Trimming the fat from wrist pins.

RonA

RonA

Active member
Joined
Feb 26, 2008
Messages
11,080
Is the preferred method to machine an internal radius like the tip(ogive) of a bullet? I don't hear much about wrist pin failures so it seems like a bit of thinning might not hurt.
 
I agree, but I don't know how much they will accept taken off. Why not machine them out of 7075?
 
joefarmer said:
I agree, but I don't know how much they will accept taken off. Why not machine them out of 7075?
Click to expand...

Elastic modulus of aluminum alloys = 1/3 that of alloy steels. The thing will bend like a noodle if machined to the same physical dimensions. You could make it a solid but the center core doesn't help much with stiffness.

Can't see that working in a diesel, at least not in one bigger than a weedwacker.
 
nwpadmax said:
Elastic modulus of aluminum alloys = 1/3 that of alloy steels. The thing will bend like a noodle if machined to the same physical dimensions. You could make it a solid but the center core doesn't help much with stiffness.

Can't see that working in a diesel, at least not in one bigger than a weedwacker.
Click to expand...
Thanks Mr Resident Metallurgist! ;) How about Ti-something?
 
joefarmer said:
Thanks Mr Resident Metallurgist! ;) How about Ti-something?
Click to expand...


Titanium alloys are a step up from Al but still only half that of steel. Amazing stuff, those old antique iron based fuddy duddy no-bling alloys.

Steels = ~29 million psi
Ti alloys = ~16 million psi
aluminum alloys = ~10 million psi

If ya wanna get nerdy, the deflection of the wrist pin is always directly proportional to the section moment of inertia (I) and the Young's modulus of the material (E). The formula for a solid round bar is this:

I = pi * D^4 / 64
and the deflection of a simply supported beam is:
y = Load * length^3 / 48*E*I)

So, as I and E go up, deflection goes down, and your wrist pin bearings are happier. And your wrist pin thanks you and agrees to function for another day and shrugs off fatigue.

The biggest thing to notice in all of this is that the stiffness (and all related stresses) are dependent on D (diameter) to the 4th power.

This means if you are going to try to go smaller diameter, your only choice is a steel pin. If you wanna go lightweight wall, you need D as large as you can get away with in order to deal with the thinner wall or more flexy material.

Titanium does not have stellar wear/bearing properties. It is grippy and not slippery, and more tricky to get a nice surface finish on. So if you were going to try that route, I'd be looking for a surface treatment real fast.

I just did a quick Google search and the Ti pins are out there (motorcycles, VW TDIs, couple others). Diamond-like carbon (DLC) coated.
 
They do rifle barrel rifling in a 5R design meaning they have radius' at the bottom and top of the groove (basically anywhere there were previously sharp edges) to decrease the deformation of the bullet as it enters the rifling. The same basic priciple should prove to be stronger than a simple straight edge flute or groove.
 
Idaho CTD said:
They do rifle barrel rifling in a 5R design meaning they have radius' at the bottom and top of the groove (basically anywhere there were previously sharp edges) to decrease the deformation of the bullet as it enters the rifling. The same basic priciple should prove to be stronger than a simple straight edge flute or groove.
Click to expand...

Doing that would reduce stress risers, which if you have a part prone to brittle fracture, would be a big deal. Or in fatigue. In this case you have the fully reversing cyclic stress, which is a worst case scenario.

As for simple ductile fracture, I think it's less clear how that would turn out. Only FEA would tell.

I don't have a lot of experience in engine building but I have not heard of outright pin failure in diesels. Even if it did fail, how would you sort it all out...since there will likely be a couple hundred other chunks of broken chit in there when that happens.
 
nwpadmax said:
Titanium alloys are a step up from Al but still only half that of steel. Amazing stuff, those old antique iron based fuddy duddy no-bling alloys.

Steels = ~29 million psi
Ti alloys = ~16 million psi
aluminum alloys = ~10 million psi

If ya wanna get nerdy, the deflection of the wrist pin is always directly proportional to the section moment of inertia (I) and the Young's modulus of the material (E). The formula for a solid round bar is this:

I = pi * D^4 / 64
and the deflection of a simply supported beam is:
y = Load * length^3 / 48*E*I)

So, as I and E go up, deflection goes down, and your wrist pin bearings are happier. And your wrist pin thanks you and agrees to function for another day and shrugs off fatigue.

The biggest thing to notice in all of this is that the stiffness (and all related stresses) are dependent on D (diameter) to the 4th power.

This means if you are going to try to go smaller diameter, your only choice is a steel pin. If you wanna go lightweight wall, you need D as large as you can get away with in order to deal with the thinner wall or more flexy material.

Titanium does not have stellar wear/bearing properties. It is grippy and not slippery, and more tricky to get a nice surface finish on. So if you were going to try that route, I'd be looking for a surface treatment real fast.

I just did a quick Google search and the Ti pins are out there (motorcycles, VW TDIs, couple others). Diamond-like carbon (DLC) coated.
Click to expand...

here are my tool steel H13 pins with DLC coatings ,


P1040863.jpg
 
H13:
Carbon.............0.37 - 0.42
Manganese.......0.20 - 0.50
Phosphorus...... 0 - 0.025
Sulfur...............0 - 0.005
Silicon..............0.80 - 1.20
Chromium.........5.00 - 5.50
Vanadium.........0.80 - 1.20
Molybdenum......1.20 - 1.75

4340:
Element Weight %
C.......0.38-0.43
Mn.....0.60-0.80
P.......0.035 (max)
S.......0.04 (max)
Si......0.15-0.30
Cr......0.70-0.90
Ni......1.65-2.00
Mo.....0.20-0.30

H13 is a classical steel for hot-working dies (meaning it holds its properties up to higher temps and thus the increased alloy content). I'm curious about how this one will handle fatigue vs. the more typical 4340.
 
Last edited:
Cool metallurgy info, I just use why I know has been working in the past of high hp engines , . My pins were about $80 a piece, and 100 grams lighter

The bob weight on my dragster is 1000 grams lighter then stock.
This allows us to get rid of the 27 lbs harmonic balancer on the front of the engine. The counter weight is a reason people are having trouble with the front and rear main in a Dmax engine.
I do not plan on loading this engine below 4500 rpms , and will use the tuning to kill torque below that level.
 
Last edited:
4340 shows a heat capacity of 0.11 BTU/lb-°F and thermal conductivity 309 BTU-in/hr-ft²-°F in the ASM spec.

H13 shows heat capacity as 0.114 BTU/lb-°F and thermal conductivity 169 BTU-in/hr-ft²-°F in the ASM spec.

I read that to mean the H13 pins will the same heat input but transfer heat slower than the 4340. Making them slower to heat up and slower to cool off. Right?
 
joefarmer said:
4340 shows a heat capacity of 0.11 BTU/lb-°F and thermal conductivity 309 BTU-in/hr-ft²-°F in the ASM spec.

H13 shows heat capacity as 0.114 BTU/lb-°F and thermal conductivity 169 BTU-in/hr-ft²-°F in the ASM spec.

I read that to mean the H13 pins will the same heat input but transfer heat slower than the 4340. Making them slower to heat up and slower to cool off. Right?
Click to expand...

Not sure what that has to do with the price of tea in China, but yeah.

Generally speaking, the more alloy content you put in something, the worse it transfers heat. For example, the tin and zinc added to copper to make bronze and brass, takes a lot of the conductivity out. In this case you can likely blame the Cr content in H13.


BTW Brandon, your avatar showed up in Urban Dictionary....y'all ain't right :hehe:
 
You must log in or register to reply here.
Top