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Old 14-10-2018, 01:17   #1
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Damaged transmission flange

Hi, thought I might start a new thread as it's a new problem
Thanks to everyone who helped with my shaft coupling to damaged shaft issue (now fixed) but now we have a lovely new solid coupling, we can see that the reason the last owners had a flexible coupling, was possibly to cover for the fact that the flange on the transmission's output flange is damaged

When I offer the coupling up to the flange, it does not go flush at all. When I hold a straight edge to it all around, and rock it back and forth, there appear to be raised surfaces around all of the bolt holes. What could cause that? Way over-tensioning? Or maybe ran loose and the bolts have tugged on the metal?

Sorry for the photo, was taken in a mirror, access is fairly good otherwise.

Seeing as the only fix I can see for this is a new transmission, can I employ clever flat filing across the bolt holes to try and flatten it? Chances of success?

Actually, looking at the photo myself, I bet it was caused by loose bolts riding around the edges?, I can flatten that yeah?
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Old 14-10-2018, 02:26   #2
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Re: Damaged transmission flange

I would remove the flange and turn it flat and trim edges on a lathe.


Plus it will balance it as well, reducing vibration.
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Old 14-10-2018, 05:13   #3
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Re: Damaged transmission flange

Thanks paulajayne, it may well come to that, in fact a new transmission may end up being the result but! If I can fix it, I would be pretty pleased I'm not after dodgy or perfect, just a solid good.
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Old 14-10-2018, 05:27   #4
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Re: Damaged transmission flange

I’ve dealt with this kind of thing over most of my professional life. If the metal around the bolt holes is raised then just touch it with a grinder, I would recommend the flapper wheel type disk. I would try to just take the damaged area around the bolt to just below the main surface.

The thing to remember is that the torque is transmitted by the flat area between the bolts I.e. the flange. The pilot (the round bit in the middle) keeps it centered. The bolts just hold it together they should not be transmitting torque, if they are then they are in shear and will have a short life.

Looks like there is plenty of life in that part yet.

P.s. I wouldn’t expect that part to be balanced but it would be prudent to do any modifications evenly ie grind all the holes evenly.

Good luck...
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Old 14-10-2018, 05:58   #5
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Re: Damaged transmission flange

I think the judicious use of a flat file might rescue the flange, followed up with some emory paper held on a flat surface to finish off with. Unless you're already skilled with a grinder, you can easily turn bad into really bad.

The best option is to pull the gearbox, remove the shaft and skim the flange in a lathe.

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Old 14-10-2018, 06:13   #6
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Re: Damaged transmission flange

Doug you’ll do more damage with a file due to the fact that’s hard to remove the metal locally around the hole. It’s easy with a small diameter grinding or flapper disk. There is actually very little damage to the flange it’s self, it’s just the holes. Even a dremal may work satisfactory and would work in situe. There’s a lot of unnecessary work having the flange removed and turned down. But it’s ultimately up to the OP to digest the info look at what they have and reach their own conclusion.

Assuming of course that everyone in the world has years of practice with metal working and fabricating tools... I left school with this knowledge not sure that is happening today...
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Old 14-10-2018, 06:51   #7
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Re: Damaged transmission flange

If the 'flex coupling' was plastic without metal inserts, over-torqueing could cause raised areas around the bolts of a few thousands.

A flat file used judiciously, that is held strictly parallel to the flange face and drawn or pushed so that it only cuts the high spots around the holes, could work. Experience is needed, especially in knowing when to stop.

All the holes appear wallowed out to me, though that may just be the chamfer on the surface. If they are wallowed out deeper, or through the full depth it may be difficult to keep the bolts tight; if that is the case it may be possible to drill them and the companion flange out to the next size up, metric or imperial, depending on which takes out the smallest amount.

Also, are you sure it isn't the area circled in red in your picture below that is holding the flange from seating in the radius? If it turns out that it is, that will have to be machined off as well.

Obviously for the most accurate job, the solution would be to remove it and have it repaired on a lathe, but a reasonably skilled machinist should be able to get them to seat with hand tools.

If the flange is distorted you may still have problems with alignment though...
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Old 14-10-2018, 07:36   #8
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Re: Damaged transmission flange

Quote:
Originally Posted by Warby12 View Post
Thanks paulajayne, it may well come to that, in fact a new transmission may end up being the result but! If I can fix it, I would be pretty pleased I'm not after dodgy or perfect, just a solid good.
Hang on. The output flange just has ridges around the bolt holes. just take a ridge reamer to the holes. $8 tool and a minute of time. worst case if the flange is really damaged, Just replace it. Every transmission flange I've seen is bolted on in some way. Just replace it, unless there are other transmission issues. (even then its more likely a rebuilt.)
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Old 14-10-2018, 07:53   #9
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Re: Damaged transmission flange

Quote:
Originally Posted by Steven UK View Post
I’ve dealt with this kind of thing over most of my professional life. If the metal around the bolt holes is raised then just touch it with a grinder, I would recommend the flapper wheel type disk. I would try to just take the damaged area around the bolt to just below the main surface.

The thing to remember is that the torque is transmitted by the flat area between the bolts I.e. the flange. The pilot (the round bit in the middle) keeps it centered. The bolts just hold it together they should not be transmitting torque, if they are then they are in shear and will have a short life.

Looks like there is plenty of life in that part yet.

P.s. I wouldn’t expect that part to be balanced but it would be prudent to do any modifications evenly ie grind all the holes evenly.

Good luck...
I agree. I am retired 43 years mechanical engineer, most of it as chief engineer building custom machinery.

I would add to the above excellent advice that the bolts must be set using a torque wrench. The manufacturer may list the torque. Otherwise you can find suggested torque for the bolt sizes on line. Insufficient torque can lead to the bolts in shear or premature fatigue failure of the bolts. This is one place in particular where SS is not suitable because of the inferior capacity. Use only medium to high strength alloy bolts. If you are in doubt pick socket head cap screws. These are usually grade 5 or better.

Typical torque chart.
https://www.portlandbolt.com/technic...-torque-chart/


I don’t know the details of the rest of your drive train. If it is all rigid, engine to prop shaft, it is naturally difficult to align and the thrust is carried entirely by the transmission bearings and motor mounts. We have a CV joint aft of the twin disk transmission that carries all thrust and accommodates large misalignment.

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Old 14-10-2018, 08:13   #10
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Re: Damaged transmission flange

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Originally Posted by jimbunyard View Post
If the 'flex coupling' was plastic without metal inserts, over-torqueing could cause raised areas around the bolts of a few thousands.

A flat file used judiciously, that is held strictly parallel to the flange face and drawn or pushed so that it only cuts the high spots around the holes, could work. Experience is needed, especially in knowing when to stop.

All the holes appear wallowed out to me, though that may just be the chamfer on the surface. If they are wallowed out deeper, or through the full depth it may be difficult to keep the bolts tight; if that is the case it may be possible to drill them and the companion flange out to the next size up, metric or imperial, depending on which takes out the smallest amount.

Also, are you sure it isn't the area circled in red in your picture below that is holding the flange from seating in the radius? If it turns out that it is, that will have to be machined off as well.

Obviously for the most accurate job, the solution would be to remove it and have it repaired on a lathe, but a reasonably skilled machinist should be able to get them to seat with hand tools.

If the flange is distorted you may still have problems with alignment though...
Your right the holes are ‘egged’ out due to being loose at some point, but it won’t matter because the flange is centered on the pilot or spigot. As long as the bolts can hold the flange together it’s not going to harm the joint at all. The bolts just clamp the two faces together, friction on the face does the important part. If the two faces slipped and used the bolts to drive the shaft the they would be slipping back and forth when forward and reverse were engaged... and causing the ‘egging’.

That being said if the flange is cheap just buy one... but it would have to be very cheap. Better to buy a dremel or similar and a torque wrench.

P.s. I’m very cheap...
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Old 14-10-2018, 08:26   #11
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Re: Damaged transmission flange

Quote:
Originally Posted by Steven UK View Post
Your right the holes are ‘egged’ out due to being loose at some point, but it won’t matter because the flange is centered on the pilot or spigot. As long as the bolts can hold the flange together it’s not going to harm the joint at all. The bolts just clamp the two faces together, friction on the face does the important part. If the two faces slipped and used the bolts to drive the shaft the they would be slipping back and forth when forward and reverse were engaged... and causing the ‘egging’.

That being said if the flange is cheap just buy one... but it would have to be very cheap. Better to buy a dremel or similar and a torque wrench.

P.s. I’m very cheap...
Egged holes mean loose bolts and the torque was transmitted by the bolts alone. This is bad. It will damage things and potentially strand you. Read my previous post please. Get the install instructions for your coupling and follow them.
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Old 14-10-2018, 08:32   #12
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Re: Damaged transmission flange

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Originally Posted by Nicholson58 View Post
Egged holes mean loose bolts and the torque was transmitted by the bolts alone. This is bad. It will damage things and potentially strand you. Read my previous post please. Get the install instructions for your coupling and follow them.
Please read my previous post... I think we’re actually on the same page
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Old 14-10-2018, 09:15   #13
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Re: Damaged transmission flange

Quote:
Originally Posted by Steven UK View Post
Your right the holes are ‘egged’ out due to being loose at some point, but it won’t matter because the flange is centered on the pilot or spigot. As long as the bolts can hold the flange together it’s not going to harm the joint at all. The bolts just clamp the two faces together, friction on the face does the important part. If the two faces slipped and used the bolts to drive the shaft the they would be slipping back and forth when forward and reverse were engaged... and causing the ‘egging’.

That being said if the flange is cheap just buy one... but it would have to be very cheap. Better to buy a dremel or similar and a torque wrench.

P.s. I’m very cheap...
I too like to do things in the most economical way.

Of course the driving of the shaft is carried out by the friction between the two mating flanges created by the tension of the connecting bolts. The problem with egged or wallowed holes comes over time, when the engaging and disengaging of the gear will eventually cause the bolts to loosen, because of the movement allowed by the oversized holes, even when properly torqued.

As for the 'raised surfaces' around the bolts, my interpretation is not that they are ridges caused by loose bolts, but 'halos' of differing levels around the holes, highest closest to the edge of the hole, and tapering to the flat of the face of the flange around the majority of the flange. This can be caused by overtorquing two metal flanges together with a malleable surface in between them, such as in the case of having a 'flexible coupling' between the flanges, as the OP originally did, and is not uncommon when bolting together cast iron parts with a thick gasket between them.

Attached is the OP's picture with an attempt to illustrate my meaning. Imagine the red circle around the bolt hole at .002" 'proud', the yellow circle at .001" 'proud', and the green at the same level as the rest of the flange, and you can probably see why a flat file would be a better tool than a Dremel or flap wheel.

I'm still curious about the damaged area in the bolder red circle. It certainly looks to me like the concentricity is compromised there...
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Old 14-10-2018, 12:20   #14
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Re: Damaged transmission flange

When finished, your alignment needs to be within .002". The flange surface needs to be exactly 90° from the shaft. To get that result by using hand tools , you really need to remove the flange so you can measure the angle by putting one leg of a square down the shaft hole.
But if you have the flange off, you might as well have a machine shop square it. And if both flanges have egg shaped holes, they really need to be redrilled to the next larger size.
There is a cheap way. I don't do it, but there are flexible spacers that install between the flanges to make up for misalignment. Boatyards that don't know how to make a proper alignment use them. It is possible to get the bolts, with egg shaped holes, tight enough to stop movement, but it won't last forever.
Or maybe buying new flanges would be the easiest/cheapest.
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Old 14-10-2018, 16:21   #15
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Re: Damaged transmission flange

Once again, I can't thank you all enough for your help and advice. Being able to get opinions from all of you is something of imeasurable value! I'm learning a lot too. From the advice on the other thread, I went on and found this great movie from 1942 on flat filing and have a much better understanding of what you were saying.

With this pain in the butt problem, I would love to pull the transmission but of course dont want to. Money, delay, money.... The engine is a Perkins 4108 which we have lavished a lot of care on, new heat exchanger, wiring, engine mounts, gauges, senders and switches, injectors, fuel and and injection pump. We treat it like royalty and it runs really well. The transmission is of an unknown brand. I did a post on it years ago and the best everybody could come up with was some sort of hurth copy.

So, I'm going to try flapper disc and flat file. I am good with a grinder, having previously refitted a steel boat. The bolt holes could be pulled in by over tightening on a flexible coulpling, but the damage around the edges looks like loose bolts to me.

The area circled in red is weird but it is a depression, like chipped off

Becuase the flange is a flat surface I hope I can cup out the area around the holes without damaging it, easier than working with a round propellor shaft!

I will source the best bolts I can and tourque them. Also if I can get it going, I always check these and other bolts before I start the engine or at least weekly if we are travelling, I keep a couple of spanners in the engine complartment, so if they keep moving, then I'll know it's not fixed.

Fingers crossed and thanks!!!!
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