Pile extractor uses direct-drive servo valve to maintain safety

Even in heavy-duty construction machinery such as equipment for extracting steel piles from the ground, delicate hydraulic balancing circuits are needed to keep the rams level and maintain safety. Moog's direct-drive servo valves provide the accuracy and reliability essential to make this possible.

Pressure within the foundation industry is increasing to find means to extract previously driven steel piles without creating any noise or potentially damaging vibrations. Such a device needs to be compact, versatile and very powerful.

Dawson Construction Plant's X1000 Universal Pile Extractor consists of two powerful hydraulic cylinders, each capable of generating up to 4000kN of extraction force through a set of hydraulic taper-wedge jaws. These jaws are engineered so as to generate increasing clamping force in direct proportion to any increase in extraction force. The clamp has been designed to accommodate most steel H and U-sheet pile sections without modification.

Robin Dawson, Chairman of Dawson Construction Plant, says: "Contractors do not plan to use our equipment until they are desperate, preferring instead to try alternative methods of extracting piles - there are a variety of these, based on vibratory methods. However, we believe our system, in addition to being the most reliable solution, and silent, is almost always the most economical."

How it works

In operation, two small hydraulic cylinders are activated so as to close the taper-wedge clamping system onto the pile. Once a predetermined clamping force has been reached the UPE electronic control system automatically begins the extraction stage of the process, whereby the two 4000kN extraction cylinders are actuated. These cylinders can extract the pile at a rate of up to 3m/min, depending on the level of resistance and the nature of the power supply.

Once the cylinders have reached the end of their operating stroke, the clamping system automatically releases and the extraction cylinders retract to their starting position.

Hydraulic power is supplied from a fully portable diesel-engined power pack, which also houses the electronic control system. This control system can be operated with manual or fully automatic control.

With such great forces attempting to remove embedded piles, it is easy to see how the system could quickly become unbalanced. In fact if the two hydraulic rams were allowed to operate without a control system to keep them in balance, the whole system could easily tip over, as was demonstrated in early tests. To prevent this from happening, two independent safety systems are incorporated in the equipment.

Safety systems

First, a simple emergency safety cut-out is in place, based on a valve. A proprietary mercury switch is used to measure the angle of tilt and, should the whole assembly tip beyond five degrees, the power system is shut down.

However, this is a 'last resort' safety mechanism and the design calls for a much more accurate and sophisticated balancing system based on the use of an electro-hydraulic control system. This is used to keep the positions of the two levelling hydraulic rams synchronised so that a potentially dangerous situation cannot arise.

The balancing system is based on a Moog direct-drive proportional valve (DDV). An alternative to a conventional hydraulic piloted proportional valve, the DDV is said to be more tolerant of the harsh environment found in the construction industry. The use of the DDV allows delicate accuracy and control of up to five microns to be achieved in heavy-duty equipment, such as the X1000 Universal Pile Extractor, or in metal forming machines.

Dawson Construction Plant Project Manager Steve Desborough comments: "We love working with Moog's engineers. They are always able to come up with an innovative solution to our design problems, and the reliability of the company's products is second to none."

DDV advantages

At the heart of most modern hydraulic control systems is the proportional valve and at the heart of the best proportional valves is the bidirectional, permanent-magnet linear force motor. Thanks to the inclusion of a pair of high-energy rare earth magnets, the Moog DDV is claimed to achieve significantly higher performance (full flow achieved in 12ms) than other proprietary valves. This has resulted in its use in a wide range of demanding applications. Indeed, the advanced combination of permanent magnets and electromagnetic coils results in spool driving forces considerably larger than those that can be obtained by solenoid coils alone, with very low power consumption.

In operation, the electromagnetic coils act as switches that divert the permanent magnetic flux paths, effectively acting as an amplifier but without drawing additional current from the drive electronics. The result is that the spool position control is stiff and well able to counter the flow forces that are created on the spool when the valve is passing high flow rate at high pressure. In contrast, conventional proportional valves face a conflict between stiffness and drive current that cannot always be resolved when both high flow and high pressure performance is demanded.

One high-profile project in the UK where the X1000 has been deployed is at Canary Wharf. Here, contractor Dawson-WAM selected the Pile Extractor to remove more than 1500 sheet piles at a prestigious development site. Some six years earlier, the 22m Corus LX25 piles had been driven as pairs to refusal with a seven-tonne hydraulic drop hammer. Because of the location, extraction has had to be undertaken safely, without vibration, and with low noise levels. The additional challenge was that the piles were sitting in 10m of dock water, so a pre-fabricated frame was utilised, sitting on the dock bed, to enable the X1000 to work above water level.

The contractor was confident that the 1000 tonnes of extraction force would ensure that the piles came out reliably first time. And so it proved, with a pile being fully extracted every 30-35 minutes.