Screw Torque Required To Compress A Spring

[valmeidan]

I was wondering how I would determine the torque setting required to compress a spring x amount of distance. I am using a M2.5 screw, and the compression spring is already determined, so I know I need to compress it about 2mm, however I looked up the torque value for the M2.5 and it says around 6inch lbs, or around 0.9NM. I am not sure if that is the point of yield and shear on the screw?? Anyhow, how can I know if I stick to that spec that the spring will compress 2mm required for given force I need? SS screwing into copper..

 

[valmeidan]

Also I was wondering if maybe a belleville washer would be better for this application for low profile, if anyone has any experience with spring washers.

 

[MintJulep]

Spring force is proportional to the length of compression.

Screw advance is proportional to thread pitch.

So I'd count turns rather than try to get there with the tenuous relationship between torque and force.

 

[valmeidan]

The thing is I can deflect this to full compression which is about 2.6mm. I just want to make sure the bolt head doesnt break off to achieve this. If I could control the length of the screw by torque value that would be better, then one could just tighten until they hit a given rotational torque value which would also compress the spring the needed amount.

 

[rb1957]

a simple relationship is P = 5*T/d or (of course) T = (ftu or fty)/Amin*0.2*d

Quando Omni Flunkus Moritati

 

[Tmoose]

Hi valmeidan,

"then one could just tighten until they hit a given rotational torque value which would also compress the spring the needed amount."
I'd read MintJulep's post again.

As much as I love and respect torque as a fastener tightening metric I can not deny that the tolerance on your spring compression is likely to be +/- 25% even after I developed a torque spec empirically.

 

[dhengr]

Valmeidan:
Torque is notoriously unreliable as a means of gaging what you are trying to do, because of all the things that can alter the torque from screw to screw. Given that you already know the spring you are using, you should know what force it takes to compress the spring 2mm. Compare this needed force to the tensile strength of the SS screw and to the thread stripping strength of the copper (or thread length required), apply an appropriate factor of safety to each, and torque should not likely control the final design. You can either use the screw thread pitch (turn of the nut, number of turns per mm), as MintJulep suggested above, as a means of measuring the screw advancement, or you could come up with a spacer gage which would stop at a fairly light torque setting when the washer had advanced 2mm. This might be as simple as a half cylindrical tube of xx length which fit around the spring and provided a spacer btwn. the base plate and the underside of the washer.

 

[SnTMan]

Also I was wondering if maybe a belleville washer would be better for this application for low profile, if anyone has any experience with spring washers.

Likely take a stack of bellevilles to get 2 mm deflection in that apparent size (2.5 ID). If deflection is what you're after. I don't think anybody really knows what "this application" is.

Regards,

Mike

 

[drawoh]

valmeidan,

I estimate that an M2.5X0.45 thread in A2-70 stainless, can handle 300lb. Unless you are using a weird screw of some sort, the strength of the heat should be equivalent. Don't use hex socket button or flat head cap screws.

The force to compress a spring part way down should be way below this.

--
JHG

 

[valmeidan]

drawoh??


"""I estimate that an M2.5X0.45 thread in A2-70 stainless, can handle 300lb. Unless you are using a weird screw of some sort, the strength of the heat should be equivalent. Don't use hex socket button or flat head cap screws.

The force to compress a spring part way down should be way below this"""

so why then are the recommended torque ratings so low on screws? I found a spec on an M2.5 screw @ .9n/m = like 7 Inch Lbs....

 

[rb1957]

"I need to compress it about 2mm" ... ok, so you know the force you'll put into the screw to do this, right? (F=kx, no?)

"I looked up the torque value for the M2.5 and it says around 6inch lbs, or around 0.9NM. I am not sure if that is the point of yield and shear on the screw??" ... well depending on what you're looking at this might be "just" the recommended installation torque. In any case, knowing F from above), then you can use T = 0.2*Fd to estimate the torque required to compress your spring.

"how can I know if I stick to that spec that the spring will compress 2mm required for given force I need?" ... you can't do this, spec values don't know what you're trying to do. if you want to compress the spring, then start there and figure out where this takes you.

personally i'd be worried about how the copper is going to react this load ... if you're screwing into a tapped hole, then i see the threads in the copper being much weaker than the SS bolt.

Quando Omni Flunkus Moritati

 

[SnTMan]

..however I looked up the torque value for the M2.5..

Care to reveal your source?

Regards,

Mike

 

[drawoh]

valmeidan,

Tightening Torque: T=CDF...

...where C is the friction factor, usually 0.2 for an unlubricated joint, D is the major diameter of the screw, and F is the force.

When I do the calculation for an unlubricated M2.5X0.45 A2-70 screw at 90% of yield, I get a torque of around 5lb.in. There are some FAQ's on this forum with more sophisticated calculations for this, but this is the ballpark.

I am getting 7lb.in for an A2-80 stainless steel screw. Good luck finding one.

Are you an engineer? This is pretty basic machine design.

--
JHG

 

[drawoh]

valmeidan,

Two more things.

Torque to tighten screws is very, very approximately 90% friction. This point has been noted above, but not explicitly. Screw torque does not translate accurately to tension force. If you want to control tension, you are better off measuring deflection.

I design a lot of springy stuff. Any spring that can be compressed by a similar sized screw, is very much more flexible than the screw. The screw is being loaded in direct tension. The spring is being loaded in torsion or in bending, and it is long and skinny. I have designed screws and nuts that were meant to compress stiff springs. People have expressed concern about the forces and torques on the screws and nuts. There is no concern. The screw or stud is many orders of magnitude stiffer than the spring.

--
JHG

 

[valmeidan]

drawoh, thanks for your help. I figure it is best to just control the deflection distance with a stop point of some sort on the cnc plate to reflect the deflection load at a given compression, instead of trying to figure out the screw torque required to compress the screw to the same distance, as there is too many factors including friction as mentioned etc. Thanks again

 

[rb1957]

we said early on in this that the best way to control compression of the spring is by controlling the rotation of the fastener. knowing the thread pitch this will give a very accurate result. of course a dial gauge would work just as well.

you raised a reasonable question (i thought) about the strength of the screw. i think the critical strength is going to be the copper thread, i understand your installation as a tapped hole in copper.

Quando Omni Flunkus Moritati