Control Reliability - Control Compliance & Chain Break Detection

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Control Reliability - Control Compliance & Chain Break Detection Powered By Docstoc
					Michigan Department of Energy, Labor & Economic Growth




 Michigan Occupational Safety & Health Administration
      Consultation Education & Training Division


                   Onsite Consultation
              Abatement Method Advice for:

             CONTROL RELIABILITY
            Control Compliance And Chain
     Break Detection For Mechanical Power Presses

                               Prepared for Michigan
                              Department of Labor and
                                 Economic Growth

                                  A. Mark Sutton
                                 District Manager
                               Detroit Regional Office
                                  (313) 887-7960


               Note: This handout is not inclusive of all standard rule
         requirements that apply to rule requirements for Control Reliability




                                                                          OSC-6078 (Rev. 8/05)
OSC-6078


                               JUST WHAT IS CONTROL RELIABILITY?

  HOW ARE MECHANICAL POWER PRESS CONTROLS DIFFERENT THAN OTHER TYPES OF
                               CONTROLS?


MECHANICAL POWER PRESS CONTROLS ARE DESIGNED TO ACT A SPECIAL WAY WHEN THEY
FAIL. IF THE CONTROLS FAIL THEY MUST:

   1)      NOT PREVENT THE NORMAL STOPPING ACTION FROM BEING APPLIED TO THE
           PRESS WHEN REQUIRED.

   2)      PREVENT INITIATION OF A SUCCESSIVE STROKE UNTIL THE FAILURE IS
           CORRECTED.

   3)      DETECT THE FAILURE BY A SIMPLE TEST OR BE INDICATED BY THE CONTROL
           SYSTEM.


   When is Control Reliability Required?

   When 2-Hand control, or presence sensing device, or Type B gate is used as means of “Point of
   Operation Guarding” on a “Part Revolution Clutch Press” and when operator’s hand(s) enter the point
   of Operation.


                         HOW DO YOU CHECK FOR CONTROL COMPLIANCE?


   The Quick (But Not Complete) Check Is To Look For The Following Eight Items:

   1) Dual safety valve.

   2) Minimum of 4-5 clutch/brake control relays.

   3) Air pressure switches in clutch/brake line and counterbalance line.

   4) Magnetic motor starter.

   5) Disconnect lockable in “off” position.

   6) Stroke selector switch

   7) Guarded palm buttons.

   8) Ground fault indicator lights.

   The more complete check is to use a check list, which goes through each rule for control compliance.




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OSC-6078


What happens when rotation to the rotary limit switch is lost and is not detected by a chain break
or motion detection due to it’s absence?

If rotation to the rotary limit switch is lost, the press control now thinks it is making an infinity long stroke,
but in actuality the press is now in a run-a-way mode and could injure an operator. It is also important to
note if motion detection is used with a drive system that used more than one chain. If any chain that is
part of the system breaks, it will cause loss of rotation to the motion detection device.

What are some of the ways of detecting a chain break or loss of motion?

     1)      Mechanical type
             A. Spring loaded rotary limit switch with position switch.
             B. Spring loaded idler with position switch.
             C. Proximity switch to detect presence of chain.

     2)      Electronic type

A. Loss of motion device using a resolver or encoder or proximity switch with a tachometer gear.

NOTE: The fastest way to check for chain break or loss of motion system is to check the control
schematic for call out.

A.        SPRING LOADED ROTARY LIMIT SWITCH

                                  Mechanical type of chain break detection


                               PIVOT POINT
                                                 CAM

                                                                                      SPRING




                                    PRESS




                                              POSITION SWITCH


B.        SPRING LOADED IDLER

                                              CAM




                                      PRESS                                      POSITION SWITCH




                                                                          SPRING FORCE



                                                                               SPRING



                                                                                                            2
 OSC-6078


                           Mechanical Type of Chain Break Detection (con’t)

 C.       Proximity Switch to Detect Presence of Chain.



                                                          CAM
                                                                                    PROXIMITY
                                          PRESS                                     SWITCH




                           ELECTRONIC TYPE LOSS OF MOTION SYSTEMS



          RESOLVER                           ENCODER                        TACHOMETER GEAR

                                                                                             WIRE TO
                                                                                             CONTROL
                                                                                             BOX
WIRE TO                                                                                     PROXIMITY
CONTROL                             WIRE TO                                                 SWITCH
BOX                                 CONTROL BOX

                                                                                          GEAR




                             CONTROL
                             BOX                  CONTROL BOX CHECKS
                                                  FOR PROPER OUT PUT
                                                  SIGNAL FROM SENSOR




 Are There Other Systems That Can Cause The Same Effect As Loss Of Rotation To The Rotary Limit
 Switch?

 Yes, some programmable (solid state) rotary limit switches that have not been designed specifically for
 mechanical power presses can fail in an unsafe fashion (no control reliability designed in. Evidence of
 control reliability would be the presence of dual relays that are added into the emergency stop circuit of
 the press).




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  OSC-6078



PART 24. Mechanical Power Presses and The Occupational Safety & Health Standards are revised in
the December 3, 1974 Federal Register, allow “hands in the die” operation with presence-sensing
devices, two-hand control devices, or a type “B” gate device when the press control system includes
1910.217 (b) (13), control reliability and (b) (14), brake system monitoring.

R 408.12429. Control Reliability.
Control reliability is described by OSHA as follows: “When required by rule 2461 of this part, the control
system shall be constructed so that a failure within the system does not prevent the normal stopping
action from being applied to the press when required, but does prevent initiation of a successive stroke
until the failure is corrected. The failure shall be detectable by a simple test, or indicated by the control
system. This requirement does not apply to those elements of the control system which have no effect on
the protection against point of operation injuries.”

To fully appreciate the goal of the control reliability requirement, one should also be familiar with the
prerequisite requirements.

R 408.12449. Clutch/brake air valve failure.
“Air cultch controls shall be designed to prevent a significant increase in the normal stopping time due to
a failure within the operating valve mechanism, and to inhibit further operation if such failure does occur.
This requirement shall apply only to those clutch/brake air-valve controls manufacturing and installed on
or after August 31, 1971, but shall not apply to presses intended only for continuous, automatic feeding
applications.”

R 408.12454. Accidental Grounding.
“All clutch/brake control electrical circuits shall be protected against the possibility of an accidental ground
in the control circuit causing false operation of the press.”

R 408.12424. Component Failure.
“Electrical clutch/brake control circuits shall incorporate features to minimize the possibility of an
unintended stroke in the event of the failure of a control component to function properly, including relays,
limit switches, and static output circuits.”

The language of these requirements sets the pace for control reliability. The ground protection and control
component failure requirements ask that no one single failure in the control should cause a false or
unintended operation. The air valve requirements, and control reliability, go one step further. A faulty air
valve must not only stop the press safely, but further operation must be inhibited. A faulty control with
control reliability must still apply normal stopping action when required, but in addition, must prevent
initiation of a successive stroke until the fault is corrected.

Regardless of the complexity of any particular press control system, it is the single stroke clutch control
portion of the press control that demands attention when control reliability is required. Pressure switches,
stop buttons, motor interlocks, etc. are required, but they must operate only occasionally. These devices
should be checked by an actual test on a regular basis. However, the run buttons, relays, static output
circuits, clutch and brake air valve, and rotary cam limit switches must perform each of the many millions
of cycles during the life of the press. OSHA doesn’t expect those operations to all be flawless. Quite the
contrary, failures are anticipated; however, control reliability requires that a failure be recognized, and that
the failure does not prevent a normal stopping action, and that initiation of the successive stroke is
prevented.

During a single stroke of a press, a reset control sequence occurs at the start of the cycle, where a check
is made that run buttons, run relays, stopping cam limit switches, air valve checking limit switches, and
motion detector, etc. if and as used , have all dropped out, tripped, or rest to their normal condition. In the
case of static output circuits, the checking should be continuous at all times except when the cycle is in
progress. In other words, this reset control sequence checks the circuit, to prevent initiation of a stroke if a
component has failed. The methods and circuits used vary with the designer. Some circuits follow a


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OSC-6078


symmetrical redundant approach, others are non-symmetrical. All include some form of self-checking, and
are designed with fail-safe in mind, fail-safe at least in the sense of deenergization to stop, or safe
condition.

Prior to the adoption of 1910.217, and its original effective date in August 1971, simple redundancy was
often used by press control circuit designers to improve control reliability in the clutch control. Since 1970,
the use of redundant circuit design has become widespread and more complex. By doubling up relay
contacts, limit switch contacts, relays or other components, the odds against circuit failure are vastly
improved.

Redundancy is also used in a mechanical sense, such as in dual air valves, and also to improve rotary
cam limit switch drive reliability. When a chain drive for the rotary cam switch is used, it should be a
double chain, based upon the principle that two chains will not fail at the same time. An improvement of
this concept would be the use of separate rotary cam switches with separate drives. A failure of either
switch to turn would stop the press.

These are examples of improving the odds in favor of control reliability. Redundancy in itself is not a
100% guarantee. Some form of fault detection is required.

Another method of improving control reliability is by the use of checking circuitry, not necessarily all
electrical. A simple example is incorporated in the double solenoid valves used in clutch and brake
controls. The two valves are redundant; however, without a checking arrangement, the failure of one
valve may not be easily detected.

In one form of a parallel flow dual valve, designed specifically for press use years ago, oversize exhaust
poppets are used in conjunction with a surge tank and somewhat restricted air source.

When one side of the valve fails, air is exhausted through the other side to stop the press, and at the
same time blow down the air in the surge tank. Loss of pressure is sensed with a pressure switch, to shut
down the main drive motor and stop further press operation.

In another form of a parallel flow dual valve, a balanced spool in the valve body monitors simultaneous
valve operation until the valve is reset. Air in the system is exhausted through the other valve to stop the
press.

In one form of a series flow dual valve, extensions are provided on the valve stems to operate limit switch
contacts. These contacts are wired into the clutch control circuit, in a manner that causes the control to
check on the proper functioning of the valves, and prevent initiation of the successive stroke upon failure
of either valve.

Regardless of the valve chosen, the proof of the matter lies in actual performance. Not all dual valves are
suited for all press applications. A test should be made periodically under safe conditions by intentionally
maintaining one side of the valve energized at the end of the stroke, determining how much additional
stopping travel occurs, and determining whether the press can be restarted before correction of the fault.
The test should be repeated for the other side of the valve. The results prove whether or not the valve
used is reliable.

If it takes a reliable air valve to stop a press, then it takes a reliable control of that air valve as well. One of
the most vulnerable parts of the system used to control the air valves is the press control rotary cam limit
switch drive. Care must be taken to insure the reliability of this drive. Several methods are used to
accomplish this, including the redundant double chain drive, and the use of two separate drives
mentioned earlier. Checking methods are also used. Chan breakage can be checked, by using a spring-
loaded idler sprocket to actuate a limit switch, so that when the chain breaks, the switch will open, and in
turn stop the press. Another method incorporates a limit switch, built into a hinged bracket that supports
the rotary cam limit switch. When the chain breaks, a spring under the hinged bracket moves the bracket,
operates the limit switch and stops the press.


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OSC-6078


For years, some press builders have monitored the rotations of the press control rotary cam limit switch
with a motion detector. This can be a zero speed switch, coupled to the switch shaft extension through a
gearing arrangement to speed it up, or it can use a pulse generator built into the rotary cam limit switch,
or mounted on the shaft expansion, to signal an electronic device whenever the cam shaft is rotating. Use
of the drive failure motion detector has a distinct advantage over a chain fault detector, in that it must
operate each stroke of the press, and therefore, can be checked for pick up and drop out each stroke of
the press.

In the clutch control circuitry, normally open and normally closed contacts of relays and limit switches are
used, as required by the particular circuit, to check for and insure a proper sequence of operation. By a
careful selection of control components and use of their contacts blended with redundancy and/or circuit
checking, a circuit designer can build reliability into the clutch and brake controls, so that the first
component failure is recognized. It is doubtful, however, if a press control will ever be built that will
recognize every failure of each individual contact in the circuit.

OSHA defines a brake monitor as “a sensor designed, constructed, and arranged to monitor the
effectiveness of the press braking system.” A brake monitor system is to monitor the performance of the
brake on each stroke and automatically prevent the activation of a successive stroke if the stopping time
or braking distance has deteriorated beyond the predetermined safe stopping limits.

Stopping time of a press is affected by many items other than brake deterioration. These include:
 1. Clutch/Brake air system pressure.
 2. Clutch/Brake air inlet and exhaust system flow characteristics as affected by dirt, oil, moisture, rust,
    shock, etc.
 3. Clutch/Brake spring adjustment.
 4. Counterbalance air pressure and flow characteristics.
 5. Slide die weight.
 6. Press speed.

Stopping Time should be determined at maximum operating speed and all other conditions normal. This
time should be increased by a braking deterioration factor when calculating allowed stopping time to
avoid nuisance faults. Some method is then necessary to assure that the actual stopping time or braking
distance is not allowed to increase beyond the allowed limits. This method is known as “Brake
Monitoring.”

One important aspect of stopping is on the downstroke where the safety of the operator requires reliability
of stopping should the operator reach into the point of operation. Another perhaps more important aspect
of stopping is at the top of the stroke, it is impractical, if not questionable for the operator whose hands
have been free during the entire upstroke. Since normal “single stroke” stopping occurs at the top of the
stroke, it is impractical if not of questionable value, to “test” the brake system each and every stroke on
the downstroke.

Brake monitoring in its simplest form is an over-the-top limit switch as shown. This is simply a stopping
position over travel sensor at the top of the stroke. To insure control reliability the switch should be part of
the press control rotary cam limit switch and should be checked for operation each stroke of the press.

Another form measure the real time of stopping, starting with the loss of the run signal and ending when
slide motion has halted. This would be a full time system since it measures every stopping time,
regardless of stroke position. Hence, it can be operational in inch or continuous as well as single stroke.
The motion sensing arrangement can take different forms, usually some type of electronic pulse
generator shown here as a multiple vane, a gear, or a rotary shutter. Normally these are incorporated on
the limit switch drive shaft, and are used additionally for motion detection to assure the continued
operation of the limit switch drive. In any event, some from of stopping time or distance monitor must be
incorporated into the single stroke control, and if the stopping limit is exceeded, further operation of the
press must be prevented until the condition is corrected.




                                                                                                          6
OSC-6078


When the Brake Monitor includes readout of the stopping time, it can be used as the basis for determining
safety distance. The stopping time is measured from the instant a run button is released, a stop button is
pressed, or a hand or other part of the body enters a presence-sensing field during the press down
stroke. It includes operating time of the sensing device control, the press control, air exhaust from the
clutch and brake and braking time of the slide. The test should be made with a typical upper die in place
and with proper air pressures for the clutch/brake, and any air counterbalance system. The air valve
exhaust muffler(s) should be clean, the brake should be adjusted properly, and the brake linings should
be in good shape. Portable stop time measuring instruments can also be purchased or rented from
several companies. They generally have a digital readout showing time directly in milliseconds.

The 90 position of the crank or eccentric shaft was selected for checking stopping time because at the
point the slid of a mechanical press is moving downward at maximum speed and it was felt that in this
position the stopping time would be longest. Tests on some presses indicate longer stopping times at top
stroke then in mid-stroke, and when this is the case, the longest stop time should be used for determining
the safety distance.

OSHA regulations require each two-hand control device, such as run buttons, or a sensing field, if used,
be separated from the point of operation by a distance sufficient to insure that the operator’s hands can’t
be moved into a hazardous area before the dies close or the slide motion stops. This distance is referred
to as the “Safety Distance.”

Two-hand control and presence sensing devices can be used only on presses with part revolution
clutches, such as the air operated friction clutch, or on machines with equivalent operational
characteristics that permit stopping the slide at any point during the stroke.

If an operator, using two-hand control for safeguarding, starts a stroke and then sees that the work is not
positioned properly, he might release one or both run buttons to interrupt the die closing action and
impulsively reach into the die area to reposition the material. If the buttons are too close to the point of
operation, a hand might move between the dies just as they are closing, resulting in a severe injury. The
same thing could occur if a presence sensing device is being used for safeguarding and the light curtain
or sensing field is too close to a hazardous area.

Because of this, the safety distance formula for these devices requires that two-hand control mechanisms
and presence sensing fields must be separated from point of operation hazards no less than a distance of
63” multiplied by the stopping time of the slide in seconds, measured approximately 90 position of
crankshaft rotation. Again, if stopping time at top of stroke is longer, I believe it should be used for the
calculation. The calculation is really simple. If a third of the second is required to stop a slide after the
control system receives a signal to stop, the minimum safety distance is one third of 63”, or 21”. This must
be increased, however, by the same braking deterioration factor used when calculating allowed stopping
time for the brake monitor.

This formula for the Safety Distance between devices and point of operation hazards is based on an
operator being able to move a hand toward the dies at a speed of 63 inches or 1.6 meters per second.
This speed was determined by Dr. Oscar Lobl in Germany during tests made in 1935, and has been used
on presses in Europe for about 40 years, even though some tests have proven it too slow.

Since presses with part revolution clutches and presses with full revolution clutches have different
operational characteristics, there are different safety distance formulas for the devices that are suitable for
use with each type of clutch.

On a press with a full revolution clutch, it isn’t possible to stop the slide at midstroke or any other
intermediate point in the cycle because when the clutch is engaged, it can’t be disengaged until the
crankshaft has completed a full revolution and the slide a full stroke. Since there is no control of the clutch
during a stroke, neither two-hand control nor presence sensing devices can be used for point of operation
safeguarding. Two-hand trip can be used, however, if the speed of the press, the clutch construction, and
the hand button location insure that the dies are closed before an operator can move either hand from a


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OSC-6078


button into the point of operation. To accomplish this, the minimum safety distance formula for two-hand
trip takes into consideration the time in seconds required for each revolution of the crankshaft, the
number of engaging points in the clutch, and the 63” per seconds possible speed of hand movement into
the dies.

Full revolution clutches have from one to fourteen engaging points, and a clutch may not become
engaged the instant that the two-hand trip operated. If there is only one engaging point and the trip is
actuated just after the rotating section of the clutch has passed the engaging point, practically a complete
revolution of the rotating member must be made before the clutch becomes engaged. This revolution,
plus the half revolution of the crank necessary for complete die closure, may give a total time equivalent
to 1-1/2 revolutions of the crankshaft between tripping and die closure when the clutch has a single
engaging point. Clutches with a greater number of engaging points have shorter time intervals that can
elapse between tripping and engagement because less rotation is necessary before another engaging
point is reached.

If the press speed and the number of clutch engagement points are known, I recommend use of the chart
prepared by American Allsafe Company or Rockford Safety Equipment Company for finding minimum
safety distances. One was published in August, 1975 Safety Newsletter of the National Safety Council
Power Press & Forging Section and is faster and easier than doing the arithmetic for each press.

Today’s market does offer modern full revolution clutch presses that do comply with the requirements for
control reliability including checking circuits and top stop monitoring. However, unless the clutch is of
multiple jaw type with 14 engaging points, a two hand trip is usually impractical for safeguarding at press
speeds less than 120 SPM. It is rarely feasible with clutches having only one engagement point.

While there are definite formulas used for determining the safety distances between two-hand control
devices and point of operation hazards, there is no specific requirement for the separation distance
between the buttons or other actuating mechanisms operated by the right and left hands. This is because
buttons are not always mounted in the same plane and there are quite a variety of shields, baffles, and
other mounting arrangements used to insure that an operator uses two hands, and not a finger and elbow
of the same arm, or knee or hip in place of a hand. When run buttons are mounted on the same or
parallel surfaces, a minimum spacing of 27” between the centers of the button mushroom heads is
recommended. Shorter spans can be safe, however, if the spacing is greater than the distance from
elbow to fingertip of the longest armed operator.

In addition to the separation distance between buttons, it is important to remember that the buttons must
be protected against unintended operation by rings or other suitable shields. OSHA requires that each
hand control shall be protected against unintended operation and arranged by design, construction and/or
separation so that the concurrent use of both hands is required.

Presses in many instances, depending on the type of job being performed, are subject to shock and
vibration. Those elements of the press control that are mounted directly on the press can be affected by
this shock and vibration, and in cases where nuisance tripping can occur, shock mounting of the
equipment must be considered. Components used in a press control system should be selected to
minimize down time, as well as contribute to better control reliability. Flush, recessed, NEMA 12
construction, and oil tight devices should be used with all external panels whenever possible.

It is easy to understand that a press control system that meets all the OSHA requirements is very
sophisticated when compared to many pre-OSHA press control systems, and it does cost more. The
space required for these controls is certainly larger, and the trouble shooting of control failures of this
equipment requires better trained electricians. Enforcement of the Safety Separation Distance
requirements requires better supervision. Despite these problems we have little doubt that the improved
control reliability of these controls will contribute a great deal to safety, and they are well worth the time
and effort that has gone into their development.




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