Over the years, technology has advanced toward the development of larger and faster hammers. These developments included rams that were raised by ropes through human or horse muscle, those hoisted by steam winches or powered by steam, air, or hydraulic pressure, and finally the diesel hammer. Double acting hammers were developed in order to speed up operations.
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Impact pile driving hammers consist of a ram and an apparatus that allows the ram to move quickly upwards and then fall onto the driving system and pile. The ram must have a mass and impact velocity that is sufficiently large to move the pile.
A properly functioning hammer strikes the pile in quick succession. It transfers a large portion of the kinetic energy of the ram into the pile. The stroke of a pile- driving hammer is usually between three and ten feet (900 to 3000 mm).
External combustion hammers burn the fuel that provides the energy for the operation outside of the hammer itself. These hammers have external power sources such as the crane, steam boilers, air compressors, and/or hydraulic power packs. The energy moves the ram upward, and in some hammers, downward as well.
The drop hammer is the oldest type of pile driving hammer in existence. The hammer is connected to a cable that is attached to a winch on the crane. The hammer is raised to the desired stroke. The winch has a clutch on it that allows the operator to release the hammer, which falls by its own weight and strikes a pile cap and the pile.
The available energy per blow of a drop hammer is the product of the ram weight times the drop height. Standard ram weights range from 500 to 10,000 lbs (2.5 to 50 kN). Typical drop heights are approximately 4’ (1.2 m). For a given hammer, varying the drop height can vary the energy per blow. Drop hammers are typically used on very small projects and for small piling.
Advantages of Drop Hammers
Single acting air/steam hammers are essentially drop hammers, except that the hoisting cable is replaced by pressurized air or steam (motive fluid). The ram is usually a short, stocky block of steel that is connected at its top to a piston. It is guided by columns or inside guiding enclosure.
Single acting air/steam units from Vulcan and Menck are commonly used to drive piles offshore.
The operating rate of single acting air/steam hammers range between 35 and 60 blows per minute. This rate is comparable to most other hammer types that lack downward assist.
Advantages of Single Acting Air/Steam Hammers
Disadvantages of Single Acting Air/Steam Hammers
The desire to create a hammer that applies blows in rather quick succession for increased productivity led to the development of double acting air/steam hammers. For an increase in blow rate, the stroke was shortened. In order to maintain a comparable energy rating, the ram was accelerated during its down stroke using active pressure.
Advantages of Double, Differential and Compound Acting Air/Steam Hammers
Disadvantages Double, Differential and Compound Acting Air/Steam Hammers
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A hydraulic hammer incorporates the use of an external energy source to lift the hammer to the top of its stroke. For the single acting hydraulic hammer, the free-falling piston develops the actual energy induced into the pile, much the same power stroke as a drop hammer or a single acting air/steam hammer.
These hammers substitute hydraulic fluid for air or steam, and it is applied to the piston to move the ram. A hydraulic power pack provides the pressurized fluid to operate the hammer. Hydraulic impact hammers can be single acting, double acting, differential acting, or other variations. Most but not all hydraulic hammers employ the use of an electric valve operated with a variable timer. The timer allows for flexible control of the output energy. Others use a purely hydraulic system to control the valve and thus the cycling of the ram. Some are powered using the hydraulic power units on the crane or excavator.
Most hydraulic hammer manufacturers claim high efficiencies for their hammers. Although there are many improvements in hydraulic hammers that enable a more efficient drop, the main reason for the higher efficiencies is that they have some kind of downward assist to equalize the hydraulic flow during the hammer cycle.
These hammers burn the fuel that powers them inside of the hammer itself. The diesel hammers are the only constituent of this class, although other types of fuel are being used.
An open-end diesel hammer consists of a long slender piston (the ram), which moves inside a cylinder. The cylinder is open at its upper end, thus allowing the ram to partially emerge from the cylinder. Since the ram falls only under gravity, the OED is also called single acting.
There are many manufacturers of open-end diesel hammers throughout the world. The hammer ratings vary from 5 to more than 300 kip-ft with ram sizes between 1 and 35 kips.
Advantages of Single Acting (Open End) Diesel Hammers
Disadvantages of Single Acting (Open End) Diesel Hammers
Upon impact, the ram pushes the impact block, hammer cushion, helmet, and pile head rapidly downward, allowing the cylinder to fall under gravity. The impact block separates from the ram within a very short time and the pressure of the combusting air-fuel mixture will cause further separation as the ram is forced upward.
A closed end diesel hammer consists of a long slender piston (the ram), which moves inside a cylinder. The cylinder is closed at its upper end, thus causing the ram to compress the air trapped between ram and cylinder top. When the ram falls, it is subject to both gravity and the pressure in the bounce chamber. For this reason, the CED is also called double acting.
There are two prominent manufacturers of closed end diesel hammers distributing their products in North America: Bermingham and MKT Geotechnical systems. The hammer ratings vary from 5 to not more than 80-kip-ft, with ram sizes between 1 and 10 kips. The MKT hammers are built using liquid fuel injection. Bermingham’s closed end models use atomized fuel injection.
Advantages of Double Acting Diesel Hammers
Disadvantages of Double Acting Diesel Hammers
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