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EMV: General

Vibratory Hammers > EMV: Excavator Mounted

Excavator Mounted Vibratory Hammers (EMV) - General
The vibrations transferred from the vibratory hammer to the pile reduce or remove the inner soil stresses so only small forces on the pile are needed to penetrate the soil. The weight of the hammer itself is normally enough to provide for the penetration. However by mounting on an excavator, the speed of penetration can be widely increased.

The manoeuvrability of an excavator and the fact that the hydraulic system of the excavator usually is enough to drive the vibratory hammer further contribute to the advantage. However, some essential facts must be taken into consideration when an vibratory hammer is planned run on an excavator:

What does the vibratory hammer require and what can the excavator provide.


Hydraulics and Engine Power

To drive a hydraulically operated machine a certain oil flow and oil pressure is required. And to deliver the required hydraulic flow and pressure a certain power is needed. The necessary power - called hydraulic power - is calculated from the formula:

Effect (kW) = pressure (bar) x flow (l/min) (1)
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If a powerpack or machine driving a vibratory hammer does not have at least the hydraulic power of the vibrator (the power calculated from the formula), it cannot drive it according to its specifications. The maximum centrifugal force and amplitude cannot be achieved. The result is that the vibratory hammer only functions as a smaller vibratory hammer.

The Hydraulics of an Excavator
Excavators are provided with a variable pump. Therefore, in excavators the oil flow decreases when the oil pressure increases and vice versa. This is to make it possible to move the buckets quicker under lower load.

If - as an example - the specifications of an excavator mention a hydraulic oil flow of 300 l/min and a hydraulic oil pressure of 340 bar, these values are not achieved at the same time. As a result, excavators are in general provided with smaller engines than formula (1) indicates.

Driving an Vibratory Hammer
To drive a vibratory hammer to its specifications, the prescribed hydraulic oil flow and oil pressure is needed. These values are mentioned in the data sheet specifications of the vibrator. For an excavator mounted vibrator, these flows and pressures must be provided by the excavator. The data sheet specifications of the excavator tells if the requirements can be met.

The oil flow required by the vibrator will be constant, but the oil pressure will vary
. The required oil pressure increases with the resistance met by the hammer. The more resistance it meets during the installation of the pile, the higher oil pressure is needed. When the highest allowable oil pressure is met (under very hard vibration), a pressure relieve valve mounted to protect the hammer will open. The hammer has reached the limit of its capacity - its maximum centrifugal force and amplitude.

Engine and Hydraulic Pump
To provide the vibratory hammer with the required oil flow and pressure, the hydraulic pump of the excavator needs sufficient power from the engine. The needed power is calculated from the formula (1), but is often specified in the data sheet of the vibrators as "hydraulic power".

Available Power From an Excavator
An excavator with a vibratory hammer needs power to drive the vibrator, but also power to operate the boom. Besides, there is energy losses to the ventilator on the cooler, in couplings between pumps and engine and so forth. In praxis only about 70 % of the energy output is available for the vibratory hammer.

Therefore, if a vibratory hammer has an required oil flow and oil pressure of 150 l/min and 350 bar respectively, the needed energy for the vibratory hammer is 88 kW (hydraulic power - formula (1)). As the available energy for the vibrator from the excavator engine is only about 70% of its full energy, the (rated) output of the engine must be 125 kW to drive such a hammer to its specifications.



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