Torsion Spring Resources 5

[FeX32]

Hey guys,

Was wondering if anyone knows of some good resources (detailed) on torsion spring design? I found this:

http://www.mitcalc.com/doc/springs/help/en/springs.htm

I'm looking for anything I may be able to attain extra insight on, even research papers and such.
Thank you,

 

[desertfox]

Hi

Try this

http://www.roymech.co.uk/Useful_Tables/Springs/Springs_Torsion.html

 

[FeX32]

Thanks desertfox.
The reason I ask is due to some discrepancy between measured coil to coil clearance and ID change from the relations.

 

[desertfox]

Hi

From my experience torsion springs and other springs for that matter are not an exact science, while we normally specify the numbers and dimensions the spring maker will vary the dimensions, number of coils etc. to obtain the desired spring rate or load at a given deflection.

 

[FeX32]

Indeed. I guess it varies more than I thought.
I will attain statistical data from the manufacturer.

 

[tbuelna]

FeX32-

When you say "torsion spring", is this the specific configuration of spring you mean?

There are many other types of torsion springs, such as torsion bars, clock springs, etc.

I would agree with what desertfox said about the wide variation in properties with the type of torsion spring shown in the picture above. If your design requires a spring that provides consistent, precise and reliable performance, this type of spring is not a good choice. The only reason to use this type of spring is that it is easy to package. If your design allows, you might be better off using a compression spring.

Regards,
Terry

 

[FeX32]

Thanks Terry.
The type I refer to is that which you refer except for 2 details. a) coil to coil clearance must always be >0, the pics show 0. b) the tangs are much shorter.

if your design requires a spring that provides consistent, precise and reliable performance, this type of spring is not a good choice.

Can you elaborate? We generally use a statistical analysis, but recently we have been trying to update the modeling so that we can say for example; spring A will be stressed 70-75 % yield with confidence (instead of using 72-82 %). As well, the statistical deviations in for example clearance (coil to coil, or other) I would like to improve the confidence margin.
If there is any information in this regard someone can provide (to compare to what I have), I would be appreciative.

 

[desertfox]

Hi FeX32

You could look at BS1726 and BS13906 which look at tolerancing and design of torsion springs and the latter specification gives a figure for allowable stress of 70% tensile strength and doesn't refer to yield strength.

http://www.leespring.com/downloads/uk/leespring_engguide.pdf

The link above is to a design guide of a spring manufacturer.

 

[israelkk]

FeX32

I do not understand what are you looking for? Are you trying to design a spring? If so, can you provide what are your exact requirements for the spring i.e. mechanical requirements, dimensions, etc., Why are you requesting that the coils will not touch each other?

 

[Tunalover]

The following has all the calculators you'd ever want about springs, including torsion springs:


Tunalover

 

[FeX32]

Thanks for the responses.
Sorry for the delay guys. Been a busy last few days.

desertfox, thanks for the manufacturing design reference. I will have a look to see what I can learn from it.

tunalover, also thanks for the reference. Looks like there is a lot of information there.

israelkk, Good questions. Actually, I am evaluating current spring design methods then applying what I learn to a new product.
Avoiding coil contact is ideal for several reasons. Wear avoidance (due to high frequency oscillation), but even more so is the design physical constraints. One being the fact that we use the springs in 'wind-up' as opposed to 'un-wind'. This causes the spring to elongate and decrease the coil to coil clearance. Also, since the spring pre-load is high, this leave small room for error on clearance calculations.
An example design criteria would be pre load 2.7Nm, mean coil diameter 43mm, spring height must stay under 20mm. Coil diameter usually 2.52-2.77mm. Spring rate of 8Nmm/deg.

One other reason I am evaluating current design methods is statistical error. When the manufacturer produces these springs for production, if there is low clearances in the design we have to take into account manufacturing errors more closely. For example if there is a coil clearance of 0.1mm in the design then it is likely that some of the springs when put in the assembly will lock up at a certain pre-load.
Has anyone come across something similar? Usually we use a basic statistical error (based on 3 standard deviations) to compute the likelihood of problems. Anyone know of better methods?

Thanks,

 

[israelkk]

FeX32

Am I correct that you mean "wire diameter usually 2.52-2.77mm"?
Spring wire commonly comes in standard wire diameters. What are the actual wire diameters in the 2.52-2.77mm range? Are you using the inch system standard wire diameter or the metric system standard wire diameters?

Your spring data lucks the following minimum data:
1. Extra deflection added to the "pre load 2.7Nm" for every cycle of oscillation.
2. How much cycles of oscillation the spring is designed for?
3. Shape of spring arms (may affect the spring file cycle)

 

[FeX32]

Thanks israelkk.

Yes you are correct, it is wire diameter. The actual wire diameter is 2.52mm exactly and 2.768mm, I'd prefer the smaller wire dia.
They are standard metric wire diameters.
The extra deflection the spring is designed for is +- 7 deg. deflection at up to 300Hz.
Cycles should be infinite.
Spring arms are simple tangs each about 8mm in length.

 

[israelkk]

FeX32

Spring wire material and heat treatment?

 

[FeX32]

Material: CrSi Alloy Per A1000-99
No heat treatment.

 

[FeX32]

If it helps, yield is about 2000 MPa for that material.

Cheers,

 

[FeX32]

Oh and also, I'd like to be able to compute the "free-length" (axial) for such a spring. Say you end up with 6 coils in the end and compute that is should fit axial into its cavity. How do I know what its axial free length is when manufactured?
I noticed it's usually 10-25% longer than a nominal position with decent distance between the coils.

 

[israelkk]

FeX32

According to ASTM A 1000-99 the minimum ultimate tensile strength (U.T.S.) of Cr-Si spring wire 2.5 mm diameter is 1965 MPa. For larger diameters the U.T.S. is even smaller. Spring wires strength is defined by the U.T.S. and not yield strength. For design, only the minimum value should be considered because there is no guaranty that for future production batches the U.T.S. will be higher.

Quick check calculations show that for a spring:

Maximum 20 mm free length
43 mm mean diameter (~45.7 mm O.D.)
Wire diameter 2.5 or 2.75 mm (standard wire nominal dimensions, the standard tolerances on the wire diameter is +/- 0.02mm).
Preload torque of 2700 N-mm
Extra deflection of 7 degrees during work
Material: Chrome Silicon per ASTM A 1000-99

there is no solution for a spring with a spring rate of 8 Nmm/deg.

There may be a solution with "much higher" spring rate using 2.75 mm wire diameter but, the coils must almost touch each other at deflected position.

To use the 2.5 mm wire diameter, the standard manufacturing process can not be used, if free length should allow a gap between the coils when the spring is deflected. Add to this that the spring rate will be again "much higher" than 8 N-mm/deg.

A spring 2.5 mm wire diameter with a spring rate a little higher than 8 N-mm/deg will leave almost no gap between the coils when the spring is deflected.

The coil cycle life is unlimited according to the calculations but my concern is the tang bends. The force on the tangs tends to "open" the tang therefore, the inner radius of the tang suffers much higher stresses than the coils added to other manufacturing issues that will lower the life cycle of the tangs.

Can you provide your location (state, country)?

http://israelkk.googlepages.com/home

http://www.webspawner.com/users/israelkk/

 

[FeX32]

Thanks israelkk !

What if I change the required preload to 2Nm?

I'm in Canada. Industry is Tier 1 Automotive.

 

[israelkk]

FeX32

In this case the 8 N-mm/deg nominal spring rate (normal manufacturing tolerance is +/-10%) is not available. The rate will be higher.

The gap between coils at maximum deflection is theoretically 0.15 to 0.2 mm. This may not be enough to insure that all coils are apart from each other.

In this case only the 2.5 mm wire diameter is a close candidate.

However, if the preload is only 2 N-m the load at the maximum deflection will be ~25% lower than if the preload is 2.7 N-m? If the specifications of the system are not rigid you are better off with enlarging the space for the spring.

In my previous post I mentioned that using standard manufacturing process the requirements are not achievable but using non orthodox (and more expensive) processes may yield good results with large enough gap between coils.

Another issue is the chance of spring getting into resonance in axial movement of the coils that may use clashing of the coils due to the 300Hz loading.

Did you take into consideration that the spring outside diameter decreases by more than 5.7 mm due to the large deflection of the spring ~260 degrees for 2.5 mm wire diameter and larger deflection for smaller wire diameters.