Re: What is a \"wet-type\" clutch?
It is in most cases multi plates clutch. It has 5-10 metal disks connected to "bell" (engine side) by splines on the outer side, and similar number of friction material covered metal disks connected to "outgoing shaft" (by splines in centre of disks). They are mounted in enclosed housing, filled with ATF oil, and rotate with engine. From "shaft" side is mounted hydraulic cylinder, which press plates together when clutch drive. It rotates together with clutch bell, and has oil pressure feeding through shaft. It is simple explanation of its construction.
Metal plates rotate together with engine, when plates connected to shaft stop together with it. When you want to carry power through it, you activate valve (usually via solenoid) to start gradually feed pressure to cylinder. Hydraulic power is generated from pump, driven from engine. That means - clutch is open when engine stops.
Why it is maintenance free?
-First, it has bigger contact area than single plate dry clutch, even with significally smaller diameter than dry clutch (2-3 times).
-Second, it is lubricated when slipping during engaging clutch.
-Third, it is well cooled during slipping, so contact temperature is few times lower, compare to dry plate, and avoid burning friction material.
-Fourth, pressure between plates is bigger, thanks to hydraulic pressure. Dry disk clutch has spring-generated pressure, which is limited, because disengaging force will be too big on clutch pedal and on axial bearing on engine crankshaft. Wet, multi plates clutch not generate any axial pressure force to crankshaft. During time dry clutch plate gets thinner, and spring pressure force decreasing causing first occasional slipping during transferring peak torque and later normal torque.
-Fifth, it is harder to reduce pressure if you are riding your leg on clutch pedal. Pedal is soft, and when you occasionally press it, clutch disengages.
Why it, sometimes, lasts short?
- Not enough oil level.
- Wrong oil type.
- Worn rings on pressure transfer path, and not enough contact pressure - especially visible/feelable on low engine RPM.
-Worn hydraulic pump, clogged filters etc.
Conclusion
Power reversers on any type tractors and backhoes use this system, but with 2 clutches (one for each direction). Geared ones, not hydrostatic ones, of course.
Comfort clutch for use, excellent in performances, more complicated than dry plate clutch, and EXPENSIVE to repair.
Anyway, I regret I haven't it on my tractor(s).
Disadvantages
It is almost impossible to start engine by towing with this type of clutch.
Not possible to leave tractor in gear when engine stops. Need very reliable parking brake, and retirement blocks under wheels.
Some tractors use multi plates disk clutches for engaging MFWD. On them usually pressure force is generated by load spring, and hydraulic pressure is use to disengage. This system is always used on tractors with front axle braking by engaging FWD (on 25 MPH tractors in Europe, don't know for US). Similar system is used for on load differential locks.
The best example where this clutch type is used are New Holland TND series. System is called Dual command. In Europe (don't know for US), TND even have two stage powershift for FWD direction.
Hope this helps. /forums/images/graemlins/wink.gif
