This Appendix provides
nonmandatory supplementary information and
guidelines to assist in the understanding and use
of 29 CFR 1910.217(h) to allow presence sensing
device initiation (PSDI) of mechanical power
presses. Although this Appendix as such is not
mandatory, it references sections and
requirements which are made mandatory by other
parts of the PSDI standard and
appendices.
1. General
OSHA intends that PSDI continue
to be prohibited where present state-of-the-art
technology will not allow it to be done safely.
Only part revolution type mechanical power
presses are approved for PSDI. Similarly, only
presses with a configuration such that a person’s
body cannot completely enter the bed area are
approved for PSDI.
2. Brake and
Clutch
Flexible steel band brakes do
not possess a long-term reliability against
structural failure as compared to other types of
brakes, and therefore are not acceptable on
presses used in the PSDI mode of
operation.
Fast and consistent
stopping times are important to safety for the PSDI
mode of operation. Consistency of braking action is
enhanced by high brake torque. The requirement in
paragraph (h)(2)(ii) defines a high torque capability
which should ensure fast and consistent stopping
times.
Brake design parameters
important to PSDI are high torque, low moment of
inertia, low air volume (if pneumatic) mechanisms,
non-interleaving engagement springs, and structural
integrity which is enhanced by over-design. The
requirement in paragraph (h)(2)(iii) reduces the
possibility of significantly increased stopping time if
a spring breaks.
As an added precaution to
the requirements in paragraph (h)(2)(iii), brake
adjustment locking means should be secured. Where brake
springs are externally accessible, lock nuts or other
means may be provided to reduce the possibility of
backing off of the compression nut which holds the
springs In place.
3. Pneumatic
Systems
Elevated clutch/brake air
pressure results in longer stopping time. The
requirement in paragraph (h)(3)(i)(C) is intended
to prevent degradation in stopping speed from
higher air pressure. Higher pressures may be
permitted, however, to increase clutch torque to
free “jammed” dies, provided positive measures
are provided to prevent the higher pressure at
other times.
4. Flywheels and
Bearings
Lubrication of bearings
is considered the single greatest deterrent to their
failure. The manufacturer’s recommended procedures for
maintenance and inspection should be closely
followed.
5.
Brake
Monitoring
The approval of brake
monitor adjustments, as required in paragraph
(h)(5)(ii), is not considered a recertification, and
does not necessarily involve an on-site inspection by a
representative of the validation organization. It is
expected that the brake monitor adjustment normally
could be evaluated on the basis of the effect on the
safety system certification/validation documentation
retained by the validation organization.
Use of a brake monitor
does not eliminate the need for periodic brake
inspection and maintenance to reduce the possibility of
catastrophic failures.
6.
Cycle Control and
Control Systems
The PSDI set-up/reset
means required by paragraph (h)(6)(iv) may be initiated
by the actuation of a special momentary pushbutton or
by the actuation of a special momentary pushbutton and
the initiation of a first stroke with two hand
controls.
It would normally be
preferable to limit the adjustment of the time required
in paragraph (h)(6)(vi) to a maximum of 15 seconds.
However, where an operator must do many operations
outside the press, such as lubricating, trimming,
deburring, etc., a longer interval up to 30 seconds is
permitted.
When a press is equipped
for PSDI operation, it is recommended that the presence
sensing device be active as a guarding device in other
production modes. This should enhance the reliability
of the device and ensure that it remains
operable.
An acceptable method for
interlocking supplemental guards as required by
paragraph (h)(6)(xiii) would be to incorporate the
supplemental guard and the PSDI presence sensing device
into a hinged arrangement in which the alignment of the
presence sensing device serves, in effect, as the
interlock. If the supplemental guards are moved, the
presence sensing device would become misaligned and the
press control would be deactivated. No extra micro
switches or interlocking sensors would be required.
Paragraph (h)(6)(xv) of the standard requires that the
control system have provisions for an “inch” operating
means; that die-setting not be done in the PSDI mode;
and that production not be done in the “inch” mode. It
should be noted that the sensing device would be
by-passed in the “inch” mode. For that reason, the
prohibitions against die-setting in the PSDI mode, and
against production in the “inch” mode are cited to
emphasize that “inch” operation is of reduced safety
and is not compatible with PSDI or other production
modes.
7.
Environmental
Requirements
It is the intent of
paragraph (h)(7) that control components be provided
with inherent design protection against operating
stresses and environmental factors affecting safety and
reliability.
8.
Safety
system
The safety system
provision continues the concept of paragraph (b)(13)
that the probability of two independent failures in the
length of time required to make one press cycle is so
remote as to be a negligible risk factor in the total
array of equipment and human factors. The emphasis is
on an integrated total system including all elements
affecting point of operation safety.
It should be noted that
this does not require redundancy for press components
such as structural elements, clutch/brake mechanisms,
plates, etc., for which adequate reliability may be
achieved by proper design, maintenance, and
inspection.
9.
Safeguarding the
Point of Operation
The intent of paragraph
(h)(9)(iii) is to prohibit use of mirrors to “bend” a
single light curtain sensing field around corners to
cover more than one side of a press. This prohibition
is needed to increase the reliability of the presence
sensing device in initiating a stroke only when the
desired work motion has been completed.
“Object sensitivity”
describes the capability of a presence sensing device
to detect an object in the sensing field, expressed as
the linear measurement of the smallest interruption
which can be detected at any point in the field.
Minimum object sensitivity describes the largest
acceptable size of the interruption in the sensing
field. A minimum object sensitivity of one and one
fourth inches (31.75 mm) means that a one and
one-fourth inch (31.75 mm) diameter object will be
continuously detected at all locations in the sensing
field.
In deriving the safety
distance required in paragraph (h)(9)(v), all stopping
time measurements should be made with clutch/brake air
pressure regulated to the press manufacturer’s
recommended value for full clutch torque capability.
The stopping time measurements should be made with the
heaviest upper die that is planned for use in the
press. If the press has a slide counterbalance system,
it is important that the counterbalance be adjusted
correctly for upper die weight according to the
manufacturer’s instructions. While the brake monitor
setting is based on the stopping time it actually
measures, i.e., the normal stopping time at the top of
the stroke, it is important that the safety distance be
computed from the longest stopping time measured at any
of the indicated three downstroke stopping positions
listed in the explanation of Ts. The use in the formula
of twice the stopping time increase, Tm, allowed by the
brake monitor for brake wear allows for greater
increases in the downstroke stopping time than occur in
normal stopping time at the top of the
stroke.
10. Inspection and Maintenance.
[Reserved]
11.
Safety System
Certification/Validation
Mandatory requirements
for certification/validation of the PSDI safety system
are provided in Appendix A and Appendix C in
this standard.
Nonmandatory supplementary information and
guidelines relating to certification/validation
of the PSDI safety system are provided to
Appendix B to this standard.
[53 FR 8364, Mar. 14,
1988; 61 FR 9227, March 7, 1996]
