Alternator tension arm cracked... replacement part number?

[Neal@F14]

Started the annual condition inspection on my RV-6 today and found a crack in the alternator tension arm bracket. I've got the 35A externally regulated Nippon Denso (Honda Civic) alternator. The original tension arm is lightweight aluminum, blue anodized.

Vans website lists a tension arm, part# VA-152-1 but there is no picture available. Is this the correct replacement? Or do I need to buy the whole "Boss Mount Alternator Bracket Kit" (Part#ES ALT-BKT) instead. The description for the "case mount" alternator bracket kit (VA-235-PC) says that you need the VA-152-1 tension arm, so it seems to suggest that this tension arm is specifically for the case bolt mounted alternator brackets. Mine is a boss-mounted alternator. Nothing on the webpage says that the VA-152-1 tension arm fits boss mounted alternators, so I'm confused as to which item I should order.

[Neal@F14]

I ended up ordering the entire bracket kit due to discovering a hairline crack across the main boss-mount bracket too. 400 hours and both bracket components cracked

[Mike S]

You might want to dig a bit into the cause---------this sounds like there may be a misalignment issue, or a balance issue with the alternator or maybe a bent pulley etc.

[Neal@F14]

The tension arm bracket cracked sometime during the last 35 flight hours / 7 months. I have no idea when the main boss bracket cracked. Its crack is so fine that I almost missed seeing it completely. Total time since new is 412 hours, which seems to be a typically expectable time for a ND alternator to last. I'll remove it this evening, but my gut feeling it that I'll likely find loose/worn bearings in the alternator. It was still charging fine though... Now would be a good time to upgrade to a 55/60A alternator too, since with all lights and radios and autopilot on, it was getting close to maxxing out the full 35A of the old one.

[Neal@F14]

Well the bearings are in good shape, belt and pully alignment are fine, but the arm was under a fair amount of tension/pressure and the bolts holding the main pivot bracket were only finger tight. I think that was the culprit. The crack in that main bracket was only from one of the bolt holes out to the edge of the bracket. Looks like I caught it just in time before things got really worse. That's why we do periodic inspections, right?

[az_gila]

The boss mount bracket is a known weak point. Lycoming made the part some 50% thicker with the same part number -

Old and new -



There is also a Lycoming SB out there somewhere that talks about using shims for a precise fit of the alternator inside the U-shaped bracket. If there is any play the bracket will be stressed when the single bolt is tightened.

[Neal@F14]

My tension arm broke in a manner similar to this: http://www.vansairforce.com/communit...99&postcount=6
but not quite as dramatic... the broken section was still captive under the alternator tension bolt and washer, and the alternator was still in place and had adequate tension on the belt.

The main boss bracket's alignment and spacing to the pivot points on the alternator were all good... it was the 4 bolts that hold this bracket to the engine case that were loose. This is also a blue anodized aluminum part, possibly machined from billet, and is not a true U-shaped piece like the steel brackets, but is a U with a tail and looks kinda like this: ___|__|

This was the alternator brackets supplied with Van's firewall forward kits for the RV-6 in the early 2000's.

[Ironflight]

I've had those bolts loosen up on me back in my Grumman days, and it messed up the rest of the bracketry and the alternator. Ever since then I have used safety-wired bolts on those parts. And usually the little keepr plates as well (belt and suspenders).

[Neal@F14]

Mine only had the keeper plates with the bent-over tabs holding on the bolt heads. My bolts aren't drilled for safety wire so I was considering using some LocTite Blue on them when I re-assemble with the new bracket when it arrives.