flat spring stock (source) (ti possibly?) 6

[JimBlackwood]

Hello all. New guy here. I build prototypes (usually mechanical but it varies) with concentration primarily in the automotive and agricultural fields. Right now I need to make a pair of leaf springs for a set of hinges to bias them in one direction and could use some help in finding a source for a suitable material in a small quantity.

The spring leaf will be 3/4"wide and can be about 1/16-1/8" thick. It will be approximately 6" long but that dimension can be varied for fine tuning. It will be clamped on one end up to within about 1" of the hinge pivot with the other end resting on the opposing hinge arm. The main issue is range of motion as the hinge will need to move through as much as 30-40°. The springs will be exposed to the elements so a corrosion resistant material is preferred such as a good grade of stainless or possibly a nickel-titanium alloy.

Regardless of the alloy chosen the range of allowable thicknesses should allow the spring to be tuned to the load so all I really need is a source that sells sheet or strip spring stock in an appropriate material. A strip only 1 ft long would be enough and I would have little need for more than 6ft of it.

Thanks for your consideration, I'm hoping this will be an easy one.

Jim

 

[dhengr]

Jim:
Don’t you want a torsion spring, about the hinge pin? Isn’t 30-40° of deflection/rotation in a concentrated length of a leaf spring a failure mechanism? Clamping it to within about 1" of the hinge pin might be too close, and cause this excessively tight flexure region. I guess this would work if the leaf spring acted as a long enough cantilever, with one end fixed and the other end springing, and moving on/along the other hinge arm, to allow the hinge to move 30-40°, if that’s what you mean. Why wouldn’t almost any available spring steel work as long as you could play with length, width, thickness, and the like? This would prove the concept, give or take a few pounds of force on the hinge. Then start looking for an exact material, based on what you have learned. Sorry, I don’t have a source off the top of my head. Google spring steel.

 

[JimBlackwood]

Correct, and correct. But the cost is greater to build the spring twice than it is to build it once using the very best materials. Ergo the search for a premium grade corrosion resistant strip. It will indeed be a cantiliver/slipper arrangement. If stainless is used a 2 or 3 leaf construction is possible to allow greater flex. If NiTi is used that would not be necessary. Clearly that construction is to be preferred.

Jim

 

[JimBlackwood]

I guess I will go with the. 031" thick full hard 301SS strip from McMasterCarr and a multi-leaf design. I expect 3 leaves will be just about right but that will be easy to tune.

Jim

 

[btrueblood]

That'd be my first choice too.

 

[JimBlackwood]

Turned out to be not a very good choice however as the 301 is entirely too soft even in the full hard spec. So, exactly the result I was trying to avoid. Fifty bucks spent on material I can't use and another delay.

And I'm back to the original question: Does anyone know where I can buy a small quantity of Nitinol strip?

Jim

 

[NickE]

too soft, or not stiff enough?

You might want to try Sandvik 7C27Mo2, it's a modified 420 stainless available in fully quenched and tempered condition. Usually around 56HRc, have you determined what hardness you need?

is the spring permanently deforming? you're going to be hard pressed to find anything stronger than fully hard spring stock.

The Ni/Ti alloys you are talking about are really expensive, and they have a lower modulus than steel.


Nick

 

[JimBlackwood]

Thanks Nick. Stiffness is the issue, hardness is really not a concern. The 301 deforms so easily that there is not even any point in cutting out the blanks. Apparently I didn'get what I thought I was. I should have known better, I've worked with 301 before, but in heat treated form I expected much better strength. Honestly this feels more like annealed to me. It is 3/4" x. 031" strip packaged in a 6" roll and it comes off the roll with what looks like a 1 or 2 ft. radius. I think banding strap does better than that. To me that doesn't qualify as a spring. I'd be better off cutting up an old band saw blade. Except that it'd probably rust.

I appreciate the suggestions. The problem is that not only am I not familiar with those particular alloys but I really have no idea where to buy them in small quantity. And I really want to avoid additional heat treatment. Plus I'd prefer to avoid another repeat so anything I can do to improve my odds is a good idea.

OK I know the nitinol is going to be pricey, but maybe affordable in the quantities I want as sheet or strip stock. One square ft is plenty. As for the modulus being less, possibly that is a function of the specific alloy. No doubt you have seen the eyeglass earpieces that you can twist into a tight little loop and they spring back? No stainless or any other steel alloy that I have ever seen can do that and I have my doubts about beryllium copper being able to match it.

That is overkill for my application but it means that I should be able to find something that will work. I have no specifications for the eyeglass earpiece material however and without it I cannot find the range. What I do know is that stainless grades used for knife blades is stiff enough to work. Whether it will flex enough without deforming or breaking is the question.

So it'sback to the original problem then.

 

[NickE]

301 stainless cannot be heat treated, only cold work strengthens it. you obviously didn't get fully hardened.

Teh 7C27Mo2 is purchased in it's fully hardened condition, no need to heat treat.

you are confusing strength with stiffness, and throwing some hardness in there to boot.

440C (common knife blade stainless) is also used as a spring material.

The stiffness of 301, 7C27Mo2, 440, 1008/1010, etc.. are essentially the same, engineers call that modulus.

Nick

 

[mfgenggear]

JimBlackwood

1st option
buy some used leaf springs.
use abrasive cut off disk & then cut it to your requirements.
or plasma cutter if available.
try to minimize the heat.

2nd option
google spring steel
a ton of hits
buy from you local material dealer.
recommend AISI 5160 Steel

Mfgenggear

 

[MikeHalloran]

It's starting to smell like you've painted yourself into a corner, geometry-wise, such that buying spring temper unobtainium is not going to get you out of it. ... and I'm not convinced that Nitinol would help, either.

This is the point at which, if you had an engineer, you would ask him or her to model the problem in such a way that it is soluble with known materials, and then adjust the dimensions toward... well, whatever your dream requires.





Mike Halloran
Pembroke Pines, FL, USA

 

[NickE]

Jim,

Just so you know, I ran a prototype spring lab for around 5 years, and I sourced all my own materials, drew some 10-20% of the prints, manufactured components myself on occasion. Just because I went to school and they taught me the difference between stiffness, strength, and hardness; doesn't mean I don't use my hands as well as my head to do my job. I just prefer to build a prototype that almost works the first time. Usually my vendors were the ones we used for production, but I did cultivate several vendors of exotic alloys in very small amounts... It was really easy, for me.


Nick

Nick

 

[israelkk]

"....I do respect theoretical design which is why I came here with the question in the first place.... "

If you really do and you know what you are doing then you should have known that at least you should give the required torque (in lbf*in) from the spring when deflected the 30-40°. Not giving this "little insignificant" data is an insult. This is why I didn't responded to your post at the first place. Not to mention other "minor details" like as how many deflections it will have to endure, or how long it may stay deflected, the exact environment including temperatures.

So, why are you so disappointed when it "....Turned out to be not a very good choice however as the 301 is entirely too soft even in the full hard spec. So, exactly the result I was trying to avoid. Fifty bucks spent on material I can't use and another delay.,,,".

How at the first place did you come to the conclusion that "The spring leaf will be 3/4" wide and can be about 1/16-1/8" thick." or decided to go with "....I guess I will go with the. 031" thick full hard 301SS strip from McMasterCarr and a multi-leaf design. I expect 3 leaves will be just about right but that will be easy to tune...." without any calculations?

Luckily, it may cost you only 50 bucks and some man work hours. I have seen hundred thousands of US$ goes to the drain when the designer of a system in a complex project "neglected" to calculate this "small insignificant spring inside his system" before completing modeling and producing manufacturing drawing, just to later on to find that the space left for this "small insignificant spring" was too small and the whole design and development process has to start all over again.

 

[JimBlackwood]

Now I have been designing and building prototypes for upwards of 45 years and many have been the times that I've helped an engineer out of the proverbial corner, and many have been the times that I have helped them improve their designs before the patent apps were filed. So I don't think it is necessary to prove whose methods work best. You do things your way and I'll do them mine. But when you need my help I do not think I have ever behaved so badly that I couldn't pass along some tidbit of inside information.

But the spring is the thing. The hinge does not need a redesign. It will work as intended and look elegant doing it once a proper spring material is in hand. I prefer to order small volume materials online, so can you name a vendor? Post a link?

And Nick, respecting your experience with springs, if you were going for the highest loading with the greatest range of motion in the thinnest material given the size required,what alloy would you select, say for a marine environment? Understanding of course that it has to be possible to actually purchase the material. Who would you buy it from? URL?

Jim

 

[mfgenggear]

Jim
what part did I miss?
for corrosion resistance why not just plate over spring steel?
I read your post, I surely gave an answer with the least amount of resistance.
and for an R & d project. Leaf Springs are heat treated to the correct hardness & met lab requirements.
trying to buy material off the shelf I don't believe will be easy to obtain.
that is why I suggest it.

"Not trying to start a fight with you guys but the condescending attitude strikes a nerve. I guess it's one of the things engineers do so well."

"so do none engineers"
The fact is you got answers (responses), just not what you were not expecting.
I would let it go & take the advise & hire some one to help.


Mfgenggear

 

[JimBlackwood]

Ah yes, the spring rate. I knew all along that would be wanted but sometimes life has higher priorities. I will get that though, perhaps even this evening. It will not be particularly high, certainly not enough to paint me into a corner, but I will let you know.

As for all of these high dollar options that you boys keep recommending, are you beginning to understand why I considered the nitinol as viable? The critical parameter here is flexure which that material has in spades. By the time I buy two or three alloys, hire another engineer who specializes in flat spring alloys, send the parts out for plating, and deal with all of the peripheral expenses that nitinol is starting to look pretty cheap.It also has about the best corrosion resistance you can buy.

But not to belabor the thing, I understand you guys have all been trained in lock step engineering and that is fine. It serves industry well enough. That isn't the only game in town though and it isn't the only way to look at materials. Close I'll admit and you do a good job of quantifying the characteristics right down to the molecular level but it's like an old sage once told me about computers. The volume of information is so vast that of necessity you have to either specialize in one area or else concentrate on a more superficial depth of knowledge of the whole. Clearly you have chosen the former while I have chosen the latter. It's fair to say we are all smart people here but our strengths vary widely. But now if you wanted to know about patent law or any of a host of other things that I could mention then I'm your guy. Even have more than typical knowledge of metalurgy, and if need be I am capable of finding my way through this on my own. But, I thought I'd check out the site, see if there was any common interest, that sort of thing.

But I digress. I'll see about those spring loadings.

Jim

 

[JimBlackwood]

I have results. Recall if you will that I said the spring pressure requirement would not paint me into a corner? Hold that thought. The hinge supports a load of maybe 60 lbs spread over a length of 4 ft and shared by two hinges. The hinge pin is located just off the balance point and is adjustable. The weight urges the hinge in the desired direction and the function of the spring is simply to insure that the hinge moves in the desired direction during articulation of a second set of hinges spaced 18" from the first set. Gravity alone should be sufficient but the addition of a set of springs biasing the hinge in the correct direction adds certainty to the movement of the hinge as well as countering unexpected wind loading. So as to the question of how much biasing force is needed, the specification is whatever I decide it should be. And what is my decision? As much biasing force as I can reasonably get out of readily available materials formed into the specified shape and dimensions. I realize this is backwards from how you guys like to do things but tha ts the result nonetheless. Ergo, no corner to be painted into. Empirical Engineering at it's finest.

Now I have some more good news. Although I had allowed for as much as 20-40° of articulation of the hinge (better to err in that particular direction) the actual minimum hinge articulation is in reality under 10°. Again, no corner.

Jim

 

[israelkk]

The phrase "spring pressure" is same as saying a "bull milk", it doesn't exist!!

If

"... I decide it should be. And what is my decision? As much biasing force as I can reasonably get out of readily available materials formed into the specified shape and dimensions... " is your mantra.

Then how it co-exists with

"... I guess I will go with the. 031" thick full hard 301SS strip from McMasterCarr and a multi-leaf design. I expect 3 leaves will be just about right but that will be easy to tune...."

and the conclusion

"...Turned out to be not a very good choice however as the 301 is entirely too soft even in the full hard spec...".

This is exactly

"....As much biasing force as I can reasonably get out of readily available materials formed into the specified shape and dimensions...."?

Not to mention how the spring deflection specifications tends to change from 40 degrees to 10 degrees on the fly.

This is no way to present an engineering problem and expect to get intelligent responses.

 

[JimBlackwood]

So. Nick or anyone else, do you have a vendor and an online address for me? In spite of all the talk nothing has changed. That is still the objective.

Jim

 

[israelkk]

"...In reality it is the result that matters..."

I completely agree with you and as you said here are your results:

"...Turned out to be not a very good choice however as the 301 is entirely too soft even in the full hard spec...".

And as regard to Nitinol that you mentioned as a possible alloy for your spring I would recommend you to check and learn that this is a Shape Memory Alloy (SMA) which is soft in the room temperature and gets it strength only when heated to a specific temperature after it was previously "trained" under the same higher temperature. I assume you meant Nickel alloys such as Inconel X-750 alloy. But since you do not really care about engineers and engineering lingo you freely use alloys names as "Nitinol" and invented engineering terms such as "Spring pressure".

I know that you do not care and I have no hope to change or convince you. But I do care about new young engineers that may learn from bad design approach too.