Seeking low thermal expansion

[bobneumann]

I'm adding an hydraulic loader to my lawn tractor. The tractor already has a factory hydraulic system, and the factory specifies 15w-40 as hydraulic oil.

By adding multiple extra hydraulic cylinders, I'm increasing the total volume of hydraulic fluid in the system, and I'll definitely be approaching the maximum capacity of the current reservoir, especially at higher operating temperatures. I'd like to avoid having to rebuild/expand the hyraulic reservoir.

Would synthetic 15w-40 motor oil have lower thermal expansion than conventional? I know that synthetic has better heat transfer properties, so the oil cooler may perform marginally better with the synthetic oil. But does synthetic expand more or less than conventional when it gets hot?

Thanks in advance.

 

[930.engineering]

Would synthetic 15w-40 motor oil have lower thermal expansion than conventional?

No, any motor oil will behave more or less the same.

I know that synthetic has better heat transfer properties, so the oil cooler may perform marginally better with the synthetic oil.

How do you 'know'?
.

 

[bobneumann]

No, any motor oil will behave more or less the same.




How do you 'know'?
.

Allow me to rephrase: "What I am googling leads me to believe that synthetic oils have better heat capacity, which leads me to believe that synthetic oils are better at heat transfer, which makes me think the oil cooler would perform marginally better."

This guy says that the coefficient of thermal expansion for synthetic motor oils is around 7% lower than for a conventional motor oil. But even he says that it's a back-of-a-napkin kind of analysis. But in his same analysis, I see that pure mineral oil has a lower coefficient than the synthetic PAO's he's referring to. So I don't know if he got his numbers mixed up, or if I'm failing to follow his logic.

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[Bill_W]

As long as it is a fluid and not gas there is no thermal expansion.

 

[FowVay]

As long as it is a fluid and not gas there is no thermal expansion.

In this application the expansion of the fluid would come from aeration. If the reservoir capacity is not changed then it should not affect the reservoir's design capacity. The addition of hydraulic cylinders is not a problem but if bigger pumps are installed then it would be something to consider.

 

[55hambone]

As long as it is a fluid and not gas there is no thermal expansion.

Then why does a transmission dipstick have a line for full when cold and another for full when hot?

 

[Boomer]

So that is why mercury goes up the tube in a thermometer when warmed? No expansion?

 

[Bill_W]

Then why does a transmission dipstick have a line for full when cold and another for full when hot?

Metal expands…

 

[bobneumann]

In this application the expansion of the fluid would come from aeration. If the reservoir capacity is not changed then it should not affect the reservoir's design capacity. The addition of hydraulic cylinders is not a problem but if bigger pumps are installed then it would be something to consider.

I know aeration can have a very pronounced effect, but I can't do much to control aeration. Meanwhile, pure thermal expansion has an effect as well. I'm trying to learn if the type or brand of oil affects the pure thermal expansion.

The problem with adding cylinders to a hydraulic system is that, due to the volume/displacement of the rod itself, an hydraulic cylinder contains more oil when it's extended than it does when it's retracted. That differential has to be made up for by excess capacity in the reservoir. I think I may have just enough excess capacity, but it's close enough that I need to be sure I'm using whatever oil has the least thermal expansion.

 

[bobneumann]

As long as it is a fluid and not gas there is no thermal expansion.

That's incorrect. That's why there are charts like ASTM D-1250 Petroleum "volume corrections factors". But the chart is only for the more raw types of crude and base stocks and fuel oils.

I'm looking for a volume correction factor for synthetic 15w-40 vs. conventional 15w-40.

Bob

 

[Bill_W]

Oops and thanks for your correction. Liquids do expand with temperature.

 

[High Performance Lubricants]

You will absolutely get thermal expansion on the oil. I don’t believe you will be able to make a significant change based on oil type. They will be very similar. PAO’s will definitely have better specific heat and thermal conductivity than mineral oils but unfortunately that happens at temperatures much higher than you will actually see in operation.

 

[DriveHard]

Are you sure you need the reservoir capacity of all the cylinders extended at the same time? If most stay retracted, you only need the capacity to fill those cylinders that will be used concurrently.

Adding fluid volume and cylinders will reduce your overall reservoir temps, increase dwell time (reduce aeration), and therefor reduce any expansion your system will see from temperature and aeration.

During times of known high temp operation, is there a cylinder that could be extended to store fluid?

 

[kschachn]

Never mind. I see someone got the point.

 

[kschachn]

You will absolutely get thermal expansion on the oil. I don’t believe you will be able to make a significant change based on oil type. They will be very similar. PAO’s will definitely have better specific heat and thermal conductivity than mineral oils but unfortunately that happens at temperatures much higher than you will actually see in operation.

Listen to this.

 

[bobneumann]

Are you sure you need the reservoir capacity of all the cylinders extended at the same time? If most stay retracted, you only need the capacity to fill those cylinders that will be used concurrently.

Adding fluid volume and cylinders will reduce your overall reservoir temps, dwell time, and therefor expansion your system could see from temperature and aeration.

During times of known high temp operation, is there a cylinder that could be extended to store fluid?

That's some great outside of the box thinking! I could mount a mirror image cylinder for each of the 4 new ones, and have the mirror image cylinder set to do the opposite action. That would result in a zero net volume change!

Great idea. I'm not sure how easy it will be to implement. and it may look a little odd. But it WOULD work. Thanks for food for thought.

 

[DriveHard]

Reply

That's some great outside of the box thinking! I could mount a mirror image cylinder for each of the 4 new ones, and have the mirror image cylinder set to do the opposite action. That would result in a zero net volume change!

Great idea. I'm not sure how easy it will be to implement. and it may look a little odd. But it WOULD work. Thanks for food for thought.

My job is R&D hydraulic system design, happy to help!

 

[930.engineering]

This guy says that the coefficient of thermal expansion for synthetic motor oils is around 7% lower than for a conventional motor oil. But even he says that it's a back-of-a-napkin kind of analysis.

Unglossedly, 7 percent of almost nothing is still almost nothing. Also, he made the assumption
that different viscosity base stock are used, which is perhaps the main reason for the difference.

As long as it is a fluid and not gas there is no thermal expansion.

Not true, as even fluids expand with heat. Guess how convection works. That said, the difference
in thermal expansion or even thermal conductivity and capacity between motor oils is negligible
particularly in this application. In an hydraulic system you don't want to see temperatures where
it would matter. I'd worry about the seals used way before.
.

 

[IndyIan]

That's some great outside of the box thinking! I could mount a mirror image cylinder for each of the 4 new ones, and have the mirror image cylinder set to do the opposite action. That would result in a zero net volume change!

Great idea. I'm not sure how easy it will be to implement. and it may look a little odd. But it WOULD work. Thanks for food for thought.

I always thought that both sides of the piston in a double action hydraulic cylinder were filled with fluid? Or supposed to be at least. Is it just volume of the piston rod on the in the retract side of the cylinders adds up to a significant amount?

 

[JimPghPA]

That's some great outside of the box thinking! I could mount a mirror image cylinder for each of the 4 new ones, and have the mirror image cylinder set to do the opposite action. That would result in a zero net volume change!

Great idea. I'm not sure how easy it will be to implement. and it may look a little odd. But it WOULD work. Thanks for food for thought.

While it would work, it would be more complicated, less reliable because of additional places for leaks, and weigh more and cost more than simply adding a larger or second reservoir. Heck you could add a simple tap (or two, maybe also one above the fluid level to maintain the same air pressure above the fluid) onto the side of the existing reservoir to connect to the side of an additional one. Or just T or Y fit on the existing lies(s) of the reservoir to enable the additional. Reservoirs are low pressure or even atmospheric ( + a little hydraulic fluid head pressure and or feed retract pressure or suction ). It is not like they have to be built to withstand the maximum pump output pressure.

I think you could MacGyver something up with a piece of pipe of decent ID, or some other containment. The vibration from what it is mounted to is probably of more concern than anything the fluid will throw at it.