Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here


75W90 Synthetic oil Blackened in Aluminium Gearbox

75W90 Synthetic oil Blackened in Aluminium Gearbox

75W90 Synthetic oil Blackened in Aluminium Gearbox

Hello, I am running an Aluminium Spur gearbox with 75W90 Synthetic oil around 200F temp in lab endurance test continuously.After 400 hrs the lubrication is failing.The oil is completely blackened out. The oil condition test shows that Phosphorus and Boron particles are burned out. There was significant sulfur content increased in the oil over the test duration. The housing innner surfaces are permanently blackened . Please let me know the reason for the increase in Sulfur content and housing blackening.Whether 75W90 Synthetic is not correct choice for this application. Thank You.

RE: 75W90 Synthetic oil Blackened in Aluminium Gearbox


you might like to look into this one
So, you check the base spec. of your oil whether it's an API GL-4 or a -5 and then also the additives content.
On the blackening of the oil itself, you might contact the suppliers tech support on this, whether it is really worn out.

RE: 75W90 Synthetic oil Blackened in Aluminium Gearbox

if your goal was that the oil in the test should last 400 hrs or perhaps longer, it obviously failed.

given the fact that it is completely dark indicates that there may be either thermal breakdown of some of the baseoils used or there may be heavy oxidation. both can lead to oil insoluble products that will form a layer on the gearbox housing and the gears themselves. additives for gear oils are usually a combination of sulfur and phosphor. when those additives are used to prevent gear damage their composition may change and as a result the previous ratio of sulfur to phosphor may also change.

could you be somewhat more specific about the type of oil used? "synthetic" is a very broad term that can encompass all types of fluids and fluid combinations and not all of them would be suitable as a base for gearlubricants.

RE: 75W90 Synthetic oil Blackened in Aluminium Gearbox

Are the working surfaces of the spur gear teeth in good condition? No evidence of scuffing/scoring damage? What type of lube oil system does the gearbox use? Is it just splash or is it pressure fed? Have you analyzed the gears for scuffing/scoring? Is there any evidence of fretting damage on aluminum housing surfaces like bearing bores?

RE: 75W90 Synthetic oil Blackened in Aluminium Gearbox

~ that paper, and elsewhere: There's oils that are made to produce that 'blackening', it's a layer of additives and made to protect surfaces / improve on wear resistance. Point is to check, whether one needs that or else whether it was simply bought "in the bundle".
On the sulphur, the check would be to look what kind of sulphur containing additives were in the oil in the first place and then, whether these additives disintegrated or by use changed their chemical state.
Could you pls. add information about the gear / rpm running of your application, and the specificalities of the failing of the lubrication?


RE: 75W90 Synthetic oil Blackened in Aluminium Gearbox

The oil turning black might be due to debris generated from contact fretting of aluminum surfaces. Gearbox lube oil can also be degraded due to the transient "flash temperature" rise experienced within the hydrodynamic oil film at the gear tooth contacts. While the OP noted the lube oil in temp was "around 200degF", the flash temp within the oil film at the gear tooth contacts will be higher, and possibly much higher under certain conditions. The less efficient the gear tooth contact process is, the greater the flash temp rise within the oil film will be.

Many of the lube oil additives described above are designed to produce a surface layer on the steel gears that inhibits adhesion between the contacting surfaces under marginal lubrication conditions, but only for limited periods of operation.

Here is an article that might help: http://www.machinerylubrication.com/Read/652/black...

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close