Will an uneven hose length affect a parallel cylinder system?

[Andres9292]

I have 2 hydraulic cylinders that work in parallel to lift a steel ramp. We are replacing some old hoses that are damaged and are hard to reach. The current hoses have a tee that divides the flow right at the middle and then two hoses that have the same length go to each cylinder. But the possition where the tee is right now is probably the cause of why it got damaged. We are planning on replaciong the hoses with a short one (0.5m) and a long one (4.5 m) that will go to each piston to reposition the tee. Some quick calculations give me that the cylinder with the longer hose would have a 16 psi greater pressure loss compared to the other cylinder. Will this affect the system or is the pressure difference negligible and will work fine?

 

[IRstuff]

Doesn't that kind of depend on what the total pressure is?

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[hydtools]

Depends on flow rate and hose size. Upsize the hose to make pressure drop due to hose length negligible. Flow area varies with the square of diameter.

Ted

 

[Andres9292]

i dont know the total pressure. It must be close to 270 psi by measuring the time it takes for the cylinders to fill up, the volume of the cylinders and the 3/4 hp hydraulic unit used and solving for the pressure using a 70% efficiency. The flow rate is about 4.25 gpm using the same time and volume measurements and the hose is 3/8 in. Using a 3/8 in hose gives me a 16 psi pressure drop and using a 1/2 hose gives me a 4 psi pressure drop. Is the 16 psi already negligible or should i go ahead and use a 1/2 hose?

 

[hydtools]

Go to 1/2 inch. That will keep delta p down with low temperature use.

Ted

 

[IRstuff]

16/270 = 5.9%
4/270 = 1.5%

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[LittleInch]

Two things for me,

1) your pressure calculation assumes I think that the 3/4hp motor is working flat out. It may well not be as this is mainly dependant on the load. Many hydraulic units are PD pumps so will pump a set volume of fluid with power changing with the pressure required to allow the fluid to enter the cylinder. So your pressure could be anything from a few psi to hundreds. Hence how big a difference a few psi pressure drop makes will be variable. To make any decision not knowing what the ACTUAL pressure is over the full range is not a good plan.

2) what is the consequence of different pressures on the load. Will it start to deflect the load (ramp) or will the load essentially balance out the forces and require a higher force if the one which is now more extended than the other sees more load.

But if the opposite occurs then the discrepancy in the two cylinders will get worse and worse until one side twists or jams the load.

The original designers clearly though very equal hosing was a good idea. If you want to move the Y piece all I can suggest is that you still use the same identical length of hose, but just curl the other one up rather than make it shorter.

But it's all about the load / force on each piston and how it is distributed.

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[Andres9292]

Heres a picture of the ramp. Im not really sure if it would deflect the load or balance out. What do you think?

 

[LittleInch]

I think any out of balance forces or one piston extending a little bit more than the other will twist the ramp. Now whether that would be self correcting is difficult to see but I don't think so. If that deflection causes one side to jam the other will just keep going and twist it some more.

Not easy to see what the weight of the ramp is or how long the arms are when extended.

The original design worked with equal length hoses. Why risk it for the sake of another 4 m of hose??

Like I said - if you can't measure the pressure then you really don't know what percent increase is. Might be OK, might not be.

Nice double deck garage though! Is there a bit of the ramp missing?

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[Andres9292]

Thanks for all the help! I really appreciate it. The ramp is actually lifted to the point its completely horizontal, then a second actuator (a garage door opener) slides the ramp inwards. Thats why it looks shorter.

 

[LittleInch]

Then you really really don't want one side to be even a little bit higher than the other.

SO are going to make any changes to your design? Or give the uneven hoses a go? Let us all know.

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[3DDave]

It looks to me like they just go full stroke; even if one is slow the other will catch up.

 

[LittleInch]

"It looks to me like they just go full stroke; even if one is slow the other will catch up."

But that's my point - will this twist the door?

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[3DDave]

It may twist in the intermediate; probably doesn't matter.

 

[hydtools]

There will not be uneven loading if only the ramp is being lifted. At least not enough to be of concern.

Ted

 

[Andres9292]

We found there were aditional issues with the system. The plastic tank of the hydraulic unit was broken and was the place where oil was leaking. We believed the leak was in the hoses that we were unable to reach/inspect. We will replace the tank first, then test. If the problem persists we will then consider replacing the hoses after doing some aditional testing. The cylinder do in fact go full stroke, sorry if i forgot to mention that. The ramp in fact is the only thing that will be lifted.

 

[akkamaan]

So your system pressure is maxed to 300-ish psi??
Is the ramp raising evenly with uneven weight distribution?
Here is a test you can do with the current 50/50 hoses.
If we assume the piston area is 4 square inches you can simulate the 16-4 = 12 psi pressure difference by adding "50 lbs" (equal to approx 6 usg/22 L of water) of mass above one of the cylinders to create more load pressure on one side.

Instead of using different sized hoses, I would go with the same size
If you use the 0.5m/4.5m hoses you can fine-tune the excessive flow (lesser pressure) in the short hose with a proper flow rated needle valve. It might even be enough with a sharp bend/kink on the short hose.

How often do you cycle this indoor ramp? I can imaging not very often and then change of the oil temperature/viscosity won't be a factor for the needle valve.

 

[Andres9292]

Thanks akkaman! that test is really useful, i hadnt thought of that! I had also thought of adding the needle valve, thanks for the suggestion!

 

[Andres9292]

Hey guys, just bringing you an update. We ended up not having to change the hoses. After we replaced the tank and fill it with hydraulic oil, we tested and the right cylinder was not extending. It turns out the cylinder chamber that isnt being used had a leak in the o-ring/seals. Additionally both of the cylinders had plugs in the unused chambers. Since the cylinder that was leaking got filled with oil in the unused chamber, it couldnt extend while the other cylinder compressed the air in its chamber. We proceeded to remove the plugs and that allowed the system to work again. Even so, we proceeded to remove both cyllinders and replace all o-rings/seals to stop present/future leaks. The system is now working perfectly with a new metal tank, new oil, and repaired cylinders. The hoses didnt need to be changed.

 

[LittleInch]

Thanks for the update. At least you know now what to do if you did change the hoses!

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Also: If you get a response it's polite to respond to it.