Modern engines can be driven at extremely low rpms. The trend is toward ever increasing engine torques. Wind-tunnel-optimized bodies are creating less wind noise. New calculation methods are helping reduce vehicle weights and weight-saving concepts are boosting engine efficiency as well. The addition of a fifth or a sixth gear can also reduce fuel consumption. Thinner oils are making precise shifting easier. In short: The sources of noise are increasing and natural damping is decreasing. What has remained is the principle of the internal combustion engine whose cyclical combustion processes excite torsional vibrations in the drive train – the unpleasant consequences of which are gear rattles and body booms.
Drivers who are accustomed to increased comfort no longer accept such background noises. The job of the clutch is now more important than ever – in addition to engaging and disengaging, it must effectively insulate the engine’s vibrations. Physically, this is easy to solve: The mass moment of inertia of the transmission must be increased without increasing the mass to be shifted. This dampens the engine’s torsional vibrations and brings about the desired comfort level. The process reduces load on the transmission at the same time.
It's all in the name
LuK was the first manufacturer in Europe to develop and sell a dual-mass flywheel in large-scale standard production that was able to realize this physical principle. The name says it all: The mass of the conventional flywheel was simply split in two. One part continues to belong to the engine’s mass moment of inertia, while the other part now increases the mass moment of inertia of the transmission. The two decoupled masses are linked by a spring/damping system. One clutch disc, without a torsion damper, between the secondary mass and the transmission handles the engaging and disengaging functions. A favorable side effect is that the transmission is easier to shift because of the low mass to be synchronized, and there is less synchronization wear.
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1 Starter ring gear
2 Primary fl ywheel mass
3 Arc spring
4 Plain bearing
5 Flange
6 Floating friction device
7 Primary cover (cross-sectional view)
8 Secondary fl ywheel mass
The division of the conventional flywheel into two discs results in a primary flywheel mass with starter ring gear on the engine side, and a secondary fl ywheel mass with vent slots for heat transfer, which increases the mass moment of inertia on the transmission side. The two decoupled masses are connected via a spring/damper system and supported by a deep groove ball bearing or plain bearing so they can rotate against each other. Two molded sheet metal parts laser-welded to the outer edge form the ring-shaped grease cavity, in which the arc springs with spring guides are located. Sealing is provided by the diaphragm. Designed as a diaphragm spring, the wings of the flange engage with the arc-shaped pressure springs. The flange can be made of rigid sheet metal, include a damper or be designed as a slip clutch in order to act as peak torque limiter. An additional friction device, a bearing floated on the hub, is carried by one of the retaining plates and is able to generate the required friction at large torsional angles. As the DMF has an integral spring/damper system, a rigid clutch disc without torsion damper is frequently used. Usually a diaphragm spring clutch, positioned via centering pins, is used as a clutch pressure plate.
1 Primary flywheel mass
2 Arc springs
3 Flange
4 Secondary flywheel mass
5 Flange with inner damper
6 Centering pin
Arc spring
The spring damping system must fulfi ll two contradictory requirements.
1. Under normal operating conditions, the cyclic irregularities of the engine produce only small working angles in the damper. In this operating range, low spring rates combined with low damping capacity are suffi cient to attain optimum vibration damping.
2. Typical load changes (e g. full acceleration), cause load change vibration, which leads to considerable noise. This can only be offset by a torsion damper, which has an extremely low spring rate and, at the same time, high damping capacity.
Aside: the compact DMF
The compact DMF is the ideal solution for the limited installation space in FWD vehicles. The particularly spacesaving version of the clutch assembly incorporates the Dual-Mass Flywheel, clutch pressure plate and clutch disc. The module is supplied pre-assembled, ready to be mounted on the crankshaft. The crankshaft bolts can be tightened through openings in the diaphragm spring of the clutch pressure plate and the clutch disc. This also simplifi es processes for the customer: instead of matching three individual components and two sets of bolts, there is now only one full-service package.
The benefi ts of the LuK Dual-Mass Flywheel at a glance:
• Excellent driving comfort• Absorbs vibration
• Neutralizes noise
• Fuel reduction thanks to low engine speeds
• Increased shifting comfort
• Reduced wear of the synchronizing components
• Overload protection of the power train
Environmental benefi ts:
• Owing to the outstanding noise behavior in the lowrev range there are fewer shifting actions, the average engine operation speed decreases• Overall effi ciency is improved, leading to lower fuel consumption and pollutant emissions
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