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Bad Vibrations (from an alternator)


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3 hours ago, spicemouse said:

Haha, you actually made me go and check! 

06_not_cork.jpg.189a21b7ba15d3dc2fad30ceddf4961d.jpg

Steel plate thankfully. They were added to allow more mounting points to the wood. 

I wondered why the engine itself wasn't moving about very much in your video; looks like the broken bracket is resting (wedged) on the timber bearers. There isn't a lot of space, and I wonder if the engine had been hitting the timber before the bracket broke?

When repaired I would expect to see that part of the bracket going up at 45 deg to be much deeper, such that very little of the oil filter would be visible in the "after" image.

 

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Over the years numbers of people have experienced problems with engine mounting systems. To use the adjusting nuts on a flexible mount on any but very minor differences in height is a very poor solution from an engineering point of view. They will fail sooner or later and the higher up the mounting point the engine sits the sooner will be the fail. In this instance because of the use of  hydraulic drive there is no need for a drive line height to be matched and the engine can sit on the plates on top of the 'rubber' element of the mounting. It is worth exploring the use of Tico pads and mounting bolt isolators coupled with substantial engine beds to avoid the problems with marine fleximounts. We use Tico on the Ford BSD3 engine in Wyrd and this engine is notorious for resonant vibration and nodding couple dance routines. It works and is a better mounting solution than a large vibrating mass suspended on thin threaded rod and held in place by nylocs. There are no significant increases in engine noise or vibration through the hull and the engine sits rock steady at all speeds.

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10 hours ago, ditchcrawler said:

As you have a hydraulic drive so no shaft alignment to worry about would it not be better to lower the engine on those 4 flexible  mounts rater than sitting so high on the threads

Whilst it is a good thing for the engine to be as low as possible on the mounts I think that in this case it's not possible.

The engine with its mounts is to wide for the space between the engine bearers as shown by the picture where the mount is touching the wood.

You are left with two choices, either raise the engine mounts further and dropping it down the bolts by adding a thicker steel spacer under the mounts or re-engineering the engine beds so that they clear the engine. If this is not done then sometime in the future you will end up back in the same place you are now with a broken mount where it has been banging against the engine beds.

 

06_not_cork.jpg.189a21b7ba15d3dc2fad30ceddf4961d.jpg

Edited by Loddon
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22 hours ago, spicemouse said:

I should also add the mounting points on the wood support were replaced 2 months ago, which might be why it looks like a recent install. 

 

 

 

Just looking at this picture :

 

06_not_cork.jpg.189a21b7ba15d3dc2fad30ceddf4961d.jpg

 

 

There appears to be a 'line' running up along the wooden bearer is this just a shadow, or is it a crack ?

 

The steel riser plate doesn't appear to attached to the wooden bearer and looks as it the fixing is relying on the bolt going thru the steel plate and into the wooden bearer.

 

However, at the angle the picture is taken it looks as if the bolt may be running into the 'split' in the wood and will be offering very little support if it is.

 

With the mounting bracket (seemingly) fouling on the wooden bearer you may well have repeat problems.

 

In view of the comments in the preceeding couple of posts, it appears to have been a pretty poor installation, was this done by a reputable company ?

 

Just a thought - whilst you are fixing it / having it fixed, it may be worth the time and expense to have a 'proper' installation done and remove the liklehood of a re-occurance.

Edited by Alan de Enfield
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Apart from cost, I don't see why a couple of two bulks of hard wood sat on each bed to raise the mount base would not be perfectly acceptable. It is for rigid mounted engines and as there is no prop thrust on this one the duty is far less onerous. I can't see the engineering point of those two steel plates apart from as crude distance pieces. I can see no evidence of any extra securing fixings in them.

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Again, lots of good stuff in here for me. 

3 hours ago, Alan de Enfield said:

There appears to be a 'line' running up along the wooden bearer is this just a shadow, or is it a crack ?


When I got the boat my survey said that the timber is cracked, however it might last another 10 years before this is a big issue. Replacing it would have been ideal, unfortunately the workshop was quoting me more than I could afford.. In the end we opted for replacing the mounting points and adding the steel plates (which do have additional pins into the timber). 

You can see how much higher original setup was:  

image.png.0756d2e59725eeb7aa33b15e90bc6afd.png

 

 

14 hours ago, ditchcrawler said:

As you have a hydraulic drive so no shaft alignment to worry about would it not be better to lower the engine on those 4 flexible  mounts rater than sitting so high on the threads

The mounts do have to be at a minimum height I think in part due to the position of this alternator support arm:

 

image.png.4427795eacba40caf8cdc3617df20bda.png

You can see that when attached it drops about 5cm below the L beam. 

 

As others have said, the main reason it was all thought to be acceptable was that this is a hydrault drive, so the vibrations shouldn't be as big an issue. 

 

3 hours ago, Alan de Enfield said:

Just a thought - whilst you are fixing it / having it fixed, it may be worth the time and expense to have a 'proper' installation done and remove the liklehood of a re-occurance.

I'm stuck in a location without elsan, bins, or water, and now no hot water, so I think I'll have to take whatever fix I can find to get me moving again. I do agree thought that if the cause of this is the setup, then the setup does have to change. I'm not one for just sitting on something till it breaks. 

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15 hours ago, ditchcrawler said:

As you have a hydraulic drive so no shaft alignment to worry about would it not be better to lower the engine on those 4 flexible  mounts rater than sitting so high on the threads

 

In our installation with a 1.8 and an hydraulic drive, the engine is bolted right down to the steel bearer.  Never had any vibration problems.

P1210771.JPG

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Rather than weld over an existing crack you would be far better to fit a new bracket completely.

Welding cracked steel, unless done properly with the cracked area fully veed from both sides, will leave elements of what is a stress failure in the base metal. This will just cause failure in a relatively short period.

If you cannot get a replacement from Beta then get a local engineering firm to cut the entire broken part of the angle beam and weld on new pieces of metal.

Which ever way you choose to weld/repair this, the 2 mounting faces will need to be machined flat before refitting. If you don't they will be understrees just from being bolted to the engine!

Pm me if you want help (  engineer by trade)

 

 

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22 hours ago, adrianh said:

Rather than weld over an existing crack you would be far better to fit a new bracket completely.

I'd much prefer a new part, but Beta aren't getting back to me and it would be a new bracket welded to the L-beam. 

 

 

22 hours ago, adrianh said:

Which ever way you choose to weld/repair this, the 2 mounting faces will need to be machined flat before refitting. If you don't they will be understrees just from being bolted to the engine!

I don't understand this part, will the existing mounting face have warped? 

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6 minutes ago, spicemouse said:

I'd much prefer a new part, but Beta aren't getting back to me and it would be a new bracket welded to the L-beam. 

Email adrian@betamarine.co.uk assuming he is still there, he managed to source parts and the original hand drawn circuit diagrams for my 1993 Beta.

The engine mounting is the same on my beta the newer ones have individual mounts.

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On 27/06/2021 at 12:35, spicemouse said:

Again, lots of good stuff in here for me. 


When I got the boat my survey said that the timber is cracked, however it might last another 10 years before this is a big issue. Replacing it would have been ideal, unfortunately the workshop was quoting me more than I could afford.. In the end we opted for replacing the mounting points and adding the steel plates (which do have additional pins into the timber). 

You can see how much higher original setup was:  

image.png.0756d2e59725eeb7aa33b15e90bc6afd.png

 

 

The mounts do have to be at a minimum height I think in part due to the position of this alternator support arm:

 

image.png.4427795eacba40caf8cdc3617df20bda.png

You can see that when attached it drops about 5cm below the L beam. 

 

As others have said, the main reason it was all thought to be acceptable was that this is a hydrault drive, so the vibrations shouldn't be as big an issue. 

 

I'm stuck in a location without elsan, bins, or water, and now no hot water, so I think I'll have to take whatever fix I can find to get me moving again. I do agree thought that if the cause of this is the setup, then the setup does have to change. I'm not one for just sitting on something till it breaks. 

The installation is odd,  but I don't see anything actually "wrong " with it.  I am convinced that the problem is with the stress resulting from the long bolt at the bottom mounting putting strain on the mounting lugs. The break will be at the point where that stress was concentrated. Get it welded and use two shorter bolts, put the new part on order for your peace of mind. 

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6 minutes ago, Loddon said:

Email adrian@betamarine.co.uk assuming he is still there, he managed to source parts and the original hand drawn circuit diagrams for my 1993 Beta.

The engine mounting is the same on my beta the newer ones have individual mounts.

I'm in contact with them  (and have spoken to Adrian in the past), but I think I might need to follow up with them to get a response. 

 

2 minutes ago, Sir Nibble said:

  I am convinced that the problem is with the stress resulting from the long bolt at the bottom mounting putting strain on the mounting lugs. 

Sorry, I'm not clear where you mean. 

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Sorry for the delay, been off scribbling in microsoft (TM)  paint.

Alternators should mount either like this on a solid lug with a long bolt.

image.png.cbfa00a27429fa483272114a98a2ea1c.png

or like this, with two separate lugs and seperate bolts.In both these drawings I have highlighted a sliding bush in yellow which is pressed into the alternator casting and slides through as the bolt is tightened to take up any clearance. 

image.png.4091413465a62b78c37db2e5a75773e6.png

If the different types are mixed and the long bolt from the top example is used in the bottom example with two separate mounts this happens.

 

The bolt squeezes at the black arrows, the yellow bush slides through and the two mounts are distorted by stress until a fracture occurs (red arrow) where the leverage from the point of pressure concentrates stress.

 

 

image.png

Edited by Sir Nibble
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Re machine the bracket after welding.

If you have a new arm fitted to the angle it is impossible to keep it flat and parallel with the other bracket. 

A proper job requires a plate with web welded in place  on the angle  with a small allowance in position/ thickness so that both the new engine mount pad and the old one on the other end of the angle can be machined over to give a flat surface.

All steels distort when welded.

The alternator bracket mounted of the side of the angle is going to put a torsional load back onto the bracket that broke in addition to the load from the engine weight.

This load is going to be cyclic depending on the alternator output and may be the reason this arm failed 1st

A redesign of the bracket,etc so that it has 2 gussets down from the angle to the engine mount plate ( ie wider plate with gusett each end ) would reduce the load on each Gusset

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