Differential Drag

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boilermaster

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71 fastback
Question for you all today,

Rebuilding my 9'' diff, all new bearings, re using ring and pinion.

How much drag to put on the assembly when checking pattern and how and where to apply it.

Also what effect will not having a drag on the assembly have on the pattern ?

No guessing please, or my uncle did it this once, looking for something factual here.

Even in the very fine tutorial by 72H do I see any information of how much drag to apply or what effect it may have on a pattern.

Boilermaster

 
You don't need a lot of drag, just enough so you can see what contact pattern you have. A screw driver held or pryed against the ring hear will give you enough. Yes, you do need to apply some drag, otherwise you won't be able to create and see what the gear contact pattern is. If you're not seeing a good pattern in the compound you're using apply more pressure to the ring gear.

 
I have a piece of an old plastic cutting board I use to wedge in there along with some WD40 and then spin the pinion with a breaker bar or my cordless drill. You'll know you have it wedged in there too far if you can't turn it and know it isn't far enough if you aren't getting any pattern whatsoever. If the pinion shim and backlash are remotely close it doesn't take much pressure to get a pattern. You can do it with just hand pressure too but I don't have a good holding fixture and I find it difficult to do it that way.

 
Good idea on the wedge, I don't have a fixture either, and trying to hold the housing in place while turning the pinion and prying against the ring gear is a challenge.

 
When I did mine a little while back, I set the drag per the manual which was in inch pounds and was dependent on if using a new crush washer or shims or reusing what you had iirc.

Then I checked the pattern.

Want to say the drag was somewhere around 25? Inch pounds but don't have it in front of me.

 
I have a piece of an old plastic cutting board I use to wedge in there along with some WD40 and then spin the pinion with a breaker bar or my cordless drill. You'll know you have it wedged in there too far if you can't turn it and know it isn't far enough if you aren't getting any pattern whatsoever. If the pinion shim and backlash are remotely close it doesn't take much pressure to get a pattern. You can do it with just hand pressure too but I don't have a good holding fixture and I find it difficult to do it that way.
I kind of like the plastic wedge, will find something I can affix to one of the mounting holes and have some kind of adjustment.

No fixture and the extra hand will be very helpful.

made one marking and did not like the pattern, will make another with

plastic in place and see if it changes the pattern any.

will then make necessary adjustments.

Used to do this at the dealership, however it has been 20 something years since I have a 9'' differential.

Boilermaster

 
You don't need a lot of drag, just enough so you can see what contact pattern you have. A screw driver held or pryed against the ring hear will give you enough. Yes, you do need to apply some drag, otherwise you won't be able to create and see what the gear contact pattern is. If you're not seeing a good pattern in the compound you're using apply more pressure to the ring gear.
This is, more or less, how I've always done it. Basically, increase pressure until you don't see a change in the pattern. If the bearing preload is set correctly, it doesn't take a lot of pressure to get a consistent pattern.

When I did mine a little while back, I set the drag per the manual which was in inch pounds and was dependent on if using a new crush washer or shims or reusing what you had iirc.

Then I checked the pattern.

Want to say the drag was somewhere around 25? Inch pounds but don't have it in front of me.
I think he's asking about preloading the ring gear for checking the contact pattern on the gears. You're right on target for the pinion bearing preload spec. The manual calls for 8-14 in-lb for used bearings and 20-30 in-lb for new (on diffs using a crush sleeve).

 
problem is the case spread and pinion preload is different for all the gear sets outside of the original ford parts.

then you have used or new bearing settings.

if you have original ford gears and new bearings then the shop manual has the correct information otherwise you have to go by the paperwork on the aftermarket gears

that was why i didn't include it.

when i get home tonight i can look it up.

you need the case spread and the pinion preload information, but if you snug everything up you should get a accurate idea of your contact pattern.

basically once you set the preload and case spread you alter the contact pattern with the spacers on the pinion nose and check your back spacing stays in spec..

since you are reusing parts, make sure you checked the runout on the crown, if she is out of spec then the original gear set is trash.

 
You don't need a lot of drag, just enough so you can see what contact pattern you have. A screw driver held or pryed against the ring hear will give you enough. Yes, you do need to apply some drag, otherwise you won't be able to create and see what the gear contact pattern is. If you're not seeing a good pattern in the compound you're using apply more pressure to the ring gear.
This is, more or less, how I've always done it. Basically, increase pressure until you don't see a change in the pattern. If the bearing preload is set correctly, it doesn't take a lot of pressure to get a consistent pattern.

When I did mine a little while back, I set the drag per the manual which was in inch pounds and was dependent on if using a new crush washer or shims or reusing what you had iirc.

Then I checked the pattern.

Want to say the drag was somewhere around 25? Inch pounds but don't have it in front of me.
I think he's asking about preloading the ring gear for checking the contact pattern on the gears. You're right on target for the pinion bearing preload spec. The manual calls for 8-14 in-lb for used bearings and 20-30 in-lb for new (on diffs using a crush sleeve).

I think you're right

 
for stock eom gears

max runout of back face .003"

diff bearing preload case spread

new bearings .008-.012

old bearings .005-.008

pinion preload crush spacer

old bearings 8-14 INCH pounds(not foot)

new 20-30

 
for stock eom gears

max runout of back face .003"

diff bearing preload case spread

new bearings .008-.012

old bearings .005-.008

pinion preload crush spacer

old bearings 8-14 INCH pounds(not foot)

new 20-30
Well People,

Even an old dog can get schooled.

I will NEVER re use a ring and pinion again, it is just not worth the frustration.

Even though the ring and pinion were pristine and I had no bearings that were even close to failing it is darn near IMPOSSIBLE to get a perfect pattern on used gears.

I would be willing to bet that if you do, you are going to be in trouble.

Kind of had to reverse engineer the whole thing.

My original intent was to just replace the open case with limited slip case.

Really glad that I measured some with the unit before I took it down.

I measured backlash, and adjuster hole locations and took a quick pattern check and made a little drawing of the contact patterns.

I was soon to realize that I could easily change the pattern on the coast side but not the drive side (that is where most of the wear is going to occur.

What I learned was to read between the lines (patterns).

there are spots where the pattern WAS, IS, and never has been and a place where the pattern never will be.

All I could do was make sure that the pattern was centered between the ROOT and TOP sides of the coast side of the teeth, which it was

and not pay too much attention to whether it was towards the HEEL or TOE.

The pattern on the DRIVE side tended to be pretty much covering the whole tooth except for the fact that there was NO contact at the ROOT or the TOP on the HEEL end and the same at the TOE end.

Where the pattern was NOT told me I was somewhat centered from root to root to top and also from toe to heel.

Now for the out of the box part.

After I had the initial pattern with the goldish colored marking compound, I took a heat gun and attempted to dry it out and then painted some white compound where the pattern was NOT located.

This did very little on the coast side, but it did help show on the drive side, show where the pattern was NOT.

It was a combination of color difference and amount of compound build up.

As it stands, I don't know where the pattern is, but I know where it is not, and I don't know where it should have worn in to.

Ended up with the same pinion shim thickness and both of the screw adjusters are changed by 3 holes, left being 3 looser and right being 3 tighter.

Case spread .008'' pinion tq= 10 lb.in. and case runout is barely .001''

If I were still working flat rate, I would be a poor boy.

Boilermaster

 
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