Tech Bulletin: Let Standard Transmission &
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What is the
DMF's function? It is designed to isolate torsional crankshaft
spikes created by diesel engines with high compression ratios. By
separating the mass of the flywheel between the diesel engines and the
transmission, torsional spikes can be isolated, eliminating potential
damage to the transmission gear teeth.
Engine-side flywheel damper springs The damper springs
that are visible on the engine side of 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 torsional
frequencies match (have the same amplitude), severe damage can occur to
the transmission if not isolated. DMF's isolate the torsional frequency
match between the engine and transmission to an r.p.m. range below the
operating range of the engine (usually between 200-400 r.p.m.). These
damper springs only work hard when the engine passes through 200-400
r.p.m. at vehicle start up and shut down.
DMF damper springs and/or damper springs nylon retainers usually fall
because the diesel engine is not running correctly. Bad fuel injectors,
worn pistol 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 DMF to fall
within the operating range of the engine. This forces the damper springs
to work continuously, resulting in failure.
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 DMF's.
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 fall if the clutch pilot bearing
is destroyed by a worn transmission input shaft (see Pilot Bearing) or
if the rated load/towing capacity of the vehicle is exceeded.
Pilot Bearing The pilot bearing supplied with most
DMF's 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.
Diagnostic Charts For Dual Mass Flywheels
Damper springs and/or damper springs
nylon spacers are failing on the engine side of the DMF after
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.
Clutch slips after short service life.
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 and rotate the outer portion of the DMF relative to the
inner portion of the DMF. It does not matter if you pry
clockwise of counter clockwise. NOTE: All DMF's have eight
to eleven degrees of free rotational movement before the
friction ring engages. You are checking for movement after
initial free play.
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.
DMF center bearing fails after short service life.
Check the DMF for missing dowel
pins. Failure to use the O.E.M. dowel pins will set up a vibration that will destroy
the center bearing. Check the DMF friction surface for heat cracks and/or hot
spots. Check the pressure plate for hot spots and the disc friction material for
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 also designed as a wear component and will fail if O.E.M.
vehicle capacity is ignored.