Hi, Can anyone tell me why there is a 4way gearbox in my NewHolland L779 skidsteer connected to both hydrostatic drive motors as well as the hydraulic pump and engine? The hydrostatic drive systems I've seen on smaller equipment just couple the engine to a pump sometimes through a gearbox and the pump supplies the pressure to the wheel drive motors directly. If someone could explain the theory of operation of the 4way gearbox to me I'd be very much appreciative. The manual just skips over the subject. On another matter, I have to rebuild the Perkins 4.203.2 diesel prime mover. In the OEM parts manual, I notice that there are no exhaust valve guide seals listed or shown, only those for the intake valves. Is this common practice for diesel engines? What do you suggest? Thanks in advance for your time!
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The only machines i can comment on are Bobcat ones. I have seen reduction boxes that the drive motors bolt to. Naturally to lower the speed and increase torque. I wonder if your setup is for two speed? You can then travel longer distances faster but you reduce your pushing power.
As for the engine, it seems weird to me too. The diesels i have worked on have valve stem seals on inlet and exhaust valves. I can see that the exhaust isn't as big of a deal as inlet though as the inlet always has a vacuum against it that will draw in engine oil. The exhaust will have pressure, essentially pushing the oil back up the valve stem.
With that said, i'd still rather have seals on both. It helps prevent enging oil being burnt out the exhaust and possibly carboning the valves and seats up more than usual.
As for the engine, it seems weird to me too. The diesels i have worked on have valve stem seals on inlet and exhaust valves. I can see that the exhaust isn't as big of a deal as inlet though as the inlet always has a vacuum against it that will draw in engine oil. The exhaust will have pressure, essentially pushing the oil back up the valve stem.
With that said, i'd still rather have seals on both. It helps prevent enging oil being burnt out the exhaust and possibly carboning the valves and seats up more than usual.
Thanks for the reply Tazza. I come from an automotive background so the lack of exhaust valve guide seals seemed odd to me. The parts vendor confirmed that this engine doesn't use them. Maybe someone else with a perkins 4.203 could comment? Regarding the gearbox, it is in fact a 2 speed, but that doesn't explain why it's connected to both wheel motors. From looking at the exploded view, there is a splined shaft with a bevel gear on it. The shaft appears to drive the concentrically located hydraulic pump directly. The hydraulic schematic for the machine shows the pump as a two stage pump with one section powering the boom and the other section powering the wheel drive motors. The bevel gear I mentioned engages two other bevel gears at 90 degrees, 1 on either side of the splined shaft bevel gear to drive shafts which are coupled to the wheel drive motors for whatever unknown reason. My guess is that each wheel drive motor has its own internal pump which is driven from this gearbox and that the hydraulic schematics in the manual are incorrect. I have been unable to obtain a correct service manual for the L779, as the ones sold for it appear to be for a L775 which I'm told is different. Any comments or suggestions would as always be appreciated.
there are parts drawings at Messicks.com if that would help.
Thanks Jerry. I found answers and am posting here in case someone else encounters the same issues. Regarding the 4way gearbox, the schematic in the L779 operators manual appears to be incorrect. A NewHolland rep advised that the L779 shares the same transmission and drive system configuration as the smaller L35. Pumps are incorporated into each wheel motor and driven through the 4way gearbox. The concentrically mounted pump on the end of the gearbox is for the loader functions only. As for the lack of exhaust valve guide seals on the Perkins 4.203.2 diesel engine, the Perkins parts rep told me that seals aren't used on the exhaust valve guides because unlike earlier models which used identical guides for both intake and exhaust valves, over the course of their production run Perkins switched to longer exhaust valve guides which left insufficient clearance for seals, which if used would cause the rocker arms to bind and break off the adjusting screw tips. Therefore, the exhaust valve guide seals are to be omitted. Thanks again to all who took the time to post.
Jyuma
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For what it's worth... I recently rebuilt a Camaro 3.4 liter gas engine and it didn't use exhaust valve guide seals... only intake. There was plenty of room to accommodate seals on the exhaust guides but they just weren't used and weren't included in the new head-set.
I believe Tazza hit the nail on the head as to the reason.
