 'piston side' that exerts the greater force for a given pressure. The larger the surface area of the 'piston side', the greater the force the ram can exert. This pressure is provided by a hydraulic pump that has a much smaller surface area than the ram.

The best example of all this is the car jack. This consists of two cylinder pistons: one, the 'pump', with a relatively small diameter; and the other, the 'lift ram', with a relatively large diameter. Pumping the pump causes the lift ram to rise slowly and with much more force than the pump, and it will travel a much shorter distance. Nevertheless, with the expenditure of very little effort, a comparatively weak person can eventually jack up a very heavy car.

If the surface area of the 'ram piston' is ten times greater than the surface area of the 'pump piston', the distance travelled by the ram will be ten times less, but it will produce ten times more force.

Pascal's Principle provides a simple formula – force (F) is equal to the area of a piston (A) multiplied by the pressure (P):

F = A x P

For example, if a ram with a diameter of 25mm has to lift 1 metric tonne (1000kg or 2205lb), the internal pressure on the piston will have to be 200bar (20,000,000Pa or 2900lb per square inch). If the internal diameter of the ram cylinder (and thus the surface area of the ram piston) increases, the pressure required to support the same load will be less. For instance, if you quadruple the diameter of the ram cylinder to 100mm, the required pressure within it drops to only 12.5bar (1,250,000Pa or about 180lb per square inch) to support the same 1 metric tonne load – the relationship between pressure and area increasing exponentially rather than directly.

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Which rams are best?
This depends on the requirement. A fast-acting ram, with a comparatively smaller diameter, would be less powerful than a powerful ram with a larger diameter, which would move relatively slowly.

What's a hydraulic circuit?
This consists of a hydraulic pump that sucks oil from a storage tank and pumps it around a series of pipes, control levers and rams or motors, finally returning back to the oil tank to begin again.

In a simple circuit where there is one ram controlled by a single control lever, if the lever is not being used, the oil is pumped straight through the control valve back to the tank. If the operator moves the control lever in one direction, the valve inside diverts the oil to the ram, thus pushing the ram in a given direction. The oil on the opposite side of the ram piston will then be displaced back into the tank. The same thing happens in the other direction if the operator pulls the lever back the other way.

Is hydraulic pressure efficient?
Typically only 60–70%. In comparison, a car engine is nearly 90% efficient.

You can find out more from the Scrapheap Challenge advisers about the battering ram and hydraulic demolition solutions.

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