What is the importance of brookfield viscosity?

[kilou]

Hi,

I'm comparing two 75W80 API GL4+ manual transmission lubricants. The two are very similar in term of kinematic viscosity at 40 and 100°C but there is a difference in the brookfield viscosity:

Oil n°1: 44000 mPa.s (cP) at -40°c
Oil n°2: 25000 mPa.s (cP) at -40°C

What is exactly the meaning of this brookfield viscosity? What is it important for? And most importantly, would oil n°2 cause increased wear with the lower BF viscosity of 25000 if the transmission calls originally for an oil with a BF viscosity of 44000 or is the BF viscosity only a measure of cold gearshift performances?

Thanks a lot

 

[ARCOgraphite]

Lube # 2 will have better cold weather performance and economy. Prob made of majority synthetic base stocks. The lube needs to warm to 100 cP to get in its working range, Heat comes from internal "friction" in the churning lubricant through shearing.

 

[SubLGT]

Do a google search for "ASTM D-2983" and you will find lots of details regarding how Brookfield vis is determined.

 

[labman]

Those figures may represent the difference in the viscosity index rather than the test method.

Gardner-Hold, eflux, and Brookfield are popular methods of measuring viscosity. When I worked as a resin chemist for Rinshed Mason, I mostly used Gardner Holt. They use special glass tubes, about 3/8'' ID and 4'' long. You fill the tube almost full and time how long it takes for the bubble to go from one line to the other. I forget whether the the time in seconds is stokes or centistokes. Multiply by the specific gravity to get poise. You can either use a stop watch or compare it to a standard. The set of standards I used stopped at Z-6. Z-7 and up existed.

Eflux, or however you spell it, uses a cup with a hole in the bottom or a small tube. Fill the cup and time how long it takes to empty. It was popular in the paint plant where you could dip the cup in a vat of paint and time it.

The above 2 ways are fairly low shear and vary with the specific gravity.

Most lube base stocks are fairly Newtonian, constant viscosity at different shear rates and shear history.

The Brookfield uses a spring and pointer to measure the torque required to turn a disk in the material. It is unaffected by the gravity of the material. You have a selection of disks including one that is more of a rod. You also have different speeds. You can select a disk and speed to give a good reading on a wide range of viscosities. You might need to start the Gardner Holt test on a -40 degree oil on Monday to finish it before the weekend.

One huge advantage is measuring viscosity at different shear rates and histories. Unlike base stocks, many materials' viscosities vary with shear rate and shear history. Put a big disk in and turn the speed up, and you may get a lower viscosity, or have it drop with time. There are even materials that the viscosity goes up under shear. A Brookfield viscosity should always be run at a specified shear rate or the rate given.

 

[crinkles]

Originally Posted By: labman
Those figures may represent the difference in the viscosity index rather than the test method.

Gardner-Hold, eflux, and Brookfield are popular methods of measuring viscosity. When I worked as a resin chemist for Rinshed Mason, I mostly used Gardner Holt. They use special glass tubes, about 3/8'' ID and 4'' long. You fill the tube almost full and time how long it takes for the bubble to go from one line to the other. I forget whether the the time in seconds is stokes or centistokes. Multiply by the specific gravity to get poise. You can either use a stop watch or compare it to a standard. The set of standards I used stopped at Z-6. Z-7 and up existed.

Eflux, or however you spell it, uses a cup with a hole in the bottom or a small tube. Fill the cup and time how long it takes to empty. It was popular in the paint plant where you could dip the cup in a vat of paint and time it.

The above 2 ways are fairly low shear and vary with the specific gravity.

Most lube base stocks are fairly Newtonian, constant viscosity at different shear rates and shear history.

The Brookfield uses a spring and pointer to measure the torque required to turn a disk in the material. It is unaffected by the gravity of the material. You have a selection of disks including one that is more of a rod. You also have different speeds. You can select a disk and speed to give a good reading on a wide range of viscosities. You might need to start the Gardner Holt test on a -40 degree oil on Monday to finish it before the weekend.

One huge advantage is measuring viscosity at different shear rates and histories. Unlike base stocks, many materials' viscosities vary with shear rate and shear history. Put a big disk in and turn the speed up, and you may get a lower viscosity, or have it drop with time. There are even materials that the viscosity goes up under shear. A Brookfield viscosity should always be run at a specified shear rate or the rate given.


whoa that brought back memories of a bit of rheology seminars for sludge and tailings...

 

[kilou]

Thanks for your replies! So in summary it cannot hurt to use a gearoil with a lower brookfield viscosity at -40°C if all the other specifications and rating are the same?

 

[labman]

If it doesn't thicken up as much at -40, it may not thin out as much at higher temperatures. Go for it.