Question to a Mechanical Engineer

Gecko · Feb 5, 2017

I am currently playing around on a new type of pinch bolt for my Pani.
Material is Ti Grade 5.
Can anybody of you guys calculate what's the max allowed torque in Nm before it breaks? Would be awesome.

Outer diameter is M8, inner diameter M5! So wall thickness is less than a milimeter

endodoc · Feb 5, 2017

I'm confused. These are for the fork bottoms/feet?

Gecko · Feb 5, 2017

To be a bit more specific: I'm trying to find a nice way to install the air ducts from "CNC Racing". The bolts that came with the mounting kit are a joke.

I'm too scared to torque the above pinch bolts to 19Nm...

Rogue · Feb 5, 2017

I asked a friend who knows these things.

Grade 5 titanium M8 bolt
Max torque is 16nm, dry threads.
Max torque is 14nm, lubricated threads.

endodoc · Feb 5, 2017

Yet the spec from NCR/Polini for M6 engine cover (clutch case etc)bolts (Ti) is 13nm. 14nm lubricated for an M8 fastener in my opinion is a bit light. Then again I'm not a metallurgist. I'm looking at torque specs in the Mercedes F1 assembly manual. All fasteners are Ti and the torque specs for M8 fasteners are all higher than 14nm. I'm baffled how people are snapping fork bottom pinch bolts. I have pulled this front wheel trying different rotors and axles at least 15 times with the same original Ti fasteners from Ohlins always torqued to spec and never had an issue. I do not know how you snap an M8 bolt at 19nm. Again, I'm not by any means an expert.

jock39racer · Feb 5, 2017

Trade Name UNS Titanium Industry Specifications Chemical
Composition Min.Tensile
(KSI) Min.Yield
(KSI) Hardness Tensile
Modulus
Grade 5 UNS R56400 AMS 4905, 4911, 4920, 4928, 4930, 4931, 4932, 4934, 4935, 4954, 4963, 4965, 4967, 4993, AMS-T-9046, AMS-T-81915,AS7460, AS7461
ASTM B265(5), B348(5), B367(C-5), B381(F-5), B861(5), B862(5), B863(5), F1472
AWS A5.16 (ERTi-5)
MIL SPEC MIL-T-81556 AI 5.5-6.75 max
C 0.10 max
Fe 0.40 max
H 0.015 max
N 0.05 max
O 0.20 max
Ti Remaining
V3.5-4.5 130 120 16.4 114 GPa
Tensile Strength, Ultimate 950 MPa 138000 psi

T=2/3 ·π ·r3τ

where

T=torque
π=3.141592654
r=shaft radius (3.4mm)
τ = shaft material shear strength
= 2/3* 3.141592654*(0.0034)3(950000000)
= 78.2Nm

Assumptions:

1. Standard temp = 15Deg C
2. .... isn't Chinese made
3. M8*1.25

endodoc · Feb 5, 2017

What he said.

youngR · Feb 5, 2017

Gecko said:
To be a bit more specific: I'm trying to find a nice way to install the air ducts from "CNC Racing". The bolts that came with the mounting kit are a joke.

I'm too scared to torque the above pinch bolts to 19Nm...

Very nice!

Off topic but Rear Titanium Nut is $250!!

Question. You probably know the info, How much does Titanium Nuts front and rear weigh compared to others?

endodoc · Feb 5, 2017

Well I guess it would have helped if i looked at the first pic. Yea those hollow bolts may be a bit iffy at 19nm. Urs just got for it. What could possible go wrong?

Gecko · Feb 5, 2017

@jock39racer: Thank you very much. I couldn't figure out a way using Google. But that's for a non-hollow bolt I guess.

@EndoDoc: Yeah, that's the thing. I cut M5 threads into the head of the bolts that go down into the shaft with the M8 thread. I can't remember how may times I tighten these bolts on the Panigale and Monster S forks. I never had any issues. In this case, I am frightened about shearing the bolt. After some tests on a lathe I applied 15Nm, I think thats okay.

@youngR: front nut is heavier than OEM (which is made out of Aluminium). Rear one I will weight for you tomorrow, I don't have one in my hand right now.

jock39racer · Feb 5, 2017

jock39racer said:
Trade Name UNS Titanium Industry Specifications Chemical
Composition Min.Tensile
(KSI) Min.Yield
(KSI) Hardness Tensile
Modulus
Grade 5 UNS R56400 AMS 4905, 4911, 4920, 4928, 4930, 4931, 4932, 4934, 4935, 4954, 4963, 4965, 4967, 4993, AMS-T-9046, AMS-T-81915,AS7460, AS7461
ASTM B265(5), B348(5), B367(C-5), B381(F-5), B861(5), B862(5), B863(5), F1472
AWS A5.16 (ERTi-5)
MIL SPEC MIL-T-81556 AI 5.5-6.75 max
C 0.10 max
Fe 0.40 max
H 0.015 max
N 0.05 max
O 0.20 max
Ti Remaining
V3.5-4.5 130 120 16.4 114 GPa
Tensile Strength, Ultimate 950 MPa 138000 psi

T=2/3 ·π ·r3τ

where

T=torque
π=3.141592654
r=shaft radius (3.4mm)
τ = shaft material shear strength
= 2/3* 3.141592654*(0.0034)3(950000000)
= 78.2Nm

Assumptions:

1. Standard temp = 15Deg C
2. .... isn't Chinese made
3. M8*1.25

Gecko said:
@jock39racer: Thank you very much. I couldn't figure out a way using Google. But that's for a non-hollow bolt I guess.

Hi Mate,

Yes that was a poor assumption on part. Sorry.

Required Torque to snap is 90.804Nm (give the internal dia = 5mm).

Note:
The importance of correct bolt tightening cannot be over emphasized. Determining the correct torque can however present problems.

Approximately 90% of the applied torque is employed in overcoming friction, 50% at the bearing face of the nut and 40% between the mating threads. It can therefore be seen that only something in the order of 10% effort is employed inducing axial load in the bolt.

Due to many variables that affect the torque-tension relationship like human error, surface texture, and lubrication the only way to determine the correct torque is through experimentation using required lubricants and actual parts under actual joint and assembly conditions.

Gecko · Feb 6, 2017

jock39racer said:
Gecko said:

@jock39racer: Thank you very much. I couldn't figure out a way using Google. But that's for a non-hollow bolt I guess.

Hi Mate,

Yes that was a poor assumption on part. Sorry.

Required Torque to snap is 90.804Nm (give the internal dia = 5mm).

Note:
The importance of correct bolt tightening cannot be over emphasized. Determining the correct torque can however present problems.

Approximately 90% of the applied torque is employed in overcoming friction, 50% at the bearing face of the nut and 40% between the mating threads. It can therefore be seen that only something in the order of 10% effort is employed inducing axial load in the bolt.

Due to many variables that affect the torque-tension relationship like human error, surface texture, and lubrication the only way to determine the correct torque is through experimentation using required lubricants and actual parts under actual joint and assembly conditions.

Click to expand...

In this case it wouldn't be a problem at all! More than 75% safety margin.
Oehlins front fork axle pinch bolts is rated for 19Nm.
I alway go a bit lower because I use Lithium grease on all Ti bolts that go into Aluminium parts of the bike. Also I always grease the bolt shafts to avoid problems with corroding holes.

The reason for my stupid bolt head design is that it wouldn't be possible to properly torque the pinch bolts anymore if the air ducts would be installed the way CNC racing designed it.

990Glen · Feb 6, 2017

The working torque for standard Ti 5 8mm bolt can be found here
http://www.hyperbolt.com/hyperbolt/document/Hyperbolt_Titanium_Specifications.pdf
and is only 12-14 nm when lubricated.
Weakening the bolt structure with the reduction of section, not to mention the internal
5mm tapping which will create stress concentrating areas in the thread roots (this will not
be as prounounced in the external, and I would expect "rolled threadform") will push it down much lower I would expect.
I think that you aren't leaving yourself much safety margin, if any, taking the core out of these bolts. Obviously, once you screw in the second bolt, it will act as a support, but you
may pop them before hand when nipping up.

Gecko · Feb 6, 2017

990Glen said:
The working torque for standard Ti 5 8mm bolt can be found here
http://www.hyperbolt.com/hyperbolt/document/Hyperbolt_Titanium_Specifications.pdf
and is only 12-14 nm when lubricated.
Weakening the bolt structure with the reduction of section, not to mention the internal
5mm tapping which will create stress concentrating areas in the thread roots (this will not
be as prounounced in the external, and I would expect "rolled threadform") will push it down much lower I would expect.
I think that you aren't leaving yourself much safety margin, if any, taking the core out of these bolts. Obviously, once you screw in the second bolt, it will act as a support, but you
may pop them before hand when nipping up.

You might be right. On the other side I did some checks and everything was fine. Anyway, I'll try to find another better solution.

But I am getting more and more confused!! 12-14Nm is nothing. Ducati has same torque values for most Ti Bolts on the SL as Steel bolts on S. I know they were thinking about making a supercomplicated torque table, but it never happened.

My expirience:
Ti M6 to 13-15Nm, no problem
Ti M8 to 22Nm, no problem, 14Nm? Naahhh... even if cut instead of rolled thread
Ducati SL top yoke M8 bolts: 22Nm, no problems

(I always clean bolts and re-lubricate them before installation)

990Glen · Feb 6, 2017

I think I would be making a handfull of sacrificial ones and testing them to failure
before use. Better than ending up failing on the bike and possibly tearing out threads
when it goes.

Good luck dude, I admire your persistance with the weight loss

Snk1199 · Feb 6, 2017

For the sake of weight loss, the addition of air ducts should be abandoned in the first place... But I get it and would not discourage Gecko (The MAN) from further expeditions into the unknown!

jock39racer · Feb 6, 2017

Gecko said:
You might be right. On the other side I did some checks and everything was fine. Anyway, I'll try to find another better solution.

But I am getting more and more confused!! 12-14Nm is nothing. Ducati has same torque values for most Ti Bolts on the SL as Steel bolts on S. I know they were thinking about making a supercomplicated torque table, but it never happened.

My expirience:
Ti M6 to 13-15Nm, no problem
Ti M8 to 22Nm, no problem, 14Nm? Naahhh... even if cut instead of rolled thread
Ducati SL top yoke M8 bolts: 22Nm, no problems

(I always clean bolts and re-lubricate them before installation)

Hi mate,

It is imperative for people to understand that the torque settings set by the manufacturer are not so much around the yield limits of the fastening device, but how much friction the bolt/screw/dowl will create between the two materials. In this case you only have to create a force that will stop the axle bolt from moving. There is a lot of surface area between the axle and the axle housing.

Keep going mate, I envy your passion for Titanium.

bradp51 · Feb 6, 2017

youngR said:
Very nice!

Off topic but Rear Titanium Nut is $250!!

Question. You probably know the info, How much does Titanium Nuts front and rear weigh compared to others?

2 x OEM rear Axle nuts are 229.6 grams

2 x Ti CNC are 138.5 grams

Front axle in Ti is 23.5 g the OEM is 13.6 grams

Question to a Mechanical Engineer

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