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Here's a view of the right side end of the assembly.
The white gear is in constant mesh with the printer's drivetrain, and is the clutch-controlled input to the feed shaft and the black gear.
With the clutch de-energized, the white gear freewheels to no effect.
With the clutch energized, the white gear is coupled to the feed shaft and to the black gear. The feed shaft can transport paper, and the black gear can supply input rotation to Tray 2's pickup assembly through a large tandem idler that's behind it. The black gear's hub is a one-way roller-bearing clutch; if counter-rotated, the black gear will freewheel.
The paper feed assembly only deals with paper from Trays 2 and 3. When printing from Tray 1, the paper feed assembly is bypassed entirely.
There's only one bronze bearing that might be prone to accumulate sludge. Remove the pinch-pressure spring and one e-clip, and the bearing can be removed for cleaning.
See this post regarding swollen rubber rollers.
The electromagnetic wrap-spring clutch is rugged and reliable, but it has a big job to do, especially on machines with an add-on tray. The clutch supplies rotation to the feed roller, Tray 2's pickup assembly and Tray 3's pickup assembly. Any hesitancy or slippage in the clutch will result in mysterious, random paper jam incidents. As mentioned earlier, printing from Tray 1 bypasses the paper feed assembly entirely, so that can be helpful for troubleshooting.
Any slippage of the black gear's one-way roller bearing clutch will retard Tray 2 and Tray 3 pickup assembly operation, also resulting in mysterious, random paper jam incidents.
Outright replacement of the assembly is the quick solution, but the units seldom, if ever, fail outright. They become flaky with age due to contaminants in the clutches.
The black gear's clutch can be flushed with Varsol in a parts washer tank, and blown dry with compressed air. The shaft it rides on must be immaculately clean. On reinstallation, lubricate the clutch with WD-40.
The electromagnetic clutch can be dismantled and cleaned. Its components must all be perfectly free of dust or lubricant of any kind.
A LaserJet 4100 paper feed assembly is P/N RG5-5086.
A LaserJet 40X0 paper feed assembly is P/N RG5-2651.
They look interchangeable to me, but I haven't tried it to confirm whether they are or not.
No service replacement components are available for either one.
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