10-4 Magazine - For Today's Trucker

September 2005 - Performance Zone

DEALING WITH BAD VIBES
By Performance Specialist Bruce C. Mallinson

Have you ever broken a crankshaft? Are you about to break one? Has your truck or engine dealer ever explained to you what the vibration damper on the front of your crankshaft does, and how it protects your crankshaft from breakage? They probably haven’t, and I’m sure you know someone who has broken a crank somewhere between 700,000 and 1,000,000 miles.

Over the last decade or so, we have seen an increase in broken crankshafts. This was never a problem in the 70’s and 80’s. If you have been following these articles since their beginning, you would have read that on the mechanical Big Cam Cummins, the recommended change interval of the vibration damper is 380,000 miles. So the rule was, during an in-chassis rebuild, you should replace the vibration damper (prior to lower press fit liners, we usually had to rebuild engines at around 300,000 to 400,000 miles). Today, because of steel top pistons and other engine advances, rebuilding usually takes place around 700,000 to 1.2 million miles.

But waiting until over a million miles to replace your vibration damper is too long. Manufacturers of the fluid dampers recommend changing it at 500,000 miles. Now, how many miles are on your damper? If it’s over half of a million miles, you should think about replacing it. After all, spending $490.00 now is a whole lot cheaper (and smarter) then waiting for your crankshaft to break. Now that you know how long they typically last, let’s talk about what a vibration damper actually does.

The main purpose of a vibration damper is to control harmonic vibration, which is caused when the injector fires and the piston, on the compression stroke, is driven to the bottom of the stroke. The force the connecting rod places on the crankshaft actually twists the crankshaft. Once the piston is at the bottom of the stroke, the crankshaft actually flexes back the opposite direction. This constant forward and backward flexing creates a harmonic vibration that can break the crankshaft, snap the bolts that holds the flywheel to the crankshaft or break the front of the camshaft and/or the accessory drive shaft. Everything doesn’t break at once, but when one of these items does break, it’s telling you that the vibration damper is defective. But what makes a vibration damper go bad?

Vibration dampers consist of two pieces – an inner ring of steel and the outer casing. All you can see is the outer casing, which houses the inner ring, which floats in a silicone fluid. The inner ring of machined steel, while floating in the silicone, reacts to the twisting of the crankshaft and tries to keep the crank from rebounding. Have you ever opened a tube of silicone, used it once, put the cap back on, and put it back in your toolbox, only to find a year later, when you want to use it again, that the entire tube has gotten hard? Well, the same thing happens to the vibration damper. The silicone inside the damper gets hard after a period of time and the inner ring can’t move, which causes the harmonics to return. Again, the forces in the crankshaft begin to attempt to twist it apart.

The reason the dampers can live so long is because they are airtight, however if a small amount of air is allowed to enter the damper, the silicone gets hard in a short time, just like that tube of silicone. Vibration dampers also create a lot of heat from the searing effect the inner ring has on the silicone. As you already know, heat and air will dry out the silicone. Once the silicone gets hard, the damper is junk.

Vibration dampers are not balancers. The main purpose of a harmonic damper is to control harmonic vibration, not necessarily to balance the engine’s rotating assembly. We recently developed a new mercury-filled balancer that goes on the crankshaft behind the vibration damper. If you’re like me and you want the best for your truck and engine, the mercury-filled balancer can be installed when the vibration damper is being replaced. The balancer is held on to the crankshaft by the bolts that hold on the damper. So instead of removing the damper a second time to install the balancer, you might as well install both items at the same time.

The mercury-filled balancer will remove fifty percent of the vibrations in an engine. I have one on my 1989 T-600 Kenworth that pulls my RV, and that engine has absolutely no vibration in it from idle to 2500 rpm. We also have the mercury-filled balancers for Harley Davidson Motorcycles, and it’s shocking to see the results in that application.

I had a balancer on my 1999 Wide Glide and that motorcycle did not vibrate at all. Then, I purchased a 2003 Ultra Classic. On that bike, my hands would go numb from the vibration just a couple of hours into a trip. Just last week I removed the chain case cover and installed a mercury-filled balancer on the crankshaft and the transmission and now my Ultra Classic Harley Davidson does not vibrate. These balancers are also available to go between the brake drum and the wheels – you will never have to balance your tires again. I have them on my 1995 Dodge Cummins pickup and let me tell you, that is one smooth running truck.

Vibrations and noise can wear on your body and make a day behind the wheel seem like eternity. By taking the vibe out of your ride and making it smoother, your truck will live longer and so will you! If you have any questions, I can be reached through Diesel Injection of Pittsburgh by calling (724) 274-4080. You can also visit our website at www.dieselinjection.net for more information about this and other performance-related topics.