The example I'll use here is the registration clutch in the HP LJ 5Si. It's made by the Ogura Clutch Company. (Their website has a lot of interesting information.)
Characteristics and Failure Mechanisms
The clutches work by applying an electromagnetic field to a steel rotor that forms one face of a disc clutch. When the electromagnet is energized, the rotor becomes magnetized and attracts its mating clutch plate to itself, thereby engaging the clutch.
The clutches are meant to engage instantaneously when energized, and remain engaged with no slippage until de-energized. Typically, the clutches become troublesome when contaminants accumulate on the clutch faces, interfering with prompt, slipless engagement. Symptoms of this are often intermittent. The registration clutch in the 5Si can cause paper jams, misregistration and misregistration accompanied by paper fold-over. Intermittent paper jams are the most common symptom of clutch trouble.
Outright electrical failure such as a winding going open is extremely rare, and easily confirmed, if suspected, by a continuity test with an ohmmeter. The winding of the 5Si's registration clutch has a DC resistance of about 203 ohms. Any given clutch's winding will exhibit some characteristic normal value of DC resistance, depending on its design parameters. An 'open circuit' reading indicates a winding failure -- replace the clutch. A resistance reading significantly lower than normal would indicate an internal short circuit in the winding due to an insulation failure; cause for clutch replacement as well. Again, though, winding failures are rare.
Clutch Service
You'll need a small mechanic's vise to press the clutch back together after dismantling it.
The clutch is held together by two bronze washers that are press-fitted onto the ends of its rotor. Note the following points:
>The washers have a dead flat side, and a slightly chamfered side. The dead flat side goes outboard.
>The washers' outer faces are flush with the ends of the rotor. There is a fair bit of axial play in a properly assembled clutch. That play is normal and necessary.
>Methyl hydrate is a good cleaning solvent here as it leaves no residue whatsoever. The gear is often filthy and needs to be scrubbed with Varsol. Follow that with a methyl hydrate scrub and a blow-off. Never leave the gear to soak in solvent for any period of time.
Pry off the bronze washers and the clutch is apart. Clean the gear as noted above. The magnetic ring on the inner face of the gear is there to cause prompt disengagement when the clutch is de-energized. If it has come free of the gear, replace the clutch. It might be possible to bond the magnetic ring back onto the gear with CA adhesive, but I've not had occasion to try that yet.
Clean the plate, the rotor and the rotor's plastic sleeve bearings with methyl hydrate. (Replacement sleeve bearings are not available; if the bearings are badly worn the clutch must be replaced.) Ideally, your shop should have an air compressor for blowing off components to ensure dust-free assemblies.
When all is satisfactory, press the bronze washers back on one at a time. With the washers on flush with the rotor's ends, you're done. The clutch is fit to go back in service.
Permanently Assembled Clutches
Permanently assembled units are much better protected from contaminant incursion than the serviceable unit dealt with above, but they are by no means 'sealed'.
It's possible to flush them somewhat with a squeeze squirter bottle of methyl hydrate, then thoroughly blow them dry with compressed air. Were that to improve the operation of a suspect clutch, it would at least tell you that the clutch must be replaced.
MONDAY, MARCH 15, 2010
Addendum
'Had a clutch failure last week in an HP LJ 8000; it was the second pass roller's clutch in the paper input unit. It gave quite a set of symptoms:
- No paper jams, oddly enough, but paper folding noises just upstream of the second pass roller.
- Affected pages were correctly registered at the leading edge, had a blank band about 3" from the leading edge and were misregistered at the trailing edge (print going clear to the edge and beyond). The blank band area showed evidence of creasing.
The clutch must have been slipping at engagement time, so a sheet's leading edge would be stuck for awhile at the second pass roller while its trailing edge kept advancing. By the time the clutch engagement finally took, the paper had gotten a fold in it, and it got fed through the regisrtation roller and transfer stage that way. On reaching the fuser, the fold would get pulled out flat and the sheet would fuse and exit normally.
I dismantled the clutch and its clutch faces didn't look to be badly fouled, but one of the rotor's plastic sleeve bearings came out with the rotor, suggesting a lack of clearance in the bearing. That may have been adding sufficient drag to cause the clutch to slip.
No comments:
Post a Comment