[modeleng] Re: Lubricating oils

G'Day Jesse,

I'm glad that at least one person enjoyed my rant on bearings. I hope all 
will forgive my typo's and misspelt words, as my spell checker in Outlook 
Express has died again, and this time I'm not going to waste my time trying 
to fix it.

As for being refered to as "Dr Phill".... I think I have seen a couple of 
minutes of that show, and don't know if it is a complement or not.... 
<LAUGH>

Ok, lets answer your questions...

Sintered bronze bearings came about as a solution to an age old problem. The 
problem is, lubricating bearings that are damn near imposible to get to, and 
are imposible to set up remote oiling for. One of the most obvious 
situations is for the spigot bearing mounted in the flywheel of your car's 
engine. The sucess of the sintered bronze bearing is because it is porous. 
It is made by granulating bronze to a small size, then putting it into a 
former and heating it only just enough to melt the surfaces of the granuels. 
This enables them to bond together and still retain it's porous nature. The 
porousity enables the bearing to retain oil for lubrication, for very long 
periods of time. Ten to fifteen years is not not unusual an unusual life for 
a sintered bronze bush. The porous nature of the sintered structure also 
permits the foreign bodies to be lodged out of the way from being in contact 
with the shaft.
The problems with sintered bronze are as large as the benefits. It is very 
soft, so is easily deformed, when compared to a normal plain bronze bush. 
Once they start to compress, the access for the lubricant is restricted, and 
they start to wear dramatically due to both lack of lubrication, and, the 
embedded foreign bodies are no longer tucked away, but are now protruding. 
The softness of these bearings limits the loads that can be placed upon them 
to less than a 1/3 of that permisible with a standard plain bearing. The 
other problem with sintered metals, is that the surface of every granual has 
an oxide layer. This layer, in the harder sintered bronze materials, can 
cause bluntening of your tooling if you try to machine it. They also require 
hardened steel pins/shafts to be used, otherwise the oxides will wear the 
shaft.
Sintered metals are not designed to be machined. This is because the forces 
involved in machining cause compression of the grain structure and smearing 
of the grains across the pores. If you must machine it, use very sharp 
tooling with a positive rake with light cuts and fine feeds, as this will 
help reduce the problems caused by machining.

Next question..... Anyone? <GRIN>


Cheers,

Phill.


----- Original Message ----- 
From: "Jesse Livingston" <fernj1@xxxxxxxxxxx>
To: <modeleng@xxxxxxxxxxxxx>
Sent: Friday, June 05, 2009 2:09 AM
Subject: [modeleng] Re: Lubricating oils


> Blimey Dr.Phill,
>
> That was a good lecture on bearing materials and I enjoyed and was
> intellectualized by it .  I assume that by PB you mean phosphor bronze and
> not powdered bronze like "Oilite®".  Sometimes I worry about the powdered
> "sintered" bronze bushes I have used as the material seems to dull HSS 
> tool
> bits for some reason.  I know it holds oil well , but still I wonder if it
> is not abrasive to some extent.  You left that material out of your 
> lecture
> professor so would you mind elaborating on the subject?
>
> Oh, congratulations on having the entire set of Audel's Mechanics and
> Engineers Guide from 1921.  My set shows considerable wear as my father
> probably used them when he was attending Mississippi State A & M college,
> plus I use them extensively when designing new projects for magazine
> articles.  The covers are in pretty fair shape, but the index pages are
> loose in some volumes.
>
> Jesse in still drizzly Troy, TN USA
>
> Jesse,
>
> I picked up all 8 books as a set when I purchased them on Ebay, and in
> perfect condition. I did notice later, that they showed up on ebay as
> individuals, but would have been expensive to buy like that.
>
> As for oil draining from stationary bearings...... Yes you, and Audel, are
> absolutely correct. But the drainage rate is dependant on 3 factors.
> Temperature of the oil, size of the drainage oriface, and, time. In our
> close tollerance bearings, compared with a 6" bearing with a free running
> fit giving (for heavy loads) of 10 thou' clearance, drainage is very slow
> and is unlikely to lose enough oil to stop boundary film lubrication in 
> the
> time it takes to refuel, unload passengers, take on water, chat up the
> passenger kid's aunty etc. Yes, I was terrible when I was single. I had a
> built in compass the would point me at every single woman within 100
> yards.....
>
> As most of you will know (from reading this lists postings) that I am
> getting ready to build a Juliet as a teaching instrument for my eldest son
> as he builds one. Going through the original articles in the ME magazine,
> has shown several areas of poor engineering. This is in the area of 
> bearing
> surfaces and materials. LBSC wrote the articles for the complete novice, 
> so
> I was not expecting high tech bearings. Or was I? Perhaps I was, as why 
> else
> am I going to change them. Things like crank pins, eccentrics, cross head
> guides, expansion links, die blocks, and valve gear pins, will now be
> hardened and polished high tensile steel. Modern hardening steels are
> relatively inexpensive (compared to LBSC's time), easily obtainable, and
> easily heat treated at home. The advantages of making these small changes,
> show up over the long term as they will wear (as will their matching
> componants) at around 10%, or less, of those materials originally 
> selected.
> This works because the difference in the materials becomes greater. Look 
> at
> the bearings in your cars engine. They are really soft compared to the 
> crank
> shaft. An even better example is the cam shaft and it's bearings. Hardened
> steel on (what is essentially) white metal bearings. Your camshaft wears 
> at
> a much lower rate than the crank shaft, and not just because it is turning
> slower, but because the difference between the dissimilar metals is 
> greater.
> The use of a bronze bush, is to be able to carry a larger load on the
> bearing than a softer material can accomodate without deformation, at the
> same size. The softer material will give you a better bearing, but must be
> much larger to take the same load without going out of round. GM will work
> Ok on polished mild steel pins/shafts etc, just.... PB is designed for
> polished hardened steel pins ONLY. It will destroy a mild steel pin at the
> same time as wearing itself. But when used on a really hard material, will
> last a very long time. GM also prefers hardened steel pins.
>
> Bearing wear occurs due to 2 main factors.
> 1) bearing being overloaded and deforming. This happens to the softer of 
> the
> 2 materials first. The case 2 takes over.
> 2) foreign bodies. Foreign bodies can be any thing from dirt, soot, dust, 
> to
> bearing material particals. These items behave in 2 ways. Firstly, if they
> are small and/or sharp enough (dust, soot, and bearing fragments) they
> become embedded in the softer material of the bearing and then act as a 
> lap
> on the harder surface. This then accellorates the process as it removes 
> more
> of the harder material, which in turn also becomes embedded in the softer
> material and hence wears away the pin/shaft. Secondly, if they are large
> and/or blunt they act as grinding paste and wear away the softer material 
> at
> a faster rate than they do the hard material.
>
> This is why really hard pins/shafts with PB bushes is favoured as a plain
> bearing. The PB is hard and strong enough to withstand high loads and some
> abbrasion. While the really hard pin/shaft is highly resistant to lapping
> when polished. If unpolished, small pieces will be torn from the surface 
> of
> the pin/shaft and be embedded into the PB to ask as a lap.
>
> There is one last factor for the selection of bronze on hardened steel....
> Coefficient of friction. Bronze has a very low coefficient of friction, 
> and
> so has hardened and polished steel.
>
>
> Oh dear..... It looks like I digressed a bit here..... Sorry Gents. I seem
> to have digressed to giving the lecture "bearings 102", so I'll leave it
> there.
>
>
> Cheers,
>
> Phill.
>
>
>
>
> ----- Original Message ----- From: "Jesse Livingston" <fernj1@xxxxxxxxxxx>
> To: <modeleng@xxxxxxxxxxxxx> Sent: Thursday, June 04, 2009 9:27 PM 
> Subject:
> [modeleng] Re: Lubricating oils
>
>
> As I am sure our resident ME Phill knows, oil will drain from a stationary
> bearing or at least that is what my 1921 Audel's manual says.
>
> Phill, did you ever get the entire 8 books in that Audel's series you 
> found
> on ebay?  If you are only missing #8, don't worry as it deals with
> electricity and not steam.
>
> Jesse in rainy Troy, TN USA
>
> Ron,
>
> With really small bearings, you can take advantage of the excess 15W40. 
> The
> basic rule is, the bigger the bearing (diameter) the thicker the oil. The
> smaller the bearing the thinner the oil. This is because the surface 
> speeds
> are generally in the same range, but the clearances are shrinking as the
> bearing decreases in size. As the clearances decrease, and the RPM 
> increases
> to maintain the surface speed, the shear stress of the oil becomes 
> greater,
> and hence the pwer being absorbed also increases. This is counteracted to
> some degree by the heat generated by this process, which thins the oil, 
> but
> not enough unless it is over heated. If the bearing is turning slowly, 
> then
> you are often better of using a thicker oil. Very few 3.5" and 5" gauge
> loco's are driven at scale RPM. This is partly due to the scale effect of
> miniaturization. Basically, properties don't change, just the quantities.
> Therefore, things like the expansion rate of steam is the same, regardless
> of the size of the engine. THEORETICALLY, a miniature staem loco, can go
> just as fast as the prototype. Unfortunately, they can't due to ballancing
> dynamics, mass stability, etc. etc. They just fly of the track, before 
> they
> come anywhere near reaching thier potential. Which brings us back to the 
> RPM
> of operation. At 3/4" scale, the scale opperating speed is 1/16 of the 
> full
> size. So assuming you are racing around the track at 5 mile an hour, which
> is pretty typical for what I have seen, then you are doing a scale speed 
> of
> 90 miles an hour..... Hmmmm..... I doubt whether many full sized shunting
> engines did that speed...... Full size speed here, was 50 miles per hour 
> for
> goods services. That makes a scale speed of 3.125 miles per hour. That's a
> pretty slow walking speed.... If you are operating at full size RPM,
> approximately 300 RPM, then you are better of using a thicker oil than a
> thin one. The boundry film pressures are lower at 300 RPM than at 600 RPM.
> and hence the thinner oil used for 600 RPM will not generate enough 
> pressure
> in the boundary layer to keep the 2 components apart. That's when wear
> starts.....
>
> Slideway oil is good stuff, on your machinery. But, not always for
> everything else. Take the tacking agent for example. It "tacks" the oil,
> only after it has stood still for a while. It has no useful effect while 
> the
> bearings are moving. If you let it sit and "tack off" and then start 
> moving
> the bearing again, it imeadiately reverts to it's normal liquid nature 
> until
> it stand still for a while again. So the tacking agent can make clean-up
> after running harder. But the other aditives are excellent, for our use, 
> as
> long as they don't get to hot. I know that most of the tacking agent don't
> like heat.
>
> I hope that gives you all more food for thought.
>
>
> Cheers,
>
> Phill.
>
>
> Re: Lubricating oils
>
>
> Some interesting replies so far! The preference seems to lean towards
> heavier oils, perhaps with a tackiness additive.  I was surprised to hear
> that some folk use steam oil for everything!
>
> I've always been reluctant to use motor oil in anything other than a car
> engine, as I don't know what effect the detergents have on bronzes etc. 
> Mind
> you, my current car is a diesel which uses the most expensive synthetic 
> oil
> imaginable
>
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