01 July 1997
3-1. IMPACT HAMMER PILE DRIVING SYSTEM. Impact hammers are hammers which drive the pile by
first inducing downward velocity in a metal ram, as shown in figure 3-1. Upon impact with the pile
accessory, the ram creates a force far larger than its weight, which, if sufficiently large, then moves the pile
an increment into the ground. An idealized version of this force is shown in figure 3-2.
a. Hammers. The characteristics, strengths, and weaknesses of various types of impact hammers
are shown in table 3-1. A listing of impact hammers presently manufactured is shown in table 3-2. This
table is divided into three parts. Table 3-2a is an instruction for the use of the tables. Table 3-2b is in the SI
unit measurements, and table 3-2c is in the English units. Impact hammers can be divided into two
categories, external combustion and internal combustion.
fuel that provides the energy for the operation of the hammer outside of the hammer itself. These
hammers have external power sources such as the crane itself, steam boilers, air compressors, and/or
hydraulic power packs to provide the energy to move the ram upward, and in some hammers, downward as
well. The various types of external combustion hammers are detailed below.
(a) Drop Hammers. The drop hammer is the oldest type of pile driving hammer in existence.
A typical drop hammer is shown in figure 3-3. The hammer is connected to a cable which is attached to a
winch on the crane. The hammer is raised to the desired stroke. The winch has a clutch on it that then
allows the operator to release the hammer, which falls by its own weight and strikes a pile cap and the pile.
Drop hammers are mainly used on very small jobs and for small piling.
(b) Single-Acting Air/Steam Hammers. These hammers use steam or compressed air to
raise the ram. At a point in the upstroke, the valve is moved and the ram floats to the top of the stroke; the
ram then falls by its own weight and makes impact. These hammers are generally referred to as
" ir/steam"because they can be operated by air or steam; a few are operable by only one or the other. A
typical single-acting air/steam hammer is shown in figure 3-4; the operating cycle is shown in figure 3-5.
Many air/steam hammers contain a device to change the upstroke valve turnover point as shown in
figure 3-6. This device enables the hammer to operate at two energies, a capability which is especially
important during the installation of concrete piles.
(c) Double-Acting Air/Steam Hammers. These hammers are similar to the single-acting
hammers except that, upon upstroke valve turnover, they apply steam or air pressure to the top of the
piston. This enables the stroke to be shorter, as it accelerates the ram to the desired impact velocity more
quickly than with single-acting hammers. This makes a higher blow rate possible, which is advantageous in
some situations. A typical double-acting air/steam hammer with a description of the operating cycle is
shown in figure 3-7. Double-acting hammers are especially popular in driving sheet piling where vibratory
hammers cannot penetrate the soil or where they are favorable economically.
(d) Differential-Acting Air/Steam Hammers. These are similar to double-acting hammers
except that the air or steam is constantly pressurized under the piston. This allows for a simpler valve
configuration than with a double-acting hammer with similar operating characteristics. A typical differen-tial-
acting hammer is shown in figure 3-8. Its operating cycle and characteristics are shown in figure 3-9.
(e) Hydraulic Impact Hammers. These hammers substitute hydraulic fluid for air or steam, and
it is applied to the piston to move the ram. 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 very flexible control of the output energy. A typical