Leaky backhoe, normal?

[GreensvilleJay]

TOH, those pictures work for a cylinder with good seals, but in my example, leaky seals will allow the oil to go from the 'left' side of the cylinder to the 'right' side. The ram pushes the oil past the leaky seal to the other side.

 

[TheOldHokie]

TOH, those pictures work for a cylinder with good seals, but in my example, leaky seals will allow the oil to go from the 'left' side of the cylinder to the 'right' side. The ram pushes the oil past the leaky seal to the other side.

Those pictures have ZERO to do with seals. They are simple solid geometry calculations that illustrate the decrease in free space in the cylinder as the ram retracts. That is accounting for both sides of the piston.

The cylinder held 119 cu-in of oil when fully extended. When retracted 1.5" that's down to 114 cu-in of oil. Where did that 5 cu-in of oil go?

Dan

 

[GreensvilleJay]

out one of the ports, all over the screen......????

 

[TheOldHokie]

out one of the ports, all over the screen......????

Bingo.

Dan

 

[hagrid]

For example, say a cylinder was put together without any seals. Wouldn't the piston be free to move in whatever direction the force on the cylinder rod wanted it to push it?

Yes.

 

[TheOldHokie]

Yes.

Care to elaborate?

Dan

 

[hagrid]

Indeed.

If the piston/zylinder interface is not "fluid tight" then fluid is free to transfer from one side of the piston to the other in response to a push/pull force applied to the working end of the ram.

 

[TheOldHokie]

Indeed.

If the piston/zylinder interface is not "fluid tight" then fluid is free to transfer from one side of the piston to the other in response to a push/pull force applied to the working end of the ram.

So these cylinders which do not have a "fluid tight" seal between piston and cylinder never worked?
They do in fact work and quite reliably. The entire cylinder is pressurized and the base end of the rod becomes the working cross section for application of the hydraulic force which extends the ram. This design is still used today - Myers single acting snow plow cylinders are an example.

Dan

 

[hagrid]

I'm confused about the use of the single acting cylinder reference? Double acting cylinders require a fluid tight piston seal otherwise they behave like shock absorbers.

 

[TheOldHokie]

I'm confused about the use of the single acting cylinder reference? Double acting cylinders require a fluid tight piston seal otherwise they behave like shock absorbers.

Did you take the time to read the lengthy thread? I like your cryptic style but that cryptic yes in response to Henro in the middle of that exchange left me wondering what you had in mind.

So just to clarify a DA cylinder requires a fluid tight piston to work properly in both directions. But with a leaky or missing piston seal it will behave as a displacement type SA cylinder. Agreed?

Dan

 

[Henro]

I'm confused about the use of the single acting cylinder reference? Double acting cylinders require a fluid tight piston seal otherwise they behave like shock absorbers.

At first I did not see the point, but now I do. I'M A BELIEVER! LOL

The important point to consider is that when the cylinder rod enters the cylinder it must displace some oil. If the oil has nowhere to go, the rod cannot enter the cylinder.

If the control valve is good, so it blocks flow for the most part (granted they all leak to some degree) the rod will only enter the cylinder at whatever rate the control valve leaks fluid. Same for a leaking fitting or rod seal, etc.

Dan uses the single acting cylinder as an example of how a cylinder can function without a piston seal at all. I missed this at first, but now see it.

Essentially, a double acting cylinder that had no piston seal would act like a single acting cylinder (I think). I even imagine without the piston/cylinder seal, the the double acting cylinder might actually extend regardless of which hose fed fluid to the cylinder...

Dan please correct this post as necessary...

Edit: I was typing as Dan was replying...never seem to cross the finish line first!

 

[TheOldHokie]

At first I did not see the point, but now I do. I'M A BELIEVER! LOL




Essentially, a double acting cylinder that had no piston seal would act like a single acting cylinder (I think). I even imagine without the piston/cylinder seal, the the double acting cylinder might actually extend regardless of which hose fed fluid to the cylinder...

Henro your mind does go some interesting places. That made me think for a moment.

If its truly a DA cylinder the spool is also DA so the base end port would be open to tank preventing that.

Otherwise yes but probably not before it retracted.enough for the bypass flow to build base end pressure greater than rod end.

Edit: see what you get for speaking too quickly. The rod could not retract for the reasons just discussed

Dan

 

[hagrid]

I concede the SA cylinder operating theory as presented. Just couldn't reconcile how it was being wielded to 'splain why the OPs bucket zylinder was losing force after the control spool was put into neutral.

 

[Henro]

Henro your mind does go some interesting places. That made me think for a moment.

If its truly a DA cylinder the spool is also DA so the base end port would be open to tank preventing that.

Otherwise yes but probably not before it retracted.enough for the bypass flow to build base end pressure greater than rod end.

Edit: see what you get for speaking too quickly. The rod could not retract for the reasons just discussed

Dan

I love how simple things sometimes become less simple than at first thought!

 

[TheOldHokie]

I concede the SA cylinder operating theory as presented. Just couldn't reconcile how it was being wielded to 'splain why the OPs bucket zylinder was losing force after the control spool was put into neutral.

It was being wielded to suggest the bypass is most likely in the spool not the cylinder. The amount of fluid loss needed to cause the thumb to loose clamping force is probably pretty darn small. An interesting data point would be to hang some weight on the teeth and time the drift rate.

Dan

 

[OrangePower]

Quick update. I went through and tightened ALL the fittings I could see. They mostly turned just a little. I had the hoe in storage position (I think) and when I took a picture of it to put it on CL, I noticed the bucket was still off the ground. Caught me by surprise cause I wasnt looking for it and or expecting it. Looks like something is holding now, at least better than it was. Have not tried the thumb as of late, nothing to carry with it. I did not see any oil leakage at the fittings or get any on me, so, it was either sucking air or the oil leak was clean and fine. X-mas is supposed t be 60 degrees so I will get out and play with the thumb then.

 

[TheOldHokie]

Quick update. I went through and tightened ALL the fittings I could see. They mostly turned just a little. I had the hoe in storage position (I think) and when I took a picture of it to put it on CL, I noticed the bucket was still off the ground. Caught me by surprise cause I wasnt looking for it and or expecting it. Looks like something is holding now, at least better than it was. Have not tried the thumb as of late, nothing to carry with it. I did not see any oil leakage at the fittings or get any on me, so, it was either sucking air or the oil leak was clean and fine. X-mas is supposed t be 60 degrees so I will get out and play with the thumb then.

Where in VA?

Dan

 

[leveraddict]

Quick update. I went through and tightened ALL the fittings I could see. They mostly turned just a little. I had the hoe in storage position (I think) and when I took a picture of it to put it on CL, I noticed the bucket was still off the ground. Caught me by surprise cause I wasnt looking for it and or expecting it. Looks like something is holding now, at least better than it was. Have not tried the thumb as of late, nothing to carry with it. I did not see any oil leakage at the fittings or get any on me, so, it was either sucking air or the oil leak was clean and fine. X-mas is supposed t be 60 degrees so I will get out and play with the thumb then.

So you just bought this machine and your selling the backhoe on craigslist?

 

[OrangePower]

So you just bought this machine and your selling the backhoe on craigslist?

No, the whole tractor. Sorry, didnt mean to confuse.

 

[leveraddict]

Skeetz has a BX. Then got a B2601. Pretty sure he's not a happy camper? Since your not happy with the BX I have a feeling the B isnt gonna thrill you!