In my case, a supposedly matching dust bag was installed by the DM, but who has not fit despite correct type designation. Sucked a sandy soil, ie all the sand was blown into the engine, resulting in a matter of seconds a bearing damage is established.
-> Total loss + Warranty extinguished (since no original dust bag used).
Even if the dust bag is not torn, is by design to be expected that fine particles from entering the bearing and thus resulting in wear - also known as planned obsolescence.
If you so choose a design, in my opinion, the pre-filter is to be interpreted before the fan so that there passes or the bearings are encapsulated accordingly so that no dust can penetrate this dust.