Seastar Pro bolt failure

[9D280]

Originally Posted by jwalker

The tapping size for an M8 thread is 6.8mm.

if you be drilling a hole for an M8 "sandard" thread...... the core dia of the bolt needs to be smaller to clear that.

Originally Posted by jwalker

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.

Eh? I was simply comparing the core diameter of an M8thread with the cross section of a nominal 8mm dia bar (the top of an M8 bolt) as it was in shear.
But agreed, a dead end hole is not brilliant for a multitude of reasons..... On steering applications are we not just through bolting two holes together with a nut at the other end of the bolt, or does this yoke have a "dead end" threaded hole design? It looks like a "nut'n'bolt connection on the Seastar sales brochure PDF sketch.....

Originally Posted by jwalker

So in both cases the thread root diameter is significant.

In certian scenarios, yes, but if for example you are bolting into a differnet material to your bolt or your hole depth to diameter ratio is way too small (usually <2.5Dia) the thread engagement becomes quite significant as well, so you don't strip the threads .....

Originally Posted by jwalker

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.

Agreed. Around 0.6mm bigger root Dia on an M8X0.75. and yes, M8 certianly sounds a touch(!) on the small side for that job.


Trailerbloke,
let us know how it goes..... And if this one snaps, if you can get a pic of the break asap after the "bang" will help the diagnosis......

[jwalker]

Originally Posted by jwalker

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.

Originally Posted by 9C280

Eh? I was simply comparing the core diameter of an M8thread with the cross section of a nominal 8mm dia bar (the top of an M8 bolt) as it was in shear.
But agreed, a dead end hole is not brilliant for a multitude of reasons.....

Sorry, on re-reading my comment I can see it will be easily misunderstood and you thought I was talking about a blind threaded hole. I realised you were comparing a bolt and a screw for strength but I believe, in the steering application, they both have approximately the same strength. My point, in this application, is that you must never use a bolt where the top of the thread screws fully into the steering arm because you cannot guarantee that the bolt is properly clamping the pivot bush or spherical joint. But this will ensure a section of thread is above the steering arm and therefore it will have much the same strength as a screw. I can see a situation where a bolt may be less good because the rigidity at the top of the bolt may concentrate any bending into the thread area.

On steering applications are we not just through bolting two holes together with a nut at the other end of the bolt,..

Nope, the steering arm has a thread in it into which the bolt screws after passing through the steering pivot or spherical joint. I do think, if there's space, it's a good idea to fit a full nylock nut onto the end of the bolt where it emerges through the steering arm because any seizing of the pivot will tend to rotate the screw/bolt. The nylock nut will act as a locknut. If the bolt/screw is cut off flush with the nylock nut and the threads are greased on assembly, this will also prevent corrosion of the protruding thread and ease disassembly.

[Limey Linda]

Have just looked again at the pics. of the screw. That may be a cheap rolled thread screw which can set up internal stresses leading to failure. IMHO the ideal set up is a shoulder bolt thru. the steering arm bushing with a heavy dished tension washer ( belville,berilium) ) under the head, threaded into the tiller arm with a bronze washer and then backed up with a nylock nut. That is what I have. You may have to play around with packing washers and shim washers to get it right. Oh, put some oil on it now and again.

[9D280]

Originally Posted by jwalker

My point, in this application, is that you must never use a bolt where the top of the thread screws fully into the steering arm because you cannot guarantee that the bolt is properly clamping the pivot bush or spherical joint. But this will ensure a section of thread is above the steering arm and therefore it will have much the same strength as a screw. I can see a situation where a bolt may be less good because the rigidity at the top of the bolt may concentrate any bending into the thread area.

Yep, I definely mistread that one, apologies for causing confusion. Both points are valid, even tho' we were talking about different things! I was basing my comments on my old engine where the tiller / arm connection was made using non - threaded holes & a nylock nut on thre end of a bolt which was free to rotate in both sides of the connection - three washers as well - top, between & bottom of the connectioon as well to allow it to rotate. The "pivoting" on engine tilt was achieved by the steering bowden cable inner core rotating in the steering tube.

Originally Posted by jwalker

Nope, the steering arm has a thread in it into which the bolt screws after passing through the steering pivot or spherical joint.

Looks like I may have tripped over some "modiffied" steering gear in my time.

Originally Posted by jwalker

I do think, if there's space, it's a good idea to fit a full nylock nut onto the end of the bolt where it emerges through the steering arm because any seizing of the pivot will tend to rotate the screw/bolt. The nylock nut will act as a locknut. If the bolt/screw is cut off flush with the nylock nut and the threads are greased on assembly, this will also prevent corrosion of the protruding thread and ease disassembly.

I'll second that.


It's funny how you never have problems like this with rope steering!!!

[Cookee]

I have been racing with Seastar Pro for over 10 years now with up to 280hp and over 90mph - never had a failure and you do get a nyloc nut with it as well. Our raceboat steering has had some quite hard work as well!

[Mollers]

Originally Posted by Cookee

I have been racing with Seastar Pro for over 10 years now with up to 280hp and over 90mph - never had a failure and you do get a nyloc nut with it as well. Our raceboat steering has had some quite hard work as well!

I have Seastar on the new boat and always reckoned it to be one of the the best systems available.

[Trailer Bloke]

Originally Posted by Mollers

I have Seastar on the new boat and always reckoned it to be one of the the best systems available.

I agree I have been using seastar for years as well and they are the best, however I think if you use them like we do maybe we should change the bolts regularly!

I have attched pics of the two fractured bolts (remember these were two different set ups)