A vibratory hammer with a variable eccentric moment can be started and stopped without vibration. For this, the eccentrics are placed in a zero position with an adjustment motor (with opposite centres of gravity, resulting in a cancellation of the eccentric force). Only after the vibratory hammer has reached full speed, the eccentric moment is set, as a result of which the vibratory hammer will start to vibrate. It is possible to set the eccentric moment at a value from 0 to 100%. The operational rpm of these vibratory hammers is higher than that of low frequency vibratory hammers. Where a low frequency vibratory hammer will rotate with approximately 1500 revolutions per minute, a high frequency vibratory hammer will rotate with approximately 2300 revolutions per minute.
Driving in urban area
Due to this high rotational speed – as a result of which the vibratory hammer works further away from the soil's resonance frequency – and due to the smaller amplitude, these vibratory hammers are less harmful to the surroundings. Also when vibrating a foundation element into the ground or when extracting a foundation element, you can use the adjustment motor to influence the eccentric moment and therefore the amplitude. This will allow optimum adjustment of the vibratory hammer. Conventional vibratory hammers have a constant eccentric moment. When passing the critical frequency area during start-up and stop, the constant amplitude will cause disturbing negative vibrations in the boom of the crane and in the soil; not just at the spot, but with a considerable perimeter distance.
The high frequency (HF) vibratory hammers generate vibrations at a rev of 2000 to 2300 tpm. Compared to a low frequent vibratory hammer, a HF hammer produces less vibrations in the nearby surroundings. Tests have demonstrated that the vibro level of a HF hammer -measured at a distance of 2m from the driving element- equals the level of vibrations produced by a low frequency hammer at a distance of 16m!
When using a high-frequency vibratory hammer with variable moment (VM) with a adjustable moment of 0 to max 2300 rev. per minute, critical vibrations in start-up and stop are eliminated. After passing the critical frequency area, the eccenter weights are automatically adjusted and synchronised to the current eccentric moment.
Therefore, when driving in urban areas, it is best to use a vibratory hammer with variable moment.
During the start-up of the vibratory hammer to the max. frequency of 2300 rev. per minute, the eccentric moment is '0'. The eccentric weights cancel each other out and no vibrations (amplitude) are generated.
When reaching the max. frequency, the automatic adjustment of the eccentric weights takes place, the so-called 'in moment' amplitude.
As the vibratory hammer already reached its max. frequency, there is no question of passing a critical frequency. Disturbing vibrations during start-up and stop are eliminated.
The adjustment of the eccenter weights is completely automated. As manual (remote) control for shifting the eccenter weights is possible, the most ideal amplitude can be generated for each soil type.
It is not necessary to readjust the vibratory hammer when stopped and re-started. The hammer will automatically select the last setting. With VM, it is possible to maintain the hammer at its maximum frequency while bringing the eccentric moment to 0. The hammer can be brought to standstill without causing any vibrations.
Vibrating with VM works automatically and safe. It is an ideal equivalent of the pressing method.
Variable Moment in action: ICE20RF (Resonance Free) working in China