Dual mass flywheel

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Cut open dual mass flywheel with clutch and starter ring gear

The dual mass flywheel (DMF) is part of the drive train of modern vehicles (cars, buses, commercial vehicles) and is used to reduce torsional vibrations .

background

In four-stroke reciprocating engines , the periodic sequence of the four strokes (intake, compression, working, exhaust) in combination with the firing sequence of the individual cylinders leads to irregularities in the rotation of the crankshaft and the (connected) flywheel . Since a drive train is a structure capable of torsional vibrations (with characteristic natural frequencies ) due to the inertia and rigidity it contains , the rotational irregularities introduced by the engine inevitably lead to torsional vibrations. These can be reduced constructively.

In the past, only couplings with torsional dampers were used to reduce torsional vibrations . In 1985, the first two-mass flywheel from LuK was launched on the market. While the conventional flywheel of an internal combustion engine consists of the parts of the crank drive, the flywheel and the clutch , a dual-mass flywheel is divided into a primary flywheel (engine side) and a secondary flywheel (gearbox side), which are connected to one another by spring-damper units.

The heart of these two-mass flywheels are so-called bow springs, which, due to their extreme length compared to clutch damper springs, isolate the vibrations that occur much better. Whereas in the first generation four bow springs were mounted at around 80 ° each, the newer generations usually have two bow springs or two spring sets (= springs coiled in opposite directions ). of about 170 ° each are used.

If a motor without a dual-mass flywheel is driven through speeds that are critical for resonance, the torsional vibration amplitudes may become excessive. These can both trigger the harmful rattle of the gears and also stimulate the body structure to vibrate via the bearings of engine and drive train components, which can propagate into the passenger compartment as humming, humming, howling noises etc.

construction

With the dual mass flywheel (DMF), the conventional flywheel is divided into the primary flywheel and the secondary flywheel. A torsional vibration damper (usually a combination of plain bearings and (in grease) mounted coil springs ) connects the two centrifugal masses in a torsionally soft manner .

The dual mass flywheel consists of:

  • Primary flywheel
  • Secondary flywheel
  • Rotary plain bearing
  • Coupling coil springs
  • Possibly. Damper elements

Mode of action

The division into the primary flywheel on the engine side and the secondary flywheel on the gearbox side increases the mass moment of inertia of the rotating gearbox parts. Through a targeted “soft” adjustment of the spring units, which connect the two centrifugal masses, the resonance frequency of the DMF is shifted well below the idling speed of the engine and the stimulating engine orders . This decouples the engine from torsional vibrations from the drive train. The DMF is then operated in the so-called supercritical state and acts as a mechanical low-pass filter : The rotational irregularities of the engine are "swallowed up" by the DMF or only transmitted in a very weakened manner. Thanks to this machine-dynamic "trick", a damper element can be dispensed with when using a DMF .

The critical resonance range is only passed through quickly when the engine is started. This is particularly important because in the area of ​​torsional resonance there is no torsional vibration decoupling, but rather an amplification of the same. Since operation in this speed range can destroy the DMF, many engine controls have integrated a function that prevents this by specifically switching off the engine. In order to be able to use the decoupling properties of a DMF in the broadest possible operating range of the engine map, springs with multi-level spring stiffnesses (spring rates), e.g. B. used by the use of two concentrically arranged single springs.

Studies have shown, however, that the position of the DMF's natural frequency is speed-dependent. The cause of this are the centrifugal forces that lead to the coil springs being pressed against their outer bearing. The frictional forces that act then prevent these coils from deflecting, which leads to a stiffening which results in an increase in the natural frequency position. However, as long as the changed natural frequency is below the engine speed, no resonance occurs.

advantages

The torsional vibration decoupling produces the following positive effects:

  • Vibration decoupling over the (entire) speed range
  • High level of noise comfort (smoother gearbox operation, less stimulation of the vehicle structure)
  • Gearbox protection by reducing torque peaks
  • Reduction of fuel consumption, since it is lower speed (= in the lower speed range ). can be driven
  • Longer life of the drive train components
  • Little space requirement (roughly the same as that of a two-plate clutch)

disadvantage

  • Lower mileage compared to a standard clutch
  • High repair costs
  • Thermal load
  • Limited torque

Alternative repair solution

At the beginning of 2007, the automotive supplier Valeo developed an alternative to the DMF, called 4KKit. This is a set of parts consisting of a rigid flywheel, clutch disc with a twisting mechanism similar to the DMF, as well as pressure plate and release bearing.

Manufacturer

The basic idea of ​​the ZMS has been further developed in different variants by several manufacturers and implemented in products:

swell

  1. ↑ A. Nicola, B. Sauer: Experimental investigation of the dynamic behavior of torsion-elastic drive elements. In: Automobiltechnische Zeitschrift. (ATZ), edition 02/2006, Vereinigte Fachverlage Mainz.