Crankshaft Pulley Damper

Name:

Crankshaft pulley damper

Also known as:

Torsional vibration damper (TVD)

Crankshaft damper

Crank pulley

What is it?

Not only does this drive the accessory belt, powering your alternator, water pump, and power steering pump, but it is also a balanced mass that harmonically cancels out undesirable vibrations in the cranktrain.

The crankshaft in an internal combustion engine does not rotate at a constant velocity. If it did we would not need a large flywheel to store up rotational inertia and smooth out the rotation. The crank is accelerated by (in the case of the 2JZ) one power stroke every 120 crank degrees. The effort that this stroke produces varies as the crank angle changes. Simulataneously, the other cylinders will be drawing in a fresh intake charge, compressing it, or expelling spent exhaust gases - i.e. resisting the power stroke. Therefore, the crank is "nudged" around in a series of close, but individual, impulses of energy.

Any mechanical system will have a natural, or resonant, frequency. A systems' resonant frequency is that at which it will vibrate in free air on its own if given a single impulse of energy. For a practical example, twang a ruler over the edge of a desk. The vibration that it vibrates at is its resonant frequency. You can see how this varies as the mechanical system changes by shortening the overhanging length of the ruler - the frequency goes up the shorter the ruler gets.

Another effect of the natural frequency is that if, instead of a single impulse, you can input a series of impulses which are timed at the same frequency as the natural frequency then the vibrations will very quickly raise to the point where they are out of all proportion to the input. However, the moment that the frequency of the input impulses moves away from the natural frequency the vibrations quickly come back under control. This effect is called resonance and is most usually encountered when you have an out of balance wheel. The wheel has a natural frequency by virtue of its out of balance. The input impulse is the speed of the wheel. At a certain narrow range of road speeds, you will find that you can feel a vibration through the steering wheel. This is the speed at which the wheel goes into resonance.

So how does all this relate to the crank pulley?

We have already mentioned that the cranktrain is being driven by a series of energy pulses. The cranktrain, like any other system, has a natural frequency. The input frequency can be approximated to the engine speed, at certain engine speeds the cranktrain will tend to go into resonance and vibrate out of control. These vibrations can have effects ranging from discomfort, noise, poor misfire diagnosis and other ECU sensor problems, to the physical failure of the crankshaft.

Cranktrain vibration comes in more than one flavour. First of all consider the crankshaft as a watch spring, oscillating back and forth (but generally turning in one direction). This is torsional vibration. Secondly, consider the crank as the ruler from earlier on. Except this time instead of a single desk edge as a support we have a series of supports in the form of the main crank bearings. Also, instead of a single energy input we have all four strokes of the four stroke cycle all acting at different places along the crank in different degrees at different times. These forces can cause the crank to bend along its length. This makes for a very complex system (both in torsion and bending) but it will still exhibit resonances in both.

However, resonance can work for us, and this finally brings us to the purpose of the crank damper.

The torsional vibration damper (TVD) consists of a mass of a known intertia which is attached to the crankshaft via a piece of rubber of a known stiffness. The mass can spring torsionally around the crank centreline. The inertia of the mass and the stiffness of the rubber are precisely tuned so as to go into resonance in an equal and opposite manner to that of the cranktrain. Therefore they cancel each other out.

A bending damper has a mass which is added to the end of the crankshaft via another piece of rubber. This type of damper nods up and down and cancels out bending vibrations along the crankshaft.

These masses, for convenience, are usually added to the front crank pulley.

Pros:

The most common reason to remove the damper is to reduce weight to lighten the car, or to reduce inertia to make the engine rev more freely (inertia by definition resists changes in rotational speed). However to do this you have to sacrifice the damping actions of the stock pulley. You cannot retain the same damping effects and reduce the mass at the same time. This is akin to having your wheels balanced using stick on balance masses, and then binning them and replacing them with lighter masses. The wheel will obviously go out of balance again. Likewise if you remove the damping pulley, the crankshaft will go into resonance.

In short – it revs up a bit quicker and the engine has marginally less rotating mass to move.

Cons:

As mentioned above it can be as little as noise and vibration or as severe as total crankshaft failure through fatigue. Fatigue is the failure of a part through repeated application of loads which are far smaller than those which would cause failure in a single application. These are most severe when the loads are reversing or alternating (like bending a paperclip back and forth). This sits nicely with the kind of vibration seen in undamped cranktrain systems.

In short – your crankshaft could eventually break.

-Digsy

nice read, very interesting.

Very interesting. I'd like to test this on my N/A engine, long term, with a solid pulley.

Solid pulleys are cheap and so are NA engines. Being a non interferance engine though, it could be as simple as a crank/bearing set replacement.

Quick question: removing the dual mass flywheel will be the same problem, as removing the damper pulley... as the crank has to be balanced on both sides !

Quick question: removing the dual mass flywheel will be the same problem, as removing the damper pulley... as the crank has to be balanced on both sides !

That's not entirely true. The flywheel is there to smooth out the torque impluses before they get to the transmission. If you consider that the flywheel is rotating at an almost constant speed, then the further towards the front of the crank you get the worse the torsional vibrations are. If you "lighten" (read: reduce the inertia of) the flywheel then you allow some of the crank torsionals to "leak" past the flywheel and into the transmission. This why a lot of people who do this kind of mod report a lot of transmission noise afterwards.

Just being picky, although I use the word a few times in the article above, the function of the pulley (and the flywheel for that matter) in the 2JZ has nothing to do with "balance" - just dampening or smoothing of torsional vibrations. As far as balancing goes the 2JZ (like any other inline 6) is 100% internally balanced. Certain engine layouts do sometimes have balance masses added externally to the crank pulley and flywheel: Inline 3's, V6's and cross plane crank V8's, for example.

Delete pls