Fixed concrete block
The fixed concrete block is a
proven method and for certain installations the most suitable. In these cases
the generator set's underframe is bolted directly to the concrete block.
Concrete block size is recommended to extend 400–500 mm (16–20") beyond
each side of the assembled generator set. The surface of the block is usually
above bed level so that the difference “h” is 100-200 mm (4–8") and forms
a pedestal. The depth of the concrete block is calculated in the following
manner:
Generator set total weight
D = –––––––––––––––
d x W x L
D = Concrete block depth in
meters
d = Concrete density in kg/m3
(use 2400 kg/m3)
B = Concrete block width in
meters
L = Concrete block length in
meters
After the concrete depth required
for installation weight and stability has been determined, check that the
ground is able to bear the total weight (engine plus concrete block) and
withstand the forces concerned. It may not be possible to reach a firm ground
layer, e.g. hard clay, compacted sand and gravel or rock, without digging
unreasonably deeply. In such a situation the load must be spread over a large
area on a concrete slab.
Floating concrete block
A floating concrete block is an
effective alternative to a fixed concrete block. In order to determine the size
of the concrete block, apply the same calculation as for a fixed concrete
block. The block is cast beforehand in a wooden mold. There must be an air gap
of approximately 25 mm (1") around all four block sides. The gap at ground
level must be sealed with a non-bonding material to keep out dirt and water,
but nevertheless also provide flexibility. Place watertight insulation material
beneath the floating block. This insulates the generator set and the block, and
significantly reduces vibrations to the surroundings. All equipment connected
to the generator set, e.g. exhaust systems and fuel pipes, must be installed
with flexible connections.
Installation procedure on
concrete blocks
When the concrete block is
poured, recesses must be made for the mounting bolts i.e. anchors or similar.
Place removable wooden boxes at each mounting bolt location. The size of the
box must be adapted to the size of the bolt used for installation. Remove the
boxes when the concrete has almost cured. Make sure the upper surface of the
concrete block is fairly even and fault free.
The illustration shows the method
with a normal anchor bolt. The depth d must be slightly greater than bolt
length L. This is so that the bolt can be dropped into the hole for the
grouting, and so that the installation can be rolled into place without the bolts
being in the way. Install a steel packing plate at each mounting point above
the hole and on each side of the bolt.
Fixed mounts
Fixed mounts are normally used on
engines installed on a frame as illustrated. The driven unit may be a
generator, a water pump or a compressor.
Vibration insulating mounts
In order to reduce noise levels
and absorb vibrations transferred to the installation foundation, the frame is
equipped with flexible mounts as illustrated. The flexible mounts must be
placed so that the load is distributed evenly. The location of the mounts is
determined by calculating the total weight and center of gravity of the
installation, then the mounts are arranging evenly around the unit's center of
gravity. The number of flexible mounts is normally between 4 and 6. It is
important to know the engine's critical rpm and the load that will be operated
at the intended installation so that the correct flexible mount hardness and
layout is chosen.
Location of flexible mounts
(W1 x L1 ) + (W2 x L2 )
L = ––––––––––––––––––
(W1+ W 2)
L1 = The distance from the
installation's front to the center of gravity for the engine and radiator
W1 = Engine and radiator weight
with coolant
L2 = The distance from the
installation front to generator center of gravity
W2 = Generator weight
L = The distance from the
installation front to the entire installation's center of gravity
To be noted when selecting
flexible mounts
On the whole, a flexible mount is
preferable to other configurations. A flexible mount is optimally designed if
the natural frequency from the vibration system, of which the engine and mount
elasticity form part, is at least 40 % lower than the engine's lowest occurring
frequency. A low natural frequency requires soft flexible mounts. However, such
mounts have the disadvantage of allowing significant deflection from external
forces that may occur with inclined installations or shock loads. On 4 cylinder
engines it is practically impossible to install a fixed connection between the
engine and the underframe when taking into consideration impulse forces of the
second order. A prerequisite for correct flexible mounts is that the foundation
must be significantly stiffer than the flexible elements, otherwise the
foundation would act as a extra spring.
The elements must be arranged so
that they bend when they are affected by the forces that arise during
operations. There must be sufficient free movement between the engine and the
chassis. Flexible mounts that fit Volvo Penta engines can be found under “Order
specification-optional equipment” in the Sales Support Tool, Partner Network
for each engine type. In order to withstand vibrational deflections from the
flexible attachment, all pipes, cables and lines must also be of flexible
design. Rubber mounted radiators are acceptable if the engine is mounted fixed.
(If both the engine and the radiator are flexibly mounted, there is a risk of
the fan coming into contact with the fan ring). When the engine, or the engine
and transmission, is installed on the flexible mounts, make sure that the
underframe is level, parallel and even. Also make sure that the mount hole
patterns are correct and the holes sufficiently large in order to avoid uneven
load stress and twisting of the mounts. Twisted rubber mounts affect sound
dampening and vibration characteristics negatively and drastically shorten the
service life of the rubber.
Uniform load on mount elements
Locate the mount elements so that
the load is distributed evenly. This is achieved by balancing the distribution
of the forces that act on the mount elements by changing the distance between
the mounts or by changing the number of mounts. In most cases, the best
alternative is to alter the number of mounts. If engine and transmission
centers of gravity and weights are known, it is possible to calculate the
forces acting on the mounts as illustrated. The following result for mount
load:
The location of the total center
of gravity SMG (engine and transmission weight) in relation to the transmission
center of gravity, can be calculated using the following equation.
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