May 23, 2012
Vibration is the enemy of the driveline. A clutch with the proper dampening properties can ensure long system life.
By Denise L. Rondini, Executive Editor
There have been many advancements in the internal combustion engine over the years. But one thing that has remained constant is vibration, says Prashant Kulkarni, engineering manager – Clutch Division, Eaton Corp.
In a recent technical paper Kulkarni said, “All of these engines create different firing pulses in order to operate and it is those pulses that cause oscillations and subsequent vibration.”
That vibration then moves throughout the entire driveline, “through the clutch, the transmission, down the driveshaft and to the axles.”
If vibration gets to be excessive it can break components like synchronizer pins, transmission and U-joint gears, he explains. “It can even [impact] gears down in the axle or any other component that is directly in the torque path of the driveline.”
Today’s high-torque, low-speed engines compound the problem, Kulkarni says, because as engine speed goes down, the amount of vibration that needs to be dampened increases.
“The engines of today are churning out gear- and teeth-rattling torque and vibrations that far exceed the capacity ratings for transmissions, driveshafts and axles,” he says.
As a result, the damper in the clutch has to be precisely designed, and, in fact, is the most critical part of a clutch, according to Kulkarni.
The stiffness of the damper will have an impact on the entire driveline. Kulkarni uses a broomstick and Slinky to illustrate his point. “If I’m holding a broomstick on one end and someone else is holding it on the other end and I shake my end, the other person will feel every oscillation.” However with a Slinky, he says the other person will not feel the oscillations because the Slinky acts like a soft spring.
He is quick to add that while a damper cannot be as soft as a Slinky, “It has to have enough travel designed into it to soften or dampen torsional vibration yet be strong enough to absorb the torque required to power the driveline.”
Remember that no one clutch fits all engines. You have to consider a several factors to find just the right cushioning balance, he says.
Those factors include:
If you went with a one-size fits all approach, you would have to make compromises on those items, Kulkarni says.
Clutch manufacturers have various configurations of springs to meet the needs of all different types of engines.
“It comes down to a design effort where we make the damper this soft or stiff to transmit this much torque with this number of springs,” he says.
Through the years there has been a proliferation of dampers and clutches. In the late 1990s, engine makers opened the flywheel bore from 8.5 in. to 10 in. According to Kulkarni, this gave clutch designers valuable additional space to dampen vibration. Some manufacturers, including Eaton, made damper springs larger and heavier to provide more travel.
Kulkarni explains the natural frequency of the driveline is determined by the stiffness and the mass of the components. Each drivetrain depending on its torque and other criteria will have a natural frequency, and a change to any component will alter that frequency. He says the only way to control the frequency of the driveline is to alter the clutch damper.
“Because of the calculations we have, we can determine how soft the damper needs to be so that the truck is never in a situation where it will be operating at the natural frequency.” This is called driveline resonance and will cause damage to the driveline components and can result in sudden failure, he says.
“You do not just put in an arbitrary stiffness. It is highly engineered and designed to ensure [your customer gets] the appropriate durability of [his vehicle’s] driveline,” Kulkarni says.
“Designing a clutch damper in such a fashion that you reduce driveline resonance, you can eliminate a lot of reliability risks for the entire driveline.”