Pull-backs and restraints are another physical
protection approach. Both are restrictive and have
limitations. For that reason, operators hate them.
Both devices shackle the operator to a machine and
Yet another approach is the two-hand down/foot-through device. (The two hands have to be placed
palms down onto two pads and the foot pedal has
to be engaged before the ram comes down.) In
some cases, this will work. However, this method
raises ergonomic issues, and it is very slow. This is
not what fabricators want in a busy, production-driven shop.
Figure 2 provides a glimpse at these more traditional safety measures.
The next level of safeguarding available to fabricators centers around electrosensitive protection devices. The most familiar safety tool in this category
is the light curtain.
Light curtains started as a simple product detection device before evolving into a more complete
machine guarding product. Early safety light curtains used incandescent lamps strung together with
a corresponding line of light detectors—basically
a reflector. Today they are photoelectric presence-sensing devices that protect against access into hazardous points and areas.
Most fabricators are familiar with light curtains
because they have probably seen them in the form
of yellow bars on the front of the press brake (see
Figure 3). This type of system comprises a light
curtain transmitter and receiver. They are wired into
a safety monitoring relay and a pair of magnetic motor starters, which are part of the machine’s control
circuit. A light curtain creates a sealed work area using beams of invisible infrared light that are spaced
about 20 mm apart. If the operator reaches through
the protected area, this press will either stop moving, or it will not begin the next cycle until he pulls
his hand away.
Safety light curtains also can safeguard personnel
in the vicinity of point-of-operation hazards. This is
done with an LED transmitter and receiver. Any interruption of the plane of light by an object equal to
or larger than the minimum object sensitivity initiates an output signal. That could be a hand, a finger, or a misplaced tool. This causes the machine to
stop, or it doesn’t allow a cycle until the blockage is
removed. The safety distance between the light curtain and the machine depends on the application,
the type of light curtain, and the machine’s stopping
With this type of setup, parts can bend up to the
light curtain’s sensing field. In this case, it stops the
cycle before it’s finished. One of two things can be
done to prevent this from happening:
• Just before the upper die touches the part, the
light curtain can be automatically shut off for the
balance of the cycle. This is referred to as muting.
This is safe and recognized by ANSI B11.19.
• If the light curtain is equipped with a floating
blank feature, the part can bend up through the
sensing field without stopping the cycle. This allows the light curtain to be active throughout the
entire light curtain cycle, upstroke and downstroke.
Floating blanking is usually selective for one or two
beams, depending on the thickness of the part being bent. Simply put, if the operator is not blocking
more than one or two beams at a time, as the workpiece swings up as it’s formed, the light curtain still
functions, and the workpiece itself presents a barrier to the point of operation so that the machine operator can’t get his hands where they don’t belong.
OSHA has a set of regulations for light curtains:
• The machine must be able to stop the
movement of the ram anywhere in the stroke.
• The stopping time of the ram must be known.
• The stopping time of the ram must be
monitored for deviation in stopping time on
• The minimum distance the light curtains can
be located to the pinch point must be known.
• The light curtains must be control-reliable.
• The machine stop circuit, with which the light
curtains are interfaced, must be control-reliable.
• The light curtains must be self-checking for
proper operation on each stroke.
• The operator should have no easy way to
disable the safety system without special tools.
• If the safety system is disabled, the operator
should have a clear indication that it is disabled.
• The operator and setup person should be
properly trained in the operation of the safety
Laser Focus on Safety
The newest entry into the press brake safety
category is the AOPD. This gives operators close
proximity to the point of operation (see Figure 4).
An AOPD is best suited for applications in which a
box is being formed or a workpiece has flanges and
in which the light curtain effectiveness is diminished
because of excessive blanking or muting.
Physical barriers (top), restraints (middle), and two-button operation are some of the more traditional
means of press brake safeguarding.
Light curtains, which many have undoubtedly seen
positioned on each end of a press brake’s bed, cover
a large work area and stop the ram should someone’s
hand enter the work area.