Small Diameter Wrist Pin Life

[axegrinder]

In high power combinations (700 to 900 NA), do .748 and .787 diameter wrist pins require pressure oiling through the rods to live? This seems like one of those areas where everything around the pin must be engineered perfectly to work. I would'nt think the strength of a thick wall, short pin itself would be the problem.

[Adger Smith]

Piston flex is also a problem with the short pins & some pistons. Like you say, design is the key.

[Adger Smith]

After thinking about what a Pro Stock truck customer that I worked with went through on pins. I would be real apprehensive of putting the small pins in anything with a big bore and over 800 hp.

[axegrinder]

After thinking about what a Pro Stock truck customer that I worked with went through on pins. I would be real apprehensive of putting the small pins in anything with a big bore and over 800 hp.

Was he running an X-forging piston?

[panic]

With .100" wall thickness and constant length (for simplicity), here's the bending resistance of some common pin sizes expressed in "stiffness units":
.748 = 33 (units)
.787 = 39
.875 = 56
.927 = 68
.990 = 84
1.000 = 87
1.030 = 96
1.094 = 117

This produces the greatest proportionate effect per dimension change.

Thick pin wall isn't as much help as you might suppose. With 1.000 OD and common length, here's the bending resistance of some wall thicknesses:
.039 = 41
.060 = 59
.083 = 76
.090 = 81
.100 = 87
.120 = 98
.140 = 108

Increasing the WT produces less than 1:1 effect on the stiffness.

You're also limited in length to the rod's pin eye width.
Here's bending resistance by total lengths (across the locks) with other factors removed.
1.50 = 260
1.60 = 210
1.70 = 180
1.80 = 150
1.90 = 130
2.00 = 110
2.10 = 90
2.20 = 80
2.30 = 70
2.40 = 63
2.50 = 56

[Adger Smith]

Interesting what .140 dia.,.020 wall thickness and .500 length does to overall strength.

[axegrinder]

With .100" wall thickness and constant length (for simplicity), here's the bending resistance of some common pin sizes expressed in "stiffness units":
.748 = 33 (units)
.787 = 39
.875 = 56
.927 = 68
.990 = 84
1.000 = 87
1.030 = 96
1.094 = 117

This produces the greatest proportionate effect per dimension change.

Thick pin wall isn't as much help as you might suppose. With 1.000 OD and common length, here's the bending resistance of some wall thicknesses:
.039 = 41
.060 = 59
.083 = 76
.090 = 81
.100 = 87
.120 = 98
.140 = 108

Increasing the WT produces less than 1:1 effect on the stiffness.

You're also limited in length to the rod's pin eye width.
Here's bending resistance by total lengths (across the locks) with other factors removed.
1.50 = 260
1.60 = 210
1.70 = 180
1.80 = 150
1.90 = 130
2.00 = 110
2.10 = 90
2.20 = 80
2.30 = 70
2.40 = 63
2.50 = 56

Thanks for the info panic. So looking at your numbers, a 24% increase in diameter (.748 to .927) more than doubles the stiffness. But a 20% increase in wall thickness (.100 to .120) only increase stiffness by 12.6%. And the total length makes a huge difference, explaining why the high end guys use very short pins.

Would squashing of the pin need to be taken into account seperately? And wouldnt wall thickness help this problem at a pretty high percentage?

[Adger Smith]

Now let's talk materials. C-350, C-300, M2, H-13 H-11, 9310, ect. That is where the real strength is and what the material does in compression and flex. A well know T/F team buys a lot of M2 pins so we used them in an A/F dragster engine I helped build. It's best asset is it is crack resistant. From what I understand a lot of the P/S teams use C-300 or C-350 material. The material and coating is what cost you on pins. I've found it is well worth the expense, esp in blown or NOS applications. I've seen too many shelf stock piston pins fail when the power levels go way up.

[RednGold86Z]

So looking at your numbers, a 24% increase in diameter (.748 to .927) more than doubles the stiffness. But a 20% increase in wall thickness (.100 to .120) only increase stiffness by 12.6%. And the total length makes a huge difference, explaining why the high end guys use very short pins.

As with any bending stress, it's all (OK, 80%) about where the material IS (how far from the center axis) in order to resist that bend. Don't quote that 80% - just sayin' it's important.

[Walter R. Malik]

With a piston pin, the ultimate strength condition is usually NOT the key.
"Toughness" IS ... how strong is it when it distorts and how much distortion can it endure before failing ?
Also, when it distorts, how much does that affect the piston ?

[axegrinder]

Now let's talk materials. C-350, C-300, M2, H-13 H-11, 9310, ect. That is where the real strength is and what the material does in compression and flex. A well know T/F team buys a lot of M2 pins so we used them in an A/F dragster engine I helped build. It's best asset is it is crack resistant. From what I understand a lot of the P/S teams use C-300 or C-350 material. The material and coating is what cost you on pins. I've found it is well worth the expense, esp in blown or NOS applications. I've seen too many shelf stock piston pins fail when the power levels go way up.

C-350: 8.08 grams/cc
9310: 7.80grams/cc
Titanium: 4.65grams/cc

Looks like the Pro Stock guys are going in the opposite direction of lite weight materials.

[Kevin Johnson]

I have been curious as to whether an interference fit of two concentric hollow pins would allow a pre-stressed net combination that could use a thinner combined wall thickness that would exceed the material properties of a single hollow pin of that wall thickness. Might be too exotic/expensive for engines but the lower mass might be useful for spacecraft. Just a thought.

[Warp Speed]

Now let's talk materials. C-350, C-300, M2, H-13 H-11, 9310, ect. That is where the real strength is and what the material does in compression and flex. A well know T/F team buys a lot of M2 pins so we used them in an A/F dragster engine I helped build. It's best asset is it is crack resistant. From what I understand a lot of the P/S teams use C-300 or C-350 material. The material and coating is what cost you on pins. I've found it is well worth the expense, esp in blown or NOS applications. I've seen too many shelf stock piston pins fail when the power levels go way up.

C-350: 8.08 grams/cc
9310: 7.80grams/cc
Titanium: 4.65grams/cc

Looks like the Pro Stock guys are going in the opposite direction of lite weight materials.

A wrist pin is not one of the areas to save weight in. Hence the direction you mentioned!

[clshore]

Although the form of pins is symmetrical (hollow cylinder), the bending loads it sees are not.
Gas loads are basically along the axis of the conrod between pin and bigend.
Inertia loads do contribute a significant radial load, but are far less at the pin end than bigend.

Assuming that we retain the cylindrical outer bearing surface, the inner profile could be optimized to better distibute the material to handle
the loads, like an I beam, optimized to handle bending loads, except in more than just one direction.
Basically, increasing the MOI, and radius of gyration.

Imagine a flattened oval inside shape, where the thickest parts are at top and bottom.
And the form could also be tailored/tapered to the position along the pin, since the outer edges don't really contribute much to the bending stiffness, but must still
resist flattening and distortion under loads from the piston.

The result is a complex internal shape.

Of course, you could not run these as floating pins (unless you somehow contrived a linkage to keep them aligned, but for what purpose?).

Fabricating such pins could be done with specialized grinding tools, or perhaps EDM.

(Maybe someday we can fab them with Additive Manufacturing aka 3D printers

Carter

[DCal]

Over the years I have seen numerous designs that were meant to stiffen the pin while attempting to keep the mass down. Some which had ribs formed lengthwise internally, some had two ribs formed where the unsupported portion lays between the pin bosses and the rod end, and some that had tapered ends but looked like washers were welded on and some that were machined that way. In the end we're mostly using straight wall pins, shorter and thicker as Panic has posted.