Seastar Pro bolt failure

Trailer Bloke · 10 June 2008, 21:12

We have had the coupling bolt between the tiller arm and the steering yoke sheer off on a seastar pro steering.

This has occured twice now on two different set ups Honda 150 and DF300 and is exciting when it happens!

Is there anyway of preventing this (apart from changing the bolts every few month!)

The bolt in question is a fine thread (a 14mm spanner fits the head?)

Any suggestions / comments

doggypaddle · 10 June 2008, 21:21

Is it a stainless steel bolt? Perhaps you could replace it with a High tensile steel one?
If its metric use one with 12.9 stamped on the head, imperial six radial lines or a letter T show a high tensile bolt.
I think most stainless steel bolts are relitevley low tensile strength compared to HT bolts.

Tim M · 11 June 2008, 06:33

I was talking to a chap who runs 2 charter RIBs with Suzuki 300s on the back, who was worried about this happening as I think it's happened once with him. Suzuki's advice was simply to check the bolt regularly and change if you suspect anything dodgy. It's something I've become paranoid about now.

Trailer Bloke · 11 June 2008, 09:47

Quote:

Originally Posted by doggypaddle

Is it a stainless steel bolt? Perhaps you could replace it with a High tensile steel one?
If its metric use one with 12.9 stamped on the head, imperial six radial lines or a letter T show a high tensile bolt.
I think most stainless steel bolts are relitevley low tensile strength compared to HT bolts.

These are stainless with a very fine thread - the thread has to be the same as it is tapped into the tiller arm

Trailer Bloke · 11 June 2008, 09:50

Quote:

Originally Posted by Tim M

I was talking to a chap who runs 2 charter RIBs with Suzuki 300s on the back, who was worried about this happening as I think it's happened once with him. Suzuki's advice was simply to check the bolt regularly and change if you suspect anything dodgy. It's something I've become paranoid about now.

Tim

How do you check the bolt? Isnt it either OK (and looks fine) or broken

Did Jimbo tell you I have got a hydrolic steering in my garage for your new boat by the way?

9D280 · 11 June 2008, 09:56

Quote:

Originally Posted by Trailer Bloke

The bolt in question is a fine thread (a 14mm spanner fits the head?)

by my reckoning you're holding the steering torque of a 300 HP engine by a single M8 bolt in shear........ if it's a screw (threaded all the way to the head) then the actual diameter doing the work is probably nearer 6.5mm!

I'd suggest make sure its a "proper" bolt with a non threaded section in the hole (pack it with washers if you have to) and if possible, go up a size of bolt.

Trailer Bloke · 11 June 2008, 11:18

Quote:

Originally Posted by 9D280

by my reckoning you're holding the steering torque of a 300 HP engine by a single M8 bolt in shear........ if it's a screw (threaded all the way to the head) then the actual diameter doing the work is probably nearer 6.5mm!

I'd suggest make sure its a "proper" bolt with a non threaded section in the hole (pack it with washers if you have to) and if possible, go up a size of bolt.

Good suggestion and yes you are right about the amount of 'meat' actually in the bolt.

There is however the restriction is the seastar yoke has a bearing in it that cannot be enlarged so I will drill the engine tiller arm out and then put as large HT bolt through as the bearing will take- M8 or maybe even slightly larger imperial size- it looks as though I will be off to the bolt shop!

doggypaddle · 11 June 2008, 12:24

just changing to a HT bolt will make all the difference, stainless are as soft as the proverbial

assuming its 6mm diameter fine its probably m6x.75 or 1mm pitch.

9D280 · 11 June 2008, 13:29

errrrr that depends on your grade of stainless!

Trailerbloke,
if you are planning to drill the arm out for a bigger bolt, but can't drill out the yoke 'coz of the bearing, was the fastener you had in there "rattling" in the yoke?

I assume from your response it was a "threaded all the way" bolt? - Got any pics of the break? I have a theory, but if I post it now the thread will fill with discussions based on a guess!

philb · 11 June 2008, 14:17

we have the same setup, SS Pro with DF300, a pic would be helpful, better go and have a look asp

doggypaddle · 11 June 2008, 19:24

[QUOTE=9D280;252081] errrrr that depends on your grade of stainless!

I did say "most" stainless bolts are softer than the HT steels

The strongest commonly available SS bolts could find were A4-80 which are 800MPa where as the standard stainless bolts in most of the chandlerys were 400MPa a 12.9 non stainless bolts are easy to get and correct me if i am wrong are 1200Mpa?
400 Mpa is really soft!

I guess a bigger bolt would be best, going from a M8 to a 7/16 or even M10 adds loas of cross sectional area.

Trailer Bloke · 11 June 2008, 19:29

Quote:

Originally Posted by 9D280

errrrr that depends on your grade of stainless!

Trailerbloke,
if you are planning to drill the arm out for a bigger bolt, but can't drill out the yoke 'coz of the bearing, was the fastener you had in there "rattling" in the yoke?

I assume from your response it was a "threaded all the way" bolt? - Got any pics of the break? I have a theory, but if I post it now the thread will fill with discussions based on a guess!

The bolt was tight and not rattling in the yoke

I have not got a pic of the break but it was a clean fracture where it started to go into the engine tiller arm. This was exactly the same place it went on the Honda150

I have attached a pic of a 'new' bolt the old one got dropped

interestingly it was a 'stress' fracture in that there were lines around the bolt (like tree rings) rather than a shear fracture

codprawn · 11 June 2008, 21:24

I might be missing something here but is there a threaded part of the bolt going through a nonthreaded hole?

Trailer Bloke · 11 June 2008, 21:50

Quote:

Originally Posted by codprawn

I might be missing something here but is there a threaded part of the bolt going through a nonthreaded hole?

This is correct the threaded bolt goes throgh a bearing on the steering yoke

codprawn · 11 June 2008, 22:11

That is seriously bad practice - they want shooting!!!

Trailer Bloke · 11 June 2008, 22:26

Quote:

Originally Posted by codprawn

That is seriously bad practice - they want shooting!!!

Thats the standard bolt that comes with the Seastar Pro set up

jwalker · 11 June 2008, 22:56

Just to be pedantic so you know what you're buying; a bolt has a plain section and a threaded end, a set screw has a thread for the full length.

Knowing the tensile strength is not of much value when the bolt is in shear. Knowing the shear strength and the fatigue characteristics will be worthwhile in that application.

Quote:

Originally Posted by Codders

That is seriously bad practice - they want shooting!!!

Would you like to explain yourself Codders?

9D280 · 12 June 2008, 12:10

Codders, Jwalker
I think we are all thinking along similar lines.......

Shear strength of the material aside, M8 thread has approx 6.4mm "core" diameter (that looks like a standard M8 thread) = 32 sq mm cross sectional area. M8 bolt with nominal OD (non threaded bit) of 8mm has a sectional area of 52 sq mm. Equates to roughly a 35% drop in your "working" area just by using a screw.

Now on to fatigue, which was my theory. If you have the threaded section in the area that is in shear, the mere fact there is a thread will gives you an instant stress raiser (the thread root). As the bolt will probably be rotating slowly in the hole every time you turn (even if it doesn't rattle round with the engine vibrations), the stress will be moving round the diameter over time. The "tree like" rings are the tell tale signs of a creep fracture. Was there a kind of "slightly lumpy" bit in the centre? If so that's where the remaining material exceeded it's tensile limit and suffered a brittle fracture. I wouldn't expect "tree rings" from a simple shear. If it was done up "tight" there was probably some tensile stress along the length of the bolt as well.

So, I would suggest:
1) measure your yoke hole (the one with the bearing), If you need to drill out the arm to match it, do so.
2) Get a proper bolt (not a screw), a couple(*) of washers and a nylock nut to fit your hole.
3) Do it up but not drum tight (hence the nylock). Make sure it isn't rattling, but not silly tight - same idea as your trailer bearing nut.

(*) may need more to pack out to the thread

jwalker · 12 June 2008, 14:45

Quote:

Originally Posted by 9D280

... M8 thread has approx 6.4mm "core" diameter (that looks like a standard M8 thread) = 32 sq mm cross sectional area.

The tapping size for an M8 thread is 6.8mm.

Quote:

M8 bolt with nominal OD (non threaded bit) of 8mm has a sectional area of 52 sq mm. Equates to roughly a 35% drop in your "working" area just by using a screw.

A bolt would need some very careful setting up for that to be the case. Normally the bolt will not be screwed right into a threaded hole otherwise you would not know whether your tightening is jamming the thread end into the hole or applying clamping pressure. So in both cases the thread root diameter is significant.

In the steering application, the thread in the steering arm is usually a fine pitch so the root diameter will be larger than that on a standard threaded bolt. An M8 thread, course or fine pitch, is not usual in a steering arm.

Trailer Bloke, I suggest you try again being sure to use a high tensile bolt and make certain it is tightened properly. It is important that the bolt thread is not bottomed out in the steering arm otherwise the bolt will be subjected to a bending stress and constant flexing can lead to fatigue. Stainless steel is poor in cyclic flexing situations.

Trailer Bloke · 12 June 2008, 15:58

Quote:

Originally Posted by jwalker

The tapping size for an M8 thread is 6.8mm.
A bolt would need some very careful setting up for that to be the case. Normally the bolt will not be screwed right into a threaded hole otherwise you would not know whether your tightening is jamming the thread end into the hole or applying clamping pressure. So in both cases the thread root diameter is significant.

In the steering application, the thread in the steering arm is usually a fine pitch so the root diameter will be larger than that on a standard threaded bolt. An M8 thread, course or fine pitch, is not usual in a steering arm.

Trailer Bloke, I suggest you try again being sure to use a high tensile bolt and make certain it is tightened properly. It is important that the bolt thread is not bottomed out in the steering arm otherwise the bolt will be subjected to a bending stress and constant flexing can lead to fatigue. Stainless steel is poor in cyclic flexing situations.

I have now got a 'new' bolt from Teleflex that has a non threaded part and then a very long thread, the bolt is not shiney stainless but looks like HT engine bolts.

The bolt that was in the engine (and broke) was from Teleflex but was shiney stainless and was threaded all the way up