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this fastener spacing information in pdf format.
One of the great advantages of our spiral design is that the ratio of
force to deflection is linear. That is, if it takes one pound to deflect
the gasket 1% of its diameter, it will take 25 pounds to deflect the
gasket 25% of its diameter. This allows fastener spacing to be easily
calculated to fairly close tolerances.
If you’ve already chosen a gasket, these formulas can be used
to determine fastener type and spacing in your application. However,
if you’re not sure whether to employ a standard, moderate or low
force gasket, these formulas can give you information that may help you
determine which gasket will be the most cost effective.
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Spacing Calculator. (Internet Explorer is Required.)
Application Information
Formula 1 below can be used to
calculate fastener spacing for all our gaskets, with the special considerations
shown for certain types of gaskets. This formula determines fastener
spacing along a straight edge. Corners require tighter spacing because
of the increased stress placed on the cover at those points.
Fasteners
In this context, fasteners, include screws and any other means of attachment
that will exert enough force to compress the gaskets 25% of their diameter.
Care must be exercised to ensure this is true. The force each fastener
must exert is equal to F0 as shown in Formula 2 (below).
Calculating Fastener Spacing
The Formulas
The formulas shown here are derived directly from classical beam problems.
Sample calculations are shown below in Example 1
and Example 2.
Note: We multiply by 80% as a safety factor.
Click to use our automatic Fastener
Spacing Calculator.
Variables
All dimensions are in inches unless otherwise noted.
L 
= length between fasteners 


Y 
= width of cover edge (Assume 1" if not distinct. 
E 
= modulus of elasticity of cover plate (psi)
= 107 psi for aluminum
= 3 x 107 psi for steel 
t 
= thickness of cover edge 
d 
= deflection of gasket
= .022" for Spira Cell
= .20D" for all other gaskets 
D 
= diameter of spiral 
F_{1} 
= minimum force of gasket on cover (pounds/inch) 
F_{2} 
= maximum force of gasket on cover (pounds/inch) 
F_{0} 
= force fastener must exert to compress gasket 25%
of its diameter. 
Force Data
"D" & Basic MultiSeal Gaskets 
Force 
F_{1} 
F_{2} 
13F_{1} + 2F_{2} 
2F_{1} + F_{2} 
Standard 
10 
35 
200 
55 
Moderate 
7 
15 
121 
29 
All Gaskets Except MultiSeal
(Honeycomb Filters: Use Moderate Force) 
Force 
F_{1} 
F_{2} 
13F_{1} + 2F_{2} 
2F_{1} + F_{2} 
Standard 
4 
30 
112 
38 
Moderate 
2 
10 
46 
14 
Low 
0.5 
2 
10.5 
3 
Example #1
Application
Assume we need to shield an aluminum box that measures 10” by 12” and
we have decided to use the groove mount SpiraShield gasket in the 04
diameter. One of our design priorities is maintainability. If possible,
we want to use quarter turn fasteners to quickly get in and out of the
box.
Variables
Material: Aluminum 

E = 10^{7} psi 
Cover thickness = .160" 

t = .160" 
Spiral diameter = D = 4/64" = .0625 

d = (.20)(.0625) = .0125 
Width of cover edge is unknown 

Assume Y = 1" 
Formulas
Calculations
Part Number 
Equation 1 
L 
Equation 2 
F_{0} 
SS04 

5.5" 
F_{0} = (38)(5.5)/3 
69.7 lbs 
MS04 

6.8" 
F_{0} = (14)(6.8)/3 
31.7 lbs 
LS04 

9.9" 
F_{0} = (3)(9.9)/3 
9.9 lbs 
Fastener Spacing
At first glance, it seems that we might be able to use as few as six
fasteners on the box if we use the LS04 gasket. However, we must consider
that the corners require tighter spacing than the straight portions
of the box. So, on the straight sections, we can use the calculated
spacing, while making sure that the corners are adequately secured.
Recommendations for each gasket follow.
SS04: Use three fasteners on each
side for a total of 12 fasteners.
MS04: Use three fasteners on the 12" side and two fasteners
on the 10" side for a total of 10 fasteners.
LS04: Use two fasteners on the 12" side and two fasteners
on the 10" side for a total of 8 fasteners.
Design Analysis
SS04: Acceptably wide spacing but requires a 70 pound fastener
load which makes the use of quarter turn fasteners impossible.
MS04: Acceptably wide spacing but
requires a 32 pound fastener load and the maximum load our quarter turn
fastener can supply is 25 pounds. However, we can take Equation
2 and work backwards using our knowledge of the maximum load of a
quarter turn fastener.
So, we take the F_{0} equation, set it equal
to 25 pounds and solve for "L".
25 = 14L/3
L=5.4"
With this additional constraint, the fastener
spacing is reduced to 5.4" which will require using three fasteners
on each side for a total of 12 fasteners but will allow us to use quarter
turn fasteners.
LS04: Acceptably wide spacing and
requires only a 10 pound fastener load which can be easily achieved using
quarter turn fasteners.
Solution
Either the MS04 with 12 fasteners or the LS04 with 8 fasteners will
work well with quarter turn fasteners. At this point, our decision must
be based on cost versus convenience factors. The MS04 gasket is more
durable, slightly lower in price and will yield slightly higher shielding.
The final determination is based on answering the question of whether
the cost of four additional fasteners, in terms of time and dollars, is
worth the benefits of using the MS04 gasket.
Example #2
Application
Assume we need to shield an aluminum box, and we have decided to use a
groove mounted "MS" series SpiraShield gasket with 6.0 inch
fastener spacing. What is the minimum size standard gasket we can use?
Variables
Material: Aluminum 

E = 10^{7} psi 
Cover thickness = .125" 

t = .125" 
Fastener Spacing = 6.0" 

L = 6.0 
Width of cover edge is unknown 

Assume Y = 1" 
Formula
Calculation
Solution
The MS05 SpiraShield gasket has a diameter of .078". Therefore,
the minimum size SpiraShield gasket which can be used is an MS05.
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