As an experienced technician, you have no doubt come across dual mass flywheels, but if you do not fully understand how these flywheels work, you are not alone. Experience has shown that many technicians do not understand the technology: therefore, this brief guide will briefly discuss what dual mass flywheels are, how they work, why they are required on some applications; why they fail, and how to diagnose possible failures of, and defects in, dual mass flywheels, starting with this question:
Unlike conventional flywheels that are fabricated from a single piece of metal, dual mass flywheels consist of two parts that are separated by various means, hence the term, “dual mass” flywheel.
While some design specifics vary between applications, all dual mass flywheels follow the same general pattern. One part (commonly known as the primary mass), bolts directly onto the rear of the crankshaft, while the second mass (commonly known as the secondary mass), is attached to the primary mass. In practice, and regardless of the means of attachment, this arrangement allows the two masses to rotate independently of each other around a support that can be either a bushing or a bearing.
Note however that the differential rotation is limited by a set of springs whose purpose it is to cushion, or absorb the effects of the differential rotation as a means to damp out engine vibrations.
While it might appear as if the flywheel on a running engine rotates at a constant angular speed, it does not actually do so, since (taking only a single piston and connecting rod as an example), the force that is exerted by the piston on the crankshaft is not constant. In practice, the piston exerts no force when it is at Top Dead Centre on the power stroke; it only starts to exert a force when it moves past TDC, and this force increases progressively until it reaches 90 degrees after TDC. Beyond 90 degrees after TDC, the force exerted by the piston decreases progressively until it reaches Bottom Dead Centre, where it again exerts no force on the crankshaft.
While the kinetic energy of a single-piece flywheel largely absorbs the vibration that is caused by the uneven application of forces to the crankshaft, if the uneven application of force is multiplied by the number of cylinders in an engine, the result is a constant vibration that is caused by a microscopic twisting and deformation of the crankshaft that cannot be completely absorbed by a one-piece flywheel.
Moreover, the deformation of the crankshaft is not constant throughout the length of the crankshaft. Since the mass and stored energy of the rotating flywheel is at the back of the crankshaft, the rear of the crankshaft is stabilised; however, the result of this is that the distortion of the crankshaft is amplified towards the front of the crankshaft. To combat this effect, most high-torque engines are fitted with harmonic balancers to absorb the vibration at the front of the crankshaft.
Although the combined effects of a harmonic balancer and a heavy, one-piece flywheel on modern high-torque engines go a long way towards eliminating crankshaft-induced engine vibrations, the development of evermore-powerful engines, and particularly diesel engines with long strokes, has resulted in increased engine vibration. This is particularly true in the case of diesel engines that develop their peak power at low engine speeds: on these applications, maximum power is generally achieved at well under 2000 RPM, which is too low for the rotation of the flywheel to absorb crankshaft-induced vibrations effectively.
While it can be argued that the more massive a flywheel is the better it is able to absorb crankshaft-induced vibrations, there are two main problems with this approach. The first involves inertia: the more massive a flywheel becomes, the more energy is required to get it moving, which means that throttle response can be severely affected as the mass of the flywheel is increased. This effect is analogous to turbo lag, which is the result of an increased volume of exhaust gas having to increase the rotational speed of a turbocharger’s compressor wheel to develop more boost pressure.
The second problem with increasing the mass of one-piece flywheels is that the more massive the flywheel becomes, the more the deformation of the crankshaft is amplified towards the front of the shaft, with the amplification being directly proportional to the increase in the flywheels’ mass. As a practical matter, this means that the mass of the harmonic balancer, and hence, that of the engine must also be increased, which in turn, comes at the cost of decreased fuel economy.
By virtue of their design, dual mass flywheels eliminate both of the problems stated above, and here is why. Since the primary mass is attached to the crankshaft directly, it follows the uneven angular speed of the crankshaft. However, since the secondary mass is not fixed to the primary mass, the cushion springs between the two masses absorb the crankshaft’s uneven rotation, before transferring the crankshaft’s rotational energy to the secondary mass, which in turn, transfers the crankshaft’s rotation to the gearbox via the clutch driven plate.
How well the cushion springs damp out the crankshaft’s uneven rotation depend on the design and arrangement of the cushion springs, the mechanical condition of the engine, whether or not there are misfires present on the engine, but most importantly, on the mechanical condition of the dual mass flywheel itself. Nonetheless, assuming that the engine is in perfect running condition and that the dual mass flywheel is not damaged in any way, most dual mass flywheel designs virtually eliminate crankshaft-induced vibrations, which is a major cause of manual gearbox damage and/or failure.
It must be noted that clutch driven plates that are designed to be used with dual mass flywheels do not have spring loaded hubs like those found on conventional clutch plates, since the cushion springs in the flywheel perform the function that spring loaded hubs in conventional clutch plates perform. Therefore, the two types of clutch plates are NOT interchangeable.
Dual mass flywheels were introduced primarily to reduce engine vibrations that damage manual gearboxes in various ways, with the degree of damage being largely proportional to the amplitude of the vibrations. However, it should be noted that not all applications are, or had been susceptible to gearbox damage caused by engine vibrations, which means that each instance of gearbox trouble such as worn, damaged, or broken input shaft bearings must be investigated on the basis of other evidence, such as mileage, use of the vehicle, driving style, and service history of the vehicle.
Nonetheless, other benefits of dual mass flywheels include smoother clutch operation, improved gear shifts, an overall reduction in vibration throughout the vehicle, and on some applications, even a marginal increase in fuel economy.
While dual mass flywheels do have some real disadvantages, the severity of any given issue depends on several factors, including the brand of dual mass flywheel in use on the vehicle, the usage pattern of the vehicle (is it used for towing or not), the vehicle mileage, how the vehicle is driven, and local driving conditions. For instance, if the vehicle is used primarily on gravel roads in dusty conditions, it is likely that the dual mass flywheel will not last as long as the flywheel in a similar vehicle that never sees gravel roads or dusty conditions.
Nonetheless, some real, objective disadvantages of dual mass flywheels include the following:
For more information about the advantages and disadvantages of Dual Mass Flywheels, listen to our 'Ask me anything' Podcast Interview with Mark Davis, Product Manager at Exedy Australia.
The most common failure modes across all brands of dual mass flywheels are free play between the two masses in the plane of rotation, and “rocking”, which is lateral free play between the two masses.
In the first case, free play indicates an issue such as broken cushion springs, or relaxed cushion springs as a result of clutch having overheated. In the second case, lateral free play is usually the result of mechanical wear or damage to the bearing or bushing that supports the secondary mass. Note however that some dual mass flywheel designs depend on there being some free play in the support bearing / bush, and this only becomes an issue when the free play becomes excessive. Note also that the amount of incorporated free play varies between designs, so be sure to consult the manufacturers’ documentation or specifications before condemning a dual mass flywheel on the basis of free play between the two masses.
Other types of failures are similar to those seen on conventional flywheels, such as excessive scoring or discolouration caused by overheating, but note that in the case of the clutch overheating on a dual mass flywheel, there is a high likelihood that the lubricant between the two masses can melt, and be flung out of the assembly. In these cases, replacement of the flywheel is the only reliable remedy, since the two masses cannot be separated to replace the lubricant.
Somewhat ironically, and despite the fact that the primary purpose of dual mass flywheels is to reduce engine vibrations, excessive vibration that may or may not be accompanied by “clunking”, “thudding”, or other mechanical noises is usually the first symptom of a failed or failing dual mass flywheel.
Bear in mind though that a clunking or thudding noise can also be caused by drive train issues such as worn or damaged U-joints, drive shaft support bearings, and damaged/broken engine and/or gearbox mounts. All of these possible causes MUST be investigated and ruled out as the cause of mechanical noises or excessive vibration before a dual mass flywheel is condemned out of hand.
However, excessive vibration is NOT always caused by worn or damaged dual mass flywheels; in fact, all other possible causes of the vibration should be investigated first, using the symptoms as diagnostic aids. For instance, a full diagnostic scan MUST be performed to either eliminate or confirm misfires as a possible cause of vibrations that as we know, can sometimes only occur at certain engine speeds.
Other possible symptoms include hard or difficult clutch operation, difficulty shifting gears, and/or a rattling sound from the gearbox in neutral while the engine is running. However, all of these issues can be caused by issues other than a failed dual mass flywheel. For instance, difficult clutch operation can be the result of a failed, broken, or damaged pressure plate, as well as by issues in the clutch master/slave cylinder(s). Similarly, difficulty in shifting gears can be caused by all of the aforementioned causes, as well as by internal gearbox issues such as inadequate lubrication, or the mechanical failure of one or more components.
Also, be aware that poorly balanced rear wheels can, and often do, mimic the symptoms of a poorly balanced dual mass flywheel, so be sure to check the balance of all the wheels on the vehicle before condemning a dual mass flywheel.
Unless you have access to specialised test equipment, such as the instruments made and supplied by LUK, who holds the patent on dual mass flywheels, there is no reliable way to assess the condition of a dual mass flywheel without removing it from the vehicle.
Even then, and assuming that there is no discolouration, excessive scoring, cracking/fracturing of any part of the assembly, or evidence of leaking lubrication present, you are limited to two basic checks.
The first check involves measuring free play between the two masses, and comparing this value to the manufacturer’s documentation and/or specifications. Bear in mind that replacement of the flywheel is the only reliable remedy if free play in any direction exceeds the maximum allowable limit.
The second check involves checking the rotation limits of the secondary mass relative to the primary mass, which requires removal of the flywheel from the vehicle, and clamping the primary mass in a vice. This can be done by inserting two bolts through holes in the mounting face, and then clamping the bolts in a sturdy bench vice. Attach a piece of steel to the secondary mass using the holes where the pressure plate is attached, and exert a rotating force to the secondary mass.
This test is admittedly crude, but if the flywheel is serviceable, there will be no free play in the plane of rotation, and it will take equal amounts of force to rotate the secondary mass (against the force of the cushion springs), through its permissible range in both directions.
Note however that this test can be improved upon by attaching a large nut to the lever so that the nut is placed over the exact centre of the flywheel assembly. Use a properly calibrated torque wrench that is fitted with an angle indicator and a suitable socket to check both the force required to rotate the secondary mass, and its angle of rotation in both directions. Any deviation from specified values must be taken as evidence that the flywheel is defective.
The point of all of the above is that dual mass flywheels are frequently diagnosed as the cause of excessive vibration and/or mechanical noises, when in fact the real cause(s) often do not involve the flywheel at all. The best way to avoid falling into this trap (and possibly losing a valued customer) is to replicate the symptoms as accurately as possible, and if needs be, to have the customer drive the vehicle during the investigation phase of the diagnostic procedure.
It must be remembered that since your customers know their vehicles best the customer is almost always better able to replicate a symptom. Translated, this means that you can save your customer a ton of money simply by diagnosing the actual root cause of a vibration of mechanical noise accurately, the first time.
