pushrod strength test - alum vs steel

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Deets55 said:
Jim,
Is there an engineering school in the area? When my son was in college he had access to all sorts of equipment. That machine you guys are referencing sounds like something that might be in a Strength and Materials lab. You might be able to get access to it if you find the right person.
Pete
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No engineering school near me. But its no big deal to send a pair of pushrods where they have this equipt.
 
A check was made of a Maney inlet steel pushrod, and the maximum buckling load was calculated at 12303 Newtons. (N). This was for tubing of 0.375" OD and a wall thickness of 0.040".

Ken, you 0.035" wall thickness tubing of 7/16" OD and same length, has a calculated max. buckling load capacity of 18548 N, although the penalty for this is an small increase in weight.

The Maney pushrod with aluminium tube of the same dimensions as the steel original, has a max. buckling load capacity of 4241 N.

By comparison, the stock Commando aluminium tube pushrod which features a tube tapered at either ends, has a calculated max. buckling load capacity of 7403 N.

In my opinion the real issue here is two fold:- 1/ is the pushrod in whatever form, capable of handling the loads, and 2/ is the material suitable for the application and the intended use.

The stock pushrod is well capable of hadling the loads generated by a stock engine over it's intended working engine speed range. To go beyond the stock engine in both components and useage, for the inexperienced can well be likened to the opening of Pandora's box !
 
Thanks, Snotzo. I do appreciate seeing real numbers as opposed to opinions. I do need to point out that the .035" tube I looked at is the standard 3/8" diameter, same as the stock Commando and most aftermarket push rods, not 7/16". I wish there was room for a 7/16" pushrod. That would really improve the performance specs of the push rod. But there's just barely enough room for the 3/8" rods.

Ken
 
A real concern (and what actually blows up these motors) is valve clash and dropping a valve. I have seen a lot of this but I have never seen an aluminum pushrod fail. I used to rev my race motors until the valves wore flat spots from rubbing each other - and this is with dual racing springs shimmed to the max (before beehive springs were available). A heavy steel pushrod may be stronger than hell but the real concern is reducing the valve train reciprocating weight to avoid the destructive valve float. Adding weight with a steel pushrod is where the real danger is. Norton motors are different than most because the rockerarm ratio is nearly 1 to 1 so the pushrod moves more and contributes more to valve float than other pushrod motors.

To match the weight of a 3/8" diameter steel pushrod tubing to a stock alum pushrod or one of my .058" wall 2024 T3 alum pushrod tubes (have been raced to 8500RPM without failing) - the steel pushrod tube would have an extremely thin wall somewhere around .018”.

Snotzo – I would like to see a buckling comparison of 3/8” OD 4130 tubing with a wall thickness of .018" - the same weight as .058” wall 3/8” OD 2024T3 high strength aircraft tubing which weighs 32 grams per foot (and not some weaker alloy such as the Stock Norton pushrod alloy that bends more easily).

EDITED - The actual weight of .058" wall alum is 32 grams (not 34) and that is equal to .018" wall steel at 32 grams (both 3/8" OD at 1 foot length)
 
Ken
I have a friend in Australia (Brisbane) who used 7/16" x 0.035" steel tube for pushrods in his Gold Star racer, and said after some considerable use he had nothing but praise for them.

Regarding steel pushrods of 3/8" x 0.035" wall thickness, for the same length as used in earlier calculations, the maximum buckling load calculates out at 11214 N against the 7/16" tube of 18548 N.

JS
we get into some pretty thin wall tubing when comparing steel to your 0.058" wall alloy, but the max. buckling load for the alloy is 5310 N, and for the steel is 6565 N
For the latter to match the alloy material weight, the wall thickness is calculated at 0.0178"
 
I think we need to step back and regroup to "the problem definition": ie. what is the problem?

The only pushrod failure I can remember was a fellow at Daytona with a Commando using carbon fiber pushrods where one just shattered. So buckling analysis or measurement, although a curiosity, is not what's at issue in my not so humble opinion. So where is the problem?

Why would someone go with a steel pushrod as opposed to the aluminum pushrod, I say it is for the stiffness. Yes,weight (mass) is certainly a factor in designing a valve train but so is stiffness. At road going speeds with stock cams, the stock Commando aluminum pushrods are apparently adequate, especially since the cams and valve train were probably designed to account for the springier aluminum. Stock pushrods also endure in many race applications.

When dealing with higher rpm, much higher than normal road use and much more aggressive cams, the aluminum pushrods will experience much greater loading and would act more like a vaulting pole, lofting the valve - more so than a stiffer steel pushrod.

See the video link below for a very graphic analogy. Consider the pole the pushrod and the athlete being the valve. The athlete is going significantly higher than the pole could ever deliver him or her with a simple arc of the straight pole. The stored energy is lofting him. Furthermore, and more significant, the athlete is going significantly higher later and is not going so high earlier when compared to a pure arc of a straight vaulting pole.

https://www.youtube.com/watch?v=UfGIZX7enN8

So with a more springier material like aluminum and higher loads of higher rpm and/or more aggressive cam profile, the valve is sitting on the seat when it should be lifting (reduced valve time-area just when the engine needs it the most) and has more of a tendency to loft the valve at peak.

Ask anyone, stiffer is better, eh?
 
Dances

the 'problem' as you perceive it did not exist in JS' original post, he was attempting a comparison of compressive strengths of pushrod tubes having the same weights but differing in wall thickness. Despite an attempt to hijack the thread by the introduction of imaginary details of the skills necessary to control some Triumph engined abortion, generally speaking I am happy that the discussion has continued in line with the original post, and feel it would be appropriate for JS to add his comment on this point

The line of your thinking would take us off along an altogether different track involving valve train dynamics, and should by rights necessitate the start of a new topic which I am quite prepared to be a participant of, but the present topic rightly should focus on the compressive strengths of the varius tubing, and presumably at some point JS will feel he has enough feed back for him to proceed on the next part of his experiment, which must at some stage involve practical engine tests to satisfy his curiosity and prove a point - one way or the other.
 
I re-checked the weight on a gram scale and the actual weight of 1 foot of .058 wall 2024 alum is 32 grams (not 34). The weight of 1 foot of .018" wall steel is also about 32 grams. (both 3/8" OD). An online calculator shows them both to weigh .069 lbs at: http://servicesteel.com/weight-per-foot-of-steel-tube/ (choose 2024 for alum guage)

Snotzo said:
....For the latter [steel] to match the alloy material weight, the wall thickness is calculated at 0.0178"
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Was this compared to 34 grams of alloy or 32 grams of alloy?

Ok now we're talking about a difference between .018" wall and .0178" wall which is negligible. We're getting down to splitting hairs here.

Snotzo - when you're comparing what you call alloy - are you talking 2024T3 or some other alloy?

Dances - your point about springiness (weight per weight) is next after strength per weight diff is established.
 
Snotzo said:
imaginary details of the skills necessary to control some Triumph engined abortion,
Click to expand...

That is a gem.

I'll restate but not dwell on the point that comparisons of compressive strengths of pushrod tubes having the same weights but differing in wall thickness is a bit academic and not based on any Norton engine problems I am aware of, but carry on. It's easily answered through engineering analysis (Euler's Column formula).

If going through the trouble of testing the material, consider a relatively stiff machine and carefully plot load versus displacement. A natural next step towards valve train dynamics.
 
Dances
like yourself I know of no pushrod problems related to Commando engines, and from work done thus far all indicators show that standard pushrods generally are well up to the job required of them, the reason for choosing otherwise being very much in the mind of the individual.
There are occasions where the choice may involve a change of material, and sometimes it is necessary to be selective of tube configuration to avoid a clash of resonant frequencies with the valve springs - as you say, delving into the realm of valve train dynamics.

JS
the 1 foot long tube is hardly relative to any Commando pushrod, nevertheless, having input the appropriate length, the mass is calculated as 32.8 grams. I do not have readily available the compressive strength of the default aluminium material, this being part of the source code that I do not have access to, but for your interest will endeavor to obtain it.
 
Dances with Shrapnel said:
....So with a more springier material like aluminum and higher loads of higher rpm and/or more aggressive cam profile, the valve is sitting on the seat when it should be lifting (reduced valve time-area just when the engine needs it the most) and has more of a tendency to loft the valve at peak.
Click to expand...

There was a time when a few Nascar racers used a springy pushrod to get more loft and increased valve lift to have an advantage on the competition. They were actually getting more power that way. Later they just used a bigger cam with stiff pushrods to get the same thing. It gets complicated.

To match the weight of the race proven 3/8" 2024 alum pushrods with .058" wall you would need a steel (4130) pushrod with .018" wall thickness.

Such a thin wall of steel could turn out to be very springy as you describe. I don't think such a thin wall has ever been race tested. Any volunteers?
 
JS
for your interest I ran a simulation of steel tube in 0.018" wall thickness, in conjunction with a flat tappet follower, a PW3 cam, steel valves and a beehive valve spring, at an engine speed of 7000 rpm.

In both the exhaust and intake positions, the pushrods performed satisfactorily, although in both positions there is a very distinct valve opening delay between the static design and the dynamic, of some 16 crankshaft degrees, occasioned by initial flex and bending of the pushrod as the first load is applied. For both exhaust and intake calculations the valve spring seated pressure was computed at 98 lbs.

At your originally stipulated test length of 1 foot, with the 0.018" wall thickness, the maximum buckling load is 2294 N.

Re your query as to the particular aluminium used in Prof. Blair's 4StHead software, the default specification is for 6061.
 
Snotzo said:
JS
for your interest I ran a simulation of steel tube in 0.018" wall thickness, in conjunction with a flat tappet follower, a PW3 cam, steel valves and a beehive valve spring, at an engine speed of 7000 rpm.

In both the exhaust and intake positions, the pushrods performed satisfactorily, although in both positions there is a very distinct valve opening delay between the static design and the dynamic, of some 16 crankshaft degrees, occasioned by initial flex and bending of the pushrod as the first load is applied. For both exhaust and intake calculations the valve spring seated pressure was computed at 98 lbs.

At your originally stipulated test length of 1 foot, with the 0.018" wall thickness, the maximum buckling load is 2294 N.

Re your query as to the particular aluminium used in Prof. Blair's 4StHead software, the default specification is for 6061.
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This getting interesting, but more complicated. Does your 2294N buckling load for .018" steel compare to the 2526N buckling load for 6061 alum that you wrote on the 1st page of this thread? And is that .058" wall alum?

2024 alum is stronger than 6061. I don't see how we can use Blair's figures and be accurate.

Thanks for showing the flex of .018" thin wall steel. I don't know how far I want to pursue and push this but its looking like alum and steel are not far apart gram for gram.
 
JS

6061 has been for some years perfectly adequate for pushrods, and still is. If you get trouble with 6061, you will also be in trouble with either 2014 or 2024, there is not sufficient difference between the three that will enable one to survive and the others not.
Splitting hairs, like your 0.018" wall thickness and my calculated 0.0178" !

I will have to re run the aluminium pushrods to see what wall thickness was used in the calculation.
 
JS
to add further to the confusion (?) I should point out that the addition of steel ends to a pushrod tube, whether it be steel or aluminium, increases the overall length of the column, and likewise changes the calculation. All my previous calculations were for typical Commando pushrods having steel ends. To make direct comparison with the compressive strength of the tubes, I have run calculations for tubes alone, and so for this reason give further results, which are intended to replace all previous. I shall give only two items from these calculations, maximum buckling load, and tube weight.

Calculation for 1 foot long steel tube of 3/8" diameter and 0.018" wall thickness. Buckle 2852 N. Weight 31.1 g
Ditto for aluminium tube. Buckle 983 N. Weight 10.8 g.

Calculation for 1 foot long steel tube of 3/8" diameter and 0.058" wall thickness. Buckle 6631 N. Weight 89 g
Ditto for aluminium tube. Buckle 2286 N. Weight 30.8 g

For the same length and diameter aluminium tube to have the same resistance to buckle as a steel tube of 0.018" wall thickness, the wall thickness in aluminium would need to be 0.1056" and the tube would weigh 47.7 g.

All pushrod tubes that are shortened for the same diameter and wall thickness, increase their resistance to buckle and reduce in weight, and if the tube diameter is increased for the same length and wall thickness, resistance to buckle is increased but so also is the weight.
 
Snotzo said:
JS
to add further to the confusion (?) I should point out that the addition of steel ends to a pushrod tube, whether it be steel or aluminium, increases the overall length of the column, and likewise changes the calculation. All my previous calculations were for typical Commando pushrods having steel ends. To make direct comparison with the compressive strength of the tubes, I have run calculations for tubes alone, and so for this reason give further results, which are intended to replace all previous. I shall give only two items from these calculations, maximum buckling load, and tube weight.

Calculation for 1 foot long steel tube of 3/8" diameter and 0.018" wall thickness. Buckle 2852 N. Weight 31.1 g
Ditto for aluminium tube. Buckle 983 N. Weight 10.8 g.

Calculation for 1 foot long steel tube of 3/8" diameter and 0.058" wall thickness. Buckle 6631 N. Weight 89 g
Ditto for aluminium tube. Buckle 2286 N. Weight 30.8 g

For the same length and diameter aluminium tube to have the same resistance to buckle as a steel tube of 0.018" wall thickness, the wall thickness in aluminium would need to be 0.1056" and the tube would weigh 47.7 g.

All pushrod tubes that are shortened for the same diameter and wall thickness, increase their resistance to buckle and reduce in weight, and if the tube diameter is increased for the same length and wall thickness, resistance to buckle is increased but so also is the weight.
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Snotzo said:

"1 foot long steel tube of 3/8" diameter and 0.018" wall thickness. Buckle 2852 N. Weight 31.1 g"

"...1 foot long ... 0.058" wall thickness ...aluminium tube. Buckle 2286 N. Weight 30.8 g

Here you say steel buckle is 2852N and alum is 2286 N. Thats 125% difference.

then you say:
"... to have the same resistance to buckle as a steel tube of 0.018" wall thickness, the wall thickness in aluminium would need to be 0.1056"

But this is nearly doubling the .058" alum diameter.

If .058" (6061) alum has 2286 buckel then why must it need to be nearly doubled to get to 2852N buckle (.018" wall steel)?

I do not agree that 6061 is similar to 2024 in strength. The 2024 is harder and stronger. In my experience, parts that work with the stronger alum alloys will bend and fail if made with 6061. They also machine differently, cut differently and sound differently. The 2024 has more "ring" to it because of its hardness.

2024 .095" wall tubing is obviously stronger and more resistant to bending than solid bar 6061 (both 3/8" dia). 6061 is butter in comparison. Simply clamping each in a vice with 1" protruding and bending it with a crecent wrench - the 2024 takes much more effort. Its not even close. I just repeated this test several times and the 2024 feels almost twice as stiff.
 
JS

I've given the results of calculations run on a very expensive piece of software. If you seriously believe squeezing a piece of tube in a vice is some sort of superior test....

I repeat what I said earlier, the difference in use as pushrod tube between 6061 and 2024 is negligible for all practical purposes. If you wish to dismiss Prof. Blair's calculations in favor of a hydraulic jack and toggle bar, go right ahead, it makes no difference at all to me.

Size for size in a compressive test, the 4130 steel tube will always be stronger, and so it will be weight for weight, no matter what type and/or grade of aluminium is used.

From here on it's up to you , either obtain the use of a materials tester, or work it all out longhand with the Euler formula, but you will still get the same results as I've given previously.
 
jseng1 said:
Here you say steel buckle is 2852N and alum is 2286 N. Thats 125% difference.
Click to expand...

That's more like 25% difference.

Testing is interesting but must be carefully executed to render meaningful results. Without a proper (and adequately stiff) testing machine and appropriate jigs and fixtures (ex. proper (near frictionless) ball ends to simulate the pushrod ends) the video results in this instance are dubious. Furthermore, the strain of one pushrod will cause eccentric loading on the other pushrod.

Take a look at Euler's column formula (and perhaps Johnson's parabolic function - test for slenderness ratio) and you should get a sense of what it is all about. In my opinion, for those who are curious, one could easily set up a spreadsheet that one could test various what-if scenarios for steel and aluminum sections & lengths.

Snotzo's analysis (through Prof. Gordon P. Blairs software) is governed by materials science and math.
 
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