By Prashant Kulkarni, Eaton Clutch Division Engineering Manager
While different versions of the internal combustion engine were being used more than 200 years ago, commercial vehicle use began in the mid-19th century with widespread petroleum production. Countless engineering advances have occurred since, yet one constant has never changed: Vibration.
All of these engine variations create different firing pulses in order to operate, and it is those pulses that cause oscillations and subsequent vibration. That same vibration then travels through the entire driveline, through the clutch, transmission, down the driveshaft, and to the axles. When the vibration gets too excessive, it can break components like synchronizer pins, transmission gears and universal joints. It can even be gears down in the axle, or any other component that is directly in the torque path of the driveline.
The clutch is the only component along the driveline that can affect vibration. That’s because it is the only soft component and the only one with ‘air.’ Everything else on the driveline is metal-to-metal—or a match-up that does not dampen vibration, but simply passes it along to the next component.
Meanwhile, today’s high-torque, low-speed engines compound the problem. 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, driveshaft and axles. Damper technology is the key solution of vibration problems. As the most critical clutch part, the damper has to be precisely designed. A damper of appropriate stiffness will positively impact the entire driveline.
The mechanics of a broomstick and Slinky spring toy illustrate damper and driveline behavior. Take two people holding the ends of a broomstick: If one shakes his or her end, the other will feel every oscillation. If a similar action is performed with two holding a Slinky, the oscillation does not travel from one end to the other. A clutch 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.
No One Clutch Fits All Engines
Several factors come into play in order to reach that precise cushioning balance:
- The amount of torque from the engine the driveline needs to support.
- The appropriate stiffness/softness of the damper to isolate vibration energy coming from the engine.
- Determining the correct size of the damper to accommodate the number of springs needed.
- The amount of friction material needed to maximize wear life.
A one-size-fits-all clutch will mean compromises made with one or all of those items. The damper will not be optimized in either its stiffness, size, amount of friction material, or torque capacity.
And even though a clutch may appear to not fail in a vehicle, excessive torsional vibration can result in other driveline component failures. A truck built prior to 1996 typically used stiffer rate 10-Spring model clutches. Vehicles shipped since are now standard with soft rate damper such as the 7-Spring or VCT to avoid excessive vibrations.
It is important to have various configurations of springs because there is only a certain amount of available space. So it comes down to a design effort where clutch designers make the damper this soft or stiff to transmit this much torque with this number of springs. That is also why, over the last 20 years, a plethora of different dampers, and subsequent clutch models, have been developed.
Beginning in the late 1990s, engine makers opened the flywheel bore from 8.5 to 10 inches. This gave clutch designers valuable extra real estate to dampen vibration produced by new engines. Eaton Clutch Division capitalized on this with the introduction of 7-Spring and VCT models. The damper springs in these models grew larger and heavier to provide more travel. It became clear the 10-Spring design was not ideal for dampening the torsional vibrations of modern engines. The soft rate 7-Spring and VCT dampers were developed, and designed to take full advantage of the 10-in flywheel bore. All major North American truck makers have since standardized on these soft rate dampers for their large bore engines.
Trucks built prior to 1996 have a 8.5-in flywheel bore that cannot accommodate the larger, soft rate dampers so Eaton still offers 10-Spring product in the aftermarket. Fortunately, the company can offer the soft rate dampers to owners of large bore engines across all torque ratings.
Eaton has invested heavily over the past two decades to develop an analytical model of commercial vehicle driveline systems. Today, we can model an engine connected to any driveline and calculate the amount of required torque and the associated natural frequencies required to have all of those parts that are connected to each other functioning properly.
The natural frequency of the driveline is determined by the stiffness and the mass of the components. A given driveline, sized for torque and other criteria, will have its own natural frequency. A change to any of the components in the torque path will change the frequency. The only way to control the frequency of the driveline is to alter the clutch damper.
Because of the calculations now in hand, 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, a condition called driveline resonance with the potential to damage driveline parts and spur sudden component failure. Designers do not impart arbitrary clutch stiffness; it is highly engineered to ensure truck operators realize appropriate driveline durability.
Eaton engineering has worked closely with all major truck manufacturers to develop the baseline for these extensive calculations. The damped clutch is right after the engine, and right before all the components that might fail due to vibrations, such as transmission input shafts, synchronizers, u-joints, and axles. 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.
Adapted from an Eaton Corp. Vehicle Group technical paper, "The Role of Mechanical Clutches in Dampening Torsional Vibration;" 800/826-4357; www.eaton.com/roadranger