Vibration Dampers 101
J.C. BEATTIE OF ATI PERFORMANCE PRODUCTS
OCT 29, 2013
I’ve been around dampers for a long time and
have traveled to numerous engine shops around the country
to physically test crankshaft twist over the past 16
years. Throughout this time, I have collected considerable
data that allows me to determine “how much damper”
a certain engine needs. When given the crank weight,
peak normal operating RPM, horsepower, rotating system
materials, rules about the damper specifications (if
racing), and the application of the engine (road racing,
oval or drag), I can make a very good prediction of
the amount of inertia weight and the type of device
your engine will need.
Let’s take a moment and think about the way a crankshaft
works. On one end, you have your flywheel, torque converter
or clutch. On the other end, there is a timing chain /
belt / gear drive, and then a small “snout”
sticking out, on to which a damper and any needed accessories
are bolted. In between the front and rear, there are main
caps and bearings that hold the crankshaft in place in
the engine. The number of main caps can differ from two
to as many as six. These main caps go over the crankshaft
and bolt to the engine block. Attached to the crankshaft,
you have the rest of the rotating assembly which consists
of connecting rods and pistons with wrist pins and rings.
This is where all of the crankshaft twist and harmful
“harmonics” truly begin.
Within a motor, something
has to be off the centerline of the crankshaft
so that as the crankshaft turns one revolution,
a piston is pushed to the top and then pulled
to the bottom
The pistons and components travel up and down, to the
top of the cylinder and then back to the bottom: one cycle
drive, one cycle driving. Think about that motion within
an engine: something has to be off the centerline of the
crankshaft so that as the crankshaft turns one revolution,
a piston is pushed to the top and then pulled to the bottom.
If this is a power stroke, where fuel is compressed and
combusted, that piston is then forced downward. That is
what actually produces your power.
Think about riding a bicycle and the way you pedal the bike
to move. The pedals themselves are like the pistons and
the rod between the pedal and the crank sprocket is just
like your connecting rods. The pedal arms have to be off
the centerline of the crank in order for you to make a
circle with the pedals and move your bike forward. Your
crankshaft and pistons can be viewed in the same light.
Because something has to be off the centerline of the
crankshaft in order to function, the leverage of that
connection to the crank is very high. That is why the
crankshaft will twist as the system is forced to rotate
when the engine is fired.
The pedals on a bike act like
the pistons and the crank arm between the pedal
and the chainring is just like your connecting rods.
While your engine is running, some pistons are being pushed
downward on a power stroke, some are being pulled down
by the crankshaft, and some are being pushed upward by
the crankshaft. Now envision this entire system happening
8,000+ times per minute! Even further, all of these different
actions are happening to the same piece of metal - the
crankshaft. These actions make the shaft twist in one
direction away from its natural home location, and when
it tries to come back to that home location, its momentum
makes it travel past its original location and farther
in the other direction.
The measured magnitude of that action is called “Degrees
of Twist – Peak to Peak” or crankshaft twist.
This is what I measure when I am damper testing. It is
this action that breaks parts and robs you of horsepower
when there is nothing to counteract and eliminate the
twist. In this system, the worst torsional vibrations,
or twist, will always occur at the farthest point from
the greatest load, or the heaviest mass. A torsional twist
is defined as a twist without a bend. If you get too much
of this twist, you will have a bend and this will cause
engine and/or crank failures. Think about twisting a piece
of rope over and over; you can make one or two revolutions
and nothing happens. After that it starts to get a wave
in it, and then as you twist more, the rope will pull
your hands closer together.
Once torsional vibrations get to the front of the engine,
something there needs to counteract that motion. This
is where the damper comes into play. The damper’s
job is to absorb and counteract as much of the twist as
possible. With the right damper on your engine, the majority
of the twist can be eliminated. However, with the wrong
damper, virtually all of the twist can remain. A damper’s
job is to rebound like the recoil of a spring. In this
case the spring is your crankshaft twisting and when it
tries to rebound past that natural state we discussed
earlier, that is when the damper needs to stop it.
You can read more about the Dangers of Power Pulleys