Aluminum Connecting Rods

[JV Puleo]

Does anyone here know what grade of Aluminum is commonly used for rods? My internet search has turned up a large amount of information on custom rods for racing engines... very little on the materials used and virtually nothing on using them in an old, relatively slow turning, long stroke engine. I have 7075 billets... but making these is a big job and I hesitate to start until I'm reasonably sure the product won't result in catastrophic failure. I do have some engineering data to calculate the stresses... the math is a bit beyond me, but I may be able to find someone to help with that.

[cahartley]

It appears you're on the right track....... 

 

https://www.ijirset.com/upload/2014/october/36_Design.pdf

 

Check out page five.

 

 

[edinmass]

I think your best bet would have Crallio make what you need. The learning curve along with metallurgy and manufacturing issues seems like the only reasonable way to do it is have a company that makes rods do it for you. Too much downside any other way.

[JV Puleo]

From a purely practical point of view you are probably right... but I am not practical. My whole purpose is to do as much as I can myself. I'm more interested in the process than the product. Besides... I have time and machines. I don't have much disposable income.

[cahartley]

Good for you JV.

I've been in a similar position most of my  life so I, too, did a lot of things myself that I probably wouldn't have done otherwise.

But I think there's more pride in saying "I made that" than "I bought that"....... 

 

[Rusty_OToole]

Aluminum rods were first used in the 1920s. I believe Pierce Arrow and Dodge tried them. There was a problem with fatigue strength that meant they had to be a lot thicker than they first figured which took away a lot of the weight advantage. Racing engines that use aluminum rods, are rebuilt frequently and the rods discarded after a few hours use.

 

If you could contact the chief engineer of a manufacturer of aluminum rods he could enlighten you. It is surprising how approachable such people can be.

[JV Puleo]

The aluminum rods in the 20s were Lynite, an Alcoa alloy. I believe they were cast – or at least they look as if they were cast. Franklin and, I think, Dusenberg were also early users. I have one of the Franklin rods in the shop to look at. I don't know what the strength of Lynite was but it apparently deteriorates over time. (Nevertheless, most Franklins must still be running with aluminum rods...at least I've never heard of them being regularly replaced.)  I don't have my figures in front of me, but I seem to remember that the tensile strength of 7075 is comparable to that of the mild steel forgings commonly available in 1910. Of course, tensile strength isn't the only salient feature that has to be considered but it is certainly a good starting point.

 

 

Edited July 11, 2017 by JV Puleo
typos, grammar & punctuation (see edit history)

[PFitz]

Franklin Club's factory drawing files show aluminum rods used in the Series 10 in early 1920's. They continued to use aluminum rods at least until 1932. Earlier connecting rod drawing numbers don't show up in the file, so I'm not sure if aluminum rods were used with earlier production.  

 

Yes, the rods and caps were supplied in Lynite. But, they were not cast, they were drop forged and then heat treated. The drawings specify "AL65", which I believe was one of the SAE aluminum alloys of that era.  If you can find an early copy of the SAE hand book, it lists many of the alloys used in early auto industry and the percent makeup of each alloy. Some of the later drawings specify Lynite 25 ST.  The Franklin Club library has a 1928 SAE handbook.  Club membership gets you access to all that and more.

 

And yes, there is thousands of original Franklin aluminum rods still doing what they were intended to do.  As far as them deteriorating with time, that's a fallacy.  If they have proper bearing clearance and lubrication as they were designed, they hold up very well to about 3000 rpm. The only weak point is the aluminum threads for the wrist pin pinch bolt, and 99% of the time any failure there is because at some point, someone over tightened the bolt and pulled the threads. Helicoiling those pinch bolt threads takes care of that very well.  

 

Paul 

Edited July 11, 2017 by PFitz (see edit history)

[JV Puleo]

Thanks... that is good info and not anything I'd have learned from peripheral view of Franklins. I like them, but I've never owned one.

 

And thanks for the reference to the SAE Handbook. I hadn't heard of that but, after reading our post, located a 1927 copy and bought it. I'd really like an earlier one... say 1910 to 1915 but I don't even know if they were published that early.

 

UPDATE: 1926 was the first year for the SAE Handbook

Edited July 11, 2017 by JV Puleo (see edit history)

[PFitz]

Your welcome.

 

FYI. I mention the 3000 rpm limit, not because the rods come apart above that, but because of the speed/load limitations of the spun babbitt process used to cast the babbitt directly onto the Lynite rods, that Franklin used. However, many of those type rods have been converted to shell bearings by now, which can take higher rpms. But even with the spun babbitt, it's rare that a Franklin rod fails, and 90+% of the time it's because of a bearing failure that caused the rod to let go.  

 

I'm sure there are modern aluminum alloys that are stronger than the old Lynite rod, but some alloys may not be easy to machine. Plus, there may be additional operations needed after machining, such as heat treating, or shot peening, for stress relief ????? 

 

Good luck with your project. I'm interested to hear how it turns out.

 

Paul

[JV Puleo]

My blanks are 7075-T6. It is already heat treated but I'm not certain what, if any stress relief is needed. It machines well - not as easily as 6061 but is easily worked on my 1930s B&S mill. I'm just now finishing two sets of timing gears of the same material that you can see in my thread in the Restoration sub-forum on my 1910 Mitchell. One of the 3 rods from this car is bent and they are all almost obscenely heavy — and very roughly made. I will never have to worry about engine speeds above 2,000 rpm - at which point the car would be traveling at 63mph.

[JACK M]

When I was serious about racing I looked into aluminum rods for one of my dirt cars.

I learned that the drag racers use them and wondered why we hadn't been using them in the dirt cars.

Answer was that a drag car only accelerates for relatively short bursts.

We accelerate and decelerate over and over which will result in a rod tending to stretch.

I am quite certain that the drag racers turn more than three thousand revs but are only pushing on the rods, not pulling and not repetitive.

And if you have never seen one of those drag racing aluminum rods they are HUGE.

 

There was an interesting read not long ago about how many times a drag motor turns in a quarter mile. It is only about 500 turns.

We always think about revs per minute, but these cars only accelerate for a few seconds.

[joe_padavano]

3 minutes ago, JACK M said:

Answer was that a drag car only accelerates for relatively short bursts.

We accelerate and decelerate over and over which will result in a rod tending to stretch.

 

^^^THIS!. My day job is aerospace engineering, concentrating in structures.  Aluminum has 1/3 the density of steel, but it also has 1/3 the stiffness.  For a stiffness designed part, properly designed aluminum and steel parts will weigh about the same.  Interestingly, aluminum also has about 1/3 the strength of high strength steel, so again, for a properly designed part that is strength-limited, aluminum and steel parts will weigh about the same.  Of course, frequently machining limitations drive minimum thicknesses and sections, which is where the aluminum parts come out ahead.  On the other hand, space limitations can often force a design to steel because there physically isn't enough room for aluminum (which would need three times the cross section for the same strength part).  This becomes obvious when you look at the big end of AL rods vs. forged steel ones.  And as Jack M. points out, aluminum has MUCH lower fatigue tolerance than steel.  Drag motors not only have short burst while running, but they typically get torn down frequently.  For a long-duration cyclical loading, you need to pay close attention to the fatigue life curve of an aluminum part. Even low loads over a long period of time will eventually cause a crack to start and propagate.

[Rusty_OToole]

Weight of the pistons is another factor. If you replace heavy cast iron pistons with aluminum of course that reduces the stress on the rods.

 

Triumph motorcycles used aluminum rods with iron or steel caps. The rod ran directly on the crankshaft, no bearing. The cap had a bearing, I don't know if it was babbit or a shell.

 

I know a guy who replaced the poured  bearings in his Indian four with home made aluminum shell bearings. He cut a piece of aluminum tube, made the bearings and scraped them to fit. That was in the fifties and it is still running with many thousands of miles on it.

 

A roundabout way of saying if you ran the aluminum directly on the crank with no bearings it would probably be ok. Just make sure you have good oil and a good oil filter. Aluminum is less forgiving of dirt than babbit because dirt and grit can get embedded in the babbit before it chews the crankshaft.

[JV Puleo]

Actually, the plan is to make bronze shells and Babbit them. It is a 1910 car so there will probably be low pressure lubrication to the main bearings and splash to lubricate the rods. The pistons will be aluminum as well... I'm guessing that the combination of rod & piston will weigh less than half what the original did... but the original parts are excessively heavy. "Crude" might be a better term. I could make the rods out of steel... it would be more work but is certainly possible. I haven't started on them yet, which is why I began this thread in the first place. If I do make them out of steel, I'd want something that is relatively easy to machine with HSS tooling. I would have used extant rods if I could find any that were wide enough at the big end to accommodate the bearing shells but trying to locate a particular antique rod by measurement is virtually impossible...unless you already know what you are looking for. Even measuring the center distance between the big and small ends is beyond the capeability of most old car guys... understandable when very few are approaching the problem from my point of view.

 

 

[Bhigdog]

An ambitious project for sure. I'm assuming manual machines rather than a 3-4 axis Haas type mill? If so, even more ambitious. If using alum my GUESS is you could use the cross section of a typical steel rod and add 25% or so fudge factor. Take lots of pix............Bob

[JV Puleo]

All manual machines... the newest of which dates from the mid-30s. All of the others are much older. But, I don't see that as a handicap. There are a great many parts in very early cars that are better made on the sort of machinery that was used to make them in the first place. A huge amount of the "advantage" to modern machines revolves around rapid production and eliminating the element of human error. That is nowhere near as critical when the largest number of like parts you'll ever make is 8... and mostly, 4 is the maximum number. I know several professionals who prefer manual machines when doing prototype and toolmaking work and this sort of project is much closer to that scenario than it is to manufacturing.

[Bhigdog]

1 hour ago, JV Puleo said:

All manual machines... the newest of which dates from the mid-30s. All of the others are much older. But, I don't see that as a handicap. There are a great many parts in very early cars that are better made on the sort of machinery that was used to make them in the first place. A huge amount of the "advantage" to modern machines revolves around rapid production and eliminating the element of human error. That is nowhere near as critical when the largest number of like parts you'll ever make is 8... and mostly, 4 is the maximum number. I know several professionals who prefer manual machines when doing prototype and toolmaking work and this sort of project is much closer to that scenario than it is to manufacturing.

 

NICE!! .......................Bob 

[JV Puleo]

Surfacing the exhaust ports on my Brown & Sharpe 2A (heavy horizontal mill with an accessory vertical head). The blocks are bolted down to the table via fixture that holds them in perfect alignment so the ports are flat across both jugs. After doing this, the entire fixture was turned around to do the intake ports.

 

[JV Puleo]

Update...

 

It is clear that 7075 aluminum is used for connecting rods.

The tensile strength of the low carbon steel forgings used in the original rods was abut 70,000 lbs. per sq. in.

The tensile strength of 7075 is abut 83,000 lbs. per sq. in. so the real question is how much strength does it loose as the temperature increases and how hot does a 4 cylinder c.1910 engine get. I would take some readings from a similar engine after running for an hour or so if I had one available but it would seem unlikely that the temp would exceed 300 degree. I'm hoping there is some useful information in the SAE handbook I ordered — the 1927 edition so it won't be exactly contemporary with the engine but may give some useful data.

 

 

[Rusty_OToole]

Why don't you fit shell bearings while you are making new rods? Everybody else goes to great expense to change their old babbit bearings to shell bearings and you can do it for free. In fact it would be easier than babbit and a lot cheaper.

 

If you use aluminum rods and pistons and have the crankshaft balanced it should run a lot freer than when new. With today's alloys if the rod is of sufficient size it should have a life of several hundred years before metal fatigue sets in. Maybe Joe can help calculate the stresses and insure you have a sufficient margin of safety.

 

Add a good oil filter like a Frantz and bearing wear will be a thing of the past.

Edited July 12, 2017 by Rusty_OToole (see edit history)

[Spinneyhill]

While the tensile strength is comparable between the steel and aluminium, the Young's (=Elastic) Modulus of Al is about 1/3 that of steel (69 vs c.200 MPa). So to keep the stiffness (deformation under stress) the same, the cross sectional area must be about 3 times greater in Al.

 

There is also the fatigue strength to consider. For 7075 Aluminium the Endurance Limit (stress below which fatigue failure never occurs) is 159 MPa (23000 ksi) or about 28% of the UTS (ultimate tensile strength). For steel, the EL is typically about 0.45 to 0.5 times the UTS.

 

The tensile yield strength of the 7075 is 502 MPa (73,000 ksi) while the yield strength of the steel is probably about half its UTS.

[JV Puleo]

I am not convinced that modern shell bearings are a good idea with a 3-main bearing crank that, even balanced, will flex much more than a modern crank. The Babbitt layer in a modern bearing is very thin, too thin, to my mind, than would be safe. Actually, I think that many people who fit modern bearings to early engines, especially brass era engines, are making a big mistake. That repair may be all well and good with a design from the mid-30s or a 7-bearing crank like Franklin had. I will fit bronze shells and get the Babbitt layer down to something like .040 to .060. Currently, the Babbitt is more than .250 thick. I'm convinced that the Mitchell company cut a lot of corners with this engine and that it proved to be very problematical. It probably is not a coincidence that the design was in use only two years during which time it underwent numerous changes. It is a fundamentally sound, if unimaginative design but the execution left a lot to be desired.

 

I am planning on an oil filter, but to do that I will need make an oil pump. The 1911 cars had one but mine was made with an external, 6-tube oiler and used a semi-total loss system. I will probably change that as well but that part of the project is pretty far down the line.

Edited July 13, 2017 by JV Puleo
correction (see edit history)

[JV Puleo]

This is all extremely helpful. I think it is reasonable to say that the 7075 will work. The issue that needs to be resolved are the dimensions of the various parts of the rod to insure its durability. I think there is room... partly because I have compared the original steel rod of the Mitchell to a Lynite rod from a Franklin given to by my friend Mike West. The salient dimensions of the Lynite rod are clearly larger but much bigger than the dropl forged Mitchell rod which is, I suspect, much heavier that it needed to be. My next problem is that I would have to estimate the weight of the piston (they aren't finished yet) and the compression ratio (which I probably won't know exactly until I can reassemble the engine although it will not exceed 5:1 and is more likely to be around 4.75:1). Presuming that the 7075 is at least as strong as the Lynite, I should be able to tentatively think in terms of the Franklin dimensions of the big and small end. There is plenty of room to increase the dimensions of the center portion. My understanding is that the weakest point of a connecting rod is the middle, at least that is what P.M. Heldt says in his 1911 Automobile Engineering textbook, Gasoline Automobiles.

 

Interestingly, Lynite was patented in 1914. I think it first appears as a piston material around 1916. I'm not sure when rods were first made of it but I suspect experimentation began almost immediately though it must have been some time before it was perfected enough for a manufacturer to adopt it.

[JV Puleo]

7 hours ago, Rusty_OToole said:

Why don't you fit shell bearings while you are making new rods? Everybody else goes to great expense to change their old babbit bearings to shell bearings and you can do it for free. In fact it would be easier than babbit and a lot cheaper.

 

I have all, or at least most of the fixtures needed to Babbit the shells. They came free with my align boring machine — neither of which I've ever used so I still have to figure that out. I've done a lot of lathe boring so I'm not particularly intimidated by that part of the job.

[stutzl6]

Stutz from 1926 on used aluminum rods in 28 the cap was steel  I am using 356 Packard rods.There may be a steel rod that you can use .Look at tractor rod like jd 2 cyl. these are large rod  . This may help to get a steel rod that will fit Thank John STUTZ L6

[Spinneyhill]

I have found one reference (scrap dealer) saying that Lynite had a UTS of 23,000 psi and consisted of aluminium 88%, copper 10%, magnesium .25%, iron 1.5%.

 

http://www.csudh.edu/oliver/chemdata/alloys.htm  shows

95Al, 2Cu, 1.5Mg, 0.8Fe, 0.2Si, 0.01Mn

Lynite, piston

93Al, 7-8.5Cu, 1.7 other elements	Lynite, 146
92Al, 9.3-11Cu, 2 other elements	Lynite, 122
90Al, 7.8Cu, 1.5Zn, 1.3Fe	Lynite crank case
89Al, 11Cu, 0.5Mg	Lynite piston
89Al, 12-14Cu, 2 other elements	Lynite, 109

 

So what version was used for con. rods?

 

[JV Puleo]

I don't know. But, I have a 1927 copy of the SAE handbook coming so I'm hoping it will be listed there. I have seen the same figure but without reference to which alloy it was. With luck, we'll know in a few days. Tomorrow I'll photograph both the Lynite rod and the original Mitchell rod.

Edited July 13, 2017 by JV Puleo (see edit history)

[PFitz]

In post #8, Franklin specified Lynite 25 ST. But they don't specify the alloy mix or strength, just hardness test.

 

Here's a thread discussing aluminum rods for Model T's. The tensile strength number mentioned for Lynite is 55K.

 

http://www.mtfca.com/discus/messages/411944/504763.html?1419742159

 

Paul

[JV Puleo]

More aluminum rods...

I took some photos and measurements in the shop today. Here are two rods... on the left we see the Lynite Franklin rod and on the right, one of the original Mitchell rods. the Franklin rod is slightly shorter but the central "I-beam" of the Mitchell rod is actually larger than that of the aluminum rod. The Mitchell rod weighs 3 lbs, 13.6 ozs. while the Franklin rod weighs 1 lb. 9.6 ozs. Even allowing for a heftier profile and a little more length, an aluminum version of the Mitchell rod would have to weigh much less than the original if it approximates the dimensions of the Franklin rod. Compression & piston pressure also have a role to play where the dimensions are concerned as well as that the 7075 T-6 aluminum is probably a good deal stronger than 1920s Lynite.

 

 

Another important consideration is the weight of the piston. Here we see my one original Mitchell piston on the right. It's nominal diameter is 4-1/4 inches and it weighs 5 lbs. 14 ozs. It is actually 7 ounces heavier than the 5" diameter iron piston on the left from a 6 cylinder Wisconsin engine. Mitchell advertised their pistons "with 5 piston rings" as an advantage, but in order to accommodate 5 of those wide rings, the piston wall is about 1/2" thick. In the center are my cast aluminum piston blanks... these are as yet unfinished but already weigh much less than the original. When finished, I expect they will be about 1/2 the weight of the original and they are quite heavy for aluminum pistons.

 

 

So... I will have to calculate the piston pressure. I can't do that accurately until the pistons are done but it can be done tentatively by estimating the weight, as long as the estimate is on the high side. There is plenty of room to increase the dimensions of the rod "I-beam" and the relative thickness of the big and small ends... the problem  is deciding on a finished dimension. I suspect we are getting there.

Edited July 14, 2017 by JV Puleo (see edit history)

[edinmass]

Great post, it's very enjoyable to see such interesting engineering and updates made to an engine. My question is this.....when machining a new connecting rod from aluminum, are there any problems or issues that can appear from strange or obscure design flaws? Does how one shapes and finishes the rod, and the actual manufacturing process can tool marks cause fracture issues or problems associated metal fatigue? It seems to me that one could spend a lifetime learning the nuances of designing and fabricating of such an engine component. Ed

[JV Puleo]

I don't see things like tool marks being an issue. but, of course, I try to eliminate them altogether. There are other aspects that these parts share with any high-stress part. For instance, sharp inside edges should be consistently avoided. A fillet, even a slight one, is far preferable. One of the curiosities of connecting rod design is that the conventional rod shape is really an accommodation to the conventional manufacturing technique (i.e. drop forging). The central beam tapering from bottom to top is unnecessary, In fact, the point of highest stress is the middle. If designed with pure engineering in mind, they should be diamond shaped, thicker in the middle than at the ends. For "machined all over" rods, there is no reason not to make them straight. The taper just complicates things. If you aren't pulling them out of a forging die, it has no purpose. If you look at the Simplex rods shown on The Old Motor in David's thread on rebuilding that engine, you'll see they are straight. Only the highest quality cars could afford the time to make the rods, or cranks or other parts this way and there was an additional advantage to this that isn't immediately apparent today. By making parts to tight tolerances all over, they were inherently balanced. The value of this was fully appreciated but it would be the late 20s (or early 30s) before dynamic balancing machines were perfected.

 

Not being an engineer, I have to cheat a bit. I spent a lot of my youth working on Silver Ghosts and Phantom Is and I've been inside quite a few RR engines of the period. I just look at something and ask myself "how would Sir Henry Royce do this?" I'm not good enough to hit that level all the time, but by trying I do get most of the way, most of the time.

[Guest]

Do you have the equipment and talent to cam grind your pistons or is that a necessity on your engine. 

When I ordered new pistons for my Pontiac they came with new style narrow rings and only three and no ring at the bottom of the piston.  Much lighter and less friction.  Sure made a difference.

[Alfa]

I hope in your design, you will be avoiding the pinch bolt style small end? The small end eye is an inherent weakness in that situation and it caused a major 'blow-up' in my 1928 Alfa 6C engine.

[JV Puleo]

I don't have a piston grinder but also don't think these have to be cam ground. Cam grinding only came in somewhat later. Early pistons are round (although also made of iron). They aren't even relieved at the wrist pins although mine will be. I'll have two compression rings and an oil control ring — which weren't invented until the 20s. Although I try to keep all my changes in the context of the working life of the car there are things that will prolong it's life and make using it more practical that I can't ignore. Also, to actually restore an engine of this vintage without making these changes would be very difficult. I've made a lot of things but don't relish trying to make my own rings and iron pistons.

 

No pinch bolts... floating wrist pins with circlips in the pistons at the ends.

[Alfa]

Excellent!!  Fascinating project that you are undertaking.

 

Adam..

[JV Puleo]

I've looked into the question of cam grinding a little further. If I understand what I've read correctly, cam grinding allows a split skirt piston to fit tightly into the bore and  prevents piston slap when the engine is warming up. I'm not terribly concerned about this. In fact, the pistons aren't even cast of a conventional "piston" alloy. They are 356-T6 which has a different, and greater coefficient of expansion. But, this is a hand-crank starting engine. I would prefer that the fit not be as tight as you would want in a modern engine when it is cold. The dimensions have been calculated so that the pistons fit properly when they are hot and a small amount of piston slap is acceptable if it is relatively easy to start.

 

It's not inconceivable that I'll want an electric starter some day... who knows whether I'll be able to crank an engine in 20 years. If I do, I'd be looking for a chain driven starter similar to that used on RRs although attached to the drive train via the coupling between the engine and transmission. I believe REO used a system like this (probably by Westinghouse or Northeast electric) on their 4-cylinder cars or trucks c.1916-1918 so, if you see a parts chassis, I may be interested in those parts to put aside against future use.

[JV Puleo]

Here is a chart from PM Heldt's Gasoline Automobiles that gives the sectional dimensions for connecting rods... except that I simply don't understand how to read it. Perhaps one of our members that has more engineering in their background can interpret it.

 

[Rusty_OToole]

We know how Henry Royce designed things. When he sat down to design the Silver Ghost engine he took the bore and stroke of the 3 leading luxury cars on the English market, and averaged them. Rolls Royce never innovated anything, he copied the  designs of others. And it was his policy to "sew" things together with multitudes of small closely spaced bolts.

 

In other words they were designed by guess and by God, by a meticulous machinist who had no imagination and no engineering training.

 

You have to address the problem of expansion of an aluminum piston in an iron cylinder. The piston is going to expand a lot from heat. If it is fitted too tight when it is cold it will seize when hot. If fitted too loose it will fit when hot but slap when cold. This is why all the effort over the years to design cam ground, slotted, strutted, wire wound and otherwise modified pistons, to get something that will fit tight when cold but not seize when hot because there is somewhere for the expansion to go.

 

Here is an unusual solution I read about in Hot Rod magazine back in the fifties. One of the editors had a hopped up Corvette with solid skirt forged aluminum pistons. These are not recommended for street use, only racing because of the piston slap when cold. His solution was to fit them to the recommended clearance, I believe .012", then knurl them. He said he ran them that way for 30,000 miles with no problem.

 

This brings up another question. Do aluminum rods grow when they get hot? I'm sure they do. Do you have to leave extra piston to head clearance to compensate? This should not matter on a 1910 engine with a combustion chamber like the Hollywood Bowl.

 

PS this is too obvious but expansion was not an issue with an iron piston in an iron cylinder because both expanded at the same rate, so clearance did not change.

Edited July 14, 2017 by Rusty_OToole (see edit history)

[Rusty_OToole]

Re PM Heldt's diagram. It seems fairly straight forward to me. You must start by calculating your explosion pressure. This will be the product of your piston area, compression, and the force produced by the exploding gas mixture. There should be a fuller explanation in the text.

 

Having found this, read across the bottom to the appropriate pressure. Draw a vertical line on the chart. Will your rod be made of carbon steel or nickel steel? Where your vertical strikes the appropriate diagonal, make a mark. From this mark, draw a horizontal line. What is the length of your rod? 8, 10, 12, or 14 inches? This is usually center to center, main bearing to wrist pin.  Where your line strikes the appropriate diagonal make a mark. Now draw another vertical line. Where this line strikes the top of the chart, you get your t number. Refer to the box diagram on the left side of the diagram to calculate the dimensions of the rod I beam and thickness.

 

The diagram at the bottom left shows the path of the line you need to draw.