Shear Quandary

R

reeac

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Went to buy a replacement shear pin for a Mariner 3.3 and was faced with a choice: did I want weak or strong? Seemed a bit tricky to me - buy weak ones and have to put up with possibly frequent failures or buy strong ones and risk damaging something elsewhere in the motor in the event of an impact. I chose the strong ones on the grounds that the one that failed appeared to be brass and therefore a weak one and it failed for no discernible reason. Why can't the manufacturer just make "medium" ones that protect but don't fail for no reason?
 
I had the same problem when I first got my 3.3.
The shear pins seem to go every few minutes; to the point of being a hazard.

I replaced mine with a strong one (a stainless pin, so I don't think it's likely to break)

Never had a problem for the last 7 or 8 years. The pointy "keel" at the bottom of the leg goes below the prop and seems to give adequate protection.
 
3 types available - brass, bronze or stainless. Weakish, medium or (very) strong.

I bought a length of brass rod (B&Q) and cut around 36 pins - rod cost around £2.50 IIRC, which is cheaper than one OEM pin.

So far no problem, son clipped something and broke one but 5 mins changed it. I would rather change a pin now and again rather than break something else like the gear box or prpoellor.
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Artificial Intelligence is no match for Natural Stupidity
 
i got fedup with changing mine, it was either broken or twisted which ruined the prop, in the end i found by accident that the stainless rivets i bought from ebay costing 2.75 for 50 had the same size pin. this has been in use for 2 years now and is still fine.

steve
 
We had a strong one on our mariner 3.3. Not exactly sure how but it recently shreded the plastic prop and was rather diff to remove, required hacksaw. New Prop £50+. Would still go for the strong version, just don't run the prop into the sand/ mud.
 
Shear pins are engineered, they're not just a bit of brass/steel/plastic/WHY that fits the hole.

Their purpose is to protect the prop when it strikes something and they are designed to shear at a predetermined load. Plastic props are more easily damaged than alluminium ones, so they require a weaker shearpin to give adequate protection.

Slap a bit of SS in and it stops being a "shearpin".

If you're going to make up your own, as suggested by Cliffe, don't forget to scribe a circumferential grove in them at least 10thou deep to give a suitable "weak" point for it to shear through; but also don't expect it to have the same performance parameters as the factory ones.
 
[ QUOTE ]
Shear pins are engineered, they're not just a bit of brass/steel/plastic/WHY that fits the hole.

Their purpose is to protect the prop when it strikes something and they are designed to shear at a predetermined load. Plastic props are more easily damaged than alluminium ones, so they require a weaker shearpin to give adequate protection.

Slap a bit of SS in and it stops being a "shearpin".

If you're going to make up your own, as suggested by Cliffe, don't forget to scribe a circumferential grove in them at least 10thou deep to give a suitable "weak" point for it to shear through; but also don't expect it to have the same performance parameters as the factory ones.

[/ QUOTE ]

That's all very well in theory but the "factory ones" that I got from the chandlers are supposed to be the correct ones , are stainless steel and don't have any scribed groove.
 
Having designed heavy duty industral shear sprockets what topcat47 says is the correct engineering method.

[ QUOTE ]
the<span style="color:red"> "factory ones" </span> that I got from the<span style="color:red"> chandlers </span> are <span style="color:red"> supposed to be </span> the correct ones

[/ QUOTE ]

That is the point "supposed to be the correct ones " but are they ?

IMHO there must be a weak point in the pin at the position the shear pin should break other wise the breaking of the pin could damage the boss of the prop.
 
Thanks for the support Roger,

Not only have I never seen a shearpin without a grove, even the SS ones have a neck of some sort, the one on my small outboard is hard plastic.

It's been while since I worked in a Marine Engineers design office but the whole point of shearpins and weak links in general is to protect the valuable assets by inserting a cheap and cheerful failure mechanism, preferably somewhere easily fixed.

I have a "weak" link down by my anchor as I'd rather lose the anchor than the vessel.
 
I don't want to prolong things too much but a quick look at my back-up Suzuki outboard reveals that its spare shear pins are stainless steel with no grooves and these pins were supplied by the manufacturer when the motor was new. I understand about weak links, stress concentrations, stress intensity factors [a parameter found in fracture mechanics] etc. but I'm just reporting what I've observed.
 
[ QUOTE ]
I understand about weak links, stress concentrations, stress intensity factors [a parameter found in fracture mechanics] etc.

[/ QUOTE ]

So do I.

But I still use a cut down split pin.
 
[ QUOTE ]
If you're going to make up your own, as suggested by Cliff, don't forget to scribe a circumferential grove in them at least 10thou deep to give a suitable "weak" point for it to shear through; but also don't expect it to have the same performance parameters as the factory ones.

[/ QUOTE ]Oops - forgot to mention TWO grooves in line with the ends of the hole through the shaft - pins will then shear cleanly leaving the two ends in the prop and the center portion in the shaft hole.
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Artificial Intelligence is no match for Natural Stupidity
 
I have fiddle d with Johnson/Evinrude 6 HP for many years. No i hafve never seen a groove or weak point in the shear pin. In this case a large nail can be cut down to length.
The pins shear in the Johnson in 2 places leaving one section in the shaft and a piece each side in the prop. I have actually got one mobile again by removing one piece of the broken pin and replacing 2 pieces that straddle the shear area.

The shear pins tend to be boken when the engine is reving too high when the motor is put into gear. The inertia weight of the prop causing the stress. So always discipline yourself to slow the engine down before puting it in gear. No matter how hard it was to start.

olewill
 
[ QUOTE ]
Shear pins are engineered,

[/ QUOTE ]

The brass ones which I used on my Mariner were certainly engineered, but they were engineered to be totally inadequate for the job. A money making scheme for the chandlers and the manufacturer.

The stainless one (without grooves) has been fine for several years. Much better than the brass ones (also without grooves)
 
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