Gear rattle is due to impact noise of unloaded gears in transmission having freedom to move in backlash region. Engine order vibrations in the presence of backlash in meshing pairs induce the problem. It is a system behavior wherein flywheel torsional vibrations, the pre-damper characteristics and transmission drag torque plays a vital role in an engine idle condition (hot & cold). Idle rattle is a severe issue, which is highly noticeable in cold condition or after 1st engine crank.
Gear rattling observed in idle condition is idle gear rattle or neutral gear rattle, specifically in cold condition is a “Cold idle rattle” and this is one of the critical noise parameters considered for entire vehicle NVH. Damper mechanism in the clutch, is used to serve better isolation (by reducing the input excitation to transmission parts) of vibrations between engine and transmission their by reducing gear rattle intensity. Engine firing order, engine downsizing, down speeding (means high peak torque at low engine speed) and other unbalance masses plays leading role in fluctuation of output torque. Clutch parameters like pre-damper stiffness, hysteresis and wind up angle affects more to the cold idle rattle phenomenon. The gear-rattle phenomenon is also depends on vehicle operating temperatures. Temperature variation affects the frictional drag of transmission, which is a most crucial parameter in clutch Pre-damper characteristics design (input parameter) to address rattle.
Through experiments, the effect of temperature variation on transmission drag torque and its effect on clutch pre-damper characteristics, which is sensitive to rattle phenomenon, was established.
Clutch designer need to understand the requirements and accordingly optimum pre-damper characteristics need to be defined which would result into acceptable idle rattle. This was demonstrated with the experiments considering different pre-damper characteristics on same powertrain. These experiments will help clutch designers to define correct selection / design of pre-damper in the clutch disc.