large deflection flat spring

[ENGINEERRMECH]

Hello Forum-
I am trying to design a flat spring used in an existing product. The spring is being redesigned due to fatigue failures in the field. The spring is 2.125" long x 0.340" deep x 0.025" thick and made of Blue Tempered Steel (1095), or Heat-treated SK85M. The spring is preloaded to a deflection of 0.625" and the maximum deflection will be 1.125." I have tried to model the problem in ANSYS, but the company I work for does not have the license for large deflections. When hand calculating the problem as a simply supported beam with one end overhanging one end the results, do not match experimental results. Any help modeling/calculating this large deflection problem would be great.

Thank you for reading this question.
Mike

 

[israelkk]

MECHANICAL SPRINGS, 1963 2nd Edition. McGraw-Hill Book Co. By A.M. Wahl page 179

 

[desertfox]

Hi EngneerMech

Can you provde a sketch? the spring is supported in two positions with one end over hanging a support,where is the load applied?

regards

desertfox

 

[ENGINEERRMECH]

Hello israelkk-
Thank you for the information.

 

[ENGINEERRMECH]

Hello desertfox-
I attached a pdf with the dimensions and loading you requested. I also added information pertaing to the analysis and experimentation that I've performed. I hope this is not too much information. Basically, I'm trying to compare my theoretical results and ANSYS results to experimental results as shown in the pdf.

Thank you for your help,
Engineerrmech

 

[btrueblood]

At least one potential problem I can see - the force gauge you are using in the experimental setup probably distorts (under-reports) load readings when the force is at a severe angle to the end of the loading pin, as shown in the picture in your pdf file.

You might want to consider a loading mechanism that duplicates the installed configuration in your part, with whatever friction boundary conditions exist there (it looks like it's a plastic housing?). If you must use the force gauge, try setting it so that the action line of the gauge is normal to the contact point on the beam at its deflected position. Or, load the spring via a calibrated hanging weight, so that the load always remains vertical at a known value?

 

[ENGINEERRMECH]

Hello btrueblood-
Thank you for the insight into loading the part with a hanging weight. I will give that a try.

 

[koza19]

I have attached a Design sheet to show you what the calculated stress are.

 

[koza19]

Sorry forgot to attach file

 

[desertfox]

Hi Engneermech

I will look and post later


desertfox

 

[ENGINEERRMECH]

Hello ckozka-
Thank you for taking the time to simulate my problem and supplying me with a pdf. May I ask what SW package you used for the simulation?

Thank you,
engineerrmech

 

[koza19]

ASD software made by UTS
Here is the site

http://www.uts.com

 

[israelkk]

ckozka

The case you analysed as appears in the Beam_Study.pdf file is different from the case as appear in ENGINEERRMECH file

http://files.engineering.com/downlo...5f-4483-b0a1-46a0f917c731&file=engr_tips.pdf.

In your case the beam is rigidly supported in one end while the actual case as appears in ENGINEERRMECH is a completely different case.

 

[israelkk]

ENGINEERRMECH

In your post

http://files.engineering.com/getfil...f5f-4483-b0a1-46a0f917c731&file=engr_tips.pdf

the 2D schematic shown on page one fits the formulation as appear on the top right side of page 3 (simple support over hanging case). However the way it appears on bottom left side of the last page (the solid picture) is not the same. In the 2D and the overhanging case the beam is supported on a small diameter pi which can be considered as point reaction. However in the actual solid the spring rests on a much larger radius which in practical supports the spring on a quite large area thereby reduces the actual beam length, resoling in larger stresses and less deflection than the calculated.

You didn't mention any requirement for number of operations (fatigue) and neither relaxtions with time under the constant preloaded conditions.

 

[btrueblood]

Ckozka's post (from a spring design calculator) has a whole bunch of warning flags on the cover page showing that the deflection is beyond the limits of "small deflections", and use of a non-linear, large-deflection formulation is warranted.

 

[desertfox]

hi Engineermech

First thing that struck me was the large deflection relative to the length of beam, ie beam theory which you have used is limited to small deflections.
Also I am slightly confused by the two set ups, the first set up generates the pre-load then the second the working load but why does the load application change position and also the supports?
Changing the support and load application points will give different spring stiffness for each situation.
Also what is the yield stress for the material? with those deflections I was wondering whether you have over stressed the spring.

Regards

desertfox

 

[ENGINEERRMECH]

ckozka-
Could the SW you used for the simulation also simulate a simply supported beam with a load at one end?

Thank you,
ENGINEERRMECH

 

[ENGINEERRMECH]

Hello israelkk-
The two 2D drawings on the first page are a simplification how the spring is loaded. First, the spring is slipped into a channel and supported/loaded as shown in the "Preloaded Experimental" drawing. Then when the trigger/handle is pressed the spring is supported similar to the "Trigger Experimental" drawing. In the "Trigger Experimental" drawing the left support is a rough/frictionless contact where the spring can move. Your observation to the size of the dowel pins vs. the actual application is true. I used the smaller dowel pins in my experiment hoping they would more closely emulate a simply supported beam equation. I'm currently trying to model with the correct radius to simulate when the handle/trigger is pressed.

As for the number of cycles we would like to see 20K cycles and the preload is constant.

Thanks,
ENGINEERRMECH

 

[ENGINEERRMECH]

Hi desertfox-

DESERTFOX:First thing that struck me was the large deflection relative to the length of beam, ie beam theory which you have used is limited to small deflections.
ENGINEERRMECH: I was worried about the large deflection, but these were the only equations I could find. Is there a number, percentage as a rule of thumb when the deflection becomes to large for these equations? Are there equations that can be used for large deflection applications?

DESERTFOX:Also I am slightly confused by the two set ups, the first set up generates the pre-load then the second the working load but why does the load application change position and also the supports?
ENGINEERMECH: The two 2D drawings on the first page are a simplification how the spring is loaded. First, the spring is slipped into a channel and supported/loaded as shown in the "Preloaded Experimental" drawing. Then when the trigger/handle is pressed the spring is supported similar to the "Trigger Experimental" drawing. In the "Trigger Experimental" drawing the left support is a rough/frictionless contact where the spring can move.

DESERTFOX:Changing the support and load application points will give different spring stiffness for each situation.
ENGINEERRMECH: I did not know this. Is this still true after reading the above explanation?

DESERTFOX: Also what is the yield stress for the material?
ENGINEERRMECH: In my experiment I've been using Blue Tempered Shim Stock, 1095 high carbon spring steel. I had to assume the temper because the vendor doesn't publish mechanical properties. I found mechanical properties at

http://www.matweb.com/search/DataSheet.aspx?MatGUID=b4a55e9a2dc7414eabcbe53f24a0c33e

with a RELATIVELY close Rockwell Hardness.

DESERTFOX: With those deflections I was wondering whether you have over stressed the spring.
ENGINEERRMECH: In the experiment and actual use the materials are permanently deflected. Could you suggest a material that might be better for this application?

Thank you very much,
ENGINEERRMECH

 

[desertfox]

If they are permanently deflected, then the equations you are using are not valid. The equations are only valid up to the yield value. Typically, you should be using, for the maximum stress in your spring, about 40% of the yield stress for that material. At this point in time I can't suggest an alternative material that might be better as the deflections for the size and geometry of the spring are too large. I think you may need to seriously think about changing the design of the spring and the geometry.

Regards,
desertfox