Thread engagement length and number of bolts of an o-ring joint 1

[SMOKES]

want to check my calcs for thread engagment and number of bolts need for 240 bar test of pump casing

Bolt M16 A4-80 55mm long no shank

UTS = 800Mpa (not sure of)

UYS = 600Mpa (not sure of)

Shear ratio is 0.75 (not sure if it should be 0.7)

UTSh * 0.75 = 600Mpa

Stress Area of bolt = 156.67mm calculated

Tapped hole CF8M cast 30mm

UTS = 485MPa (known data)

UYS = 205MPa (known data)

Shear ratio =0 .75 (not sure about ratio)

UTS * 0.75

UTSh = 363.75Mpa

Minimum Length Of engagement = UTSh of bolt *0.8* Normal diameterof bolt/UTsh of tapped hole

= 600*0.8*16/363.75 = 21.2mm

Nasa Pull out load of thread

Pullout load = pi()*Min Minor diameter of thread *UTShtapped hole* Length of engagement/3

Min Minor diamet of thread = 13.872mm

= pi()*13.872*363.75* 21.2mm/3

Pullout load =112.03kN @ 21.2mm of thread in CF8m

Ultimate tensile load of bolt = UTS * At

800 * 156.67mm = 125Kn

Max tensile load of bolt = UYS * At

600*156.67 = 94Kn

Pressure is 240 bar= 24Mpa

Pcd Of bolts = 170 mm

diameter of hole in bolted flange = 80 mm

Therefore

Area of pressure = (Pi (Pcd+bolt diamter/2)^2 - Pi(80/2)^2)

Area of pressure = 22145mm

Force = Area of pressure x Pressure

= 22145mm * 24 Mpa

Force on flange = 531.5Kn

Min number of bolts = Force on flange / Max tensile load of bolt

= 531.5Kn/ 94Kn =5.6 bolts

Bolts with a Safety factor of 2 = Force on flange / (Max tensile load of bolt/2)

= 531.5Kn/ (94Kn/2) =11.3 bolts

Therefore I would need 12 bolts

Is the Flange is CF8m and 25mm long and the bolt is 55mm long and it an groove o-ring joint? what the stiffness factor of the joint?

My calcs say C is 0.9 but i think that is off by a large margin!!

Does any one have an worked example to calculate C for a tapped and flange or a studded assembly.

 

[MikeHalloran]

For the area of pressure, I would use the gross area of the flange, or at least the area of the bolt pitchline circle. ... because you can't guarantee that the gasket will seal at its inner diameter.



Mike Halloran
Pembroke Pines, FL, USA

 

[SMOKES]

Err I did the PDC Plus bolt diamet if you look at the calcs

 

[MikeHalloran]

Indeed you did; I looked at the 80 and the 80 and jumped to the wrong conclusion. My apology.


Mike Halloran
Pembroke Pines, FL, USA

 

[SMOKES]

no worries

Trying to figure the right rule to use machinist handbook says one thing internet says another, I guess i am going to have to pony up for a standard....

 

[btrueblood]

Well, er, your post title said an o-ring joint, so I'd use the o.d. of the ring groove as a pressure area. But your calculation, even if using a gasket is still wrong:

diameter of hole in bolted flange = 80 mm

Therefore

Area of pressure = (Pi (Pcd+bolt diamter/2)^2 - Pi(80/2)^2)


Why are you subtracting the central area of the flange?

 

[Tmoose]

Bolt preload is required to be sufficient to keep the flanges from separating under full pressure.
Are the bolts in the pressurized area?
(I'm guessing the pump shaft rotating centerline is below the picture somewhere)
Is there room to remove the bolts, and allow tightening them?
If the pressure will force the o-ring into a large chamfer back-up rings are required.
What style of washers are under the bolt heads?

 

[desertfox]

Hi SMOKES

You're bolt material stress values appear to be correct in accordance with this site:-

http://www.roymech.co.uk/Useful_Tables/Screws/Strength.htm

However I would have taken the shear stress ratio between 0.5 and 0.6 of the material yield stress again in accordance with:-

http://www.roymech.co.uk/Useful_Tables/Matter/shear_tensile.htm

and in view of no other information for both the bolt and cast material.

You're method of calculation appears to be ok assuming that the thrust due to pressure is correct and of course up to this point the you're assuming that the joint flanges are rigid, whereas in practice there not, so I think buying the correct code or standard for your area is the right decision because it will take the elasticity of the joint into account.

Regards

desertfox

http://www.roymech.co.uk/Useful_Tables/Matter/shear_tensile.htm