Many workshop employees have already heard the technical explanation and had their nerves calmed, but the topic remains current.
There is a wide variety of designs and generations of the DMF. In all cases, however, the secondary mass must be able to move in relation to the primary mass. Depending on the design, this may be through either roller bearings or plain bearings. A closer look at these two types of bearing quickly makes clear why a DMF with plain bearings has significantly more axial clearance than a DMF designed with roller bearings.
A ball bearing can also absorb some of its radial forces axially. That means, on a DMF with ball bearings, that the secondary mass is seated relatively tightly on the primary mass and is axially virtually immovable. A plain bearing, on the other hand, naturally requires a certain amount of internal clearance to allow correct function and radial guidance of the secondary mass. Under certain circumstances, this may be detectable as slight tilt clearance even on a new part. In comparison to a ball bearing, axial guidance occurs only in the direction of the engine.
A plain bearing DMF can therefore also exhibit axial clearance between the primary and secondary mass, depending on the type. If you now move the secondary mass axially back and forth, you can hear the rattling noise mentioned above. This is definitely not a defect, because this situation only occurs on the workbench, or following disassembly of the transmission. During driving in real-life scenarios , the release system preload along with the clutch plate guidance on the transmission input shaft (main and pilot bearing if present) ensure that the secondary mass is stable and correctly positioned. Axial and tilting movements therefore do not occur.
Axial clearance, which is measured at the extreme outer edge when disassembled, can be a maximum of 2.9 mm for plain bearing designs. Where the DMF has already been run, this axial clearance can serve as a wear indicator. The precise actual value can be determined using the LuK special tool 400008010. Further information, wear values and the vehicles for it is appropriate can be found under the REPXPERT product catalog or here.