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(+)+(+) The Quick Changeover e-letter
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### FEBRUARY 2004
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SOME THOUGHTS ON...
MACHINE GUARDING

Safety must always be the primary priority in any plant. It must never be
compromised for any reason. Certainly any changeover process must be done
in such a way as to minimize or eliminate any danger to the people
performing it. (Reminder: Although I will never suggest anything that I do
not believe to be safe, anything can be applied in an unsafe manner. It is
up to you to verify safety in your plant and application before applying
any ideas or concepts I have discussed in these newsletters.)

One area where safety and changeover interface is machine guarding.
Guarding is used both to keep things in and to keep things out. Properly
designed guarding will prevent anything from flying out of the machine and
striking someone. For example, in a glass bottle filler, bottles will be
broken. The guards will prevent shards of glass from escaping the machine.
In addition to keeping physical things in, guards will often be used to
contain sound (vibratory feeders), light (laser coding or processing) or
fumes (welding). This need to contain things is sometimes overlooked but
must always be considered when designing guarding.

In addition to keeping things in, guarding is most commonly used to keep
things out. Principally this is people and their body parts but it may also
be used to prevent the entry of fumes, light and extraneous contaminating
materials such as dust.

A conflict inherent in any guarding system is that it needs to do all the
things mentioned above while at the same time not unduly restricting
physical or visual access to the machine for operation, adjustment,
maintenance or changeover. Achievement of these conflicting goals can
sometimes be a tall order. Below are some ideas to help in the fabrication
of guarding systems.

Materials:

There are several materials that are commonly used. Perhaps the best,
generally, is Lexan (GE’s trade name for polycarbonate). It is physically
strong, easy to cut, drill and form, and provides good visibility. It’s
main drawback is that it can be attacked by some cleaning chemicals which
can cause it to haze, reducing visibility. It resists normal temperatures
well but will soften at higher temperatures making it unsuitable for hot
operations. Though it is fairly tough and resistant to scratching and
abrasion, Lexan is a plastic and is not proof against them.

Generally Lexan used in guarding should be colorless for maximum
visibility. There are a couple of special cases. If the guard is
surrounding a high intensity light such as a laser, a welding torch or the
like, use a bronzed or smoked Lexan to prevent the light from bothering
others. Dusty operations may be another exception. Stokes, a manufacturer
of tablet presses, did a study in the 80's and found that if they used an
orange Lexan for guarding of the compression area, accumulations of dust on
the guard did not cut down visibility as much as similar accumulations on
clear Lexan. It became the standard for their machine guarding.

Plexiglas, Rohm & Haas' name for acrylic plastic is also used occasionally
for guarding as it is cheaper than Lexan. There is a reason it is cheaper.
It is just not as good for this type of application. It is more prone to
scratching, less resistant to chemicals and generally just not as tough.

Where vision as well as resistance to chemicals and physical abrasion is
needed, tempered or safety glass can be used. DO NOT ever use regular glass
for safety guarding.

Expanded metal steel or wire mesh screen is another material that is often
used. It is relatively inexpensive, very strong and easily worked. It can
be washed or steamed with aggressive chemicals without degradation. It can
also be useful where physical guarding is required but airflow cannot be
interrupted. It’s principle disadvantages are that it may not contain small
projectiles such as shards of glass. It also restricts visibility. One
trick I learned from RA Pearson, a manufacturer of case packers and related
equipment, is to paint the expanded mesh a flat black. This makes it
considerably easier to see through as the eye will focus behind the mesh
rather than on it.

Solid sheet metal guarding can also be used where visibility is not a
requirement.

An easy way to construct guarding is to use a system such as that
manufactured by Solus http://www.solusii.com/products.html under the
SpeedFrame name. These systems consist of extruded aluminum struts and a
variety of mounting hardware and accessories. The struts are cut to length
and joined as necessary like an Erector or Mechano set. When an entire
machine needs to be guarded, these systems can be used to build a floor
mounted enclosure, space permitting. This makes fabrication much simpler
than fitting the guards to a complex machine shape.

Access:

The ultimate safety guarding would be something on the lines of a steel
box, ½" thick, totally enclosing the machine and welded shut. Nothing could
go in or out. Unfortunately, the machine would be useless. Doors must be
provided and they must be big enough to be comfortable. They should be
designed to swing or slide completely out of the way and, if required,
should latch open. Where doors open vertically, they should have air
springs to assist in opening and keep them open. In some cases it may be
necessary to allow easy removal of doors but this is generally not
preferred. The reason is that if the door is removed, it must be placed
somewhere, generally on the floor, and may be damaged or misplaced. Doors
should also have positive latches to prevent them from vibrating open
during machine operation. In some cases, locks or latches requiring special
tools for opening may be recommended.

Interlocks

All doors, of whatever design must have safety interlocks. These interlocks
must emergency stop, rather than simply cycle stop, the machine when
activated. They must be difficult to defeat using a screwdriver or other
common tools. (You might be amazed how imaginative mechanics can be in this
regard). The interlocks should be positive, closing or making by spring
pressure but opening by physical action. In some cases contacts may weld
together or a spring may fail preventing the interlock from stopping the
machine. They need to “fail safe”, in other words. If there are a large
number of interlocks on a machine, it is a good idea to have a panel
showing the status of each with a red or green LED. This can save time
troubleshooting when a machine fails to start.

There is much more that can be said about machine guarding, I have merely
scratched the surface here. For example, I have not even mentioned light
curtains or other devices. My intention here was to touch on some of the
key points and to give some lines for further investigation.

The US Occupational Safety and Health Administration has some good
information on guarding at:

http://www.osha.gov/SLTC/machineguarding/standards.html

http://www.osha.gov/SLTC/etools/machineguarding/intro.html

TIP OF THE MONTH...
BRUSH DRYER

Tablet filling machines use round brushes to aid the flow of tablets. These
brushes need to be washed after each production run and then dried before
next use. One of my clients built a small machine to speed up the drying.
The machine consists of a Lexan box split lengthwise. A pair of bearings is
mounted at each end of the lower section and the shaft of the brush laid in
them. The top section also has a bearing at each end so that the shaft is
captured between 3 bearings when the box is closed. A small motor then
rotates the brush at high speed throwing off of the water and leaving the
brush almost completely dry.


Best,

John R Henry CPP

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