« Back to TDR Sample Articles

TECH TOPICS

It's Time to Inspect Your Harmonic Damper
(But please don't call it a balancer)

by David Magnoli, TDR Contributor

As my truck nears the 90,000 mile mark, I thought I would take a look at the owner's manual to see what the recommended maintenance was for that milestone just in case I were to forget, or was unaware of, some suggested repair. I always read the manual when I buy a vehicle, but often forget how much useful information is in it. Under the 90K schedule was the recommendation of "Inspect Damper". I also noticed that this same prompt was listed at 30K and 60K. My first thought was that this referred to the steering damper (4x4). Not entirely sure, I looked it up in my service manual and found that it was not steering-related at all but was instead pointing me to that often overlooked, and for the most part, forgotten item more properly called the harmonic damper that sits on the front end of the engine's crankshaft directly in front of, and part of, the serpentine-belt pulley. Like a lot of people, I suppose, I never paid much attention to this black doughnut, and did not realize that it required any sort of inspection or maintenance. It does in fact deserve our attention and in the process of doing so, I learned quite a bit about this important piece of our mighty Cummins Diesel engine.

2010 Crew Cab 2500 truck with ST Trim Package

The 5.9L stock damper uses a rubber ring between the cast hub and the
machined-steel pulley/inertia ring to dampen the firing impulses of the Cummins engine.

This picture clearly shows the degradation of this rubber, as well as the section that is missing,
which caused it to wobble on Cliff Scott's engine. This should motivate the reader to inspect this important,
but often-overlooked part, and make sure there are no signs of failure on your own truck.

Surprisingly, the index marks are still in alignment on this damper,
but this one should have been replaced some time ago.

What Is a Damper and What Does It Do?

Although often referred to as the harmonic balancer, because our Cummins is internally balanced (the crankshaft itself being balanced), the damper on our engines does not balance the engine. It damps harmonic vibrations and, as such, is a harmonic damper. Other diesel pickup engines are externally balanced and in addition to the damper, have a counter-weight either made as part of the damper, or bolted on in addition to it to provide the balancing requirement.

With each firing of the fuel/air charge in the cylinder, there is a violent but short-duration deformation or twist of the crankshaft as the force of the power stroke drives the crank against the resistance of the driveline. This occurs with each ignition event and results in the repeated twist and untwist of the crankshaft as this force alternately begins and ends, momentarily forcing the crank past its normal rotation, and then allowing the crank to rebound after each firing. This spring effect is more pronounced in a straight-six engine with its longer crankshaft than with the shorter crankshaft of a V configuration. It is also more pronounced at the front end of the crankshaft where there is less rotational resistance than at the transmission end.

While the torque-converter or clutch/flywheel will offer minimal vibration damping at the rear of the engine due to their mass, they do nothing for the front end of the crankshaft, which will experience a greater degree of twist/rebound under power. This is also more of an issue in modified high-power engines with higher cylinder pressures and the associated higher crank torque-spikes. Varying with the speed and load of the engine, these firing impulses and rebound forces can, and do, set up harmonic and torsional vibrations in the crankshaft. And because it is the front of the crankshaft that drives the injector pump as well as the camshaft, increased vibrations and harmonic twisting can negatively affect their function as well.

Our engine cranks have a natural resonance inherent to their design, which means that they tend to vibrate much more at certain specific frequencies than others. When the power pulses are in phase with this natural tendency to vibrate, this will result in lost horsepower, increased bearing wear, unstable valve and injector-pump operation as well as the effects being felt in the vehicle. Harmonics are the frequencies that are smaller, less pronounced multiples of the primary resonance.

Everything has a natural resonance at which it will vibrate. Think of a piano with its many strings. The long and thick strings will vibrate slowly. Short and thinner wires will vibrate faster and, because they are connected to a soundboard, they allow you to hear these vibrations as sound. Hit a 2x4 with a hammer and it will vibrate also at a fixed frequency determined by its size, mass, and physical shape. You can try this experiment to better understand the principles of native frequency and vibration. The next time you are standing on a single-span pedestrian bridge, such as one spanning a creek, stand in the middle of it and bounce up and down. You will feel its natural frequency as it shakes. If you time your bounces just right, and in cadence with and emphasizing this natural frequency, you can get the bridge moving up and down a surprising amount, much more so than if you were just walking across it. I am always astonished at how violently the bridge will shudder up and down if I continue this motion at just the right timing. Now imagine that in the center of this bouncing bridge you also have a 55-gallon drum filled with heavy gear-oil. Inside the drum and hanging suspended on a spring is a large, heavy steel washer that is allowed to move independently of the drum, which moves with the bridge. Now when you bounce on the bridge your motion is countered and dampened by the resistance of the weight moving against the oil. This is precisely what the engine damper on our truck is designed to do. The damper takes these short-duration shocks and absorbs them, thereby reducing the transmitted vibrations. The result is not only less stress and wear but also more usable power and a more stable valve train operation.

2010 Crew Cab 2500 truck with ST Trim Package

The pulley side of the stock damper shows the bonded rubber ring squeezing out from the gap between the hub and the inertia ring. The Service Manual clearly states that any protrusion beyond the face of the ring other than a slight convexity requires the replacement of the damper, as this indicates the beginning of the failure of the rubber.

Although there are no missing sections on this side of the damper, the fact that the ring is so far out of the gap indicates a failure of the damper.

The Stock Damper

The stock 5.9 damper, in good working condition, for the most part works well with the stock engine it was designed for. But when we add power and increase these internal forces, or as it ages, the stock damper may not be up to the task anymore. The engineers that design the stock damper do so with our enginesí stock horsepower and native frequency (RPM, crank weight, and load) in mind and try to design the damper to be effective at the most offending part of the power band by using a tuned damper designed to address a certain range of vibrations. The harmonic damper bolts to the front flange of the crankshaft and is built as part of the serpentine-belt pulley that drives the A/C, water pump alternator, and steering pump. There is a difference, however, in how this damper works on 5.9L and 6.7L engines.

On the 5.9 engines there is a cast-iron hub which bolts to the engine crankshaft. The serpentine pulley is not part of this hub, but is machined as part of the heavy, outer steel inertia ring. Between this hub and the pulley/inertia ring is a layer of rubber that separates the two and is bonded to both. It is this tuned-rubber ring that allows the damper to work by allowing the two parts to flex and move independently of each other, damping out the firing impulses and converting this energy to heat. The stiffness and thickness of this rubber ring is designed to cancel out a specific frequency of vibration, determined by fueling, RPM, engine load, etc. and is limited to a very narrow band of resonance, that which the engineers have decided is needed. Due to engine heat, age, and continuous flexing, this rubber will deteriorate over time, becoming less effective. Because the pulley is being driven by rubber, as opposed to being part of the hub itself, when the damper deteriorates, it also affects the alignment of the pulley to the belt.

Starting in 2007.5 with the 6.7L engines Cummins changed the damper design, foregoing the rubber in favor of a viscous-damped type. This damper works by enclosing an inertia ring inside a housing that contains a thick silicone fluid. This allows the damper to be more effective at a much wider RPM range than the tuned-rubber type used on the earlier trucks. Because the inertia ring is now floating in a thick viscous fluid rather than being a bonded mechanical connection, it offers a variable resistance to the firing impulses and therefore is better suited to countering them and canceling vibrations. I'll talk more about this type when discussing the aftermarket dampers. These newer dampers do not require any periodic inspection, so the rest of this article applies primarily to 5.9L equipped trucks.

Stock damper in usable condition.

This shows the stock damper in good usable condition.
Notice the rubber ring compared to the previous picture, and how it is flush with the hub and inertia ring.

The two holes at the bottom of the picture on the damper were drilled when the damper was balanced.
This damper is off of my 2001 with less than 90K miles and has seen much less severe use than the other damper,
which has seen many high-powered miles towing more than 13,000 pounds.

The Need For Inspection and Replacement

An index mark on the 5.9L dampers, in the form of an inscribed line, crosses both the pulley hub and the pulley/inertia ring so that you can see if there is any degradation of the damper, requiring its replacement. The service manual for my truck (2001) states that if this index line is more than 1/16" out of rotational alignment, the damper should be replaced. Another requirement for replacement is the migration of the rubber ring in a lateral direction, causing it to move out past the face of the inertia ring. This clearly indicates that the damper on our trucks is something that wears out over time and will need to be replaced at some point. The following story will show that this part is something that should be inspected as directed by the Ownerís Manual.

A few days before leaving to attend this yearís annual TDR May Madness event held in Pahrump, Nevada, longtime TDR member Cliff Scott was giving his 2001 truck an inspection when he noticed that the damper was wobbling when the engine was running. Upon closer inspection, Cliff noticed that not only was the inertia ring out of alignment with the hub, but there was a substantial amount of the rubber ring actually missing, as well as squeezing out of the space between the two. It is his damper I used in the previous photos.

He has close to 180K miles on his 2001 but the amount of damage tells me that this has been going on for some time, proving that the Ownerís Manual inspection recommendation should be wisely followed. Luckily for Cliff, I had a brand new Fluidampr sitting on my bench in preparation for this article and I offered it to him to use. He was able to install it and drove over 450 miles to Pahrump towing his fifth-wheel. He liked it so much I had to order another one and hope it would come in time for my review.

While I have heard stories of stock tuned dampers coming off of the hub, and have seen the bolts broken or missing, as well as the cast hub cracked, I would hope that the driver would feel increased vibrations indicating that something was amiss before that would occur. However, the possibility for such failures does exist. If the failure proceeds slowly, the driver may not notice the increase in vibrations. The point here is not intended to alarm anyone though; it is simply to remind you to ìInspect Damper,î which should include checking the torque (92 ft-lbs) on the four mounting bolts.

Even if your stock damper is not damaged, if you have substantially increased the power of your engine, your stock damper will most likely be out of its designed range. There was a substantial size difference between the 160 and the 215 horsepower dampers on the 12-valve engines, indicating that more power requires an increased level of damping. You can see this difference that was noted in TDR Issue 18, page 23.

Different size dampers.

This is a reproduction of a marginal photo. The key item; yes, there is a size difference between the dampers. 160 on the left, 215 on the right.

« Previous | 1 | 2 | 3 | Next »

Buy TDR
back issues.

TDR Issue 73

Originally published in TDR Issue 73, Aug/Sept/Oct 2011.

Become a member of the TDR to get great informative articles just like this one delivered to your door four times a year.

Subscribe today!