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Dave Clark
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Re: Spa Classic 2017 Prep Begins

Post by Dave Clark »

U bolt diameter is oversize for cutting fresh thread with a die and, even when a very good die is used with suitable lubrication the thread is likely to be ragged and therefore weak.

This is not speculation or engineering knowledge, just speaking from experience.
Big Daddy
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Re: Spa Classic 2017 Prep Begins

Post by Big Daddy »

Dave Clark wrote:This is not speculation or engineering knowledge, just speaking from experience.
I'll get paranoia at this rate because that's the second time today. My question to Neil was genuine and I would like to learn from experience of others... :thumbs:

We have probably all produced ragged threads when metal cutting with taps and dies. But why does starting with a slightly oversize bolt shank cause this problem :?: Doesn't using a split die and adjusting clamp accordingly allow for this oversized diameter :?: Is it something to do with the need for a second pass :?: What's the mechanism :?:

I try to avoid cutting threads in bolt shanks preferring to use the correct hardware instead. But sometimes it's unavoidable on specialised components. I've resorted it many times and never encountered any problems in service.

More constructive comments please... :wave:
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Hoppend
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Re: Spa Classic 2017 Prep Begins

Post by Hoppend »

What i have been taught is:

I do not know the exact production method of the u-bolt however they are load bearing parts that are not shear esthetic.

No matter how competend and careful the operator may be, cutting threads across the grain of the metal interrupts the flow of stress through the bolt. This means that not even the best lathe-turned threads are going to be, first, as strong as the original thread, and second, as resistant to fatique as as the original thread. Turning or cutting involves tearing metal which leaves rough spots which form perfect stress risers".
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Re: Spa Classic 2017 Prep Begins

Post by Big Daddy »

Very interesting - thanks Huib... :thumbs:
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Countryboy
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Re: Spa Classic 2017 Prep Begins

Post by Countryboy »

Very interesting comments gents

Shall I start a new thread or continue with the original one ......... :lol:

Interestingly, two different suppliers bolts are very different

Earl part right BM left
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Confession

Had the Earlpart ones in stock :whistle: BM ones arrived late after being sent to my old address

Neither has much of a shoulder at the end of the thread. Surely if I cut this carefully these will be stronger than the current 62 year old items?
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Dave Clark
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Re: Spa Classic 2017 Prep Begins

Post by Dave Clark »

Huib, when you refer to the original thread I am assuming you meant that it was a rolled thread,rather than one cut by any means ?

You do seem to have confirmed my post re ragged threads but with a more reasoned explanation, which points up the difference between an engineer and an amateur.
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Re: Spa Classic 2017 Prep Begins

Post by Big Daddy »

Countryboy wrote:Neither has much of a shoulder at the end of the thread. Surely if I cut this carefully these will be stronger than the current 62 year old items?
Providing these new recut U-bolts torque up properly, then everything will be fine. They are relatively low stressed parts probably made from 8.8 grade steel... :thumbs:

Huib's comments about grain flow has me thinking. That's why forged components are stronger than castings. Also suggests that a rolled thread is inherently stronger than a cut thread... :whistle:
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Big Daddy
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Re: Spa Classic 2017 Prep Begins

Post by Big Daddy »

Dave Clark wrote:...which points up the difference between an engineer and an amateur.
Enthusiasm and natural intuition of amateurs often more than compensates for lack of formal training, as demonstrated by Colin Furze and Howden Ganley... :bow:
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Hoppend
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Re: Spa Classic 2017 Prep Begins

Post by Hoppend »

Dave Clark wrote:Huib, when you refer to the original thread I am assuming you meant that it was a rolled thread,rather than one cut by any means ?
As I do not know what the production method was. And I just want to illustrate what the disadvantages can be, when cutting the thread on a part that has been produced with any thread. That is why I did not specifically mention a rolled thread.

I did some calculating and as I see it: The U-bolts are (of course) well dimensioned at 5/16 inch but I do not know the material grade so I can only compare them with low grade bolts. The entire load on the back axle is transferred though these four bolts. And I think I would always recommend using original parts and not modify them yourself. As one can never be sure what forces the axle will endure.

CB, do the other u-bolts you have provide you with enough thread? You might be able to fit some spacer bushes between the bottom plate and the nut?
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Re: Spa Classic 2017 Prep Begins

Post by Big Daddy »

Thought I was conservative Huib... :whistle:

I cannot believe modified U-bolts will fail in service providing they tighten satisfactorily to specified torque :thumbs:
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Re: Spa Classic 2017 Prep Begins

Post by Big Daddy »

Some related data on safety critical parts.

We use M12 wheel studs on Evo2 made from 10.9 high tensile steel. Actually they are setscrews (ie fully threaded) because they attach the brake discs to custom hubs before presenting threaded studs for the wheel nuts. Unfortunately only bolts with unthreaded shanks are available for M12 at the required length. So specialised non-standard hardware is required.

I had a batch machined in 10.9 material. Thread quality is satisfactory but not to regular bolt quality. However they assembled well and accept wheel nut toque of 65 lbf.ft without problem (A35 spec is 42 lbf.ft). There have been no issues at all in service, even under harsh track conditions.

There are pragmatic ways of reliably assessing component strength... :thumbs:
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Hoppend
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Re: Spa Classic 2017 Prep Begins

Post by Hoppend »

Big Daddy wrote: I cannot believe modified U-bolts will fail in service providing they tighten satisfactorily to specified torque :thumbs:
I don't think they will, however. I try to be carefull with advising people.
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Re: Spa Classic 2017 Prep Begins

Post by Big Daddy »

Hoppend wrote:I don't think they will, however. I try to be carefull with advising people.
Huib - From a genuine point of interest, which failure mode do you believe weak U-bolts would suffer :?: ... :wave:
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Neil Evans
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Re: Spa Classic 2017 Prep Begins

Post by Neil Evans »

Big Daddy wrote:
Could it be:
Bolt diameter is over-size for die thread so more than one pass is needed :?:
In this case the die nut is unsuitable as it's 'one size fits all'
Big Daddy wrote:
Or bolt diameter is under-size (as with rolled threads) so outer newly cut thread diameter is too small impairing strength :?:
My experience of modifying components with rolled threads has never produced good results. They never produce a seamless transition and as a worst scenario the nut could be half and half on the new and old threads...
Big Daddy wrote:
Destroying cadmium plated finish :?:
Is anything cadmium plated these days? BZP perhaps, but cadmium is such a huge potential pollutant that I believe it's use for such purposes disappeared years ago...
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Big Daddy
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Re: Spa Classic 2017 Prep Begins

Post by Big Daddy »

Neil Evans wrote:Is anything cadmium plated these days? BZP perhaps, but cadmium is such a huge potential pollutant that I believe it's use for such purposes disappeared years ago...
Fair enough - just showing my age... :whistle:
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