The Ford Barn - View Single Post

nkaminar · 01-24-2021, 10:43 AM

Well, the dampening pulley finally came and I installed it a few weeks go. I am going to report on the results in three posts. The first post will be some background theory that I uncovered while waiting for the dampener. The second post will explain how I installed the new pulley, and the last post will be the results.

So here is the first post: Warning: Technical discussion following.

The crankshaft experiences torsional vibration AT ALL ENGINE SPEEDS and the vibration can be several different modes at the same time. At certain rpm’s the vibration can be more extreme due to the exciting pulses matching a resonate frequency of the crankshaft. The resulting torque on the crankshaft can be 6 to 10 times the amount of torque going out the end of the engine to drive the car. For the Model A, the natural resonance of the crankshaft can be within the operating engine rpm depending on how fast the engine is revved, but there is always torsional vibration present.

The flywheel rotates at a more or less constant speed while the front end of the crankshaft can be torqued 1 degree or more. The torsional vibration can lead to a broken crank and can reduce the life of bearings, camshaft gear, cam bearings, and variation in ignition timing. The maximum torque on the crankshaft due to torsional vibration is at the rear bearing.

The stock flywheel rotates as a very constant speed because the node point (where there is no torsional movement of the crank) is right at the flywheel. For a lightened flywheel the node point moves forward so the flywheel now experiences some torsional vibration. A counterbalanced crankshaft will lower the natural frequencies somewhat but will add to the overall torsional deflection at the front of the crankshaft.

The dynamic dampener pulley consists of three parts: The hub which is pressed onto the end of the crankshaft, the inertia disk which is a heavy disk mounted to the outside diameter of the hub, and the rubber that connects the two. The dynamic dampener has its own natural frequency, the rubber acting like a spring. The natural frequency of the dynamic dampener is designed to counteract the natural frequency of the crankshaft. It applies a counter torque at the end of the crankshaft that dampens the crankshaft. There is some energy absorbed by the rubber but the main action is a counter torque due to the resonance of the dampener. The dampener has to be designed to fit the engine. It does not have to be especially large but it does have to have the proper natural resonance. Computer modeling and vibration testing is used to design the dynamic dampener. This is not something that an average Model A owner can do.

The hub of the dynamic dampener has to be a press fit on the end of the crankshaft so that the hub and the end of the crankshaft work in unison. The rubber is molded between the hub and the inertia disk and has to be the right durometer, compound, and configuration. There is a lot of science built into each dynamic dampener. The rubber compound will determine the life of the dampener and is a closely guarded secrete held by the manufacturer.

Most of this information came from an article in the “Vintage Speed & Sport Secrets Magazine”, volume 23, number 4, April 2014, which in turn is referencing a white paper by William C. Sisco for BHJ Dynamics. (Note: when I tried to visit their website I got a security risk notice.)

01-24-2021, 10:43 AM	#34
nkaminar Senior Member Join Date: Jan 2012 Location: Western North Carolina Posts: 3,901	Re: Dynamic dampening front pulley Well, the dampening pulley finally came and I installed it a few weeks go. I am going to report on the results in three posts. The first post will be some background theory that I uncovered while waiting for the dampener. The second post will explain how I installed the new pulley, and the last post will be the results. So here is the first post: Warning: Technical discussion following. The crankshaft experiences torsional vibration AT ALL ENGINE SPEEDS and the vibration can be several different modes at the same time. At certain rpm’s the vibration can be more extreme due to the exciting pulses matching a resonate frequency of the crankshaft. The resulting torque on the crankshaft can be 6 to 10 times the amount of torque going out the end of the engine to drive the car. For the Model A, the natural resonance of the crankshaft can be within the operating engine rpm depending on how fast the engine is revved, but there is always torsional vibration present. The flywheel rotates at a more or less constant speed while the front end of the crankshaft can be torqued 1 degree or more. The torsional vibration can lead to a broken crank and can reduce the life of bearings, camshaft gear, cam bearings, and variation in ignition timing. The maximum torque on the crankshaft due to torsional vibration is at the rear bearing. The stock flywheel rotates as a very constant speed because the node point (where there is no torsional movement of the crank) is right at the flywheel. For a lightened flywheel the node point moves forward so the flywheel now experiences some torsional vibration. A counterbalanced crankshaft will lower the natural frequencies somewhat but will add to the overall torsional deflection at the front of the crankshaft. The dynamic dampener pulley consists of three parts: The hub which is pressed onto the end of the crankshaft, the inertia disk which is a heavy disk mounted to the outside diameter of the hub, and the rubber that connects the two. The dynamic dampener has its own natural frequency, the rubber acting like a spring. The natural frequency of the dynamic dampener is designed to counteract the natural frequency of the crankshaft. It applies a counter torque at the end of the crankshaft that dampens the crankshaft. There is some energy absorbed by the rubber but the main action is a counter torque due to the resonance of the dampener. The dampener has to be designed to fit the engine. It does not have to be especially large but it does have to have the proper natural resonance. Computer modeling and vibration testing is used to design the dynamic dampener. This is not something that an average Model A owner can do. The hub of the dynamic dampener has to be a press fit on the end of the crankshaft so that the hub and the end of the crankshaft work in unison. The rubber is molded between the hub and the inertia disk and has to be the right durometer, compound, and configuration. There is a lot of science built into each dynamic dampener. The rubber compound will determine the life of the dampener and is a closely guarded secrete held by the manufacturer. Most of this information came from an article in the “Vintage Speed & Sport Secrets Magazine”, volume 23, number 4, April 2014, which in turn is referencing a white paper by William C. Sisco for BHJ Dynamics. (Note: when I tried to visit their website I got a security risk notice.) __________________ A is for apple, green as the sky. Step on the gas, for tomorrow I die. Forget the brakes, they really don't work. The clutch always sticks, and starts with a jerk. My car grows red hair, and flies through the air. Driving's a blast, a blast from the past. Last edited by nkaminar; 01-24-2021 at 10:49 AM.