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Ford 9N, 2N & 8N Discussion Forum
Show Parts for Model:

Re: Need advice re ring/crank bearing replacement

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TheOldHokie

04-08-2013 08:11:19
74.110.74.117



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Quoting Removed, click Modern View to see

Couple thoughts for you to mull over:
  1. What if any engineering basis is there for that "rule of thumb"?
  2. How does an old flat head B&S lawn mower engine run happily along year after year at 3600 RPM with absolutely zero "oil pressure"?
TOH




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Bulldozer

04-09-2013 01:09:43
108.184.35.167



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 Re: Need advice re ring/crank bearing replacement in reply to TheOldHokie, 04-08-2013 08:11:19  
and genrators and irrigation pump engines at zero oil pressure with light weight crankshafts and two & four cycle outboard engines and motor cycle engines.

gravity: its the gravity load on the crankshaft.



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TheOldHokie

04-09-2013 05:50:33
74.110.74.117



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 Re: Need advice re ring/crank bearing replacement in reply to Bulldozer, 04-09-2013 01:09:43  

Bulldozer said: (quoted from post at 04:09:43 04/09/13) and genrators and irrigation pump engines at zero oil pressure with light weight crankshafts and two & four cycle outboard engines and motor cycle engines.

gravity:
its the gravity load on the crankshaft.


What keeps the journal off the bearing surface in a small engine that does not have an oil pump (e.g "zero" engine oil pressure) is the same thing that produces the oil film in an engine with an oil pump. Journal bearings operate on the principle of hydrodynamic lubrication. The oil film that separates the journal surface from the bearing surface is produced by the rotation of the journal dragging the oil inside the bearing clearances around the circumference and constricting it at the narrower load point. That constriction produces a localized pressure gradient inside the bearing that is orders of magnitude greater than oil pump pressure and is opposed to the operating load (and gravity) imposed on the journal. The basic parameters affecting the film thickness are crankshaft load, rotational surface speed of the journal, oil viscosity, and running oil clearance.
You can externally pressurize a normal journal bearing but if the crankshaft is not rotating it remains on the surface of the bearing. It simply sinks to the bottom of the evenly pressurized oil clearance area. When the engine is started and journal rotational speed begins to increase a hydrodynamic pressure gradient is formed at the load point and that pressure gradient acts to push the journal surface away from the bearing surface. The journal/bearing lubrication regime transitions from full boundary mode lubrication (metal to metal) when the rotational speed is zero, to a mixed mode (partial metal to meatl contact) as the speed begins to increase and the film is partially formed, and ideally to full hydrodynamic mode (zero metal to metal contact) when the rotational speed becomes sufficient to produce a gradient greater than the load on the journal. Teh film thickness is determiend by how much the gradient exceeds the load on the journal. Oil pump pressure is not involved beyond maintaining an oil supply adequate to replenish oil lost out the sides of the journal during operation. Small engines traditionally relied on splash lubrication for that oil supply.

TOH


This post was edited by TheOldHokie at 08:17:05 04/09/13 7 times.

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Bulldozer

04-09-2013 08:23:26
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 Re: Need advice re ring/crank bearing replacement in reply to TheOldHokie, 04-09-2013 05:50:33  
Always understood that most of the journal wear occurs at start up rather than at normal operating conditions.

Ok, so why does the flatheat N have the 0-50 psig oil pressure gauge and the OHV NAA have the 0-80 psig oil pressure gauge?

Both engines have equal rated 2000 rpm and nearly equal 6.6/6.7 CR after 85000 serial #.
The oil pump capacity was increased at some point on the N series?

Larger capacity oil pump would increase the back pressure at the journal at equal RPM?

Maybe the larger capcity pump was incorporated to increase the longevity of the engine and not to increase the oil pressure on a new or new rebuilt engine?

So what is the concern about oil back pressure, since the oil pressure is not in the equation for oil film thickness?

In my opinion the N series engine needs to have minimum of 15 psig oil pressure @ 1500 rpm.

Maybe the NAA needs more oil back pressure than the N, since the gauge has a greater range or with a larger capacity pump than the N, the oil pressure is higher.

The tire mfg's are always researching & testing to come up with the ultimate thread pattern to reduce the chance of hydroplaning, which creates a thin film of water between the tire and road in heavy rain conditions. The water film supports the "gravity" weight of the vehicle, which is basically the same physics as the engine journal. Michelin Tire has a great video on U-Tube.

Oil pressure issue seems to be one of the most frequent FAQs on the N forums.

AKA "raven" on the NTC forum

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TheOldHokie

04-09-2013 09:56:53
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 Re: Need advice re ring/crank bearing replacement in reply to Bulldozer, 04-09-2013 08:23:26  
Always understood that most of the journal wear occurs at start up rather than at normal operating conditions.



That is normally the case since that is the transition from boundary to hydrostatic regime. Luging an engine alsl contributes to accelerated wear since increased load at lower RPM equates to a reduced oil film thickness and mixed mode lubrication.

Ok, so why does the flatheat N have the 0-50 psig oil pressure gauge and the OHV NAA have the 0-80 psig oil pressure gauge?

WAG - the engineers wanted a higher pumping pressure and increased flow rate to accomodate the greater demands of the OHV design. A flathead doesn't need top end lubrication but an OHV does and it needs it nearly immediately on startup. The oil pressure gauge on my 1963 Triumph equipped with a similar size and vintage Standard engine is 100 PSI and the Leyland specification is a minimum idle hot oil pressure of 30 PSI hot. With an OHV you have to move more oil, move it further, and get it there faster. An OHC is even more demanding in that regard.

So what is the concern about oil back pressure, since the oil pressure is not in the equation for oil film thickness?

It is a easily collected performance metric. The issues I just mentioned are part of that performance but pump oil pressure is also a very good indirect measure of bearing clearances. Bearing clearances ARE a factor in oil film thickness and a low pumping pressure indicates increased clearances and a reduction/loss of the protective oil film.

In my opinion the N series engine needs to have minimum of 15 psig oil pressure @ 1500 rpm.

A fresh engine with good oil clearances will easily make 40 PSI at 1500 RPM. I would say that an engine with 15 PSI @ 1500 RPM is telling you it has wide clearances and/or a worn oil pump and is nearing the end of it's service life. It's those widened clearances that will cause accelerated engine wear - not the low oil pressure.

TOH

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