Curious why tran. fluid expands when hot

[doitmyself]

Just changed the hy./tran fluid on my farm tractor and with the correct fill quantity, the fluid is halfway up the dipstick marks when cold. When warmed up, it hits the "full" mark.

Why do tranny fluids expand and contract so much with temp. changes??? Cars are like this too.

Motor oil doesn't seem to do this.

Just curious!

 

[MolaKule]

Motor oil does do this, you just don't notice it as much.

This is one of the reasons why auto manufacturers recommend checking tranny fluid when hot, so as not to overfill it.

Most materials, including fluids, expand in volume when hot and decrease in volume when cooled. This called a "positive coefficient of expansion."

 

[nobadges]

Probably not noticing as much with the engine oil because:
1. You never check it hot and,
2. Its slopping around in there and goes right up the dipstick.

Note that hamburgers have a "negative coefficient of expansion"...
They get smaller when you cook 'em.

 

[MolaKule]

quote:

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Note that hamburgers have a "negative coefficient of expansion"...
They get smaller when you cook 'em.

---

In my case, their effect is to create a positive expansion around my waist.

Another interesting aside is that water expands like amost other substances above 4 C, but from O C to 4 C the volume of water decreases, and has its greatest density at 4 C.

 

[nascarnation]

In addition to the thermal expansion, gearbox systems will entrain air during operation (often 3 to 5 %)and it will take some time to settle back out.

 

[Titan]

Molecule wrote: Another interesting aside is that water expands like amost other substances above 4 C, but from O C to 4 C the volume of water decreases, and has its greatest density at 4 C.

And THAT is why fish in deep water don't freeze! Our world would certainly be unrecognizable if it weren't for this property of water.

Molecule, do you know if oils dissolve more gas (not gasoline, but, oxygen, carbon dioxide, etc.) at colder temperatures, like water does?

 

[MolaKule]

Wow, good physics question.

Since most fluids, including gasses, are more reactive at higher temperatures, the gas has a better chance of diffusing into the oil at higher temperatures.

 

[MolaKule]

Air entrainment is simply air being whipped into oil and not being able to escape.

This is one reason we add anti-foam agents to the oil. The bubbles surface tension is reduced by the AF which causes the bubble to burst and realease air. We know air is NOT a good lubricant in most machinery.

 

[doitmyself]

Thanks for the answers. I felt kind of dumb when mola explained basic basic physics of expansion.

I was really looking for other answers such as nascarnation's "air entrainment", which makes sense also.

I thought maybe there were chemical components that caused it to expand/contract what seems like a lot more than engine oil.

Thanks again.