Attachment Hydraulic Hose Pressure Relief

[retiree-urgency abandoned]

My photo with the grapple wasn't that clear - the assembly connects to both loose ends, but I"ll drain it, remove it and connect the hoses before taking the the machine out to work. The assembly isn't in the way of picking up the attachments with the FEL quicktach. Hope that makes more sense.

 

[whatsupdoc]

These look nice and easy to connect but the price.

 

[retiree-urgency abandoned]

Yike$$$$. 3 sets would be more than my annual diesel & beer allowance.

 

[Ashhh]

Just get a female-to-female coupler to connect the hoses. This allows the pressure from thermal expansion to equalize between the lines. The T-valve idea is clever but adds unnecessary complexity and a potential mess.

 

[mdhughes]

Just get a female-to-female coupler to connect the hoses. This allows the pressure from thermal expansion to equalize between the lines. The T-valve idea is clever but adds unnecessary complexity and a potential mess.

You are the second person that I have seen on the thread say this, I don't see how this is going to make the pressure in the lines be less then if they aren't connected. It is still a closed system and the two sides of the cylinder are going to push against each other when you get thermal expantion and you will still have pressure build up.

 

[tthorkil]

You are the second person that I have seen on the thread say this, I don't see how this is going to make the pressure in the lines be less then if they aren't connected. It is still a closed system and the two sides of the cylinder are going to push against each other when you get thermal expantion and you will still have pressure build up.

I tried the connect the hoses together method once and found that I could not disconnect the couplers until I cracked one of the lines open to relieve the pressure so the only advantage for me was keeping the couplerss clean.

 

[WI_Hedgehog]

You are the second person that I have seen on the thread say this, I don't see how this is going to make the pressure in the lines be less then if they aren't connected. It is still a closed system and the two sides of the cylinder are going to push against each other when you get thermal expantion and you will still have pressure build up.

If they're not connected together, they may not be at the same pressure.

I'm a lousy artist, so I'll try to explain with words. The forces on the two sides of the piston are equal (but opposite). The force on the cylinder end is the area of the piston Ap times the pressure P. The force on the rod end is 1) the annular area between the piston and the rod Aa times the pressure P, plus 2) the force on the rod Fr. Assuming we relaxed the implement so that Fr=0, then Ap*P=Aa*P. Since Ap<>Aa, the only solution is P=0, which is what we want. The cylinder has to be free to extend slightly to accommodate the slight increase in volume. If it hits the end cap, that exerts a force Fr and changes things.

As @John T wrote (post #15), if the implement is allowed to exert force on the rod, that's going to increase the pressure in the hoses whether they're coupled together or not.

That makes sense. You're saying because the rod occupies volume the pressure increase due to heat could be mitigated by the implement cylinder moving slightly and offsetting the pressure buildup.

If the implement can move and if hydraulic pressure changes because the hydraulic fluid expanded or contracted (slightly) the extra fluid expands into the non-rod end of the cylinder and moves the implement (slightly).

 

[mdhughes]

If the implement can move and if hydraulic pressure changes because the hydraulic fluid expanded or contracted (slightly) the extra fluid expands into the non-rod end of the cylinder and moves the implement (slightly).

There isn't anywhere for the fluid to go. The pressure on both sides of the cylinder increases at the same time, it is a close loop.

If the pressure stays the same and the implement moves, then this works, but the thermal expansion is a different case.

 

[PoTreeBoy]

There isn't anywhere for the fluid to go. The pressure on both sides of the cylinder increases at the same time, it is a close loop.

If the pressure stays the same and the implement moves, then this works, but the thermal expansion is a different case.

If the hoses are connected, it's really not a closed system, not by volume anyway. If the cylinder rod is free to move, when the rod extends the total volume of fluid in the cylinder increases. There's less area, and therefore less volume/inch, on the rod end.

 

[mdhughes]

If the hoses are connected, it's really not a closed system, not by volume anyway. If the cylinder rod is free to move, when the rod extends the total volume of fluid in the cylinder increases. There's less area, and therefore less volume/inch, on the rod end.

I'm not following what you are saying.

Are you saying that since the non-rod end can hold more fluid then the rod end, that as the fluid expands it will push the piston towards the rod end as it fills the non-rod end?

 

[PoTreeBoy]

I'm not following what you are saying.

Are you saying that since the non-rod end can hold more fluid then the rod end, that as the fluid expands it will push the piston towards the rod end as it fills the non-rod end?

Yes. If the hoses are both capped, you have two closed systems, there's no room on either end for fluid to expand and the rod can't move. But if you couple them together, they share the total fluid space which can expand when the rod extends.

 

[nbryan]

Yes. If the hoses are both capped, you have two closed systems, there's no room on either end for fluid to expand and the rod can't move. But if you couple them together, they share the total fluid space which can expand when the rod extends.

Your logic escapes me.

When the rod extends (or contracts) with the hoses coupled together, it just moves fluid through the coupling to the other side of the cylinder. The pressure of that loop can still rise because it is STILL a closed loop, one now instead of 2.

The warming and expanding fluid will still increase the pressure. I'll need an idiot-proof explanation why I'm wrong, please. I accept that my brain/mind behaves like one often enough.

But your logic broke my brain and I need help, please, lol...

 

[Russell King]

Your logic escapes me.

When the rod extends (or contracts) with the hoses coupled together, it just moves fluid through the coupling to the other side of the cylinder. The pressure of that loop can still rise because it is STILL a closed loop, one now instead of 2.

The warming and expanding fluid will still increase the pressure. I'll need an idiot-proof explanation why I'm wrong, please. I accept that my brain/mind behaves like one often enough.

But your logic broke my brain and I need help, please, lol...

The area on the rod size is smaller than the cylinder side piston area. The formula for force and pressure is F=A*P (F=force in lbf, A = area in square inches and P = pressure in pounds per square inch). If you apply the same pressure on both sides the force on the cylinder side is greater than the rod side. If you connect the hoses then then the fluid is able to move from the rod side towards the cylinder side and balance the pressure but increase the pressure by thermal expansion and the force is unbalanced again and the rod will extend until the pressure balance is achieved (You may want to change the formula to be P=F/A to see that and use A1 for cylinder side and A2 for rod side, if P1 = P2 then the F/A must also be the same on both sides.

Good enough or do you need more explanation?

 

[nbryan]

The area on the rod size is smaller than the cylinder side piston area. The formula for force and pressure is F=A*P (F=force in lbf, A = area in square inches and P = pressure in pounds per square inch). If you apply the same pressure on both sides the force on the cylinder side is greater than the rod side. If you connect the hoses then then the fluid is able to move from the rod side towards the cylinder side and balance the pressure but increase the pressure by thermal expansion and the force is unbalanced again and the rod will extend until the pressure balance is achieved (You may want to change the formula to be P=F/A to see that and use A1 for cylinder side and A2 for rod side, if P1 = P2 then the F/A must also be the same on both sides.

Good enough or do you need more explanation?

Thanks, I think I get it now.
The overall volume can increase because of the volume that the rod occupies lets the piston side expand.
But I'm now guessing that if the rod is fully retracted when hoses connected that it's at max volume and will pressurize?

 

[Dustball]

If you connect the hoses then then the fluid is able to move from the rod side towards the cylinder side and balance the pressure but increase the pressure by thermal expansion and the force is unbalanced again and the rod will extend until the pressure balance is achieved (You may want to change the formula to be P=F/A to see that and use A1 for cylinder side and A2 for rod side, if P1 = P2 then the F/A must also be the same on both sides.

Good enough or do you need more explanation?

I don't agree when it comes to looping the two ports of a cylinder together.

Let's say we disconnect both hoses of a cylinder from the system and the pressure of the fluid in both hoses are at 0 psi while it's in the shade on a cool morning.

I connect both hoses together. Pressure is still at 0 psi but it's uniform throughout the loop.

Later in the day, sun's been out shining on the cylinder/hoses all day and the fluid's warmed up and has expanded. Since both hoses are connected together, the fluid pressure is the same throughout the loop.

I then disconnect the hoses from each other. The pressure of the non-rod side will decrease slightly while the non-rod size will increase pressure slightly, up until the forces on both sides of the piston equalize.

What if the cylinder is already fully extended to start with? There would be no change in pressure when the hoses are disconnected.

In the end, there's still pressurized hoses on both sides of the cylinder so there is no advantage to connecting them both together to start with.

 

[PoTreeBoy]

Thanks, I think I get it now.
The overall volume can increase because of the volume that the rod occupies lets the piston side expand.
But I'm now guessing that if the rod is fully retracted extended when hoses connected that it's at max volume and will pressurize?

FIFY