DMF function tests

DMF function tests

Overview

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The LuK testing tool allows you to perform the following tests on the DMF:

  • Measuring the freeplay angle
  • Measuring the rock
These tests, in combination with a visual inspection to check grease egress, thermal load, clutch condition, etc., allow for a reliable assessment of the DMF’s operational condition.

The freeplay angle is the angle at which primary and secondary mass can be rotated against each other until load is exerted on the arc springs. The measuring points are both end stops in the left-hand and right-hand direction of rotation. The measured freeplay serves as a wear indicator.

Caution:

Some DMFs have a friction control disc that can be felt as a hard stop in one direction. In this case apply greater force to rotate the secondary mass a few more millimetres until spring resistance can be felt, and then allow it to return. This also rotates the friction control disc in the DMF.

The term “rock” describes the clearance between the two DMF masses which allows them to be tilted towards and away from each other
 

Which test suits which DMF?

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On DMFs with an even number of threaded holes to secure the clutch pressure plate, the slotted bar can be mounted centrally, making it possible to determine the freeplay angle using a degree gauge. As this measuring method can be used on almost all DMF types, it should be the preferred method – see Freeplay measurement with degree gauge
 

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There are a few DMF types with an odd number of threaded holes for the clutch pressure plate, making it impossible to mount the slotted bar centrally. In this case, the freeplay angle must be measured by counting the teeth of the starter ring gear – see Freeplay measurement by counting starter ring gear teeth

The above distinction makes no difference when measuring rock – see Rock measurement

Freeplay measurement with degree gauge

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  • Remove the gearbox and clutch according to the manufacturer’s instructions.
     
  • Screw the appropriate adapters (M6, M7 or M8) into two vertically opposing clutch bolt holes on the DMF and torque down.

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  • Centralise the slotted bar on the adapters by using the graduations and tighten the nuts. 

    The degree gauge must be positioned centrally on the DMF.

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  • Lock the DMF using the locking dogs and, if necessary, appropriate spacers to align the locking tool flush with the starter ring gear.

    If the distance exceeds the size of the provided spacers, use additional washers.

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  • If the locking tool can only be mounted to a hole with a dowel fitted, use the adaptor sleeve provided over the dowel.

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  • Bolt the dial gauge stand to the engine block using a suitable bolt, i.e. a gearbox bolt and, if required, the adaptor sleeve can be used in a similar way to the locking tool.
     

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  • The same bolt can be used to fasten the locking dogs and the dial gauge stand if required.

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  • Fit the degree gauge locking bar to the degree gauge and the dial gauge stand and tighten the knurled screw.

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  • Use the slotted arm to rotate the secondary mass anticlockwise until the arc spring force can be felt.
     
  • Caution:
    Some DMFs have a friction control disc that can be felt as a hard stop in one direction. In this case apply greater force to rotate the secondary mass a few more millimetres until spring resistance can be felt, and then allow it to return. This also rotates the friction control disc in the DMF.

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  • Slowly release the slotted arm, allowing the arc springs to relax. Set the degree gauge pointer to “0”

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  • Use the slotted arm to rotate the secondary mass clockwise until the arc spring force can be felt.

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  • Slowly release the slotted arm, allowing the arc springs to relax. Read off the degree gauge and compare the measurement against the rated value

Freeplay measurement by counting starter ring gear teeth

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  • Remove the gearbox and clutch according to the manufacturer’s instructions.
     
  • Screw the appropriate adapters (M6, M7 or M8) into two approximately vertically opposing clutch bolt holes on the DMF and torque down.

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  • Centralise the slotted bar on the adapters by using the graduations and tighten the nuts.

    As there is an odd number of clutch bolt holes, the slotted arm cannot be fixed centrally on the DMF.

 

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  • Lock the DMF using the locking dogs and, if necessary, appropriate spacers to align the locking tool flushwith the starter ring gear.

    If the distance exceeds the size of the provided spacers, use additional washers.

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  • If the locking tool can only be mounted to a hole with a dowel fitted, use the adapter sleeve provided over the dowel

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  • Use the slotted arm to rotate the secondary mass anticlockwise until the arc spring force can be felt.
     
  • Caution:
    Some DMFs have a friction control disc that can be felt as a hard stop in one direction. In this case apply greater force to rotate the secondary mass a few more millimetres until spring resistance can be felt, and then allow it to return. This also rotates the friction control disc in the DMF.

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  • Slowly release the slotted arm, allowing the springs to relax.

    Mark the secondary mass and primary mass/starting ring gear with a line.

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  • Rotate the secondary mass clockwise until the arc spring force can be felt. Slowly release the slotted arm, allowing the arc springs to relax.

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  • Count the number of teeth of the starter ring gear between the original mark and its current position and compare against the rated value

Rock measurement

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  • Fit the dial gauge and arm to the dial gauge stand.

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  • Centralise the dial gauge on the adapter and set to the required preload
     
  • Caution:
    The measurement should be done gently. Applying too much force will result in inaccurate measurements and could damage the DMF.

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  • Gently push the slotted arm towards the engine (using your thumb, for example) until resistance can be felt.

    Keep the slotted arm in this position while setting the dial gauge to “0”.

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  • Pull the lever gently in the opposite direction (using your finger, for example) until resistance can be felt. Read off the dial gauge and compare the measurement against the relevant rated value