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A.2.8 Quick Release Pins

A.2.8.1 Quick Release Pin History

Quick release pins, also known as PIP (push in and pull) pins are pins with fast-acting retention
and release mechanisms built into the pin. They come in a variety of forms: single-acting (push
to release) or double acting (push or pull to release) with a wide range of handle designs and
other features to choose from, but generally they all utilize a spring-loaded central shaft to
actuate one or more retention balls, which are held in via swages or staking of the housing
material around the balls. Figure 5, Diagram of a Typical Quick Release Pin, depicts a cross-
section of a representative quick release pin, in this case a double-acting pin, that illustrates the
principle.

Quick release pins were originally designed for use in non-critical, remove-before-flight ground
applications on aircraft. Their speed and convenience has led to their continued adoption for
other purposes, such as spaceflight applications involving crew interfaces. Unfortunately, these
applications are far beyond the original design applications, which has resulted in a history of
failure.

Figure 5—Diagram of a Typical Quick Release Pin

A.2.8.2 Quick Release Pin Failures in the Space Program

Though several documented inadvertent releases of quick release pins were noted, no serious
documented failures occurred in the space program until 1990. In that year, NASA began
environmental testing of the extravehicular activity (EVA) Development Flight Experiments
payload, which contained quick release pins. During vibration testing, several locking balls in
the pins vibrated out of their sockets; and during cold temperature vacuum testing, the lubricant
in the pins froze and seized the pins. NASA solved these problems by using military standard
pins that were quality controlled and removing all lubrication from the pins based on their single
mission use and because the lubrication was mainly provided for corrosion protection in the first
place.

After this failure, several design changes were proposed for quick release pins to create a more
reliable pin for space use:

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• Use of four retention balls instead of two to improve retention in the event of a
single ball release.

• Double-action.

• PTFE-coated tethers.

• Welded handles and tether rings.

• Addition of dry film lubricants.

• Addition of hitch pin.

More detail is provided in PIP Pin Reliability and Design (Skyles, 1994).

The addition of hitch pins, which are self-retained shear pins that are placed through both the
interior shaft and exterior housing to prevent inadvertent actuation of the pin and act as a
secondary means of retention in the event of ball release, would turn out to be controversial.
Hitch pins proved to be difficult to install with gloved hands and presented snag hazards during
EVAs. Several instances of inadvertently pulled hitch pins were encountered on Hubble Space
Telescope (HST) servicing missions 3A and 3B.

In 1994, these and other problems prompted the Space Shuttle Safety Review Panel to establish a
policy in which quick release pins had to be treated as mechanisms in their own right, requiring
the same engineering rigor and review practices as other mechanisms. A continuing history of
problems led to the revision of the policy that prohibited quick release pins from being used in
zero-fault-tolerant applications in 2000.

In 2001, a set of pins that had incorporated these improvements exhibited some other problems
during preparation for HST servicing mission 3B. Pins with a two-piece welded spindle/button
construction were found to be prone to fracture. Such pins that were used only in contingency
scenarios were launched with hitch pins; others with planned uses were flown with a system of
hook-and-loop flags to secure the pins in the event of a structural failure.

Vibration testing of the same pins uncovered a defect that caused the retaining balls to stick in
the extended position. The ball staking and shank boring processes were inconsistent, resulting
in the protrusion of the balls to vary widely. In addition, the retention balls had a rough surface
finish. The combination allowed side loads to jam the balls into the staking under vibration
loads. An acceptance test procedure was developed to screen for this failure mode. Nine of the
forty-eight pins used in the mission, which came from multiple vendors, failed the test and five
more barely passed.

Afterward, a new vendor was contracted to build forty-six custom pins for the project. A series
of meetings and site visits by HST and NASA personnel ensured that the new hardware was built
and tested to the highest possible standards. Design enhancements included a single-piece

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spindle/button design, an H-1025 heat treat of the 15-5 PH steel used in the pins for greater
ductility, molybdenum disulfide lubrication, a drive-out feature, specially designed EVA-friendly
handles, and hitch pins. The manufacturer also performed additional verification including extra
dimensional inspections and staking tests on 100 percent of the pins. All subsequent
qualification and acceptance testing (thermal, random vibration, and “stick-ball” tests) was
successful.

In 2004, a double-acting pin on the ISS mobile transporter rail was found fractured prior to flight
due to a ductile overload from a suspected incidental impact. The investigation revealed a
deficient design of the head that allowed tolerance stack-ups between an external groove and an
internal thread combined with poor process control to produce an unacceptably thin wall. The
head and other parts of the pin were redesigned by the prime contractor and the pin manufacturer
to address the deficiencies and add random vibration testing to the part specification, creating a
new part number approved for use in certain space applications. One hundred and twenty-four
discrepant pins were replaced on the ISS.

A.2.8.3 Current Quick Release Pin Best Practices

Given this history, quick release pins are not recommended for applications that control hazards,
and should never be used in critical applications in which they experience axial loads. Properly
designed and constructed pins can be used successfully in non-critical shear applications. Even
with the most up-to-date pin designs, four failure modes have to be addressed when considering
their use, as follows:

a. Loss of locking balls.

Due to the arrangement of the balls and the subsequent need to use swaging or staking around
them, loss of locking balls is nearly impossible to eliminate with design and manufacturing. The
usual control is to provide back-up retention feature such as a hitch pin or some other means of
ensuring that the pin will not fall out under environmental conditions if the balls disappear.
Failures of the shank and head can be eliminated but depend on the pin vendors since they rely
on design practices and process controls. A continuing risk is that pin manufacturers can change
their internal designs at any time without notification, so the detailed design of the mechanism
and the change history should be investigated thoroughly prior to the use of any pin. Lastly,
quick release pin structural integrity in a particular application has to be assessed; due to the
internal features of the pin, dependable allowables are not always available, are dependent upon
application, and will not cover situations such as inadvertent contact loads.

A.2.9 Inspection

Inspection of both constituent parts and assembled mechanisms is an important part of the testing
and preparation of flight mechanisms. Some materials used are more susceptible to material

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