Dual mass flywheel

Dual mass flywheel section

A Dual mass flywheel or DMF is a rotating mechanical device that is used to provide continuous energy (rotational energy) in systems where the energy source is not continuous, the same way as a conventional flywheel acts, but damping any violent variation of torque or revolutions that could cause an unwanted vibration. The vibration reduction is achieved by accumulating stored energy in the two flywheel half masses over a period of time but damped by a series of strong springs, doing that at a rate that is compatible with the energy source, and then releasing that energy at a much higher rate over a relatively short time. The compact dual-mass flywheel also includes the whole clutch, (with the pressure plate and the friction disc).^[1]

History

Schaeffler torque converter with a pendulum absorber using the same DMF's bent springs.

The Dual Mass Flywheels were developed to address the escalation of torque and power, especially at low revs. The growing concern for the environment and the adoption of more stringent regulations have marked the development of more efficient new engines, lowering the cylinder number to 3 or even 2 cylinders, and allowing the delivery of more torque and power at low revolutions. The counterpart has been an increase in the level of vibration which traditional clutch discs are unable to absorb. This is where the Dual Mass Flywheels play a key role, making these mechanical developments more viable.^[2]

The absorption capacity of the vibration depends on the moving parts of the DMF, these parts are subject to wear. Whenever the clutch is replaced, the DMF should be checked for wear. The two key wear characteristics are freeplay and sideplay (rock). These should be measured to determine whether the flywheel is serviceable. The wear limit specifications can be found in vehicle or flywheel manufacturer's published documentation. Other failure modes are severely grooved/damaged clutch mating surface, grease loss, and cracking.^[2]

Types

Principle of dual mass flywheel.
Black: absorber springs.
Red: flywheel, crankshaft side
Blue: flywheel, transmission side

The main type is called a planetary DMF. The planetary gear and the torsional damper are incorporated into the main flywheel. For this purpose, the main flywheel is divided into primary and secondary pinion-connected masses, and between them there are four different types of bent springs:

Individual bent spring

The simplest form of the bent spring is the standard single spring.

One-phase bent springs in parallel

The standard springs are called parallel springs of one phase. These consist of an outer and an inner spring of almost equal lengths and connected in parallel. The individual characteristic curves of the two springs are added to form the characteristic curve of the spring pair.

Two-phase bent springs in parallel

In the case of two-stage spring there are two curved parallel springs, one inside the other, but the internal spring is shorter so that it acts later. The characteristic curve of the outer spring is adapted to increase when the engine is started. The softer outer spring only acts to increase the problematic resonance frequency range. When the torque increases, reaching the maximum value, the internal spring also acts. In this second phase, the inner and outer springs work together. The collaboration of both springs thus ensures good acoustic isolation at all engine speeds.

Three-phase bent spring

This curved spring consists of an outer and two inner springs with different elastic characteristics connected in series. This category of bent spring uses the two concepts together: parallel and series connection in order to ensure optimal torsional compensation for each value of torque.

Solutions to unreliability issues

Because of expensive unreliability issues with Dual Mass Flywheels^[3] there has been a trend to fit solid flywheels from lower power models of the same engine.^[4] Some mechanics have even welded the dual mass elements together.^[5] These 'fixes' have resulted in instances of gearbox damage and damage to engine crankshafts, as well as refinement and drive-ability issues.

Long established parts maker Valeo has developed a properly engineered 4-piece clutch kit to replace the DMF. The UK introduction was at the end of 2005.^[6] It is claimed to provide increased reliability for the expensive replacement operation (labour time cost and the part cost), of the dual mass flywheel. The 4-piece kit is composed of a traditional rigid flywheel and long travel damper consisting of a cover assembly, a high performance drive plate and a release bearing. Rigorous R&D testing showed the 4-piece kit performs just as well in terms of overall clutch operation, transmission protection, comfort and improves heat dissipation and durability. The transmission protection provides full engine and gearbox safety; the 4-piece kit matches the DMF for mechanical vibration reduction and therefore limits premature wear within the gearbox. The long travel damper closely matches the damping performance of a DMF. Fitting times are also comparable.^[7]

See also

• Centrifugal pendulum absorber
• Clutch
• Flywheel
• Flywheel energy storage
• Harmonic balancer
• List of moments of inertia

References

External links

Wikimedia Commons has media related to Flywheels.

• Dual Mass Flywheel.