basic hydraulic cylinder question

[RonR]

Poo-Bear,

R U saying I'm two for two???????????????

If so, I haven't been that correct in MONTHS!

Have fun,
ron

 

[Pooh_Bear]

There used to be a type of single acting cyclinder called a push pull cylinder.
I had a dump truck with one on it.
The pump pressurized one side of the cylinder only.
The other side was used as the hydrualic reservoir.
The pump was mounted directly to one end of the cylinder.
Pump, reservoir, and cylinder were all one unit.

Pooh Bear

 

[MadReferee]

Here is a link to a post from Kubmech, our resident Kubota mechanic. He explained how regen and the 3-way vs 4-way valves worked in a post from several years ago.

Regen and 3-way vs 4-way valves

 

[Dirthog]

Not necessarly most larger equipment when you dump the bucket, boom or the arm comes in on a excavator or backhoe gravity takes over and the attachment will fall faster than the pump can provide oil then you get air in to the system and a hesitation in the function that is why they use a regeneration circuit. this will happen with any system only it is not as extreme on a smaller machine where the cycle times for the system are slower and there is not as much weight involved. That is why when we have a customer complain that he has a hesitation in his machine there is the first place you check

 

[dfkrug]

Dirthog: I am not sure what you mean "one side faster
than the other". Clearly, adding hyd fluid from the side
of the cylinder exhausting fluid to the other side if the cyl
increases the speed at which the cyl moves. Dump loads
can place negative pressure on one side of the cyl, causing
boiling or cavitation during valve actuation. Increasing the
inflow rate with the exhaust oil added to the pump flow
helps avoid cavitation this way.

 

[john_bud]

dfkrug,

Maybe I'm wrong, but I thought the regen function was to slow down the action when the weight of the load is the dominant force. The regen puts some pressurized fluid on the side that is exhausting, thus slowing it down for more control. Imaging you are driving a huge payloader with a 10,000 pound lift. You pick it up to 8' with that load, then you lower it down. As you activate the control the load check in the control releases and the 10,000 pounds pushes the fluid out FAST and with FORCE. Right? Now, imaging that the exhausting side of the cylinder is given some pressure from the pump. The fluid can't rush out quite so fast, giving more control.

Where does the air come from if regen isn't used? Ok, back to the same example with out regen. As you feather the loader control to lower the load, the weight of the load is pushing the piston down - right? Ok, it's a great big load so it is pushing it down fast. The operator (you) don't want that honking big load to be going so fast so you only give it a wee bit of fluid from the pump to the other side of the cylinder. Well, the load check in the control is now free so the fluid from the cylinder can rush out FAST allowing the cylinder to drop FAST. The cylinder is dropping, but you are not giving the other side fluid at the same rate (remember you are only feathering the control in an attempt to slow it down). The cylinder continues to go down and the pressure side actually draws a vacuum. Either air leaks in or the oil boils off any dissolved air or water (cavitates), possibly both. Air in the cylinders will do bad things over time...

If wrong, please correct me! I want to know.

Thanks,
jb

 

[Dirthog]

The regen function on the kobelco excavators is just a spool in the main control valve untill you get to some of the larger 60k# or so excavators they then have a spool or poppet in the main spool which will meter the flow some but that it is not its main function. It is to keep the cylinder from cavating same with the kawaski loaders we sell and older fiat-allis we sold in fact i had to install a separate block with a relief on several of them to slow down the dump function on the bucket you did not want to be in the machine when the buckett hit the stops with out it.

 

[dfkrug]

I think that avoiding negative pressure on the dump action is
important for the large forces a large excavator experiences.
But the speed of the dump cycle is also important. If you
have large hydraulic pump flow rates, like industrial equip-ment, I doubt dump speed is such a big issue, so avoidance
of cavitation may be more important. But on my little 33hp JD
CUT, the regen position on the first spool (SCV) is
to speed up the dump. I don't think cavitation is a problem
with CUTs, IMO. The flow rate on the JD is low, so you
want faster dumps.

 

[john_bud]

Dirthog,

Thanks for the clarification. /forums/images/graemlins/smile.gif

jb

 

[MMM]

</font><font color="blue" class="small">( regen function on the kobelco excavators is just a spool in the main control valve )</font>

Is this the same as "anticavitation" valve on my industrial loader SCV? From what I've read it feeds some exhaust oil (not supply) to pressure side of cyls during dump/lower cycle until 0 press drop is achieved.