Dual Mass Flywheels

How it functions:

The Dual Mass Flywheel (DMF) is designed to isolate torsional crankshaft pikes created by diesel engines with high compression ratios.  By separating the mass of the flywheel between the diesel engine and the transmission, torsional spikes can be isolated, eliminating potential damage to the transmission gear teeth.

Dual Mass Flywheel Components:

Ø  Engine Side Flywheel Damper Springs:

The damper springs that are visible on the engine side of the DMF are designed to dampen heavy torsional spikes that occur when the diesel engine’s torsional frequency matches the torsional frequency of the transmission.  When the torsional frequencies match (have the same amplitude), severe damage can occur to the transmission if not isolated.  These flywheels isolate the torsional frequency match between the engine and transmission to an rpm range below the operating range of the engine (usually 200-400 rpm).  These springs only work hard when the engine passes through 200-400 rpm when the vehicle is being either started or shut down.

The DMF damper springs and/or the nylon retainers usually fail because the diesel engine is not running correctly.  Bad fuel injectors, worn piston rings, bad valves, etc., will change the resonant frequency of the engine.  A change in the resonant frequency of the engine can force the torsional frequency match between the engine and the DMF to fall within the operating range of the engine.  This forces the damper springs to work continuously, resulting in failure.

Ø  The Friction Ring:

The friction ring located between the inner and outer flywheel is designed to allow the inner and outer flywheel to slip.  This feature saves the transmission from damage when torque loads exceed the vehicle rating of the transmission.  The friction ring will wear out if excessive torque loads are continuously applied.  Loading the vehicle beyond the rated load capacity is often the root cause of friction ring failures in Dual Mass Flywheels.

Ø  The Center Bearing:

A sealed double row center ball bearing carries the load between the inner and outer halves of the DMF.  The leading cause of center bearing failure is often related to out of balance vibration caused by not aligning the pressure plate with the DMF dowel pins.  The center bearing may also fail if a worn transmission input shaft destroys the clutch pilot bearing or if the rated load/towing capacity of the vehicle is exceeded.

Ø  The Pilot Bearing:

The pilot bearing supplied is a caged needle roller bearing.  If it fails, the transmission input shaft must be repaired or replaced.  If the input shaft is not repaired correctly or replaced, the lack of input shaft support will result in DMF Center Bearing failure.

Troubleshooting:

Problem: Damper springs or nylon spacers are failing on the engine side of the DMF after short service life.

Solution: Service diesel engine.  Correct bad fuel injectors, poor engine compression, etc.  Make sure engine idle is set to factory specifications.  Important:  Changing the turbo boost will change the torsional frequency of the engine, which can damage the DMF!

                PFP will not honor warranty on DMF’s with broken damper springs.

                PFP will not honor warranty on vehicles with altered turbochargers.

Problem: Clutch slips after short service life.

Solution: Check the friction ring between the inner and outer halves of the DMF.  This check should be made with the DMF installed and the clutch and bell housing removed.

·         Partially insert two pressure plate bolts on opposite sides of the DMF’s pressure plate bolt holes.

·         Block the engine so that it cannot turn.

·         Place a pry bar between the two pressure plate bolts.  Try to rotate the outer portion of the DMF relative to the inner portion.  It does not matter if you pry it clockwise or counter-clockwise.

Note: all DMF’s have about ten degrees of free rotational movement before the friction ring engages.  You will be checking for movement after the initial play is taken up.

·         If the outer half of the DMF will rotate with hand pressure on the pry bar, the DMF friction ring is worn out.  The DMF will slip and must be replaced.

The friction ring is designed to protect the transmission against torsional engine spikes.  PFP will not honor warranty on DMF’s with worn friction rings.

Problem: DMF Center bearing fails after short service life.

Solution: Check the DMF for missing dowel pins.  Failure to use the OEM dowel pins will set up a vibration that will destroy the center bearing.  Also, check the DMF friction surface for heat checks and/or heat spots.  Check the pressure plate for hot spots and the disc friction material separation.  Both are signs that the truck is being used beyond it’s rated capacity and/or the clutch is not releasing properly due to a worn or faulty hydraulic clutch release system.  The DMF is designed to protect the transmission.  It is designed as a wear component and will fail if OEM vehicle capacity is ignored.

                PFP will not honor warranty DMF’s with missing dowel pins.

Problem: DMF pilot bearing fails after short service life.

Solution: Check the transmission input shaft pilot-bearing surface.  If this surface is scored or otherwise damaged, the shaft must be repaired or replaced.  Failure to properly repair or replace a bad transmission input shaft would ultimately result in failure of the DMF.