pushrod strength test - alum vs steel
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lcrken
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Very nice real world test. Wish I'd thought of it. I wrote the following on the same subject here a while back, and what you've done with a test seems to agree with the calculations.
"A little more info. When I started this, I made some engineering calculations to see what wall thickness was required to get the same column bending stiffness in pushrod tubes of different materials. I looked at steel, aluminum, and titanium. I used .049" wall 2024 aluminum as a baseline, and calculated the wall thickness required in 4130 steel and in grade 9 titanium to get the same stiffness. Then I calculated the pushrod weights with those wall thicknesses. For the same stiffness, the aluminum and titanium rods were the same weight, and the steel rod was a lot heavier. You wouldn't gain anything from using titanium, and it would be a lot more expensive. You might also have trouble getting tube in the right wall thickness. You'd need titanium with .027" wall thickness, and the closest size I could find is .035". If you used that, the rod would be heavier. With steel, you'd need .025" wall, and the closest I could find is .035", so it's even heavier. All in all, it looked to me like aluminum was the best material. The bending stiffness of thin columns is primarily a function of the modulus of elasticity, not the tensile strength. Aluminum is also better at shock absorbtion and vibration damping than steel or titanium, and I think that should also be considered. It might be that there are other considerations. Peter Williams preferred steel pushrods, and that's what Steve Maney uses, so I could be missing something. I'd have to dig into some technical papers on the design of pushrods to see if that's the case, and I haven't done that. That's why I'm trying to pick the brains of folks here on the list."
Ken
"A little more info. When I started this, I made some engineering calculations to see what wall thickness was required to get the same column bending stiffness in pushrod tubes of different materials. I looked at steel, aluminum, and titanium. I used .049" wall 2024 aluminum as a baseline, and calculated the wall thickness required in 4130 steel and in grade 9 titanium to get the same stiffness. Then I calculated the pushrod weights with those wall thicknesses. For the same stiffness, the aluminum and titanium rods were the same weight, and the steel rod was a lot heavier. You wouldn't gain anything from using titanium, and it would be a lot more expensive. You might also have trouble getting tube in the right wall thickness. You'd need titanium with .027" wall thickness, and the closest size I could find is .035". If you used that, the rod would be heavier. With steel, you'd need .025" wall, and the closest I could find is .035", so it's even heavier. All in all, it looked to me like aluminum was the best material. The bending stiffness of thin columns is primarily a function of the modulus of elasticity, not the tensile strength. Aluminum is also better at shock absorbtion and vibration damping than steel or titanium, and I think that should also be considered. It might be that there are other considerations. Peter Williams preferred steel pushrods, and that's what Steve Maney uses, so I could be missing something. I'd have to dig into some technical papers on the design of pushrods to see if that's the case, and I haven't done that. That's why I'm trying to pick the brains of folks here on the list."
Ken
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when questioning peter williams about aluminium or steel pushrods he said that they had looked at both with a stroboscoop (do not know if this is the correct spelling but you know what i mean) and that the alloy ones where flexing (absorbing ???) and so put a greater strain on the valvetrain , and advocating valve bounce ; he compared it with a jump , a bow, the pushrod launching the rocker and thus the valve and back again
this was 10 years ago so i might have misunderstood or made up my mind over the years as what he said , as english is a foreign tongue for me , but that is what i always believed
so it is not just the pressure that counts ,as in jims test , but the hammering and consequent "bending" at 4000rpm and so on
this was 10 years ago so i might have misunderstood or made up my mind over the years as what he said , as english is a foreign tongue for me , but that is what i always believed
so it is not just the pressure that counts ,as in jims test , but the hammering and consequent "bending" at 4000rpm and so on
lcrken
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Thanks for the added detail, 1up and lynx. I've read Mick's book, and heard the reference to flex in the valve train before, but not with much detail. There was also an explanation from Alan Cathcart that because of it's lower elastic modulus the aluminum pushrods compressed twice as much as the steel ones, thereby introducing too much "springiness" into the valve train. I have no reason to doubt that the steel pushrods Norton used were better in some respect, but I think the tradeoff was that his steel pushrods were much heavier than the aluminum ones. I would think that they could also have got less flex by going with heavier wall aluminum tube, which would still have weighed less than the equivalent steel ones. But as I said above, I might be missing something here. It's not that simple to analyze. The simplest way to know would be to run the same weight aluminum and steel pushrods in an engine and measure/observe the amount of flex at high rpm. Peter might have done that. I should have submitted the question for Jerry to ask him for the Quincy rally. Maybe next time.
What Jim is testing is buckling strength, and that's what I based my calculations on. The equation for the buckling limit on columns was worked out by Euler a couple of centuries ago, and is still the standard for engineering calculations. Euler also did a lot of work on the vibration properties of columnar structures, which might be more relevant to flex under forces below the buckling limit, but I haven't looked into that. The Euler buckling limit equation is very simple, and proportional to the product of elastic modulus of the material and second moment of area of the tube cross section. That makes it pretty simple to calculate the tube wall thickness required for equivalent performance with aluminum and steel tubes, and the steel comes out heavier. That makes me believe that to have steel pushrods that flex less than aluminum ones also requires them to be a lot heavier.
That was my logic train anyhow. To actually calculate the amount of deflection in the tubes requires the solution of a non-linear differential equation (which is a lot easier now with digital computers than when I was in school), and is a much more complicated calculation. It is still primarily dependent on the elastic modulus and the second moment of area, but it's not a linear relationship as in Euler's equation. And even if you did that, it wouldn't answer the question of flex under the sort of periodically varying loading a pushrod gets in actual operation. One would have to also be able to model the loads and look at the resonant frequencies of the pushrod as a spring. That's way beyond my abilities, so I have to stick with simpler calcs. I don't recall if Prof. Blair included pushrod flex in his simulations. If he did, that would be a good way to compare materials.
Still better yet would be a batch of controlled experiments along the lines of some of comnoz's recent work on valve trains. Not sure if it would be worth the effort it would take. We've got racers using both types of push rods successfully, so it can't be too critical.
Ken
What Jim is testing is buckling strength, and that's what I based my calculations on. The equation for the buckling limit on columns was worked out by Euler a couple of centuries ago, and is still the standard for engineering calculations. Euler also did a lot of work on the vibration properties of columnar structures, which might be more relevant to flex under forces below the buckling limit, but I haven't looked into that. The Euler buckling limit equation is very simple, and proportional to the product of elastic modulus of the material and second moment of area of the tube cross section. That makes it pretty simple to calculate the tube wall thickness required for equivalent performance with aluminum and steel tubes, and the steel comes out heavier. That makes me believe that to have steel pushrods that flex less than aluminum ones also requires them to be a lot heavier.
That was my logic train anyhow. To actually calculate the amount of deflection in the tubes requires the solution of a non-linear differential equation (which is a lot easier now with digital computers than when I was in school), and is a much more complicated calculation. It is still primarily dependent on the elastic modulus and the second moment of area, but it's not a linear relationship as in Euler's equation. And even if you did that, it wouldn't answer the question of flex under the sort of periodically varying loading a pushrod gets in actual operation. One would have to also be able to model the loads and look at the resonant frequencies of the pushrod as a spring. That's way beyond my abilities, so I have to stick with simpler calcs. I don't recall if Prof. Blair included pushrod flex in his simulations. If he did, that would be a good way to compare materials.
Still better yet would be a batch of controlled experiments along the lines of some of comnoz's recent work on valve trains. Not sure if it would be worth the effort it would take. We've got racers using both types of push rods successfully, so it can't be too critical.
Ken
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Were not the original push rods simply alloy tube of the same diameter over their complete length with later versions being 'blown' to increase their diameter to reduce flexiung. When I wanted some shorter 88 length blown pushrods a doxen brand new blown tubes appeared from where I cannot possibly say. Are new Andover Norton pushrods 'blown' tube???
Anyway Norton twins were NOT designed to be revved as the factory lumps were. As one Gentleman said to me decades ago regaqrding factory prepared race motors......'They produced lots of power but not for very long' Wonder which on average went BANG first...gearbox or motor?.
Decades ago our old short stroke 68 x 68 Dommy with its ex Gus Kuhn one piece crank (one of 2 made by Laystall Engineering of Wolverhampton for Gus Kuhn) would 'happily' rev to 8,000 and that had std allow blown pushrods. Of cxourse the rider reported the bars felt about 8 inches in diameter at such revs....
Anyway Norton twins were NOT designed to be revved as the factory lumps were. As one Gentleman said to me decades ago regaqrding factory prepared race motors......'They produced lots of power but not for very long' Wonder which on average went BANG first...gearbox or motor?.
Decades ago our old short stroke 68 x 68 Dommy with its ex Gus Kuhn one piece crank (one of 2 made by Laystall Engineering of Wolverhampton for Gus Kuhn) would 'happily' rev to 8,000 and that had std allow blown pushrods. Of cxourse the rider reported the bars felt about 8 inches in diameter at such revs....
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Ken
Prof. Blair did indeed include pushrod flex in his calculations.
Using the tube sizes Jim gave in his movie, and running both a steel and an alluminum rod through the 4StHead program, the steel rod is actually a much better tool for the purpose than the alloy item.
Bear in mind that while the bottom of the pushrod movement is linear, the top is describing an arc as it moves with the rocker arm.
Results of the calculations are as follows:-
Steel compared to Aluminum
46.3 grams compared to 46.8 grams
4221 Maximum buckling load (N) compared to 2526
8607 Axial stiffness, (N/mm) compared to 6725
109 Lateral stiffness, (N/mm) compared to 61
2170 Axial vibration natural frequency, (Hz) compared to 1907.6
244.7 Lateral vibration natural frequency, (Hz) compared to 182.3
The last two items are especially important, as they must not co-incide with the natural vibration frequency of the valve springs otherwise the whole pushrod valve train will become seriously disrupted while the engine is in that particular speed range.
The way usually chosen to prevent such an occurrence is to deliberately select components so that any such unfortunate match does not occur.
The rate of expansion is not included in the calculations, but is obviously going to be of concern depending whether an alloy or an iron cylinder barrel is used
Prof. Blair did indeed include pushrod flex in his calculations.
Using the tube sizes Jim gave in his movie, and running both a steel and an alluminum rod through the 4StHead program, the steel rod is actually a much better tool for the purpose than the alloy item.
Bear in mind that while the bottom of the pushrod movement is linear, the top is describing an arc as it moves with the rocker arm.
Results of the calculations are as follows:-
Steel compared to Aluminum
46.3 grams compared to 46.8 grams
4221 Maximum buckling load (N) compared to 2526
8607 Axial stiffness, (N/mm) compared to 6725
109 Lateral stiffness, (N/mm) compared to 61
2170 Axial vibration natural frequency, (Hz) compared to 1907.6
244.7 Lateral vibration natural frequency, (Hz) compared to 182.3
The last two items are especially important, as they must not co-incide with the natural vibration frequency of the valve springs otherwise the whole pushrod valve train will become seriously disrupted while the engine is in that particular speed range.
The way usually chosen to prevent such an occurrence is to deliberately select components so that any such unfortunate match does not occur.
The rate of expansion is not included in the calculations, but is obviously going to be of concern depending whether an alloy or an iron cylinder barrel is used
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The steel pushrods being used in Nortons are generally heavier than the alum pushrods - that's important with the Norton's 1 to 1.13 rocer arm ratio. I don't hear of either steel or alum failing in use. The "springiness" can be pro or con. Snotzo was the 1st to tell me that there was a time when Nascars were getting more power with pushrods that would "spring" at the nose of the lobe and actually give more lift (& more HP) than stiffer pushrods (someone else has confirmed this). I'm sure they've made changes to make up for that by now. The shock absorbing qualities of alum could be pro or con depending on how you look at it - alum rods are used in Nitro dragsters because their shock absorption helps save the crank.
Its good to hear everyone weigh in with their differing opinions. Personally - I like the bend resisting qualities of the thicker walled alum - the thicker wall is what give alum an advantage (pound for pound) in some cases such as frames and swingarms. 2024 Alum is stronger and stiffer than many other types of alum - the type of alum needs to be factored in when comparing to stock alum pushrods and steel aftermarket types.
Its good to hear everyone weigh in with their differing opinions. Personally - I like the bend resisting qualities of the thicker walled alum - the thicker wall is what give alum an advantage (pound for pound) in some cases such as frames and swingarms. 2024 Alum is stronger and stiffer than many other types of alum - the type of alum needs to be factored in when comparing to stock alum pushrods and steel aftermarket types.
lcrken
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Thanks, Snotzo. I was hoping you'd chime in with info like that. I have Blair's book, but not the software. Really good of you to run the numbers.
So, I think I have to abandon the idea that proper aluminum rods are lighter than equivalent steel ones. There might still be a problem of how to set the clearance with steel rods and aluminum cylinders, depending on the cam's ramp design. Still, it seems to work for the racers who are using steel pushrods, so maybe it isn't that much of an issue. For me, the biggest problem is where to get the pushrod ends for the thinner wall steel tubes that would bring the weight down to that of the aluminum items. I get ends from Smith Brothers and from Compcams, and they don't offer them in large enough shanks for thinner wall tubes. They work fine for the .058" and .090" wall tubing I use, but that's about it. I priced getting a batch done at a local CNC shop and heat treater, and it's way prohibitive unless I am buying by the thousands. At Jim Constock's suggestion, I've made a couple on my own from aluminum bronze to fit the thinner wall tube, but that's way too time consuming for more than one or two sets. I might try them in an iron cylinder engine 920 engine I'm hoping to get back together for Bonneville. So many things to try, so little time.
Having never had a push rod fail, it's hard to justify putting too much time into this. The only reason I started making pushrods was to be able to make them in the lengths I needed. I was also hoping to make them lighter than the stock ones, but the differences in weight are very small using the same wall thickness as the stockers.
Ken
So, I think I have to abandon the idea that proper aluminum rods are lighter than equivalent steel ones. There might still be a problem of how to set the clearance with steel rods and aluminum cylinders, depending on the cam's ramp design. Still, it seems to work for the racers who are using steel pushrods, so maybe it isn't that much of an issue. For me, the biggest problem is where to get the pushrod ends for the thinner wall steel tubes that would bring the weight down to that of the aluminum items. I get ends from Smith Brothers and from Compcams, and they don't offer them in large enough shanks for thinner wall tubes. They work fine for the .058" and .090" wall tubing I use, but that's about it. I priced getting a batch done at a local CNC shop and heat treater, and it's way prohibitive unless I am buying by the thousands. At Jim Constock's suggestion, I've made a couple on my own from aluminum bronze to fit the thinner wall tube, but that's way too time consuming for more than one or two sets. I might try them in an iron cylinder engine 920 engine I'm hoping to get back together for Bonneville. So many things to try, so little time.
Having never had a push rod fail, it's hard to justify putting too much time into this. The only reason I started making pushrods was to be able to make them in the lengths I needed. I was also hoping to make them lighter than the stock ones, but the differences in weight are very small using the same wall thickness as the stockers.
Ken
lcrken
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jseng1 said:
I'm with you there, Jim. I'm currently using the .090" wall tubes for the pushrods in the 1007 for my street bike for the same reasons. I don't expect to rev it all that high, and I like whatever extra rigidity I can get from the heavier tubes.
Ken
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Jim
once the situation was understood, Nascar re designed the valve train so as to obtain the additional lift whilst maintaining control, whereas previously the lift came from a combination of effects that launched the valve off the peak of the cam, much of this additional lift as a result of pushrod flex that unwound like a spring (which in fact it was at that point). Firstly, it was discovered that this extra lift from fling could be factored in by subtle pushrod manipulation, but later a redesigned valve train and a modified cam profile brought everything back under control .
The tubes in your test seem to be somewhat longer than those one would expect to find in an engine. As can be imagined, a short tube posesses greater inherent stiffness compared to a longer item, and in the case of tube lengths as found in a Commando engine, the intake and exhaust pushrods are of different lengths, and thus the exhaust tubes will, size for size, be stiffer. If you can find room for aluminum pushrods of 7/16" diameter but with the wall thickness adjusted to keep the same weight, you will get a very significant increase in overall strength and stiffness.
once the situation was understood, Nascar re designed the valve train so as to obtain the additional lift whilst maintaining control, whereas previously the lift came from a combination of effects that launched the valve off the peak of the cam, much of this additional lift as a result of pushrod flex that unwound like a spring (which in fact it was at that point). Firstly, it was discovered that this extra lift from fling could be factored in by subtle pushrod manipulation, but later a redesigned valve train and a modified cam profile brought everything back under control .
The tubes in your test seem to be somewhat longer than those one would expect to find in an engine. As can be imagined, a short tube posesses greater inherent stiffness compared to a longer item, and in the case of tube lengths as found in a Commando engine, the intake and exhaust pushrods are of different lengths, and thus the exhaust tubes will, size for size, be stiffer. If you can find room for aluminum pushrods of 7/16" diameter but with the wall thickness adjusted to keep the same weight, you will get a very significant increase in overall strength and stiffness.
worntorn
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Ken - Smith Bros. Racing make pushrods any length you like, cheap like Borscht, I recall about $8 ea.. The pushrods I got from them for my Special are hollow aluminium. They also make pushrods in regular 4130 Chrome Moly and heat treated 4130 Chrome Moly for extreme pressure valve spring applications, up to 700 pounds pressure. They have a variety of hardened steel ends on offer.
These are run in NASCAR, Fuel Dragsters and all over the Hot Rod and racing world.
I suspect you are like me-not enough time to-complete all of the projects. Making pushrods is one you can avoid by purchasing from Smith Bros.
Who knew that there was an entire company specializing in making custom pushrods?
http://www.pushrods.net/
Glen
These are run in NASCAR, Fuel Dragsters and all over the Hot Rod and racing world.
I suspect you are like me-not enough time to-complete all of the projects. Making pushrods is one you can avoid by purchasing from Smith Bros.
Who knew that there was an entire company specializing in making custom pushrods?
http://www.pushrods.net/
Glen
mdt-son
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Jim,
Sorry to question your experiment, but I truly believe your finding is questionable. Being a specialist in structural mechanics and stability of structures in particular, I can tell you this:
* Forces in a set-up like yours are distributed according to effective stiffness E*I . Determination of E*I (Young's modulus times area moment of inertia) doesn't require an experment. Thus, I think one rod was subjected to a higher load than the other.
* Straight line loading is paramount. I am not convinced your setup ensures completely axial loading due to the eccentric placement of specimens and the apparatus itself. Testing one by one in a professional testing machine would give more confidence in the result.
* Boundary conditions play a significant role in achieved stability load. In this case, deformation of the aluminum ends and shear load due to friction may have increased the collapse load of one vs. the other. You really need to mimic the simply supported end conditions of the genuine pushrods by providing a ball and cup type of condition at both ends, thus eliminating adhesion, end deformation and misalignment.
I would be most interested in your findings, but I also think simple calculations will provide a reliable answer. I may provide a calculation shortly.
Kind regards,
Knut Sonsteby
Sorry to question your experiment, but I truly believe your finding is questionable. Being a specialist in structural mechanics and stability of structures in particular, I can tell you this:
* Forces in a set-up like yours are distributed according to effective stiffness E*I . Determination of E*I (Young's modulus times area moment of inertia) doesn't require an experment. Thus, I think one rod was subjected to a higher load than the other.
* Straight line loading is paramount. I am not convinced your setup ensures completely axial loading due to the eccentric placement of specimens and the apparatus itself. Testing one by one in a professional testing machine would give more confidence in the result.
* Boundary conditions play a significant role in achieved stability load. In this case, deformation of the aluminum ends and shear load due to friction may have increased the collapse load of one vs. the other. You really need to mimic the simply supported end conditions of the genuine pushrods by providing a ball and cup type of condition at both ends, thus eliminating adhesion, end deformation and misalignment.
I would be most interested in your findings, but I also think simple calculations will provide a reliable answer. I may provide a calculation shortly.
Kind regards,
Knut Sonsteby
batrider
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I tried some aluminum pushrods made by Alloy-Tech (?) back in the 70's and they didn't have any steel ends. I found the aluminum did not hold up long (wear at both ends) and they were trash after a few thousand miles. Strength is only one consideration.
lcrken
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worntorn said:
Quite right, Glen. And they are nice people to deal with too. I have no problem with having them make push rods, but when I'm trying to sort out the right length for a particular engine, it really saves time to be able to make them in the shop, instead of ordering and waiting for them. Also, as I said above, they don't offer pushrod ends in the Norton cup and ball sizes to fit thinner wall tube. Their 4130 pushrods use a thicker wall tube, and weigh more than the comparable aluminum ones. I was interested in making steel rods in the thinner wall tube to keep the weight down, and Smith Bros. doesn't offer that option. I'm not sure how much more I'll pursue this. As you say, it does suck up valuable time. What I'd really like to make are pushrods with Al-SiC metal matrix tubes. I looked into that, but the diamond tooling required makes it too expensive. Smith Bros. made a few for Harley drag bikes, but gave it up because of the cost. It was too easy to break those diamond coated reamers.
Ken
lcrken
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batrider said:
Yeah, I still have some of the Alloy-Tech pushrods. They made the aluminum ones with both steel and aluminum ends. The aluminum ones were hard anodized, and as you experienced, didn't last very long. I used their aluminum rods with steel ends and had no problems. They also made 4130 rods with steel ends. I think I still have a set of those that I never used. FWIW, their aluminum rods with steel ends weigh almost exactly the same as stock rods. Their 4130 rods were much heavier, 50 g for steel vs. 29.8 g for aluminum with steel ends (weights are for the longer intake pushrods).
Ken
lcrken
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Matchless said:
I have three carbon fiber pushrods for a Commando. The fourth one came apart in the engine. They were in the Commonwealth Norton race bike back (for a very short time!) in the '80s. I don't know of anyone who is using carbon fiber pushrods now.
Ken
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