Hydraulic Vibrators for Steel and Plastic Sheet Piles, Steel Profiles and Steel pipes.
Dawson hydraulic vibrators are high frequency vibrators designed specifically for driving and extracting Sheet Piles with an excavator.
With a compactor plate in the jaws the vibrators are well suited for soil compacting.
All Dawson EMV Vibrators are all provided with a Chain Clamp – see picture and the Chain Clamp chapter Chain Clamp.
The pile is lifted to vertical with the chain clamp and gripped with the clamp of the vibrator. Properly secured, the pile is vibrated while the excavator is applying down-crowd force. The force in combination with the total weight of pile and vibrator results in quick penetration.
Vibratory Hammers consist of:
The Gear Case contains an even number of eccentrics synchronously rotated in phase and in opposite rotational directions. The resulting vertical forces are transferred to the pile.The frequency corresponds to the rotational speed.
A Suppressor Housing is mounted to the vibration case via elastomer dampers and provided with a device for attachment to the excavator, crane or rig. In a Dawson EMV the device is a swivel.
Under 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.
| Specification | Dim. | EMV70 | EMV300 | EMV450 | EMV550 |
| Static Moment | kgm | 0.7 | 4.6 | 6.9 | 8.3 |
| Frequency | rpm | 3000 | 2400 | 2460 | 2500 |
| Centrifugal Force | kN | 70 | 300 | 450 | 550 |
| Dynamic mass | kg | 410 | 625 | 1008 | 1150 |
| Total Mass | kg | 520 | 965 | 1275 | 1500 |
| Max. Push/Pull | kg | 2800 | 15000 | 15000 | 15000 |
| Clamp Force | Tonnes | 30 | 36 | 54 | 66 |
The clamps of the Dawson vibrators are standard provided with jaws for installing steel sheet piles. The smaller vibratory hammer EMV300 (centrifugal force 300 kNm) is well suited for installation plastic sheet piles. For this purpose the standard jaws can be exchanged with flat jaws.
The Saddle (the red part) of the EMV- hammers can be exchanged with a saddle for crane suspension. By having a CSV saddle (Crane Suspended Vibrator saddle) to the EMV, a very versatile hammer set is achived.
The Universal Clamp of the CSV version of the EMV550 can be exchanged with a caisson beam with two clamps. This makes it possible to install pipes from 300 mm ø to 1100 mm ø.
Apart from optionally offering CSV Saddles and Caisson Beams with Clamps we offer:
Oil Flow
The oil flow to the hydraulic motor(s) determines the rotational speed of the eccentrics. Full rotational speed requires the minimum flow. – see table. For EMV450 2460 rpm is achieved at the flow 195 l/min. Flow higher than min. flow is sent back to the tank. To save energy and fuel it is advisable to set the flow from the excavator to a little above the min. flow of the vibrator.
The max. flow is the maximum flow the flow regulator is able to “handle”. Higher flows leads to instability in the flow regulation.
Oil Pressure
The sizes of the hammer motors, i.e. the torque, secures that rated rpm and thereby the rated centrifugal force is maintained even under hard driving. The motor(s) require a certain hydraulic oil pressure, called the minimum pressure, to provide the torque the motor is designed to deliver.
At the minimum flow and the minimum pressure the vibratory hammer runs at normal speed and provides the specified centrifugal force.
The max. pressure is the safe rating of the components. Dawson hammers do not have pressure relief valves, but relies on the excavator pressure relief valve.
Dawson Vibratory Hammers – Hydraulics
| Model | Centri-
fugal Force |
rpm | Min.
Flow |
Max.
Flow |
Min. Press. | Max. press. | Hydraul.
Power |
| Dim. | kN | rpm | l/min | l/min | bar | bar | kW |
| EMV300 | 300 | 2400 | 130 | 250 | 280 | 350 | 60 |
| EMV450 | 450 | 2460 | 195 | 350 | 270 | 350 | 88 |
| EMV550 | 550 | 2500 | 256 | 400 | 280 | 350 | 120 |
The engine of the excavator chosen to operate the Vibratory Hammer must be able to deliver a power that is minimum the hydraulic power of the vibrator.
In this connection it must be noted, that only about 70% of the engine power of an excavator is available for the vibrator. The rest goes to move the stick, for cooling etc.
The hydraulic power of the Vibrator is normally indicated in the vibrator specifications. If not, it can be calculated from the following formula:
Flow [l/min] x pressure [bar]/600 = Hydraulic Power [kW]
———————————————————————–
The engine power of the vibrator-operating excavator must be at least 1.4 times the Hydraulic Power of the vibratory hammer (only 70% is available for the vibrator).
The capability and versatility of vibratory hammers for pile driving is often underestimated. For many installations vibratory hammers are more effective alternatives to impact hammers and in other cases a 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”
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 more, download the pdf-file:
“How to Select
the Right Vibratory Hammer”
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.
Frequency and Rotational Speed
Freqency (in HZ (Herz) = vibrations per second)
corresponds to
Rotational Speed divided by 60 (in rpm – revolutions per minute).
The concept Resonance is widely misunderstood.
Resonance
Every flexible body – also soil – has a natural frequency (also named The Critical Frequency). If the body is superimposed by an outside swinging force to swing at exact the natural frequency of the body, it will react by swinging with its largest possible amplitude only limited by a possible damping.
The body is said to be in resonance.
An example: If a rotor is rotating at its critical rotational speed, a small outside force imposed to the rotor may make it bend so it starts rotating like a skipping rope and thereby be in danger of breaking. 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.
Most soil has a natural frequency between 15 and 20 HZ (oscillation per second). It corresponds to a rotational speed of 900 to 1200 rpm (rotations per minute) for a vibratory hammer.
When soil is superimposed with vibrations in its natural frequency area, it will start to swing with high amplitudes. The swinging will propagate to the surrounding area. 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 amplitude and thereby the extend of disturbances depends on the damping of the soil. The damping depends on soil type, layer distribution, water content etc.The lower damping the higher amplitude.
High Frequency Vibrators run with 2500 to 3000 rpm and therefore operate conveniently away from the critical frequency of the soil. Such vibrators are normally preferred. All Dawson vibrators are of the high frequency type.
In some cases and for certain soil conditions it may be an advantage to use low frequency vibrators – i.e. hammers with a rotational speed of 1200 to 1500 rpm – as the large amplitudes of the soil may contribute to quicker penetration. However, this can only be done with free hanging vibrators (supporting machines may otherwise be spoiled) and far away from buildings and other structures that otherwise could be damaged.
