BOLT TIGHTENING TURN TO GET THE REQUIRED FORCE VALUE. 2

[skengg]

Hello all,
i need some guidance on solving a numerical problem, we are currently doing packing compression test as per API 622, picture attached below of sketch of test setup fixture.
as u can see in picture, packing is compressed by tightening bolt & nut which is putting force on spring through washer .
if i have given compressive stress value on packing (let's say 30 MPa) which we need to apply through washer by tightening bolt, can u tell me how to calculate tightening force which we need to apply on the nut and

and
how much turns should we rotate the nut to fully tighten the bolt with stress value shown above.

 

[3DDave]

The load appears to be resisted by the disc springs. In previous practice with a similar need it was only solved by pre-measuring the stack height of the disc springs at the desired force. The previous attempt, using turns of the nut, resulted in variation of 50% to 200% of the desired load.

 

[btrueblood]

With proper design of the spring stack, you might get reliable turn-of-nut numbers. Likely there isn't room in the assembly for the Belleville spring required, and it probably isn't a stock spring, but more likely a custom one required.

 

[skengg]

hello, dave
the line "The previous attempt, using turns of the nut, resulted in variation of 50% to 200% of the desired load" what's mean by that?

 

[skengg]

brtueblood,
can u provide any reference for disc spring design shown in figure, I have literature of helical coil spring design.

 

[btrueblood]

Get a copy of Roark's Formulas for Stress and Strain, edited by Warren C. Young, or Wahl's Mechanical Springs. Both have formulas for design of Belleville washers. There are also many online calculators that work, just google "Belleville spring calculator"; these mostly are located on the web pages of manufacturers of custom and stock Bellevilles.

Typical catalog stock Bellevilles (e.g. McMaster Carr) are designed to be placed under bolt heads to hold the bolts (when torqued to make the spring washer flat) at a substantial fraction of the full bolt preload/proof load in case the joint opens. For any lower load value (like maintaining a packing pressure on a non-metallic seal element), you need typically much lower than the full bolt proof load, which requires a thinner and probably larger diameter Belleville. With a good spring stack design, you can make the spring deliver a fairly constant load over a very wide range of spring stack heights. If you get really clever, you might find stock flat washers with the right thickness and diameters, and then lathe turn a simple conical punch and die to form the conical spring, at least well enough to confirm the prototype. I also have worked with a manufacturer that could turn Bellevilles from hardened rod stock.

 

[skengg]

thanks, btrueblood for information.
yea I'm looking for for all those mentioned reference.

 

[skengg]

btrueblood, i want to ask one question as the disc spring will be stacked, how to be it suppose to stack like in series or parallel or combination of both? what are the decisive factor to consider which stacking is suitable?