hydraulic flow problem

[sweetk30]

i have a old jacobson mower deck mounted to a quick plate for a skid steer .

the unit works great for what it is but keeps popping the seal on the drive motor for the blades .

its a bobcat 753 unit .

the pump is a roller stator unit ?

quick search i found the problem is common but i didn't have much time with heavy overtime at work to dig deeper . figure i would post here to see if someone knows just what i need to keep this from happening .

seems the problem is pressure build up after the shut off . and it pops the seal after a bit of use this way . thinking i need a blead off valve / bypass valve or what ever its name is .

 

[one9gt]

Have you compared the flow rates between the original machine it's meant to be on and what your 753 is running at? You can run an andjustable flow regulator to limit flown


Sent from my iPhone using Tapatalk

 

[sweetk30]

old unit power was 15-16gpm

this bobcat is the same range .

i think its the sudden OFF of flow thats popping the seal when it wants to pop .

we have used it for hr's on end with no problems. but when it pops its a mess to clean up .

 

[Olafur]

seems the problem is pressure build up after the shut off . and it pops the seal after a bit of use this way . thinking i need a blead off valve / bypass valve or what ever its name is .

If you are using valve intended for double acting ram, to drive the mover, when OFF both pressure and return line are closed.

This means, if you stop the mover the return line is blocked - - before the mover has stopped. So the rotational momentum from the mover blades are driving the motor (and the motor acts like a pump) creating pressure pulse in the return line - popping the seal. (many similar rotors have small passage from behind the seal straight to the return port -ergo same pressure behind the seal as in the return line)

I would bypass any control valves on the return line and plumb it directly to return line to tank.

 

[A_Pmech]

You need a relief valve across the pressure and return lines of the implement to allow the hydraulic fluid to circulate within the closed mower circuit as the mower spins down.

 

[Olafur]

You need a relief valve across the pressure and return lines of the implement to allow the hydraulic fluid to circulate within the closed mower circuit as the mower spins down.

That one would need to have very low setting, basically a check valve.

Possibly easier than finding a direct return to the tank.

 

[A_Pmech]

That one would need to have very low setting, basically a check valve.

Possibly easier than finding a direct return to the tank.

Most all hydraulic motors are reversible and will withstand system pressure on either port. Setting the relief valve for system pressure will act as a motor brake.

 

[AA/FC]

What would be nice to have on that setup is called a "case drain" on the hydraulic drive motor. Newer bobcats and other equipment now run a 3rd, smaller hydraulic hose which allows excess case pressure to drain back into the hydraulic oil tank. It relieves pressure on the back side of the seal to prevent seal and/or shaft damage. Case drains are only found on hydraulic equipment with rotating drive motors, not on hydraulic cylinders.

Unfortunately your older bobcat probably does not have that 3rd smaller quick disconect plumbed into the hydraulic system. However, it could easily be added. Its just a hose plumbed directly back to the reservoir. As for connecting the case drain on the hydraulic drive motor itself.... the motor probably already has the provision for that third line, it would be a oil port somewhere on the case with a pipe plug blocking it off. If not, a new drive motor will certainly have a case drain.

 

[Olafur]

Most all hydraulic motors are reversible and will withstand system pressure on either port. Setting the relief valve for system pressure will act as a motor brake.

The motor OP has shows arrow indicating flow direction. It doesn't have separate bleed line like bi-directional motors have. Chances are it doesn't like any pressure on the return line.

A simple check valve would probably do the trick and save the OP the hassle of plumbing return line to the tank.

 

[bobcatdan]

Curious , what way do you have the hydraulic couplers. Continuous flow out of the bobcat is from female coupler. You can run continuous backward, but it's technically not coreect.

 

[sweetk30]

this bobcat has a little hose and quick coupler on the pass side of the boom . the 2 BIG lines are on the driver side .

the way its plumbed now its hold the button left on the left joystick handle.

i will have to verify the hose's and get back to you . it would be great to have it work without holding the button all the time.

if i went check valve to the system to let it bleed its self down any good ones to buy ?

 

[A_Pmech]

The motor OP has shows arrow indicating flow direction. It doesn't have separate bleed line like bi-directional motors have. Chances are it doesn't like any pressure on the return line.

A simple check valve would probably do the trick and save the OP the hassle of plumbing return line to the tank.

I see that now! Good catch.

 

[Firebrick43]

The motor OP has shows arrow indicating flow direction. It doesn't have separate bleed line like bi-directional motors have. Chances are it doesn't like any pressure on the return line.

A simple check valve would probably do the trick and save the OP the hassle of plumbing return line to the tank.

+1.

Check on motor side of spool so when the spool is closed oil can flow from outlet of the motor(now acting as a pump) directly back to inlet of the motor. Make sure the checks flow rating is high enough.

 

[Heavy Metal Doctor]

+1.

Check on motor side of spool so when the spool is closed oil can flow from outlet of the motor(now acting as a pump) directly back to inlet of the motor. Make sure the checks flow rating is high enough.

Yeah. Inertia of the blades is likely building too much pressure trying to spin the motor after oil flow stops from the valve. We put the check valve on the motor assy for situations like this, usualy to save any line resistance and it's uaully easier to physicly put in the parts needed. It's pretty simple to adapt in the check between the two hose connections so that the running pressure seals it closed. When you turn the unit off the interia will pull the check open and just spin the oil in a loop without overpressurizing the motor case.

 

[sweetk30]

Yeah. Inertia of the blades is likely building too much pressure trying to spin the motor after oil flow stops from the valve. We put the check valve on the motor assy for situations like this, usualy to save any line resistance and it's uaully easier to physicly put in the parts needed. It's pretty simple to adapt in the check between the two hose connections so that the running pressure seals it closed. When you turn the unit off the interia will pull the check open and just spin the oil in a loop without overpressurizing the motor case.

were do I get this stuff ? ? ?

or what to ask for ? ? ?

 

[Heavy Metal Doctor]

were do I get this stuff ? ? ?

or what to ask for ? ? ?

Here's one generic type we use:


http://ph.parker.com/us/en/valves-check-up-to-3000-psi-dc-series

I have one that's 1/2" with JIC male / female ports -- so you can essentially put it in place between 2 JIC fittings / hose ends with no other adapters needed. But it's and OEM specific part. I can sell you one at cost, but being OEM, I'm sure it's not cheap....

 

[sweetk30]

here is the extra single smaller hose on the unit . and i forgot it was on the same side as the 2 big ones.

 

[sweetk30]

Here's one generic type we use:


http://ph.parker.com/us/en/valves-check-up-to-3000-psi-dc-series

I have one that's 1/2" with JIC male / female ports -- so you can essentially put it in place between 2 JIC fittings / hose ends with no other adapters needed. But it's and OEM specific part. I can sell you one at cost, but being OEM, I'm sure it's not cheap....

so how would this help if the return hose is blocked off when you let go of the button control ?

i under stand its a check valve 1 way flow . . .

 

[Djstorm100]

What you have is a Danfoss gear pump.

Is the extra smaller hose on the gear pump, that spins the mowing deck?

If so it need to be back to tank or tied in to a hose that goes to the tank.

-OR-

As one stated leave it capped and get a check valve and tie the input and output of the motor together. Basically you'll need 2 tee's, some hose and a check valve.
As far as companies these are the good ones.
Parker
Danfoss
Hydac
Hydroforce
Bosch Rexroth


I'm a hydraulic engineer at CAT BCP division in NC.

 

[sweetk30]

no extra smaller hose on bobcat unit .

ahh i see now 2 tee fittings and 1 valve in the middle to make a letter H in the hose setup .

any key psi point to look for ?

the deck works good at 1/2-2/3 throttle now so we dont spin it crazy with hydro on the bobcat .

 

[Djstorm100]

no extra smaller hose on bobcat unit .

ahh i see now 2 tee fittings and 1 valve in the middle to make a letter H in the hose setup .

any key psi point to look for ?

the deck works good at 1/2-2/3 throttle now so we dont spin it crazy with hydro on the bobcat .

The line that is capped, where does that go or where is it coming from?


In order to get a correct idea you need to put a tee on one end (inlet) and a gauge on the port. Then fire it up and see the pressure range you are working. Pressure for the most part is measure in BAR. I know on out skid steers they ran between 250-325 BAR. Depending on the unit.

 

[kspwski]

Like several have said, a single check will likely solve the problem. The check gets plumbed at the motor to allow flow from return line to pressure line. When the mower is stopped, the deck wants to keep moving and pressurizes the return line. This check would just allow that oil to circulate back to the other side at low pressure. The blades might take a little longer to come to a stop. I'd look for a 20-50 psi check. The lower the longer the blades spin, the higher the greater the risk of popping a seal. Check surpluscenter.com for a check and fittings.

 

[kspwski]

Like this one link

 

[Heavy Metal Doctor]

so how would this help if the return hose is blocked off when you let go of the button control ?

i under stand its a check valve 1 way flow . . .

very messy / quick sketch, but it should get the point across:

 

[Djstorm100]

You are also using a pump as a motor. The inlet and outlet are different sizes (pump). Motors have the same size inlets.

 

[sweetk30]

You are also using a pump as a motor. The inlet and outlet are different sizes (pump). Motors have the same size inlets.

its what come on the unit factory and both inlet/outlet are the same size .

 

[sweetk30]

Like this one link

checked my local shop and nothing .

so I ordered this one . 8.95 valve and 10.55 s+h . . .

 

[sweetk30]

got my check valve today . . . story goes i ordered it and i got a flat face quick coupler for a hydro hose . . and just so happens it might fit the case drain hose .

so i called them up and told them the problem and they sent me a second one out that day Friday . today Monday it showed up . and it was the correct one this time. tomorrow i will be getting be going to the hydro hose shop and have them cut/install a tee fitting in the hoses for install and test the system .

 

[kspwski]

How do you plan on hooking up to the case drain, where do you plan on the check going?

 

[sweetk30]

guys that posted before said just the check valve tee'd in line of the 2 hoses should be all I need.

 

[kspwski]

Right, check to enable flow from outlet of motor to inlet of motor. I got confused when you started talking about case drains.

 

[sweetk30]

ya i will install the way guys posted .

stopped in the hydro shop today . all fittings minus 1 . be here in the am . then cut/crimp on my almost new hoses they made for it last year . so no problems.

will report back .

glad to see so many post up this is the fix i need. i was getting a bit second guessing my self . . .

 

[sweetk30]

got the hoses cut/fitted/crimped and re-installed today with the valve in the hoses to make a tee .

first test I got backwards movement of the drive motor ? ? ?

so I said well then lets try flipping the check valve 180* . . . . .

reinstalled and back to 100% normal movement and also when you let go of the button it has a spinning sound now as it winds down . as before it was almost a dead stop .

tested it for a few min in the field next to the shop at work and turned it on/off a lot of times and no leaks and seems to be good . so I re-installed the belt covers .

and as a bonus if you hit the button the wrong way now the drive motor dosnt go CLUNK from wrong flow . . this valve lets the fluid bypass back up the system .

boss will be using it again real soon so I will post up long term use . . . . THANKS FELLOW MEMBERS !

 

[kspwski]

Perfect! Glad we could help.

 

[sweetk30]

so they say it stalls out the blades and dosnt have the power it use to before the valve was installed .

he said if we cant get the power back up on it like before then he will just pull the valve out . but I told him it will then blow the seals out . . .

i installed the valve and tried it at first . but it spun the drive motor went backwards so i flipped it and now spins correct direction .

any idea's ? on what we can do ?

maybe try and do a way to hook its return to a blead off / check valve on the return side to the 3rd hose for the case drain . ? ? ?

i just know the thing he will do is just remove the check valve and keep blowing seals now and then if i cant figure this out .

i did go up and try it my self and there trying to basicly brush hog with it . the grass is so tall and wet/thick this season . . .

 

[WittHay]

The farmer fix is having just 1 supply line from the remotes and a return line with no restrictions back to the reservoir. A small case drain line can be plumbed into the return line if needed.

Having said that, brush mower attachments on Bobcats are common and they just hook up using the 2 hoses on the remotes and are reversible. They have a relief valve and something called a cross port check valve. I can't give you the specifics, as i am not too familiar with Bobcats. i would look at some commercially made brush cutters and see exactly what valves and hose routing they use.

Cutting tall grass takes a lot of power. Have a pto driven Woods hydraulic ditch bank cutter and the tractor engine has to be at full throttle to have enough pressure so the mower doesn't bog down

 

[Firebrick43]

First do you belive its low in power? I learned a long time ago that some operators perceptions on power, noise, and speed of machinery can be influenced by other psychological issues. Certain operators at work complain about speed of their machine slowing down and we know it's not. One guy we just go to the hyd unit, act like we made an adjustment and he's happy for a month or more.

Another operator was making adjustments himself screwing things up. Management wouldn't do anything so we plumbed the original pressure reducing valve and gage that does nothing except change the Gage's reading. A second reducing valve and gage is mounted discretely that actually controls that part of the circuit . Operator can make all the changes he wants and is happy as a pig in **** yet it changes nothing.

Check could be bad out of the box or has trash in it holding it open. Not common but I have seen bad new parts. Take the input line and check off the motor. Cap the line where it would hook to the motor. Now where the check would of dumped into the return line point into a bucket and actuate the circuit. The check should close and there should be no appreciate-able flow. If their is the you have a bad check.

 

[Olafur]

Check valve shouldn't effect power at all, unless it's leaking. I find it more likely your boss is wrong about the power loss.

Quick test:
Try spinning the mover by hand. It should turn relatively freely forward, but not backwards. However this is not cut and dry, because gear rotors tend to leak little bit internally so by using some force you can probably spin the mover slowly backwards.

 

[kctyphoon]

No idea if this is relevant.. but I had a truck where the pto shaft would constantly blow the seal at work.. mechanics would fix it, and as soon as you put pressure under the unit to pull something, it would blow again. If it's the seal around the motor shaft, on my issue, that seal is really just a like a dust cover.. fleet would just keep pressing a new one in, ingonring the fact that the seals behind that where leaking too. Eventually it was fixed by replacing the entire motor. It was explained to be by a better mechanic that once the shaft develops a slight bend on it, it will just keep blowing seals. in our case, there were layers of seals behind the last one that would keep blowing.... he replaced because he felt that rebuilding the entire motor could just be a waste of time for something that was already on record being fixed 3 times previously.. took a few years to actually wind up with a mechanic willing to fix it right.

 

[427]

I agree with last post. That seal that is pushed out is dirt and water seal. Internal high pressure shaft seal is likely leaking causing external seal to push out.