Slow Dump on LA351 Loader

[Pks]

What would cause a 2 year old LA351 loader to pause during its dump cycle? It tends to do this when the lever is pushed far to the right in the rapid dump position (away from the operator in accordance with the manual & sticker). The hoses have been checked for proper routing-there're OK. The weight of material inside the bucket does not affect the problem. My suspicion is air in the dump circuit. Can anyone provide guidance? Thank you, Peter

 

[PaulB]

My LA301 loader has a slight hesitation - much less than a second - when I go from dump to rapid dump. Always has, I just assumed it was normal. I do know that when you go into rapid dump there is a set of springs and detent balls which engage and hold the spool stem into position (just like when you set the loader to float position), so maybe there is a momentary hesitation to engage a new hydraulic direction of flow??? If this is abnormal, it is the way mine came to me from the factory.

 

[MChalkley]

If air gets in the dump circuit, it won't stay there long. So, I doubt that's in, unless you've got a leak on that spool inside the block. This is unlikely, though. Are you sure nothing has changed?

Mark

 

[Anonymous Poster]

Nothing has changed as far as I know. I do remember removing the quick disconnects at the valve last summer. I had to replace the steel oil line that goes from the block in the front to the block in the rear. But that's it. Why is it that air would not stay in some portion of the lines or the cylinder? Looks to me like it's pretty well trapped.

 

[W5FL]

How long since you changed the hydraulic filter and hndraulic oil (or cleaned the strainer if it has one)?

 

[MChalkley]

All fluid sent into any cylinder, whether to force it to extend or retract, is pulled by suction from the tank (the "transmission case") by the pump and sent to the cylinder under pressure. All fluid leaving a cylinder is dumped to tank. Since air is compressible, it gets compressed on a pressure stroke to infinitesimally small bubbles, which mix with the oil and are carried to tank after a few cycles, at the most, because the air never gets a chance to expand back to any appreciable volume, since the fluid is always under relatively high pressure (from a gas standpoint), even when exiting the cylinder. However, if for some reason enough air is being regularly introduced into the system that you always have air in it, you have a major problem. Likely the fluid level is low, or a suction filter is clogged, either of which can also cause pump cavitation and ruin the pump in short order. Other causes of air introduced into the system are rare. If you think about it, any place air could get in, hydraulic oil would be forced out in great quantity when the circuit was put under pressure.

Mark

 

[Anonymous Poster]

My LA351 also has the slight pause when dumping and has as long as I can remember (3 years old). This does not occur all the time and when it does it only lasts a second or so. I assumed it was occuring when switching from regular dump to fast dump mode. I think this is just the way it works and there is nothing wrong with your machine.

 

[art]

The LA351 loader is advertised as a 2 second dump time and a 2.7 second roll back time. I do believe that that would be at near full throttle.

 

[Cowboy]

Pks
My Case loader has, a dump speed adjustment linkage. On the Case, it is located at the bottom of the dump control lever. Check your manual, or your dealer to see if your's is equipped with an adjustment.

Hope this helps.

Cowboy
BigBoyz Toyz

 

[Pks]

PaulB,

It sounds like mine is just like yours. But there are times when there is a pause, while changing from slow dump to fast. That pause is usually longer than 2 seconds. During the pause, the bucket seems to float. Meaning if the load is just right then the bucket will very slowly tend to dump. But if the load is back slightly, the bucket will slowly move back to the carry position (aka crowd position).

The pause is greatest when I'm lowering the lifting arms and begin to slow-dump. Then as the slow-dump motion begins, while the arms are coming down, I'll change my mind and shift to fast dump. That's when the pause begins. The lift arms continue to come down-no changes there. But the bucket just pauses. My guess is that it's oil starvation within the cylinders. I figure the change in demand, with the arms demanding their own amounts, creates the pause.

BUT... if I start a fast dump while the lifting arms are coming down, there is NO pause.

...goofy huh?

 

[hayden]

Sorry I missed this thread last summer when it was posted.

My LA351 did exactly this. I went to the dealer about it and they said it was normal. My LA402 does exactly the same thing, so that plus the experience relayed here makes me believe that it truely is normal.

Although I don't totally understand the cause, it's clearly related to the regenerative cycle that makes up the fast dump. A regenerative cycle is when you connect both the ports on a cylinder together and apply hydraulic pressure to both at the same time. On first glance you would think the workd would come to a screeching halt if you tried that, but here's what really happens:

- Equal oil pressure is applied to the two ends of the cylinder.

- The end with the rod has a smaller surface area to apply oil pressure against when compared to the non-rod end because of the area lost to the rod itself. This means the net mechanical force (pressure x area) on the non-rod end will be greater and push the rod outwards (dumping the bucket).

- Because the two ports are plumbed together, the oil expelled from the rod end flows to the non-rod end increasing the oil flow rate into the non-rod end.

- The increased oil flow (combination of pump flow plus expulsion flow) makes the rod extend faster than it would under "normal" conditions.

- Net result is faster dump because the rod is moving out faster, but dump force is less since it's the difference between the force on the two ends of the cylinder.

Now this may be interesting cocktail conversation, but it still doesn't explain the pregnant pause before the dump starts happening. The only thing I can figure is that oil has to get moved around inside the control differently to do the regenerative dump and that takes time, but who knows. I don't think it's air since there are no other indications of air in the system, an dif there were I woudl expect it to bleed out with repeated cycles.

Peter

 

[Anonymous Poster]

MY LA 401 DOES THE SAME THING. IT DID NOT HAPPEN WHEN THE TRACTOR (2710) WAS NEW. IT ALSO DOES NOT DO IT ALL THE TIME. IF I MOVE THE STICK ALL THE WAY OVER TO THE RIGHT (FAST DUMP )TO REMOVE STICKING MATERIAL, IT WILL SOMETIMES DUMP SLOW. I WILL THEN MOVE THE STICK LFT - RT (ROLL BACK - FAST DUMP ). SOMETIMES THIS WORKS, SOMETIMES NOT. WHEN I DO THIS THE LOADER IS NOT GOING UP OR DOWN. THE LOADER VALVE IS ALSO A 4 POSITION AND NOT THE STANDARD 3 POSITION. THE FLUID AND FILTER WAS CHANGED, SCREENS CLEANED @ 50 HRS. (NOW HAS 90 HRS.) I WOULD GUESS THAT SOMETHING IS STICKING IN THE VALVE.
FRED

 

[CraigM]

Interesting puzzle. I'd like to offer a possible explanation. Let me start by saying I'm no expert in hydraulics, so this may turn out to be way off base.

Pks says that the problem is worst when changing from slow to fast dump with the lift arms in motion. I don't know what the lift motion has to do with it, but after reading hayden's explanation of regenerative cycles, I have a thought on why the hesitation is worst when shifting from normal dump to fast dump.

With the bucket in motion on a normal dump, fluid flows from the pump to the non-rod side of the bucket cylinder. The fluid on the rod side of the cylinder is exhausted through the valve to the tank. Pressure in the exhaust side of the circuit is esentially zero. Now engage the fast dump. Wham! The moving fluid in the exhaust circuit is hit by full pump pressure in the opposite direction. The rod side of the piston now sees a force that wants to curl the bucket. This force is smaller than the dump force on the other side of the piston, so the bucket will still dump. Before engaging fast dump, there was motion happening at a steady rate to dump the bucket. Hitting the exhaust side of the cylinder with pump pressure has to interrupt this motion until the higher force available from the non-rod side of the piston prevails and gets things moving again. Think of it this way. A 300 pound football player is barreling along at full speed and gets hit head on by a 150 pounder. The big guy's forward progress is momentarily slowed until his greater strength and momentum can overcome the little guy. If fast dump is engaged from the start, then both sides of the piston see the same pressure at the same time and the dump motion starts immediately in response to the imbalanace in force between the rod and non-rod sides of the piston. Sound reasonable?

Craig
Hershey, PA

 

[Pks]

Craig,

I think you and Peter Hayden have identified the reason for the pausing.

This means that I should decide whether I want a fast dump or a slow dump BEFORE I begin the dump cycle. I think this is pointing out my wishy-washy gean.