Thanks to the N experts-We now have oil pressure!!

Welding man

Well-known Member
Location
West Virginia
Yesterday I posted about an 8N that I was working on that kept loosing oil pressure after sitting and would not pick it back up without priming. Thanks to posts from cowdog-HOBO and The Old Hokie and others, a good night sleep and a little measuring-machining and reassembly we now have oil pressure and it acts like it is going to live. First off cowdog said that he had bought a kit not quiet up to specs. I looked at the new gears and they seemed to have more backlash than I thought they should.I took them out miced the width and they were .010 narrower than the originals. Remember the originals wouldn't pump either. I rechecked the housing and could not detect any wear at all.I looked through my inventory and found a set of Ford NOS 3/4 in gears. I set them up in the mill and cut them down to .005 more than the original set.Put them in the pump tightened everything up and I couldn't turn the pump. Took the gears back out lapped them a couple of thousands and put it back together. Everything was nice and smooth. Poured a little oil in it, put the pump back on fired it up and the oil pressure went to 30 Lbs. at mid throttle 18 at idle. Finished the assembly, took her out for a ride and let it get good and warm. It now has 15 at idle and 35 at full throttle. Not great but much better than 0. Thanks for everyone's help. Goes to show 2 or 3 heads are better than one.
 
(quoted from post at 20:22:45 10/06/17) Yesterday I posted about an 8N that I was working on that kept loosing oil pressure after sitting and would not pick it back up without priming. Thanks to posts from cowdog-HOBO and The Old Hokie and others, a good night sleep and a little measuring-machining and reassembly we now have oil pressure and it acts like it is going to live. First off cowdog said that he had bought a kit not quiet up to specs. I looked at the new gears and they seemed to have more backlash than I thought they should.I took them out miced the width and they were .010 narrower than the originals. Remember the originals wouldn't pump either. I rechecked the housing and could not detect any wear at all.I looked through my inventory and found a set of Ford NOS 3/4 in gears. I set them up in the mill and cut them down to .005 more than the original set.Put them in the pump tightened everything up and I couldn't turn the pump. Took the gears back out lapped them a couple of thousands and put it back together. Everything was nice and smooth. Poured a little oil in it, put the pump back on fired it up and the oil pressure went to 30 Lbs. at mid throttle 18 at idle. Finished the assembly, took her out for a ride and let it get good and warm. It now has 15 at idle and 35 at full throttle. Not great but much better than 0. Thanks for everyone's help. Goes to show 2 or 3 heads are better than one.

If I understand your procedure that would make end clearance rather than housing wear the culprit.

TOH
 
15 at idle good and warmed up is
alright. There are some better, but
15 is ok of you consider where they
are measuring the pressure. Main
thing is it "IS" pressure and it
keeps its prime. Very glad I could
help you out!
 
It certainly appears that end clearance does play a big part in it,but the Ford gears did mesh deeper and have less backlash.The gears were soft and machined easily.
 
(quoted from post at 08:21:50 10/07/17) It certainly appears that end clearance does play a big part in it,but the Ford gears did mesh deeper and have less backlash.The gears were soft and machined easily.

I don't think gear lash is an issue. The pumping action is created by oil trapped between the teeth and the housing wall being pulled around the OD of the gears. Tip clearance is the principal issue there yet we continue to see pumps fail to maintain prime even though the appear to have good tip clearance.

That said I think you are really on to something here. The pump housing "floor" beneath the gears is also a wear area and it rather than housing walls may be the source of the by-passing problem. By cutting down the wider 3/4" gears to make them .005 wider than the new 9/16" gears you created an oversize 9/16" gear and closed up some of that end gap wear. That was a clever idea and well done. I'll have to give it a hard examination on the next pump rebuild I get in (should be coming in the mail any day now). That gentleman is having the same sort of issues you had after having his pump rebuilt and I'll give the end clearances a good hard look when the pump gets here.

TOH
 

How can something so simple be so complicated. Wait I know its to easy to condemn it and throw another on after all its just a oil pump... I will admit it can easily get by you that's why I open up all new pumps that don't mean I know what I am looking it but I am gonna look check it out and prime it...

In the mid 80's I built a ford 302 it had oil pressures problems I did not see a issue with the new republic pump I replaced it with another one had the same problem. Being hard headed I put the old original pump back on problem solved. It was a van the engine had to come out are by this time it was easier to just pull the engine instead of dealing with the oil pan while the engine was still in the van. The new pumps were sent back to Republic and to my surprise they confirmed the pumps were defective. They paid $800 for my troubles tho it was not enoufh I wanted $1200... From that point on I never looked at oil pumps as a plug and play item again... I had to come up with a way to determine what direction I needed to go...

Thanks for the follow up and to those that did not have any real diagnostic value to the post and confuse the issue...

Good Job!
 
(quoted from post at 14:11:52 10/07/17)
(quoted from post at 08:21:50 10/07/17)

I don't think gear lash is an issue. The pumping action is created by oil trapped between the teeth and the housing wall being pulled around the OD of the gears. Tip clearance is the principal issue there yet we continue to see pumps fail to maintain prime even though the appear to have good tip clearance.
TOH

I respectfully disagree. Gear lash is every bit as important as the tip clearance. Look at this pump drawing and follow the oil.

lowoilpressure3.jpg


As the gear teeth mesh in the center they form a seal. If there is more than a minimum of gear lash there, it makes an opening that oil under pressure can pass back thru, which results in the oil going in a circle instead of being pushed out the pressure port. The gear tips carry the oil around the sides and attempt to squeeze it out the pressure side, but that attempt fails if there is no seal between the gear teeth to stop it from going back to the suction side. Extra gear lash also makes an opening so that the pump can't "suck" the oil into the inlet. That suction also relies on a good seal between the gear teeth. Most loss off prime issues are from excess gear lash (including gear end play). There's probably another picture somewhere that shows the oil going back thru the center due to excessive lash, but I think you can see by looking at this one what the result of too much gear lash would do. Low or no pressure, low or no suction, loss of prime.

.
 
(quoted from post at 14:34:41 10/07/17)
(quoted from post at 14:11:52 10/07/17)
(quoted from post at 08:21:50 10/07/17)

I don't think gear lash is an issue. The pumping action is created by oil trapped between the teeth and the housing wall being pulled around the OD of the gears. Tip clearance is the principal issue there yet we continue to see pumps fail to maintain prime even though the appear to have good tip clearance.
TOH

I respectfully disagree. Gear lash is every bit as important as the tip clearance. Look at this pump drawing and follow the oil.

lowoilpressure3.jpg


As the gear teeth mesh in the center they form a seal. If there is more than a minimum of gear lash there, it makes an opening that oil under pressure can pass back thru, which results in the oil going in a circle instead of being pushed out the pressure port. The gear tips carry the oil around the sides and attempt to squeeze it out the pressure side, but that attempt fails if there is no seal between the gear teeth to stop it from going back to the suction side. Extra gear lash also makes an opening so that the pump can't "suck" the oil into the inlet. That suction also relies on a good seal between the gear teeth. Most loss off prime issues are from excess gear lash (including gear end play). There's probably another picture somewhere that shows the oil going back thru the center due to excessive lash, but I think you can see by looking at this one what the result of too much gear lash would do. Low or no pressure, low or no suction, loss of prime.

.

My analysis is a bit different and here is how I would describe it.

Involute gears are made with a root clearance defined by any number of different tooth form standards - ANSI/AGMA being a major one. There is an intentional [b:1a3d1c68ba]clearance[/b:1a3d1c68ba] left between the tip of a tooth and the root of the meshing gear.

sgw_profile.gif


Gear lash is the measure of free play between the face of two meshed gears and is not a part of the tooth form standard. It is typically measured and/or specified at the pitch circle:

220px-Backlash.svg.png


Backlash is a necessary component of almost all gear designs:

[b:1a3d1c68ba]Wikipedia[/b:1a3d1c68ba]

[i:1a3d1c68ba]Depending on the application, backlash may or may not be desirable. It is unavoidable for nearly all reversing mechanical couplings, although its effects can be negated or compensated for. In many applications, the theoretical ideal would be zero backlash, but in actual practice some backlash must be allowed to prevent jamming. Reasons for the presence of backlash include allowing for lubrication, manufacturing errors, deflection under load, and thermal expansion.[/i:1a3d1c68ba]

When designing gears and gear sets the easiest way to control backlash is with gear center spacing. The further you widen the center spacing on the gears the more you increase the lash. Moving the centers closer together reduces lash. But no matter how close you put them together, even to the point of binding, you cannot eliminate that root clearance which is there as part of the tooth form standard.

So what does all of that mean inside a gear pump? You measure gear lash by immobilizing one gear and measuring the free movement of the second one. When the gears are engaged and operating at RPM the driver tooth is fully pressed against the driven tooth and the gear lash (clearance) is all on the back side of the meshed teeth. As the gears rotate the meshed teeth are actually sliding against each other along the involute curvature of their faces which are in full contact for the entire time the teeth are engaged. In a gear pump oil is not going to bypass rearward through the engaged teeth of the operating gear set regardless of any (reasonable) amount of backlash they may have when not under load. Oil can and will easily bypass through excessive housing end or tip clearances.

TOH
 
(quoted from post at 21:29:13 10/07/17)
(quoted from post at 14:34:41 10/07/17)
(quoted from post at 14:11:52 10/07/17)
(quoted from post at 08:21:50 10/07/17)

I don't think gear lash is an issue. The pumping action is created by oil trapped between the teeth and the housing wall being pulled around the OD of the gears. Tip clearance is the principal issue there yet we continue to see pumps fail to maintain prime even though the appear to have good tip clearance.
TOH

I respectfully disagree. Gear lash is every bit as important as the tip clearance. Look at this pump drawing and follow the oil.

lowoilpressure3.jpg


As the gear teeth mesh in the center they form a seal. If there is more than a minimum of gear lash there, it makes an opening that oil under pressure can pass back thru, which results in the oil going in a circle instead of being pushed out the pressure port. The gear tips carry the oil around the sides and attempt to squeeze it out the pressure side, but that attempt fails if there is no seal between the gear teeth to stop it from going back to the suction side. Extra gear lash also makes an opening so that the pump can't "suck" the oil into the inlet. That suction also relies on a good seal between the gear teeth. Most loss off prime issues are from excess gear lash (including gear end play). There's probably another picture somewhere that shows the oil going back thru the center due to excessive lash, but I think you can see by looking at this one what the result of too much gear lash would do. Low or no pressure, low or no suction, loss of prime.

.

My analysis is a bit different and here is how I would describe it.

Involute gears are made with a root clearance defined by any number of different tooth form standards - ANSI/AGMA being a major one. There is an intentional [b:25b007cb1b]clearance[/b:25b007cb1b] left between the tip of a tooth and the root of the meshing gear.

sgw_profile.gif


Gear lash is the measure of free play between the face of two meshed gears and is not a part of the tooth form standard. It is typically measured and/or specified at the pitch circle:

220px-Backlash.svg.png


Backlash is a necessary component of almost all gear designs:

[b:25b007cb1b]Wikipedia[/b:25b007cb1b]

[i:25b007cb1b]Depending on the application, backlash may or may not be desirable. It is unavoidable for nearly all reversing mechanical couplings, although its effects can be negated or compensated for. In many applications, the theoretical ideal would be zero backlash, but in actual practice some backlash must be allowed to prevent jamming. Reasons for the presence of backlash include allowing for lubrication, manufacturing errors, deflection under load, and thermal expansion.[/i:25b007cb1b]

When designing gears and gear sets the easiest way to control backlash is with gear center spacing. The further you widen the center spacing on the gears the more you increase the lash. Moving the centers closer together reduces lash. But no matter how close you put them together, even to the point of binding, you cannot eliminate that root clearance which is there as part of the tooth form standard.

So what does all of that mean inside a gear pump? You measure gear lash by immobilizing one gear and measuring the free movement of the second one. When the gears are engaged and operating at RPM the driver tooth is fully pressed against the driven tooth and the gear lash (clearance) is all on the back side of the meshed teeth. As the gears rotate the meshed teeth are actually sliding against each other along the involute curvature of their faces which are in full contact for the entire time the teeth are engaged. In a gear pump oil is not going to bypass rearward through the engaged teeth of the operating gear set regardless of any (reasonable) amount of backlash they may have when not under load. Oil can and will easily bypass through excessive housing end or tip clearances.

TOH

You're certainly entitled to your opinion. 30+ years of real world experience working with hydraulic gear pumps in an industrial setting has taught me otherwise.

.
 
Guys, I really didn't mean to start a whizzing contest over this little pump. I am 67
years old have been in business for myself over 40 years. I worked as a Class A
Mechanic/welder in the largest coal fired power house in the USA and a Chemical Plant as
a millwright/welder and a corrective maintenance inspector before going into business
for myself. One thing that I have learned, is that things don't always work in real life
as they are designed to do on paper. Engineers do make a lot of mistakes and a lot of
times it is left to the millwrights and mechanics to figure out a way to make them work.
When I was working for the chemical plant, we had a problem that was going to shut the
whole plant down at a cost of 1.5 million dollars a day. I went to my boss, a senior
engineer, and told him that I knew how to keep the plant running and fix the problem.
We went behind closed doors and when we came out he took the plan to the plant manager
and the wheels started to turn. They took my idea,acted on it and he got the credit a
big raise and I got nothing. The people with the biggest title don't always know the
solution. Henry Fords ideas were cutting edge in the day but today they would be viewed
as simple and obsolete.Some of the best inventions in farm machinery have been
engineered by the farmer.I have made more money over the years than you can
imagine,fixing things that others said, THAT CAN'T be fixed!!!. I do not have an
engineering degree,barely made it through high school, welding school,machinist school,
and the rest I know I have learned through the school of hard knocks, trial and error
and correcting my own mistakes. All I know is that I now have an 8N with oil pressure, that didn't when it came through the doors and I will shut up and leave it at that!!
 
(quoted from post at 01:00:40 10/08/17) Guys, I really didn't mean to start a whizzing contest over this little pump. I am 67
years old have been in business for myself over 40 years. I worked as a Class A
Mechanic/welder in the largest coal fired power house in the USA and a Chemical Plant as
a millwright/welder and a corrective maintenance inspector before going into business
for myself. One thing that I have learned, is that things don't always work in real life
as they are designed to do on paper. Engineers do make a lot of mistakes and a lot of
times it is left to the millwrights and mechanics to figure out a way to make them work.
When I was working for the chemical plant, we had a problem that was going to shut the
whole plant down at a cost of 1.5 million dollars a day. I went to my boss, a senior
engineer, and told him that I knew how to keep the plant running and fix the problem.
We went behind closed doors and when we came out he took the plan to the plant manager
and the wheels started to turn. They took my idea,acted on it and he got the credit a
big raise and I got nothing. The people with the biggest title don't always know the
solution. Henry Fords ideas were cutting edge in the day but today they would be viewed
as simple and obsolete.Some of the best inventions in farm machinery have been
engineered by the farmer.I have made more money over the years than you can
imagine,fixing things that others said, THAT CAN'T be fixed!!!. I do not have an
engineering degree,barely made it through high school, welding school,machinist school,
and the rest I know I have learned through the school of hard knocks, trial and error
and correcting my own mistakes. All I know is that I now have an 8N with oil pressure, that didn't when it came through the doors and I will shut up and leave it at that!!

Welding man, the repairs you made were exactly what was needed. You reduced the end play and got a pair of gears with less backlash. Now it works. You did good.

.
 
In a perfect scenario, with correct end clearances, the gears are always meshed on the pressure side regardless of wear or backlash. However, as the gear tooth wears and thins, the tooth contact time interval increases from one to the next. At some point, the extended contact time will allow back flow between the teeth before the next pair mesh and seal.
For the what its worth department.
Happy tractoring!
TP
 
That may be true but what does that have to do with a NEW poorly manufactured non spec.
aftermarket gears that was manufactured in the land of almost right. They weren't worn
straight out of the box. Apparently they took the measurements to manufacture the new ones
off a set that were totally worn out. It's sad when the new parts you buy are worse than
the new ones you are replacing.
 
(quoted from post at 19:36:26 10/09/17) That may be true but what does that have to do with a NEW poorly manufactured non spec.
aftermarket gears that was manufactured in the land of almost right. They weren't worn
straight out of the box. Apparently they took the measurements to manufacture the new ones
off a set that were totally worn out. It's sad when the new parts you buy are worse than
the new ones you are replacing.

Since you asked here are the Ford OEM specifications for the 8N-6614 (3/4" wide) gears. I would expect the older 52-6614 to be the same with the exception of face width but I am waiting on that data.

Material: SAE 1112 stl. cold drawn Brinell hardness 175-205. Optionally, SAE1112 stl hot rolled machine straightened Brinell Hardness 140-170
Inner bore: .406/.40
Pitch dia .833
O.D. .997/.998
Gear width .724/.725 ends to be square with bore within .001 T.I.R.
Tooth data:
Pitch 12
No. of teeth 9
Addendum .0833
Depth below pitch line .1042
Whole depth of tooth .1875
Pressure angle 20 degrees
Must show .003-.006 backlash when meshed with 8N-6610 on .833 centers
Outer tooth tip corner radii .01/.03 R

A "universal" rule for gear design is the contact ratio on new gears must be be a minimum of 1.2. That means that at least one tooth is in contact with a tooth on the mating gear at all times and for some portion of time two teeth are in simultaneous contact. That means there is ALWAYS a full width face seal regardless of backlash. Increased backlash simply moves the contact point up the flank on the tooth (see image) and in a gear pump increases the volume of oil (red area) that is ALWAYS pumped backwards through the teeth by an infinitesimal amount. In that image the gears are shown with two contact points (K1 and K2) and the volume of oil being pumped from the pressure to suction side of the pump is 2X the volume of tip clearance volume plus the backlash volume. Gear pumps run at 90% plus volumetric efficiency and changes in backlash have little effect on that volumetric efficiency.

When gears get excessively worn over time all sorts of bad thinks happen - the teeth wear unevenly, and oil starts to bypass that flank seal and you have bad pumping performance. That is not what we are talking about initially. With NEW gears backlash really doesn't matter because the flank seal is good and the only difference is that nearly infinitesimal volumetric increase in reverse pumping that comes with increased backlash.

TOH

gear-mesh.gif
 
Great story that demonstrates the power of collaboration. It's also very rewarding when you can reason your way through something and recognize just because they are new it doesn't mean it's right.

Well done.
 

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