Engineering Calculation

[noelex 77]

Can someone calculate the size of stainless steel rod that would have the same yield strength as a 16mm diameter aluminium rod
The aluminium is 6061 T6 (yield strength from Wikipedia 35,000 psi)

My guess is 16 mm diameter rod in aluminium would be similar to about 12mm stainless steel.

[bobconnie]

Have to know which stainless ! easy to with a machinst handbook or on line if you know the number go for it

[noelex 77]

Originally Posted by bobconnie

Have to know which stainless ! easy to with a machinst handbook or on line if you know the number go for it

The equivalent in 316 or 304 would be great.

[delmarrey]

I'm not sure what your after but I do know that SS will lift around 3 to 3-1/2 times what aluminum will as far as typical tensile strengths. If it's for bolts/rod it will depend on weather its a rolled thread or cut thread.
There are so many variables. What do you plan to use it for?

Need more details.

[noelex 77]

It's for the axle of the bow roller. The existing axles are 16 mm stainless steel. The bow roller assembly is welded from 13mm thick 5083 aluminium on a 47 foot aluminium boat. There a 3 nylon (I think) rollers on the stainless steel axels.
I have had to cut one of the stainless axels to replace a split nylon roller.

I have some 16 mm solid aluminium rod on board and using this avoids the dissimilar metal problem of the stainless rods. The aluminium rods will be easy to inspect and replace if they bend, but it would be nice to know if the aluminium will be strong enough.
Wandering around the boatyard I see some similar boats use thinner stainless steel axels on their bow rollers and these have survived. So I was trying to calculate the equivant strength in stainless for a 16mm unthreaded aluminium 6061 T6 round bar. If this is grater than the diameter of axels other similar boats are using it should mean the aluminium rod will be acceptable.

[delmarrey]

What your most likely after would be shear strength or bending resistance. Alum 6061 shear is about 30,000 psi and I believe 316 SS is around 75,000.

But here's a chart that shows some details. Modulus of Rigidity

And yeah I'd go with the aluminum bar and dump the SS bolts. You can actually buy alum bolts. >>> http://data.axmag.com/data/201110/U3...812/index.html

[noelex 77]

Thanks I am still not sure what formula to use.
I seem to rember the strength is proportional to cross sectional area?
If so
Then by using 75000 and 30000
The equalent stainless rod would be 10.1mm in diameter

Only small boats have stainless rods of 10mm on their bow so I think this means the 16mm aluminium would too weak, or at least less strong than most boats my size are using.

[thinwater]

Originally Posted by noelex 77

Thanks I am still not sure what formula to use.
I seem to rember the strength is proportional to cross sectional area?

No, it's proportional to the crossectional moment of inertia. Strength in bending will go with the square of the diameter. Think of it as area times leverage from the centerline.

However, the yield strength of 316 SS is 35ksi and the yield strength of 6061 T6 is 40ksi; very close. Thus, the rod size does not change. The alminum will be more suseptible to metal fatigue, but only if the design is close to bending already; if the existing rod has any bend in it, go larger or stay with SS.
____________

Some folk will have trouble getting their arms around the fact that SS is no stronger than Al in this case, but it all depends on the alloy. Climbing carabiners, for example, are no larger than marine carabiners for the same rating. You see, 316 SS is one of the weaker steels (15-5 ~ 145 ksi), and 6061 is one of the stronger Al alloys (pure Al ~ 16 ksi).

[goboatingnow]

The other thing to bear in mind is wear rates. the Al rod if it is turning on poor ( i.e. high friction bears or where the bow roller bears and rolls on it will wear faster.


Stainless and a little zinc chromate (DURALAC) will solve the dissimilar metals issue
Dave

[thinwater]

Originally Posted by thinwater

No, it's proportional to the crossectional moment of inertia. Strength in bending will go with the square of the diameter. Think of it as area times leverage from the centerline.

However, the yield strength of 316 SS is 35ksi and the yield strength of 6061 T6 is 40ksi; very close. Thus, the rod size does not change. The alminum will be more suseptible to metal fatigue, but only if the design is close to bending already; if the existing rod has any bend in it, go larger or stay with SS.
____________

Some folk will have trouble getting their arms around the fact that SS is no stronger than Al in this case, but it all depends on the alloy. Climbing carabiners, for example, are no larger than marine carabiners for the same rating. You see, 316 SS is one of the weaker steels (15-5 ~ 145 ksi), and 6061 is one of the stronger Al alloys (pure Al ~ 16 ksi).

I meant the cube. It's early.

As for wear, that's a valid point. Personally, I would go with SS for a number of reasons.

[noelex 77]

Originally Posted by thinwater

No, it's proportional to the crossectional moment of inertia. Strength in bending will go with the square of the diameter. Think of it as area times leverage from the centerline.

However, the yield strength of 316 SS is 35ksi and the yield strength of 6061 T6 is 40ksi; very close. Thus, the rod size does not change. The alminum will be more suseptible to metal fatigue, but only if the design is close to bending already; if the existing rod has any bend in it, go larger or stay with SS.
____________

Some folk will have trouble getting their arms around the fact that SS is no stronger than Al in this case, but it all depends on the alloy. Climbing carabiners, for example, are no larger than marine carabiners for the same rating. You see, 316 SS is one of the weaker steels (15-5 ~ 145 ksi), and 6061 is one of the stronger Al alloys (pure Al ~ 16 ksi).

Thanks
Looks like it is strong enough then.
The wear is not a cenern as the bearing is the sides of bow roller which is part of my aluminium boat. I would rather the rod wear than the bow roller. I have lots of 16mm rod . The minimum purchase when I got it a few years ago was 5m.

[bobconnie]

Don't see any problem with the aluminum rod. All it could do is bend some and would be easy to replace as you have some aboard, use the same lube as mentioned above just for the rolling ease it provides. heck the boats alum, mite as well stay with what ya got ! thats what I would do ! just my 2 cents

[Cheechako]

The yield strength is going to be roughly the same if the SS is not work hardened. However, aluminum bends (deflects) much lower than SS. (non permanent bending that is)

[noelex 77]

The deflection may cause some binding of the the plastic bow roller, but it's a loose fit over the 16 aluminium axel and I presume the the " plastic " bow roller would also defect if given a high load.
How much deflection could be expected with a 16mm solid aluminim rod with a span of 90 mm and a load of say 1000 kg I am guessing at the load,but this seems to be a resonable amount for breaking out the anchor when deeply set.
A 55kg Rocna takes some pulling out after strong winds.i

[Cheechako]

Assuming the roller transfers a generally even load along the length... not much deflection really as both ends are supported.... does the roller have a bushing in it? I think you are good to go. Watch corrosion inside the roller though.... just curious...why dont you want to use stainless?