supply anti vibration mounts also referred to as vibration isolators for a wide
variety of industrial and mobile equipment applications.
the correct anti vibration engine mount for a diesel engine is not difficult
but an incorrect selection can have significant impacts on the installation.
Common outcomes of incorrect mount selection are metal fatigue of structures,
excessive noise and mount failure.
aspects of application information ensure the correct anti vibration mounts are
must always be remembered that vibrations are absorbed by allowing controlled
movement of the engine using resilient engine mounts. This can also be used to
dampen some of the movement when viscoelastic materials such as rubber are used
in the mount. Generally speaking, the more the anti vibration mount lets the
engine move the better the vibration isolation. There is a limit to this
alternative movement. The selection of an engine anti vibration mount can
therefore become a compromise between a number of competing criteria.
in engines come predominantly from the movement of pistons and connecting rods
up and down (mostly) and the rotation of the crankshaft. There are also
vibrations from crankshaft flex and from the combustion process. Generally the movement
of pistons and the rotation of the crankshaft have the greatest amplitude and
are the lowest frequency. The lower the frequency the harder the vibration is
to isolate so we will keep the discussion focussed upon the vibrations from
pistons, connecting rods and crankshafts as these are normally the largest and
cylinder engines are more difficult to balance as a single piston and
connecting rod make balancing more difficult. So for single cylinder engines
the vibrations generated of most concern are RPM/60 hertz (first order).
cylinder in-line engines have both pistons moving up and down in unison and so
represent the same case as a single cylinder engine. So for in-line twin
cylinder engines the vibrations generated of most concern are again RPM/60
hertz (first order).
cylinder engines can be better balanced for piston movements by using a V-twin
arrangement and this is often adopted for medium horsepower engines although
the result is an uneven firing pattern. Due to the ability to better balance
V-twins they are commonly used for twin cylinder engines.
cylinder in-line engines are inherently balanced for the piston movements by
the 120 degree arrangement around the crankshaft. In lower cost engines there
are often end to end vibrations of the crankshaft but these are addressed by
the addition of balancer shafts in more expensive engines. First order
vibrations are generally not of great concern although the second order end to
end vibrations of the crankshaft in lower cost engines may be an issue. Second
order vibrations are (RPMx2)/60 Hz.
cylinder in-line engines have two pistons moving up and two cylinders moving
down and most of the effects of piston movement are balanced out. Due to piston
velocities at certain parts of the cycle this cancelling out is not complete
and in more expensive engines a balancer shaft can be used to cancel out this
second order (RPMx2)/60Hz effect.
all engines there are vibrations generated from firing. These vibrations increase
with torque output (load). In one and two cylinder in-line engines these
vibrations are first order and are cumulative to the piston and connecting rod
effects. In three and four cylinder in-line engines the firing effects are
what does it mean in a practical sense when installing a small diesel engine.
Firstly, one and two cylinder in-line engines vibrate more noticeably at lower
frequencies than the other in-line types. For two cylinder in-line engines the
firing frequency also adds to the first order vibrations, especially under
higher torque. One and two cylinder in-line engines therefore more frequently
require anti vibration mounts with increased rebound capacity. Mackay MD
mounts, which are especially designed with increased rebound response, ideally
suit these applications. Higher rebound capacity mounts allow for more lifting
forces from piston movements hence the engine may move about a small amount
more than with other general purpose anti vibration mounts.
three and four cylinder engines the second order (and higher) vibrations are
more noticeable only because the first order vibrations are reduced. These
higher frequency vibrations are more easily isolated and the need to
accommodate rebound is reduced.
lower frequency vibrations are more “difficult” to isolate as they require an
anti vibration mount that has more flexibility. For example, a motor running at
1500RPM has a first order vibration of 1500/60=25Hz and a second order
vibration of 50Hz. If an anti vibration mount is selected that has a static
deflection of 1mm due to the supported mass then the isolation efficiency will
be approximately 60% for the 25Hz vibration and approximately 90% for the 50Hz
vibration. To achieve 90% isolation efficiency for the 25% vibration a static
deflection of about 4mm would be required.
when CBC is approached about engine vibrations in existing installations the
culprit is the installation of anti vibration mounts that are too stiff to
effectively isolate the first order vibrations. The reasoning for the stiffer
mounts being in service may be to stop excessive engine movement and problems
with space envelopes for the engines and ancillaries. Unfortunately, movement
equals isolation. There are a number of ways in which excessive engine movement
can be limited which allow basic general purpose anti vibration mounts to be
used but in general these mounting arrangements need to be selected via an
appropriate calculation criteria at the initial design stages.
speeds are an area where significant problems with vibration can be
encountered. For example for an engine idling at 500rpm a mount static
deflection of just over 10mm gives only 50% isolation efficiency for first
order vibrations and transmitted vibrations may cause significant issues. Few
available anti vibration mounts give this level of static deflection so
increasing idle speed marginally to 750rpm allows a mount with around 6mm of
static deflection to give around 65% isolation efficiency with first order
most small engines tend to run at higher constant speeds over 1,500 rpm where
the first order vibrations are 25Hz and an anti vibration mount with 3mm static
deflection will give around 85% isolation efficiency. Engines which are run at
both idle and power settings, such as medium and larger sized marine engines,
are either three or more cylinders’ with relatively good first order balance or
are twins with a vee arrangement which also have good first order balance and
hence the problem with low frequency vibrations is avoided in most cases.
provides a range of anti vibration mounts for a wide variety of applications.
Application engineering services are available from our engineering team and
are supported by our suppliers Mackay Rubber, Rosta etc.
CBC supply package for anti vibration mounts and isolators includes ranges from
the following manufacturers – Mackay, Paulstra, Fruedenberg, Silent Block,
Rosta, Lord and Trelleborg.