410d coupling flange torque setting

rockbeer

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Hi all,
I need to replace the diff - prop shaft coupling flange on my 94 410D light transporter. I'm told that the torque setting for this is critical, but I have no idea what I should tighten it to.

Can anybody shed any light please? Or, failing that, offer any opinions on how I can gauge the correct tightness in some other way?

Many thanks as always for your help.
 

television

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I will try and get hold of the info later in the day
 
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rockbeer

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That would be so much appreciated, thank you.
 

television

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So sorry there is nothing listed now for this van, only a few engine bits of info.

Would it be worth asking at a MB commercial center, they are normall quite helpful
 

Number_Cruncher

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In the absence of manufacturer data, find the size and grade of the bolt - from these, and knowledge of the material it screws into (or a nut?), there are standard tables which give estimates for torque.

These tables aren't the las word, and sometimes, manufacturer data conflicts with them, but, it's better than a pure guess.
 
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rockbeer

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Thanks for the info guys.

There's a place I can ask, failing that I'll try your table lookup idea Number Cruncher.

Cheers,
Bruce
 
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rockbeer

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So, just in case someone comes looking, I finally managed to find out the following info thanks to a very helpful guy at Mutec in Dublin:

The retaining nut itself has no specific torque setting. The procedure involves tightening the collar nut gradually until the friction torque required to spin the drive pinion is 2.5 - 3Nm (on my axle - on some the figure is 4.1-4.5Nm). This is supposed to be checked with a special retainer wrench and torque meter but I'm hoping it can be done with the creative use of a standard torque wrench.

According to the documentation, the correct friction torque must not be obtained by loosening the collar nut. Therefore it's essential not to overtighten the nut as otherwise the inner compression ring must be replaced. This requires total disassembly of the gear set.

Looks like this should be fun ;)
 

Number_Cruncher

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Sorry Rockbeer, I had *completely* misunderstood your original question.

I had assumed you were talking about the bolts which connect the prop shaft to the flange - but, you were talking about the single large nut which connects the drive flange to the pinion shaft. That's a different connection altogether!

Please ignore anything I've written above - those torque figures would NOT be applicable.

The information you've been given sounds right - by tightening this nut, you are pre-loading a pair of bearings, and, if you over-tighten, their life will be very short. If you under-tighten, the pinion gear will move around, and your axle will become noisy. Yes, it's a critical fastener.

Are you removing the flange to replace the oil seal behind it? Are you re-using your existing flange, or, fitting a new one?

If you're re-using your existing flange, one method is to mark the position of the nut relative to the pinion shaft, and re-install it to the same position. It's not a perfect method, but, it does work well.

An important point - do the friction torques rely upon a new collapsible spacer being fitted behind the front pinion bearing - or, are they based on the re-use of the existing spacer (which would be an unusual spec).

In order to apply the friction torque, measure the diameter of an accessible cylindrical part of the flange, and work out what weight on the end of a string/wire wrapped around that cylinder would give you the required torque. This avoids the use of any special tools, but still gives a good indication.

For example, say the diameter is 50mm - then, the radius is 25mm. To obtain 2.5Nm, you would need

Torque = mass * acceleration due to gravity * radius

re-arranged gives

mass = Torque / (acceleration due to gravity * radius)

mass = 2.5 /(10*25e-3)

[I've assumed g is 10 instead of the more accurate figure of 9.81, for easy reckoning]

mass = 10kg

So, a 10 kg mass off the end of such a string would give you the right torque, and you would set up the pinion nut so the mass would just drop freely - too loose and it would plummet, too tight and it would not move at all.

You'll need to do the sums again for the diameter you wind the string round.

I hope this helps.
 
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rockbeer

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Many thanks to you Number_Cruncher for that fantastic explanation.

Sorry Rockbeer, I had *completely* misunderstood your original question.

No worries, I probably didn't explain it very well and the situation isn't helped by that flange going by about 12 different names - and that's just in the Mercedes documentation.

Are you removing the flange to replace the oil seal behind it?

Partly. I'm replacing the flange because the old one is badly worn. In London a few weeks ago I started to get serious vibrations from the back axle so I had it looked at by a mechanic. He dismantled it and showed me the worn part. Then he temporarily solved the problem (to get me home from abroad) by over-tightening the retaining nut with instructions to sort it properly once I got back.

He knew what he was doing was wrong but it was the best option under the circumstances. He was satisfied there was no major internal damage to the diff at that point, but keen I should get it done right asap once I got back.

Which is where I am now.


An important point - do the friction torques rely upon a new collapsible spacer being fitted behind the front pinion bearing - or, are they based on the re-use of the existing spacer (which would be an unusual spec).

The documentation the guy in Dublin sent me seems quite clear that the spacer needs replacing IF the collar nut has been overtightened - which mine almost certainly has at this stage. It's a touch ambiguous about whether replacement is necessary otherwise, but my impression is probably not. I think the documentation assumes the flange won't ever need replacing on its own. Makes no difference in this case anyway since mine was certainly overtightened. Looks as though I'll have to dismantle the entire diff just to replace the flange. Oh joy.

I hope this helps.

It's absolutely brilliant, many thanks again. Wish me luck.
 

Number_Cruncher

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Hi Rockbeer.

I would be very much surprised if you don't also find that you need to replace the pinion bearings.

If the flange had become so loose that it was causing a vibration, usually the only way this can happen is a pinion bearing collapsing. Sorry.

I don't know the detail set-up of this axle (but MB commercial axles are usually quite well arranged); if you can remove the crown wheel and re-install it with both position and preload intact, the job might be straight forward.

One thing you'll need to do is to measure the depth of the pinion gear relative to the crown wheel bearing halves before you remove it. There will be shims between the inner rear pinion bearing and the pinion itself which set that depth. It needs to be correct to give correct meshing.

You will also need to blue the teeth, and run them together to make sure the blue wears off in the right contact pattern "Fottprinting" the gears. If the relative positions of crown wheel and pinion are wrong, you'll soon hear it.

You'll also need to do a backlash check between crown wheel and pinion - for which it's usual to use a dial gauge.

You are going to need a manual with the settings and procedures in - or, take it to someone who is very experienced in setting these gears up - if not, they'll howl noisily at you until thy wear out!
 
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rockbeer

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Thanks again Number_Cruncher.

Hmmm, beginning to think I might be in over my head here.

Re the possibility of a collapsed bearing, the mechanic who checked it out in London was very alert to this probability and checked as carefully as possible for it (short of dismantling the diff). He was, to his surprise, convinced the bearings were fine. He knew I had to travel back to Ireland and his original prognosis was that this wasn't going to happen without a diff rebuild - it was only after careful checking, adjustment and testing that he was happy to send me off, so I think I have just about got away without damage to the bearings.

I have got hold of a detailed breakdown of the procedures and settings, but it looks as though it uses all manner of specialist tools I don't have, plus I don't really have anywhere to do this job at the moment.

I think I'll take your last advice and hand it over to someone who knows what they're doing. I'll have a go at anything, but experience suggests sometimes it's best to leave it to the professionals.
 
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rockbeer

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Hi again Number_Cruncher,
Hope you don't mind, I just wanted to try and clarify this formula before I get the overalls on to see whether I'll need to dismantle the diff.

You said

For example, say the diameter is 50mm - then, the radius is 25mm. To obtain 2.5Nm, you would need

Torque = mass * acceleration due to gravity * radius

re-arranged gives

mass = Torque / (acceleration due to gravity * radius)

mass = 2.5 /(10*25e-3)

Now I look closely, I realize I don't quite understand the e-3 bit in this equation. What is e and why would you want to subtract 3 from it?

I've looked up other torque formula online and most seem to ignore the gravity thing entirely. So I got to this (working in lb-ft 'cos I'm old school that way I'm afraid ;) ):

In this case the flange circumference is 7.5 inches so the radius is 1.194, and the required torque is around 2 lb-ft (between 1.8 and 2.2 is conveniently acceptable. So:

T = F X D

F = T / D

F = 2 / 0.1

F = 20 lb

I need to tighten the nut until a 20 pound weight just moves the flange freely. Does that sound as though it might be somewhere near right? And, if you can bear it, could you explain how you would recalculate the formula if you were to introduce the gravity constant as this has me defeated I'm afraid.

Many thanks for your trouble.

Now if I'm right I just have to find something that weighs 20 pounds and is small enough to tie onto a piece of string and hang off the flange. Think it might be easier to increase the distance with an extension bar...
 
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Number_Cruncher

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Sorry, I should have explained the notation a little.

25e-3 is 25 millimetres, expressed in units of metres. The e-3 is shorthand for divide by a thousand.

Sorry, I don't do imperial (unless my arm is twisted up my back!)

If your flange circumference is 7.5 inches, that's 7.5*25.4=190.5 millimetres

So, the diameter is 190.5/pi = 60.64mm, giving a radius of 30.32mm.

>>T = F X D

Yes, this is right - in our case though, the force comes from a mass being pulled down by gravity, so, we replace F with mass*gravity

Torque (Nm) = Mass (kg) * acceleration due to gravity (m/s^2) * radius (m)

Re-arranging gives

mass = Torque / (acceleration due to gravity * radius)

using 2.5 Nm again, and 10 m/s^2 for the acceleration due to gravity gives

mass = 2.5 / (10 * (30.32/1000))

Where I've converted from millimetres to metres by dividing by 1000

mass = 8.2457

So, you need a mass of about 8 and a quarter killogrammes - or, 18 pounds

If you do use an extension arm, you'll need to account for the torque which the arm itself will impose.

If you look at Ebay.co.uk item 130305260819 - a proper tool for the job - you'll see a sliding weight on one side, and a fixed weight on the other. The fixed weight is there to compensate for the torque of the arm - you'll need to compensate for this torque in your calculations.
 
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rockbeer

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Sorry, I should have explained the notation a little.

25e-3 is 25 millimetres, expressed in units of metres. The e-3 is shorthand for divide by a thousand.

Sorry, I don't do imperial (unless my arm is twisted up my back!)

If your flange circumference is 7.5 inches, that's 7.5*25.4=190.5 millimetres

So, the diameter is 190.5/pi = 60.64mm, giving a radius of 30.32mm.

>>T = F X D

Yes, this is right - in our case though, the force comes from a mass being pulled down by gravity, so, we replace F with mass*gravity

Torque (Nm) = Mass (kg) * acceleration due to gravity (m/s^2) * radius (m)

Re-arranging gives

mass = Torque / (acceleration due to gravity * radius)

using 2.5 Nm again, and 10 m/s^2 for the acceleration due to gravity gives

mass = 2.5 / (10 * (30.32/1000))

Where I've converted from millimetres to metres by dividing by 1000

mass = 8.2457

So, you need a mass of about 8 and a quarter killogrammes - or, 18 pounds

If you do use an extension arm, you'll need to account for the torque which the arm itself will impose.

If you look at Ebay.co.uk item 130305260819 - a proper tool for the job - you'll see a sliding weight on one side, and a fixed weight on the other. The fixed weight is there to compensate for the torque of the arm - you'll need to compensate for this torque in your calculations.

Fantastic - thanks again. Managed to find something heavy enough to do the trick:

vice.jpg

So that's how you Earth guys do science :)

So with the help of Number_Cruncher, some bailer twine and a handy old 8 Kilo workshop vice I can confirm that the nut has been horribly overtightened and the diff needs to come out.

Time to call the mechanics!

Cheers for all your help, at least I know what I'm dealing with now.
 

Number_Cruncher

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>>the nut has been horribly overtightened

Oh dear!

However, if your diff is quiet, and is properly rebuilt, then, you should get away without the horrendous cost of needing a new crown wheel and pinion gear pair. Off the top of my head, I wouldn't imagine there would be much change out of £500 just for those two gears!

(For my W124, the gear pair would cost £540)
 

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I've also used a spring balance and ring spanner in the past. For larger nut sizes, the spanner's weight might need accounting for, or greatly reduced by using it in a vertical position. String and spring balance is also an option...
 

Number_Cruncher

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>>by using it in a vertical position

Yes, that's the shrewd way to use a spring balance - with the spanner vertical, it is contributing no torque.
 

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