Thinking about valve oil viscosity and slide grease migration

[gocsick]

TL/DR: You need to mix in more heavy oil than you think to thicken light valve oil an I don't think oil migration from tuning slides is an issue if you oil valves/rotors regularly.

Sorry for the wordy title... I am an academic after all

In the myriad valve oil threads a couple points have been brought up 1) Adding a few drops of mineral oil or motor oil to a thinner valve oil to make a heavy or "vintage" oil for instruments with worn valved and 2) Migration of grease/mineral oil from tuning slides to valves causing sluggish action. I am going into professor mode.. so math incoming in a few minutes

I am almost out of my Berp bio heavy oil (viscosity of 5.8 centiStokes cSt)... so I went to mix up some new stuff from lamp oil (2.5 cSt) and USP mineral oil (~40cSt). I didn't think much about it added a few drops... didn't seem to do much.. Did a proportional mixing

μ_mix =x_1μ_1+x_2μ_2=x⋅2.5+(1−x)⋅40 set μ_mix=6 my desired viscosity and got 90.7% lamp oil 9.3% mineral oil... Much more than I originally thought.. mixed it up and.... Still much too thin...

Then I remembered my polymer physics class... Flory Huggins theory... how polymer chains interact in solution.... pulled up some references and oils behave in much the same way. So that means viscosity doesn't follow an linear mixing rule but rather a logarithmic mixing rule... The physics of this is that you have an "Activation energy" associated with molecular interactions.. in this case two larger molecules like long chain paraffin oils sliding past each other..This is the mechanical energy that must be supplied for the molecules to move... If the available thermal energy per mole is RT.. R is the ideal gas constant from chemistry.... remember PV=nRT,, and T is temperature... assuming that the interactions are random (meaning our oils are not reacting or seperating in this case) we can apply some statistical thermodynamics and get that

μ ~ e^(E/RT) or the ln(μ)~E/RT

This is a common result in transport phenomena like diffusion and heat flow etc.. If the activation energy for viscosity is linearly-mixed, then the log-viscosity is linear in composition — hence the logarithmic mixing rule

ln(μ_mix)​=x_1​ln(μ_1)​+x_2(ln_μ2)

now if we plug in our desired 6cSt and the values lamp oil and mineral oil we get 68% lamp oil and 32% mineral oil by mass ... the densities are pretty close so that gives us 70.4% lamp oil and 29.6% by volume...

That is a heckin' lot of mineral oil. Mixed it up and guess what????? it is pretty close in feel to the Berp heavy oil... Color me surprised... theory and application actually match
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The practical upshot is it is easier to thin a thick grease than to thicken a light oil

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So what about migration of heavy oil or grease from tuning slides.. we need an estimate of how much oil can migrate under capilary action.. so this is highly approximate

Assuming a tuba with a 0.75" bore (1.905 cm) and a slide contact length of 6 inches (15.24 cm), here's the estimate:
Slide contact area (10% involved in migration):
~15.24 cm × π × 1.905 cm × 10% = ~9.1 cm²
Grease film thickness: 0.1 mm = 0.01 cm
Grease creep length/week: 1 mm = 0.1 cm/week
Grease density: 0.85 g/cm³

Volume=thickness×creep length×migrated area
=0.01×0.1×9.1≈0.0091 cm3

Mass=0.0091×0.85=0.00775 g=7.75 mg
Mass=0.0091×0.85=0.00775g=
7.75mg

That’s ~7.75 mg of grease migrating from one slide in one event (e.g., over several days).

For a tuba with 4–5 greased slides, that’s a total potential migration of ~30–40 mg/month if unchecked.

Typical valve oil volume in a large piston: 2–3 mL

40 mg of grease in 2.5 mL = ~1.6% by mass

That’s still well below the ~30% threshold required to significantly thicken valve oil to 6 cSt (from 2.5 cSt)

Even for a large-bore tuba, grease migration per slide is small, and regular oiling easily dilutes it. You might start to notice sluggishness only if you neglect valve oiling for weeks or overgrease your slides.

Now granted that is passive migration. what about an actuated slide???? Where you are potentially pushing grease towards the valves??

A 0.75" bore tuba slide is actuated regularly during playing (e.g., pitch compensation).

Grease is displaced 1 mm per actuation along the bore surface.

The slide is actuated hundreds of times per week (e.g., during a rehearsal-heavy period).

Even assuming 1% of visible grease is pushed into the tubing per week:

If total slide grease load is ~0.5 g, then:

Migration per week=0.01×0.5 g=5 mg

With 2–3 actuated slides (e.g., 1st/3rd), that’s 10–15 mg/week

Now we're back in the 30–50 mg/month range, but entirely from actively displaced grease — not passive migration...

So In a system with ~2.5 mL of valve oil, 50 mg of grease is 2% by mass... still not going to thicken things up too much

IF YOU OIL YOUR VALVES REGULARLY/DAILY.... you will be continually washing away or dilluting any migrated grease...

neglecting means you will have preferential evaporation... the regions without dissolved grease will evaporate first leaving behind thicker tackier oil. Which will gum up things pretty quickly..

[bloke]

Would all of that be classified is over-thinking or over as over-calculating?.

I use the thinnest oil I can manage on slides... I've tried to use lamp oil on my tuba slides, but I think they're happier with mineral oil (which is a little too thick for any valves - valves which are in good condition - but not very much too thick).

Every once in awhile I'll run the slides in and out aggressively several times each and scuff any light coating of lime or rough tarnish away simply from the aggressive in and out action, wipe them off two or three times and then I might run a clarinet swab or something of that nature down inside the outside slide tubes to remove oil suspended tarnish and lime from those as well.

This isn't science; it's just me.

These are some of the strategies I use to attempt to completely eliminate with the necessity of chem clean jobs on my own instruments.

[gocsick]

Would all of that be classified is over-thinking or over as over-calculating?.

Yes but I am an engineering professor... that is 100% the job description.

[Yahnay-san]

Another engineer here. industrial flavor. With no theoretical background whatsoever I arrived at roughly 1/3 mineral oil to 2/3 lamp oil by trial and error, with the vague notion “wow, that’s a lot of mineral oil!” Nice to see some justification for my mad science. Yeah, probably will have to muck out the valves at some point; really should be doing that regularly anyway right?

[UncleBeer]

Heh: we used to get "told off" by a traffic engineer here.

[bloke]

hmm...
Maybe I should be braggin' about my various instruments' valves.

Even the oldest one (which I bought new in 1982), mixing very much mineral oil at all with the lamp oil defines me "feeling" the valves (ie. they lag).

[MiBrassFS]

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