Sheared bolt supporting alternator

GH29 said:
Could you explain why not to use stainless steel please. Thanks.
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As I understand it - and I am not a metallurgist - stainless steel a) has a lower shear strength and b) is more susceptible to metal fatigue than mild steel. As the bolt on which an alternator swings is subject to loads that are fairly high and vary at high frequency, stainless is more likely to fail than plain steel. Of course, there are grades of stainless that would be satisfactory, but my experience was with bolts bought in a chandlery, probably 316, and two of them failed in much the same way after relatively few hours of service.
 
How many vehicle manufacturers use stainless bolts for their alternator brackets? I can assure you that vehicles run many more hours than leisure boat engines.
 
Caraway said:
How many vehicle manufacturers use stainless bolts for their alternator brackets? I can assure you that vehicles run many more hours than leisure boat engines.
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Stainless steel is not a single thing, and different grades have vastly different properties. The ones used in marine applications are mainly 316, which has good resistance to salt water, but (I understand) is not so resistant to fatigue and shearing stresses as plain steel. Bolts bought in a chandlery will almost certainly be that grade. No doubt car manufacturers use something appropriate to the application, which may well be nothing like the stainless steel we know. However I don't claim to be expert and defer to vyv_cox and Jumbleduck on material science issues.
 
AntarcticPilot said:
As I understand it - and I am not a metallurgist - stainless steel a) has a lower shear strength and b) is more susceptible to metal fatigue than mild steel. As the bolt on which an alternator swings is subject to loads that are fairly high and vary at high frequency, stainless is more likely to fail than plain steel. Of course, there are grades of stainless that would be satisfactory, but my experience was with bolts bought in a chandlery, probably 316, and two of them failed in much the same way after relatively few hours of service.
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Interesting thanks. I shall avoid them. High tensile steel sound a better bet.
 
We had an alternator bolt shear off and needed another in a hurry. The sheared bolt was one with a hex ('Allen key') socket head, but the replacement didn't quite fit as the head, which is recessed into the alternator, was very slightly larger diameter than the original. So the boat yard's helpful mechanic put it in the lathe and reduced its diameter by a mm. Some 6 seasons later it's still there and holding up fine.

This is an argument for a high-tensile steel one rather than stainless as machining stainless is very much harder to do even if you do have a lathe, and there really shouldn't be sea-water sloshing around the alternator so corrosion resistance isn't a factor.
 
jdc said:
We had an alternator bolt shear off and needed another in a hurry. The sheared bolt was one with a hex ('Allen key') socket head, but the replacement didn't quite fit as the head, which is recessed into the alternator, was very slightly larger diameter than the original. So the boat yard's helpful mechanic put it in the lathe and reduced its diameter by a mm. Some 6 seasons later it's still there and holding up fine.

This is an argument for a high-tensile steel one rather than stainless as machining stainless is very much harder to do even if you do have a lathe, and there really shouldn't be sea-water sloshing around the alternator so corrosion resistance isn't a factor.

Thanks. I enjoyed the account of your experience. I am definitely going to use high tensile
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jdc said:
We had an alternator bolt shear off and needed another in a hurry. The sheared bolt was one with a hex ('Allen key') socket head, but the replacement didn't quite fit as the head, which is recessed into the alternator, was very slightly larger diameter than the original. So the boat yard's helpful mechanic put it in the lathe and reduced its diameter by a mm. Some 6 seasons later it's still there and holding up fine.

This is an argument for a high-tensile steel one rather than stainless as machining stainless is very much harder to do even if you do have a lathe, and there really shouldn't be sea-water sloshing around the alternator so corrosion resistance isn't a factor.
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Thanks. I enjoyed the account of your experience. I am definitely going to use high tensile.
 
RichardS said:
On cars, I would be very circumspect about using stainless bolts for things like engine mounts, tow bars, brake callipers or prop shaft flanges. However, I would use them for alternator bolts.
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Agreed. One of the worst design features of the Triumph Herald is the rear suspension trunnion support, a 7/6" mild steel bolt which runs through a mild steel bush and rusts to it more or less as you fit it. The obvious answer is to use a stainless bolt and bush, but having investigated it in some detail I couldn't find any stainless bolts which were nearly as strong as the mild steel one.
 
At the risk of significant deviationm the keel bolts on my Moody are whopping big mild steel ones which are rusty in appearance, However when I saw the boat which lost its keel in the Scilly Isles there were two rows of what looked like very shiny neatly snapped stainless steel bolts.
 
jimi said:
At the risk of significant deviationm the keel bolts on my Moody are whopping big mild steel ones which are rusty in appearance, However when I saw the boat which lost its keel in the Scilly Isles there were two rows of what looked like very shiny neatly snapped stainless steel bolts.
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My Jouster's keel was help on with stainless bolts ... sixteen of them, each 3/4" diameter.
 
I had the alternator bolt sheer several times until I replaced it with a through bolt (tensile steel) with a nyloc nut on the back, since then its been okay. And just in case its long enough to be able to unscrew it from the back.
 
ColinR said:
I had the alternator bolt sheer several times until I replaced it with a through bolt (tensile steel) with a nyloc nut on the back, since then its been okay. And just in case its long enough to be able to unscrew it from the back.
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Unfortunately, a through bolt isn't possible on all engines; on my VP 2003 it goes into a threaded blind hole in the cylinder block.
 
jimi said:
At the risk of significant deviationm the keel bolts on my Moody are whopping big mild steel ones which are rusty in appearance, However when I saw the boat which lost its keel in the Scilly Isles there were two rows of what looked like very shiny neatly snapped stainless steel bolts.
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That seems to support the case for using s/s where rust might be an issue eg in contact with sea water but against using them where any load bearing is required.
 
JumbleDuck said:
Agreed. One of the worst design features of the Triumph Herald is the rear suspension trunnion support, a 7/6" mild steel bolt which runs through a mild steel bush and rusts to it more or less as you fit it. The obvious answer is to use a stainless bolt and bush, but having investigated it in some detail I couldn't find any stainless bolts which were nearly as strong as the mild steel one.
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The joys of Triumph Herald ownership. I had to Mini Travellers and a Morris Minor.
 
Caraway said:
How many vehicle manufacturers use stainless bolts for their alternator brackets? I can assure you that vehicles run many more hours than leisure boat engines.
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Almost none, the costs is the first issue, and as well mentioned here - this isn't a good application for stainless steel as it lacks the shear strength.

To the OP - it's a metric bolt so you can replace with a suitable grade 8.8 bolt , 10.9 would be stronger, but not so much in shear, only in torsion.
 
ColinR said:
I had the alternator bolt sheer several times until I replaced it with a through bolt (tensile steel) with a nyloc nut on the back, since then its been okay. And just in case its long enough to be able to unscrew it from the back.
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That sounds like the pivot bolt or the adjuster bolt. I took it that the OP's bolt was one which screwed into the block to hold the alternator mounting barcket.
 
My background is in ROV's, and every bolt on an ROV is stainless steel. In 45 years I've never seen a bolt sheer on an ROV, and that's with kit that has from 450hp down to 1hp motors.
How much is it usual for an alternator to be rated for? 1kW, which is a bit above an hp.
I'd suggest that if bolts are shearing, then it's because they weren't correctly installed. Ford probably don't use stainless because it costs more.
 
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