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3.6.3 Diametrical clearance. The greatest factor in reducing the life of
O-ring packings is the extrusion of the O-ring into the clearance gap. The
clearance consequently should be held as small as practicable with special
attention given to factors such as thermal expansions, pressure expansions,
side load, eccentricities{ type of motion, and other basic considerations of
surface finish, lubrication, and accuracy which affect o-ring life. Backup
rings or nonextrusion devices permit the use of slightly larger gaps.
Diametrical clearance is the total difference between the diameter of the bore
and the diameter of the member contained therein.
3.6.4 General limitations of O-rinqs. O-ring packings and gaskets have some
general limitations which should be kept in mind when designing hydraulic
units. Some of these limitations are shown in Figure 4.
3.6.4.1 Friction. In some units, such as surface control boost cylinders,
the breakout friction of O-rings can cause undesirable drag, which if not
are not suitable for the purpose intended. Breakout friction is caused by the
extrusion of the packing material into the surface irregularities of the
mating surfaces or adhesion, or both, of these materials (see 3.6.2). Breakout
friction of O-rings is higher than running friction. All standard packings
have some minimum friction value which cannot be materially reduced by
practical methods of design. When friction problems are encountered, a
special gland installation or a special packing should be considered.
3.6.4.2 Oscillation. O-ring packings, when used to seal rapidly oscillating
mechanisms, have not always proven successful in past installations. Special
studies must be made in designs of this type to provide for proper life and
performance.
3.6.4.3 Rotary seals. Standard packings are not specifically designed as
rotary seals. However, where infrequent rotary motion or low peripheral
velocity is required, they may be used, provided consistent surface finishes
over the entire gland are used and eccentricities are accurately controlled.
In addition, the use of low-friction nonextrusion devices have been found to
be helpful in prolonging life and improving performance.
3.6.4.4 Precaution. Glands in which the o-ring is seriously deformed or
distorted by crushing or other loads (stretching and twisting) can induce
permanent set and disintegration in the o-ring. Increased temperatures or
strains, or both, induced in a seal will cause rapid deterioration of the seal
owing to the strain aging and permanent set properties of seal compounds.
3.7 Reclaimed materials. The use of reclaimed materials shall be encouraged
to the maximum extent possible.
4.
QUALITY ASSURANCE PROVISIONS.
4.3 Responsibility for inspection. Unless otherwise specified in the
contract or purchase order, the contractor is responsible for the performance
of all inspection requirements (examinations and tests) as specified herein.
Except as otherwise specified in the contract or purchase order, the
contractor may use his own or any other facilities suitable for the
performance of the inspection requirements specified herein, unless
disapproved by the Government. The Government reserves the right to perform
any of the inspections set forth in this specification where such inspections
are deemed necessary to ensure supplies and services conform to prescribed
requirements.
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