What is torsional vibration?
This is not a collegiate level lecture. This is a workbench understanding. What is crankshaft torsional vibration? How does it relate to my engine?
It’s an interesting physics topic. The implications we witness firsthand with performance racing industry and OEM partners. Fluidampr has presented at the Advanced Engineering Technology Conference and Engine Expo-North America. Educational articles and contributions have been published by many professional magazines. These include EngineLabs.com, AERA Engine Professional, Race Engine Technology and Precision Engine. We have performed countless training seminars at industry leading parts suppliers. Have no fear. Engine harmonics is not mystical black magic or fear mongering. It's science. Ready? Let's begin.
There’s two things that kill an engine. Heat and vibration. We all know you need to keep the oil and the coolant pumping. Watch your manifold air pressure and be mindful of your boost. What about engine vibration? It is a major concern of professional race engine builders. Did you know the Cosworth CA 2.4L flat-plane crank V8 raced in Formula One in the mid-2000s had 13 vibration damping devices to help achieve 20,000rpm. Including 5 viscous dampers. One on the crankshaft. One on each camshaft. [Source: Extract of "King of speed", an article written by Ian Bamsey and published in Race Engine Technology Issue 73.] Serious engineering right there.
From our introduction we learned that there are three main crankshaft vibrations. We manage unbalanced vibration with a balancing service. Axial by the main bearing support and thrust bearings. Torsional by the harmonic balancer or damper.
The torsional vibration set up
Your rotating assembly comprises the crankshaft, rods, pistons, flexplate/flywheel and damper. This assembly is a series of masses. Mass has stiffness and inertia properties. In basic terms, if you hit it, how much will it flex? At what velocity and force? A simple analogy is a tuning fork. It has mass. It vibrates at a given frequency when you strike it. Strike it harder and it becomes louder.
With us? Great! It’s about to get real.
The rotating assembly goes in the block. Pistons fit in the cylinders. We close it off with the cylinder head. Bring on the power. Suck. Squish. Bang! Blow.
The bang we love raises cylinder mean effective pressure. A 427ci LS engine producing 450 foot-pounds of torque can generate 1,088 psi on the crank. In a mild performance diesel application we’ve seen it exceed 17,000 psi. That spike of pressure moves the piston in a sudden not so gentle way. The crankshaft undergoes stress. Every single ignition. Across every single cylinder. Repeatedly through the rpm range.
What does the crankshaft do besides rotate? It is a mass. In the ignition micro-moments during a rotation it will twist ahead of its natural rotation and rebound back. This triggers torsional vibration.
Frequency & order
Torsional vibration has a frequency measured in hertz or cycles per second. Frequency is RPM times order, divided by 60 (cycles per second). An order is how often a vibration event occurs during one revolution of the crankshaft.
In a four-stroke engine, the primary order is half the number of the cylinders. This is because only half the cylinders fire during one revolution of the crankshaft.
Other orders that set up are deviations from vibration oscillating through the crankshaft. As you move up through the RPM range, the frequency of each order increases.
For example, our 427ci LS V8 has a primary order of 4. At 6,500 RPM torsional vibration is occurring at a frequency of 433 hertz ((4 x 6,500) / 60). That means the crank is cycling through twisting and rebounding 433 times per second! That’s a single order. There’s multiple deviant orders happening at different rates too!
Torsional vibration has amplitude. Amplitude is the amount of crankshaft deflection in degrees.
You can measure two ways. One from center to peak twist. The other is the total amount of deflection from peak twist, through center, to peak rebound.
Crankshaft length in relation to the firing order, mass elastics or stiffness, and the amount of mean effective cylinder pressure applied determines amplitude. Amplitude increases with higher pressure and/or a longer crankshaft and/or taller connecting rods.
From our 427ci LS engine example, as frequency is occurring 433 times at 6,500 rpm, the crankshaft may be twisting and rebounding peak-to-peak through an amplitude of 0.7 degrees. On 2.559-inch-diameter mains that equates to 0.015 inches of movement. ((.7xw/180) x (2.559/2)).
Meh. So we’ve got some torsional vibration going on. There’s always a given level of stress on internal engine components. What’s the big deal? Where’s the failures?
Back to the simple analogy of a tuning fork. The rotating assembly is a series of system masses. Together they have a natural resonant frequency. The right pitch will put a tuning fork in motion. Likewise, the right torsional vibration frequency when aligned with natural resonance frequency will spike the twisting and rebounding of the rotating assembly.
System mass has spring stiffness and inertia. We need to watch the speed and torque behind the amplitude. Unfold a paper clip. You can bend it in short strokes nice and slow for a while. Long strokes real fast and it breaks.
At this critical moment of frequency alignment amplitude can double or triple in size. It’s happening at a high frequency. That’s most likely when the carnage will happen.
Continuing with our example, and for simplicity, let’s assume the natural frequency of the rotating assembly is also 433 hertz. When RPMs reach 6,500, torsional vibration frequency aligns with natural resonance frequency. Peak-to-peak amplitude doubles to 1.4 degrees. Now crankshaft fluctuation is 0.03 inches!
Torsional vibration doesn’t care. Over time, or sometimes very quick, it will find the weak spot in the crank and snap it. At Fluidampr, we’ve seen failures at the crank snout, at the flywheel, and every point between. For good measure vibration passes through metal-to-metal contact. The oil pump, timing gear & chain, valve train, main bearings all see added wear or failure. Accessory brackets fracture. Bolts back out. Drive belts slap or come off. Hard times if you don’t get torsional vibration under control at the source.
Enter the job of the harmonic balancer.
There you have it. A brief workbench explanation of torsional vibration. If anything, we hope you’ve gained an appreciation for why performance engines need a quality harmonic balancer. For a little deeper dive, check out "Understanding Engine Harmonics and Vibrations With Fluidampr" on EngineLabs.com.
Is there more to it? Absolutely! Is the torsional vibration of each engine model different? You bet. Does it change with engine mods? Certainly. How we measure torsional vibration and what inputs effect it are two great follow up topics.
Professional engine designers will want to take advantage of Vibratech TVD’s engineering focused educational opportunities. Vibratech TVD is the parent company of Fluidampr and a torsional vibration solutions provider to global powertrain OEMs across a wide variety of industries.