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3 The Engineer's Historic Duty in Safe Operations

3 The Engineer's Historic Duty in Safe Operations

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An Overview of Press Safeguarding


needed to protect our much smashed-up railway employees. Public

sentiment, and its consequential legislation, doubtless will, after a

while, do these things so necessary to an era of decent civilization.”

Today, the same advantages of automatic operation exist. Even so,

many pressworking operations continue to be hand-fed in a manner

that exposes the operator to serious injury should an equipment malfunction occur. It is not the intent or purpose of this book to give

advice on how to make a specific operation safe. Proper provision for

safety of personnel in the workplace is the employer’s duty.

Voluntary standards organizations call on industry experts to formulate guidelines for safe operation of power equipment. In some

political jurisdictions, government authorities make laws defining

minimum requirements for safe power press operation. Often,

these laws embody the recommendations of voluntary standards



For liability reasons, neither SME nor the author represent the

information on press safety systems contained in this work to reflect

current law, or to meet the exact safety requirements of any company or governmental regulatory body. This is a partial discussion

of some risks historically associated with pressworking, together with

some safety measures taken to minimize the danger to personnel. It is

intended as general background information, with the expectation

that progress in training, automatic operation, and safeguarding

measures will continue to reduce pressroom accidents.

Any time a hand or any part of the body is in a point of operation

such as pinch point or crush point in or around a power press, there is

a statistical probability the press or ancillary device, such as a feeder,

may actuate unexpectedly and cause an injury.

For example, the clutch and flywheel system operates with close

spacing of frictional surfaces and bearing clearances when not

engaged. Should a mechanical seizure of the clutch occur, the press

would repeat through several cycles although the clutch was signaled

to de-activate. The brake will slip if a mechanical failure locks the

clutch in solid engagement. Because of this danger, power presses are

equipped with clutch and brake control reliability monitors to provide

332 Quick Die Change

warning of clutch or brake deterioration some time before a catastrophic failure.

Power Lockout

It is a requirement that a power lockout procedure be followed

whenever any part of the body is in the point of operation during

repair or maintenance operations. The essence of power lockout is to

lock out all power sources and dissipate stored energy that could

cause unexpected movement of the press or auxiliary equipment.

When working on a die in a press, automation or other equipment

that could injure the worker must be locked out. Every person must

have his or her own lock(s). When more than one person is working on

the equipment, multiple lockout hasps are to be used so each person

has a lock on each piece of equipment requiring lockout to render it

safe for them to work on. Die setters, diemakers, and maintenance

tradespersons require several safety locks to accomplish the task of

locking out machinery that needs to be rendered safe.

Lockout Locks and Tags

Each safety lock an employee uses has a unique key. The name and

a small photograph of the person issued a lock is on a small tag laminated with plastic and securely attached to the lock. This identifies

whose lock is on a lockout point. Figure 15-1 is an illustration of a lock

tag belonging to the author when he worked as a die tryout group

leader at Ford Motor Company.

Safety Blocks

If maintenance work is done with the press in the open position,

special props called die safety blocks are used to block the slide from

moving under its own weight. The blocks must support the static

weight of the slide, attached linkage, and maximum upper die weight.

While a safety factor is included, the blocks are not designed to withstand full press tonnage load under power at any stroke position.

The block must be a snug fit in the press opening to prevent any

slide movement. Aluminum or magnesium blocks are used for lightness. An aluminum block with a large screw for length adjustment

and attached interlock plug is illustrated in Figure 15-2. Two blocks

An Overview of Press Safeguarding


Figure 15-1. Employee identification tag attached to safety locks identifies the

person working on equipment requiring lockout. (Courtesy Ford Motor Company)

are used for straightside and wide gap-frame presses. Large presses

may require four blocks. For large multiple ram transfer presses, four

blocks may be required per slide.

Figure 15-3 illustrates a press safety block placed in a storage position hanging from a hook. Larger blocks as shown in Figure 15-2 can

be placed into a compartment on the side of the press.

334 Quick Die Change

Figure 15-2. An aluminum die safety block is equipped with a captive adjusting

screw to provide a snug fit. Note the attached chain and interlock plug. (Courtesy

Rockford Systems, Inc.)

Figure 15-4 illustrates a simple yet robust type of safety block that

is low cost and easy to fabricate. A sliding sleeve holds it snugly in

place over a die setup block. A good application for this type of block

is an open-backed inclinable (OBI) or open-backed stationary (OBS)

press die. The chain going to the safety block interlock plug is welded

to the sliding sleeve. Like any safety blocking device, it must be

designed for the static load of the upper die and attached press parts

An Overview of Press Safeguarding


Figure 15-3. A gap-press safety block is shown in a storage position interlocked to

the motor run circuit. (Courtesy W. C. McCurdy Company)

and maximum upper die weight plus an engineered safety factor.

The block shown in Figure 15-4 can be slipped into a section of tubing attached to the side of the press adjacent to the interlock plug


Regarding safety block applications:

1. At least two blocks should be used and placed across diagonal

corners of straightside presses.

2. Safety block pads may be provided in the die to simplify correct block placement.

3. A Ford Motor Company standard is to provide a minimum of

two safety block pads with a closed height of 6 in. (152 mm).

4. The Ford safety blocks are sized 4.5 in. (114 mm) longer than

the press stroke. Hardwood wedges are used to fill in the

336 Quick Die Change

Figure15-4. A standard gap-press safety block is designed to slip over a die setup






remaining 1.5-in. (38-mm) space to prevent the slide from gaining inertia before contacting the blocks.

The compressive strength of the blocks must be sufficient to

support the weight of the press slide, attached linkage, and

upper die, with a safety factor added.

Since the blocks are not designed to withstand full press tonnage, a safety plug that interrupts the main run circuit of the

press is attached by a short chain. Blocks must not be placed in

the die space with the press running.

If safety block pads are provided in the die, the compressive

strength of the die in those areas must equal that of the safety

blocks in any press into which the die may be set.

Since the length of the blocks is determined by the press stroke,

the blocks should be conspicuously identified with the press

number to prevent use in a press other than the one intended.

An Overview of Press Safeguarding


9. The blocks should be viewed as a lifeboat on a ship—they

should be used for no other purpose.

Function of Interlock Plugs and Start/Stop Buttons

The plug of all safety blocks is interlocked to the motor’s run circuit. The interlock is attached with a short chain so the plug must be

removed from the receptacle to prevent the press from accidentally

cycling with the block in place. Should press cycling occur with the

block(s) in place, catastrophic press and die damage may result. Serious injury to personnel from the flying safety blocks and machine

components may also occur.

Figure 15-5 illustrates a simple motor starting circuit. The electrical schematic symbols differ in many ways from those used for electronic equipment. For example, parts of a single motor starting contactor are shown as four normally open contacts labeled M1 through

M4. The contactor is essentially a large relay. The contactor starting

coil is shown as a circle. Electrical control schematics are shown in this

way because parts of a single device, such as a contactor, are shown at

various points on the circuit drawing.

Figure 15-5. A simple three-phase motor starting and stopping circuit is shown.

338 Quick Die Change

An important safety feature is that the motor starter coil disconnects the motor from the incoming three-phase power if power is

momentarily interrupted. To restart the motor, the start button must

be manually pressed. When the motor contactor is energized, contact

M4 seals the contactor coil circuit on until a power interruption occurs

or the circuit is unsealed when the stop button is pressed.

Figure 15-5 illustrates point “X” where additional electrical controls can be wired into the motor stop circuit. Figure 15-6 illustrates a

series circuit of two pressure switches, an emergency stop button, and

a safety block receptacle. The circuit wiring may be interrupted at

point “X” in Figure 15-5, and the series circuit illustrated in Figure

15-6 connected at that point.

Often, emergency stop buttons and safety block interlock plugs

are wired into the series electrical circuit with the low air and counterbalance pressure switches. An example of a correctly installed

safety block interlock plug is illustrated in Figure 15-3. It is important

that all press wiring be done according to the manufacturer’s recommendations. Tests should be conducted periodically to ensure each

shutoff device will shut off the main motor as intended.

A typical mechanical press has one or more pressure switches for

safe operation. Air counterbalance systems have a pressure-actuated

switch that opens the main motor run circuit in the event of a drop

below a minimum value.

A similar switch, wired in series with the counterbalance switch, is

used to detect sufficient air pressure to correctly activate the clutch.

Hydraulically actuated die clamps may also require pressure switches

to ensure correct clamping pressure is maintained. The pressure

switches, which have adjustable setpoints, are of the normally open

type. The correct pressure setting must be in accordance with the press

manufacturer’s recommendation. A minimum amount of clutch actuating and counterbalance air pressure must be present before the motor

can start. Should a dangerous drop in pressure occur, either switch

must open at the correct minimum setting, shutting off the motor.

To avoid press damage from a loss of recirculating lubricant pressure, a pressure switch is usually provided to stop the press. Other

switches are often wired into this safety circuit to stop the press for

various harmful conditions, such as a lack of sufficient hydraulic overload pre-charge pressure and over-travel of the slide adjustment


An Overview of Press Safeguarding


Figure 15-6. A press electrical series circuit is shown, which features two pressure

switches, an emergency stop button, and a safety click plug receptacle. Interrupting the current flow in this circuit opens the main driver motor contactor illustrated

in Figure 15-5.

Pressing the emergency stop button also opens the motor run circuit, shutting off power to the motor. Usually, several large emergency

stop buttons are conspicuously located on the press. All buttons are of

the normally closed type and wired into the series circuit.

One or more safety blocks, plugs, and receptacles of the type

illustrated in Figure 15-3 are provided. It is extremely important that

the safety block be interlocked with the electrical controls of the press

so the press cannot be cycled with the block in it.


Many companies’ safety rules do not permit hand-in-die operation. For low-volume production jobs, the use of safety tongs, hand

vacuum tools, magnetic lifters, and simple gravity slides serve to

accomplish the desired production while avoiding employee exposure to the point of operation. Such tools and production aids are not

a substitute for proper point-of-operation guarding.

Frequent amputation injuries result in high industrial compensation costs and fines. Depending upon circumstances, employers and

340 Quick Die Change

equipment builders may be required to pay the injured employee civil


Holdout or Restraint Devices

Holdout (restraint) devices must prevent the operator from inadvertently reaching into the point of operation. Figure 15-7 illustrates a

holdout device used with hand-loading tongs.

Attachments (wristlets) are provided for each of the operator’s

hands. These attachments must be securely anchored and adjusted so

the operator is restrained from reaching into the point of operation.

Pullout Devices

Pullout (pullback) devices must withdraw the operator’s hands

should they be inadvertently located in the point of operation as the

dies close. Pullout devices must have attachments for each of the oper-

Figure 15-7. A holdout device used in conjunction with hand loading tongs:

attachments (wristlets) are provided for each of the operator’s hands. (Courtesy

Rockford Systems, Inc.)

An Overview of Press Safeguarding


ator’s hands. These hand attachments are connected to and actuated

by the motion of the press slide. A rigid framework securely attached

to the press supports the mechanism that withdraws the operator’s

hands using a slide motion. Figure 15-8 illustrates a pullout device

in use.

Precautions for Using Holdout and Pullout Devices

For low-volume production, holdout and pullout methods are

effective provided proper safety precautions are taken. An essential

factor in achieving operator safety with holdout and pullout systems

Figure 15-8. A ram-driven pullout device: if the operator’s hands are in the point

of operation as the dies close, they will be forcibly withdrawn. (Courtesy Rockford

Systems, Inc.)

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3 The Engineer's Historic Duty in Safe Operations

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