PV ratings for bearings - peak or average load?

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Anne from Little Britan
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PV ratings for bearings - peak or average load?

Post by csonics » Wed Nov 18, 2009 4:23 pm

Posted on behalf of barts:

barts
Lighting the Boiler


Joined: 02 May 2009
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Location: Menlo Park, CA
Posted: Sat Oct 10, 2009 9:28 pm Post subject: PV ratings for bearings - peak or average load?
Hi folks -

As part of my further reworking of my 2.5"x2.5" single, I'm considering boring out the (somewhat sloppy) bronze bearing on the big end of the connecting rod and lining it w/ tin babbitt. I ran some numbers and discovered that surface speed is only about 100 sf ft/min, but the peak pressure on the connecting rod is on the order of 800 lbs, which maps to about 800 psi bearing load. This is well within the peak pressure and speed ratings for tin babbitt, which I understand to be 1500 psi and 1200 sf ft/min - but if I use the product of 800 * 100 , babbitt isn't suitable as its PV rating is on the order of 30000. Are PV ratings supposed to use the average (over a revolution) load or the peak?

I'd like to use babbitt as it's easily renewed... I'm also working out how to babbitt the eccentric straps, which apparently have been taken up many times in the last 110 years and are really getting too thin to continue running directly on the iron eccentrics.... since the engine has a piston valve, the loads are low and I don't expect problems.

- Bart
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mcandrew1894
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Posted: Sun Oct 11, 2009 2:44 am Post subject:
Hi Bart,

The peak pressure which would be full boiler pressure is what I used on my engine, as "ideally" you have that until the point of cut-off, which for conversational purposes I will assume is something like 75% or longer on your engine.

1200 psi sounds high to me. My reference which was from GE, showed more like 400-500 psi for babbitt conrod bearings with drip feed. That is working very well for me at the moment, but then again, I am running bronze

If I assume your engine has a 1" journal and is 1" wide, and I back calculate from that projected area, your running 150+ psig?

That engine just may not be happy at that pressure as most singles run at lower pressure, especially the older ones. You may want to put an oilpump feed on your conrod bearing.
The little ratchet locomotive type work well for this and mine has been great as getting drip feed oil on a journal at 600 rpm is more oil covered acrobatic torture than lubrication. The ratchet pump solves this problem and is automatic and consistent.

It sounds tough, but SteamboatJack set me straight and it works great and is easy to work on and maintain. Check out "Grayling" page. No crank cross drilling required!

I haven't done babbit Bart, but as I understand it, tinning the material first is key.

Do you have any pictures of the patient?

Hope that helps,

Dave

Last edited by mcandrew1894 on Sun Oct 11, 2009 3:33 am; edited 3 times in total
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mcandrew1894
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Posted: Sun Oct 11, 2009 3:12 am Post subject:
A reference.

http://www.edtcorp.com/b.designassistan ... lation.pdf


This is another good reference.

http://www.dmr-usa.com/pdfs/190_DMR_Fib ... es__01.pdf

If this appears correct...your running about 400 rpm @ 150 psig? which is consistent with your post.

Dave
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DetroiTug
Warming the Engine


Joined: 19 Jul 2009
Posts: 73
Location: Outside Detroit
Posted: Sun Oct 11, 2009 10:31 am Post subject:
Hi Bart,

It may just be the bronze that is in there now. If it is just standard bronze or sintered, try some ampco bronze if it is not already. Material is much stronger and resistant to wear.

(Thoughts on bearings)

I've never been keen on the idea of installing roller or needle bearings in applications where the bearing never makes a full rotation, chucking back and forth a few degrees, like we see in some of the rod eyes. I've seen many bearings fail prematurely in that type of application, and generally just a replacement of ampco or similar has a longer service life. The only comparison and reasoning I can draw is the principle of the "hunting tooth" in the mesh of gears. There must be some variations and reorganization for components to wear evenly, seat, lubricate one another etc.

Just my opinion on it.

-Ron
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barts
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Joined: 02 May 2009
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Location: Menlo Park, CA
Posted: Sun Oct 11, 2009 12:21 pm Post subject:
Thanks guys - that helps a lot.

Yes, I'm running 150+ psi at 400+ rpm, and the con rod is 1" dia. x 1" width... the mains are amply sized, but the the crankpin.... I realize that 150 psi is severe duty, but I'd like design for that point if I can. Certainly, reducing steam pressure will help - but a 19' boat of perhaps 1800 lbs displacement loaded does need a couple of hp to make reasonable progress, and with this size engine that means steam pressure. The engine isn't very well balanced, so the very high rpm route to hp seems out. The rest of the engine seems quite happy w/ the loads; I'm pleasantly surprised as to how much less fuss and strain the piston valve places on the valve gear.

I'm considering better lube; the "catch as catch can" drip system seems quite adept at slinging oil into the sump underneath the engine, but little else. When I bought the engine, it was a box of parts; most of the wear surfaces showed the effects of rather casual lubrication. Given what I'm seeing, the problems may have been more w/ the design of the engine's lube system than sloth on the part of the previous engineer...

DetroiTug, I understand the concerns regarding rocking motion on needle bearings, but they do survive this rather nicely on universal joints on cars, so I'm inclined to give them a try. You suggest Ampco Bronze for the connecting rod - I assume this is one of their aluminum bronzes? Aluminum bronze does have the PV (125,000) rating to survive in this application.

You can see some pictures of the Otter and her new engine (not yet finished, but running well enough to steam for several hours) here: http://www.smaalders.net/barbie/Travels ... index.html
I need to take some more detailed pictures of the various parts and post those....

This is our home server over our DSL line, so picture loads may not be very fast....

- Bart
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DetroiTug
Warming the Engine


Joined: 19 Jul 2009
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Location: Outside Detroit
Posted: Sun Oct 11, 2009 1:05 pm Post subject:
"but they do survive this rather nicely on universal joints on cars, "

True. However driveshafts/drivetrains are often designed to where there is almost perfect alignment under typical vehicle loading. Where short service life is exampled more profoundly is when that geometry is altered. As in the case of four wheel drive trucks and "lifting" the suspension. Throwing this alignment out of some acceptable range of service. The needle bearing life appears to be much shorter. Again, this is just based on observation over the years.

That's a nice engine you have in the Otter. Must be a thrill to have some original iron spinning the prop.

-Ron
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mcandrew1894
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Posted: Sun Oct 11, 2009 6:11 pm Post subject:
Hi Guys,

Any bearing mis-applied won't last long, needle, plain or otherwise.

My needle bearings have been in for 10 years without trouble.

Ray Hasbrouck uses them in his launch engines and my friend Will's boat has had one in for 20 years.....

Horses for courses.....

Bart, if your crank needs weight and your looking for better speed and load carrying capacity, could you just make a new crank with the right weight and a hardened crankpin?.....with a good bronze, it would probably outlive you.
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87gn@tahoe
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Location: South Lake Tahoe, CA
Posted: Sun Oct 11, 2009 6:19 pm Post subject:
Bart, is there room to fit "slingers" from the main bearing ala Simpson Strickland?

Seems that Mr. Grosjean had some big-end beairing lubrication issues, until he fitted slingers to the engne... now it just keeps humming along

(bearings are babbit liners about 1.25"-1.5" x 1.25"-1.5" (not 100% sure on dimentions, may be smaller), max 220psi on safety, normally 150-175 psi @ 850rpm)

wes
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mcandrew1894
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Posted: Sun Oct 11, 2009 6:38 pm Post subject:
As yes, that 850 rpm is key!...that is whole reason a slinger works.

I calculated the pressure a the biggest slinger would give me and it was maybe .5 to 1.0 psi at 600 rpm. But it would seem to be enough.

These engines really do want to run forever, and simple consistent oiling is key...however you get it in there.


Dave
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barts
Lighting the Boiler


Joined: 02 May 2009
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Location: Menlo Park, CA
Posted: Mon Oct 12, 2009 9:40 pm Post subject:
mcandrew1894 wrote:

Bart, if your crank needs weight and your looking for better speed and load carrying capacity, could you just make a new crank with the right weight and a hardened crankpin?.....with a good bronze, it would probably outlive you.


I've been mulling this over for a while, as it would also allow me to increase the size slightly of the mains; this would let me line bore the engine once, and then fit bronze bushings w/ babbitt linings from then on. Right now the mains are actually worn down by some .005 or more than the rest of the crankshaft.

I can see how to turn a crank from solid, particularly w/ bolted on counterweights - the thought of turning a 6" diameter round into a new crank is a little intimidating- but how to harden the center crankpin? Or would a press-together construction be better?

- Bart
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DetroiTug
Warming the Engine


Joined: 19 Jul 2009
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Location: Outside Detroit
Posted: Mon Oct 12, 2009 11:22 pm Post subject:
This is how the twins crank assembles at the center, it should work for a single disc crank as well. 1.0" dia fit into 1.0" dia reamed hole. 1/4" tight key ways, and 1/2-13 set screw with another for backup in the disc. Just a piece of plate and some 4140 shafting. As long as everything fits tight it should handle more horsepower than required.

The two stub assemblies are pressed together, but I have no idea about the worthiness of those joints. they look as though they were heated etc, and the lathe work was pretty nice. Hopefully they are proper. Have a feeling these were done by Pop Arnold himself as the original owner had a few parts machined by Tiny Power at the time it was purchased new.

I've never quite understood what they were trying to do, the rear stub assembly was way too long and had threads on the end of the shaft and threads in the disc. Not favoring threads for a joint like that, I spun it down and did the reamed hole with the key instead.

-Ron

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mcandrew1894
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Posted: Tue Oct 13, 2009 3:48 am Post subject:
Hi Bart,

I was thinking of a multipiece crank like Ron's with a pressed in throw made from a hardened dowl pin. The reciprocating bearing loads are higher there.

I would do this if you can't get room in there for more pin area through either bigger diameter, width or both....if you can, save the trouble


Dave
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DetroiTug
Warming the Engine


Joined: 19 Jul 2009
Posts: 73
Location: Outside Detroit
Posted: Tue Oct 13, 2009 7:43 am Post subject:
The Hasbrouck single uses a big chamfer on the hole and 90 included taper on the shaft. Then a ring of weld to secure. The shaft and throw are both that way if I remember correctly That route, one would be relegated to mild steels it seems. Tool steels can be difficult to weld.

-Ron
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barts
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Joined: 02 May 2009
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Location: Menlo Park, CA
Posted: Tue Oct 13, 2009 9:23 am Post subject:
mcandrew1894 wrote:
Hi Bart,

I was thinking of a multipiece crank like Ron's with a pressed in throw made from a hardened dowl pin. The reciprocating bearing loads are higher there.

I would do this if you can't get room in there for more pin area through either bigger diameter, width or both....if you can, save the trouble


Dave


This is one of those over-constrained problems... the existing crankwebs are only about 7/16" thick... I can prob. sneak that up to 9/16" or so, which would be likely wide enough to hold a pressed-in crankpin securely. There really isn't width available to also make the crankpin much wider, and increasing the diameter won't help since it increases the SFM just as much as it drops the pressure/unit area. A pressed in dowel pin would be about
C47; hard enough to run w/ aluminum bronze... but the improvements here are marginal; it's not as if I can decrease bearing pressures by 50% or more; the space constraints basically force the design. I could achieve better balance w/ bigger counter weights on the crank, though.

I spent some time this morning reading some engineering design texts from the 1950s and running a bunch of numbers;; as you posted earlier the lubrication seems to be the key. If I can keep the bearing fully oiled at all times (e.g. not w/ a intermittent drip as built!) the PV limits matter only at startup; once the engine is turning at speed, it's a hydrodynamic bearing and the shaft is supported entirely on a film of oil. This is what you pointed out in your earlier post regarding lubrication, of course...

It seems as if the right approach here is to re-babbitt the mains, rebore the connecting rod big end to achieve a .002 overall clearance w/ the crankpin, and polish the crankpin as much as possible (first!). I'll fit a Torrington needle roller bearing at the wristpin, and arrange for pumped lubrication of the mains and con-rod bearings w/ some nylon tubing as per Grayling and your engine....

What kind of oil consumption do you see w/ pumped lubrication? I can always flush the sump into the firebox if needed...

Thanks for all the ideas!

- Bart
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Bart Smaalders
S.L Otter
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mcandrew1894
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Posted: Tue Oct 13, 2009 8:54 pm Post subject:
Working on it Bart....trying to get dimensions for the pump

I purchased it from American Model Engine Supply

I run 1 click per rev, but that doesn't tell you much.....I'll get back to you tomorrow with some numbers


It runs great by the way

Dave

mcandrew1894
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Posted: Wed Oct 14, 2009 5:37 pm Post subject:
Hi Bart,

IT is officially 1.5 - 2 drops per rev of the pump.

Not a technical rating to be sure....but here are some observations..shy of me tearing the pump apart and measuring the ram.

1 full revolution of the pump will pump approximately .75" down the teflon tubing. This tubing has an ID of approximately .0625"

That works out to .0023 cubic inches per revolution of the pump...approximately

Each one of my journals (3/4 diameter x 1.25 wide) takes 1 click of the pump ratchet wheel per engine rev. IIRC the ratchet wheel is 48 tooth, and a quick view of the pump on American's web site leads me to believe that is correct, then the journals of my engine use

.0023/48 = .000479 cubic inches / engine rev/crankpin journal.

At 600 rpm, that is .028 cubic inches /minute or 1.72 cubic inches /hour /journal.

3.45 cubic inches / hour total on the rod journals alone. I have the two pump unit (ML5)

I am using PB&J from GreenVelvet 50W non-detergent.

One oilsorb pad easily soaks up 3 hours worth of oil in the engine bilge. It really makes engine cleanup easy. This includes main bearings and crosshead drip which comes from the big drip feed lubricator.

I would experiment with low volumes as this pump will produce as much as 1000 psi if allowed to. That will blow lines on the pushloc fittings.

Ron does not recommend running the pump at 1 click or at high speed and he tried talking me out of using it this way, but I can say that this pump has performed very well doing just that and I am very happy with it.

It is very well built, and easy to use

Hope that helps!

Dave
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