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 to 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]
