Alternator bracket broken (Volvo D2-55)

[Jes]

In our boat the alternator bracket (5mm U-shaped profile) has been broken last year, and (after welding) it has been broken off this year again.
I have now dismounted the alternator, and discovered that the carbon brushes needs replacement (one of the carbon brushes is too short to touch the rotor).
Is it possible that a carbon brush problem can cause the alternator and alternator belt to start resonating, resulting in a broken mounting bracket?
(we have observed some resonance at some RPM's)

Any other suggestion why a steel bracket of 5mm thick breaks twice after 15 years of service (and 1.400 working hours?)

[Compass790]

Vibration & too much weight for the steel thickness would be my guess.
I would take it to a metal fabrication place & get them to make you one up out of 8mm plate. If there is enough room I would put a stiffener in the middle of the u-shaped bracket when fabricating it. That would solve the problem. You may need longer mounting bolts then but check.

[Lepke]

Metal fatigue from engine vibration and belt tension. It's poor bracket design in the first place. An alternator bracket needs to be rigid, no flexing at all. That bracket either needs to be much thicker, with gussets or stiffeners. The steel in the bracket you have is fatigued. Without support it will continue to crack. When you weld it, cracks will happen somewhere else, maybe even along the edge of the weld. And welding introduces new stresses.

[GordMay]

Originally Posted by Lepke

Metal fatigue from engine vibration and belt tension. It's poor bracket design in the first place. An alternator bracket needs to be rigid, no flexing at all. That bracket either needs to be much thicker, with gussets or stiffeners...

Indeed.

[Jes]

Originally Posted by Lepke

Metal fatigue from engine vibration and belt tension. It's poor bracket design in the first place. An alternator bracket needs to be rigid, no flexing at all. That bracket either needs to be much thicker, with gussets or stiffeners. The steel in the bracket you have is fatigued. Without support it will continue to crack. When you weld it, cracks will happen somewhere else, maybe even along the edge of the weld. And welding introduces new stresses.

Thanks for your respose. It's obvious that something went wrong. But a "simple" stiffening the construction won't help. The alternator is mounted between the two flanks of an U profile, which in itself is a proper way of mounting. Fastening the main alternator bolt will need to bend the two flanks slightly together. A "rigid" construction with a sandwich assembly will inhibit a proper fix of the alternator. Maybe the steel plate of 5 mm thickness itself was already too stiff (or damaged during construction/welding).

Still wonder if anybody else has had simular problem with these kind of engines... If the construction is really that weak, more complaints should be known.

I still suspect the carbon brushes of the alternator which appears to be worn-out (just touching the rotor). An alternator with a full energized field will load the alternator belt with 60A*14V=840W (which is slightly more than one HP). When this load is switched on-and-off with a higher frequency, can this force cause excessive vibrations of the alternator and alternator belt? Can this cause the metal to become fatigue?

[Lepke]

Any flexing of metal will cause fatigue. It doesn't matter what causes the fatigue. Too much vibration and you'll start having problems with the alternator case. You can probably buy new brushes online.