Gasket Compression and Fastener Spacing Design Equation and Calculator

General Engineering Reviews
Engineering Applications and Design

Proper cover - gasket compression design by application of spaced fasteners is a function of plate thickness, acceptable gasket pressures, gasket performance under compression, and flange dimensions.

Maximum gasket deflection occurs at the location where the fastener applies force. The amount of bowing or deflection of the cover flange depends on several factors. Bowing may be severe enough that inadequate pressure is applied at the mid location of the gasket between fasteners. This effect may be minimized by proper design of the fastener spacing, cover / flange plate thickness and stiffness, and proper selection of gasket materials.

Related Resource: What is the Right Flatness Tolerance for a Gasket Application (Video)

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The basic equation for determining bolt spacing is:

Where:

a
= Width of cover / flange plate at seam (in)
b
= Width of gasket (in)
C
= Bolt spacing (in) (see illustration below)
E
= Modulus of elasticity of cover / flange plate (lb/in2)
ΔH
= H2- H1 (in)
H1
= Minimum gasket deflection (in)
H2
= Maximum gasket deflection (in)
Pmin
= Minimum gasket pressure (lb/in2)
Pmax
= Maximum gasket pressure (lb/in2)
t
= Cover / flange thickness (in)

The equation above may be simplified by adopting the following assumptions which only slightly affect the bolt spacing results.

The Width of the gasket equals the width of the cover / flange (a=b). This is the limiting condition, as the flange plate dimension a will always equal or be larger than the the gasket width. If the gasket width is equal to one half of the flange width, the bolt spacing correction should be 1.19 times less the value obtained for a=b or a/b=1. The actual correction factor is (a/b)1/4 . Using (a=b) provides a safety factor over any other relationship between a and b.

The maximum pressure Pmax equals three times the minim pressure Pmin. For most gaskets, Pmax is normally greater than twice Pmin. Using the ratio Pmax/Pmin = 3, bolt spacing is reduced by less than 7% for Pmax to Pmin ratio of 6. Actual correction factors for other values of Pmax and Pmin are shown in table A.

Pmax / Pmin Correction

Pmax / Pmin
Correction Factor
2
1.02
3
1.00
4
.98
5
.95
6
.94

Pmin
Correction Factor
10
1.19
20
1.00
30
.90
40
.84
50
.80


Table A

 

Where:
a/b =1
Pmax / Pmin = 3

For when Pmin = 20psi

Aluminum plate (E = 107 psi)
Steel plate (E = 3 x 107 psi)

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