×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

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

Jobs

viscosity degradation

viscosity degradation

viscosity degradation

(OP)
Oil analysis results from a machine service shows the oil viscosity is ~50cSt. This has been consistent from a number of samples taken over a 12month period

The oil used is an ISO68 oil and a sample taken out of the drum prior to filling the machine reported viscosity at 61cSt. I'm told for compliance the viscosity of stocked oil is allowed to be ±10% so this is right on the lower limit. However another sample was taken ~1 week after a complete oil change and it was again down around 50cSt

What can cause this viscosity degradation? The duty on the machine is not extreme; average operating pressure is only around 60bar although there are some peaks up to 300bar, and average operating temperature is 40°C. From the data we have the oil temp has never been above 44°C

So far the oil manufacturer and pump supplier have not come up with anything useful so I would appreciate any ideas anyone has

RE: viscosity degradation

I'm surprised neither your oil supplier nor your pump supplier had the answer here. It's a fairly well known process called mechanodegradation. The high shear rate that the hydraulic system imposes on the oil causes some of the heaviest [largest/longest chain] hydrocarbon molecules to be broken into smaller molecules. This causes a permanent loss of viscosity.

Your oil datasheet might give a value for "shear stability" and this is the clue. A super refined oil with a particular viscosity resulting from the chain molecules all being around the same size will be less susceptible to shear thinning than an oil with a wider range of molecule sizes (for which the viscosity is determined only by the average molecule size).

High viscosity index oils (HV or HVLP types) have a viscosity index improver additive which is also sensitive to shear thinning. The major oil manufacturers have put a lot of effort into making their additives shear stable, if yours is a budget range oil then you might have the old style compounds.

If the resultant [shear thinned] viscosity is too low for you then you need to start out with a higher grade and let it thin down to the required value in service. If all you wanted was an explanation then you're good to go.

DOL

RE: viscosity degradation

I've heard it called shear-back, but same thing. I would think that this is more prevelant with conventional oils that have more VI improvers than high end conventionals, or even full synthetics, which don't need as much help. Another thing to look for would be contamination or high oxidation. ISZ

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!


Resources