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|  MIL-G-5514G
3.6.1 Design data. The design data contained in this section are intended to
supply the designer with the basic fundamental reasons behind the requirements
of this specification and the results which may be expected when there is
deviation from those requirements. Typical installation of staic seals is
shown in Figure 3.
3.6.2 O-ring squeeze. Referring to Table I, the o-ring squeeze it
represented by the difference between the free O-ring cross-section diameter
and dimensions A-F or E-B (as applicable).
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3.6.2.1 Type I systems. In order to produce an acceptable product that will
perform satisfactorily throughout its normal life, it is recommended that
0-ring packing squeeze and dimensions listed in Table I be used. The minimum
squeeze and dimensions shown in Table I are so established that with all
tolerances, clearances, eccentricities, side loads, and linear contraction of
the packing compound taken into consideration, there will still be a positive
interference remaining on the O-ring section throughout the temperature range
of this type system.
3.6.2.2 Type II systems. For these systems, it is recommeded that the
O-ring packing squeeze dimensions listed in Table I be used.
3.6.2.3 Change of squeeze considerations. The following items were
considered in setting up the dimensions shown in Table I for Type I systems
and must be given due consideration when deviations from these dimensions are
made. For Type II systems, these considerations maynot be directly
applicable.
3.6.2.3.1 Decrease squeeze. Decreasing the squeeze will slightly reduce
friction and breakout under low hydraulic pressure (under 500 psi) operating
conditions. When reduced squeeze is used, a better surface finish is usually
required for low-pressure sealing. The saving in friction will be neutralized
at high pressures owing to compression of the o-ring into the end of the
groove. Figure 4 illustrates this condition as well as positions of packing
in their grooves under various degrees of pressure. Breakout friction of
O-ring type packing will be higher than running friction, being dependent on
factors of surface finish, time, pressure, squeeze, etc. Particular care must
be taken to ensure that low-pressure and, low-temperature, leakage is not
encountered.
3.6.2.3.2 Increase squeeze. Greater o-ring squeeze than specified in Table I
may result in greater assembly problems, requiring larger or flatter angle
bevels, or both, at shoulders, etc., (see Figure 1). Increasing the squeeze
will also tend to increase the scrubbing and rolling of the O-ring during
operation which may in turn result in shorter packing life. The friction at
low-operating pressure will be increased. The greater squeeze may, however,
result in lowering the critical cold temperature of the unit from the
standpoint of low-temperature leakage. When squeeze is increased beyond that
shown in Table I and backup rings are required, those listed in Table I cannot
be used owing to interference.
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