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Vibrators General

Vibratory Hammers

Vibratory Hammers - General
Vibratory hammers are used for driving and extracting piles and steel profiles but also for soil compaction. They generate vertical oscillating movements with a frequency of 1200 to 3000 (20 to 50 Hz) depending on the hammer used. The vibrations 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 or a leader with crowd force, or by mounting an extra weight on the vibrator, the speed of penetration can be widely increased.

Dawson Excavator Mounted Vibratory Hammer EMV 300

The Construction of a Vibratory Hammer
Basically a vibratory hammer consists of:

  • Vibration case (gear case).
  • Suppressor housing.
  • Hydraulic Clamp(s).

Vibration case (gear case)
The casing contains an even number of eccentric weights that are rotated at high speed in opposite directions by one or more hydraulic motors The eccentrics, which are synchronised by a gear system, are phased so that the horizontal components of the centrifugal force are eliminated and the vertical components added. As a result, vertical forces varying as sinus waves are generated and result in oscillating pile and hammer movements. The frequency is equal to the rotational speed of the eccentric weights.
Suppressor housing
The vibration case is mounted to a suppressor housing via a number of elastomer dampers. The suppressor housing is provided with eye for crane or with swivel for excavator attachment. Thanks to the dampers no vibrations are transferred into crane or excavator.
Hydraulic clamp(s)
At the base of the vibration case is mounted a hydraulic clamp or a set of clamps that clamps to the pile and transmits the vibrations from the gear case to the pile.

Advantages by Vibratory Hammers

The capability and versatility of hydraulic vibratory pile driving hammers are often underestimated. For many installations vibratory hammers are better alternatives to impact hammers and in others an important and cost effective complementary tool to impact hammers.

For details about the use, principles and design of vibratory hammers, please download the pdf-file,
"Vibratory Hammers for Pile Driving and Soil Improvement" by clicking the pdf-icon:

To select the right vibratory hammer for a given job some general rules apply. For clay the amplitude is especially important. For sand the amplitude is less important, but it must in general be between 3 and 6 mm. For sand the centrifugal force is in general more important.

To learn how to select the right vibratory hammer we recommend you to download the pdf-file:
"How to Select the Right Vibratory Hammer" by clicking the pdf-icon:

The frequency of vibratory hammers
Normal frequency hammers. Rotate with a speed of between 1200 and 2000 rpm and thereby vibrate with a frequency between 20 and 33 Hz.

High Frequency Hammers. Rotate with a speed of between 2000 and 3000 rpm and thereby vibrate with a frequency between 33 and 50 Hz.

Resonance free hammers or Hammers with Variable Moment. Are in general high frequency hammers with a design that makes it possible to run up the hammer to the wished revolutions per minute without vibration. The excenters of the hammer are then adjusted to make the hammer vibrate the pile into the soil. After the pile has been installed the hammer is run down to zero revolutions without vibration.

About Rebound - What Any Operator Should Know

Rebound is the most important cause of damage to vibratory hammers. It occurs when the pile stops penetrating by hitting rock or large boulders and the vibrator - in spite of this - is kept vibrating. By rebound the hammer is bouncing on the pile as nearly all the energy delivered by the vibrator is reflected from the rock/boulder to the hammer and gives cause to damaging forces.
No operator should operate a vibratory hammer without being thoroughly informed about rebound, its nature and its damaging results.
By rebound the vibratory hammer must be stopped immediately.

For Readers Who Want to Know About Resonance.

Natural Frequencies and Resonance

The concept resonance is widely misunderstood.

Resonance. Every flexible body - also soil - has a natural frequency (also called the critical frequency). If the body is forced by an outside swinging force to swing at this frequency, the amplitude of the body becomes as large as it can be and the body is said to be in resonance.

Damping. If - as an example - a rotor is rotating at its critical rotational speed, a small outside force on the rotor may make the shaft start to swing and it will be in great danger of vibrating with so large an amplitude that it breaks. The so-called magnification factor expresses the size of the deflection (amplitude) of the body at the critical frequency. The magnification factor depends on the damping of the body. With low damping the magnification factor is high.

When soil is superimposed with vibrations in its critical frequency area, it will start to swing with high amplitudes. The swinging will propagate to the surrounding area. The amplitude depends on the damping of the soil. The lower damping the higher amplitude. The result is major or minor disturbances in the form of cracks and dislocations of the ground and often damage to nearby located buildings. The damping in soil depends on soil type, layer distribution, water content etc.

Natural Freqency of Soil. Most standard vibratory hammers operate only slightly above the natural or critical frequency area of the soil. Most soil has a natural frequency between 15 and 20 HZ corresponding to the frequency of a vibratory hammer running at a rotational speed of 900 to 1200 rpm.

Utilization of Natural Frequency. In some cases it may be an advantage to utilise a frequency of the vibratory hammer near the natural frequency of the soil. This may be the case by concrete pile extraction, but also under certain soil conditions where it can speed-up the penetration. No building, however, should be close by.

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