Chapter 15. Machine Guarding

Transcription

1 Chapter 15 Machine Guarding

2 Introduction More efforts and resources have been expanded to guard machines more than any other Safety and Health endeavor. The aggregate guarding of machines become major undertaking (plant maintenance, operation, purchasing, scheduling, etc) It is the responsibility of Safety and Health manager to implement machine guarding (enumerating problem areas, setting priorities, selecting alternatives, and ensuring compliance with standards)

3 General Machine Guarding Safety and Health manager should be knowledgeable about what makes a machine dangerous. Different machines shares the same mechanical hazards. Mechanical Hazards (in order of importance): 1. Point of operation (when the tool engages the work) 2. Power transmission (belts and pulleys) 3. In-running nip points (feed raw materials/stock) 4. Rotating or reciprocating machine parts 5. Flying chips, sparks, or parts

4 In-running nip points.

5 General Machine Guarding (Cont.) Other hazards include: electrical, noise and burn. It is the mechanical hazards that are controlled by machine guards Power transmission (belts and pulleys) is easier to guard (needed in case of maintenance) than the point of operation (must be accessible)

6 General Machine Guarding (Cont.) Rotating and reciprocating moving parts; parts that move intermittently (accidental motion) Personal protective equipment can be one means of protecting workers from flying chips or sparks, but shield or shield guard is an effective mean to protect the operator and other workers from flying chips, sparks, or (broken) parts.

7 In-running nip point In-running (in going) nip point, occurs when: or Machines equipped with continuous feed Belts contact pulleys and gears mesh In-running nip point can cause injuries indirectly by catching loose clothing and drawing the worker into the machine

8 Guarding by location or distance Guarding by location: the easiest and cleverest way is to position the dangerous parts where no one will be exposed to the dangers (machine design, locating the machine so that its belts, pulleys and drive motor are impossible to reach during normal operation) Guarding "by distance": setting up the operations sequence such that the operator does not need to be close to the danger (the press brake example) Guarding "by distance" is an acceptable method of guarding certain difficult-to-guard machines (But it is not positive control. Do not generalize the concept)

9 Press brake guarding by distance.

10 Guarding by Tagouts or Lockouts Lockout system for protection of maintenance workers while the machine is being repaired.

11 1. Safeguarding the point of operation The point of operation is generally safeguarded by guards or devices. Hand-feeding tools are helpful in eliminating the need for the operators to place their hands in the danger zone but these tools do not qualify as guards or devices

12 1. Guards Principle of machine guard: The farther away from the danger zone, the larger can be the opening in the guard without creating a hazard. Maximum permissible guard opening should depend on distance to the danger zone. Guard opening size gauge (Go/No-Go guard gauge)

13 1. Guards (Cont.) Visibility through the guard is an important matter. Orange paint, black paint, a transparent material. Transparent machine guard

14 1. Guards (Cont.) 1.1 Die Enclosure Guard: A die enclosure guard is essential to enclose the space between upper and lower dies. Adv.: small Disadv. 1) Since dies vary widely in size and shape, it must be custommade 2) Max permissible opening size is 1/4 inch (limit visibility) Die enclosure guard used with sliding die for feeding.

15 1. Guards (Cont.) 1.2 Fixed-Barrier Guard

16 1. Guards (Cont.) 1.3 Adjustable-Barrier Guard: Can be adjusted to individual applications during the setup. Should be easy enough to adjust but not so easy to gain access to danger zone.

17 1. Guards (Cont.) 1.4 Interlocked-Barrier Guard: Electrically disables the actuating mechanism whenever the guard is open.

18 Awareness Barriers: (for warning) Not recognized as a guard (does not meet the guarding criteria) It does provide a reminder of the danger (does not prevent the danger or protect the operator) Can be a simple rope or chain with a sign

19 Power press (Punch press) Inherently dangerous. Most useful for mass-production of identical parts (interchangeability). Hydraulic-powered - forging press - mechanical punch press. A set of mating dies that close on each other to cut, shape, or assemble materials. Features of the press (method of power transmission, control of the stroke, etc) determine the permissible safeguarding

20 Typical power presses: (a) open-back inclinable (OBI) model; (b) straight-sided model.

21 Press Hazards There are many press-operator work-related amputations each year. Feeding the press by hand is very dangerous, even for careful operators. In a hand-feed setup, the press and the operator are alternating actions in rhythm that cycles every few seconds or fraction of a second. Production incentives motivate the operator to higher and higher speeds as skill develops. The process involves eye-hand-food coordination every cycle. Causes of accidents: readjust a misaligned work piece in a die No hands in the dies : tongs or other tools and devices could be used to feed work piece. In addition, press guards or safety devices could be used. In metal-stamping industry, other measure could be taken.

22 Press-cycle operation sequence (no safeguarding).

23 Press Design Most presses are mechanically powered, some models are hydraulically or pneumatically powered (large hydraulic cylinder above the ram) Hydraulic and pneumatic presses are used for fastener application and working with hot metal. Press brake is a power press with a long bed that is used to bend sheet metal (not considered a mechanical power press - they use blades instead of dies) Mechanical power press is distinguished by the presence of a large heavy flywheel mounted near the ram.

24 Press Design (cont.) Two methods of engaging and disengaging the flywheel to deliver power to the ram: Full-revolution type (engagement can not be broken until one complete revolution is made, after that the flywheel is disengaged and rotate freely under the power of the motor) Part-revolution type has a friction clutch (using compressed air), that can be disengaged at time, then a brake is applied that immediately stops the ram. Advantage of the part-revolution type: can be interrupted at any time cycles quickly once engaged (no time for the operator to reach into the point of operation) Most presses are full-revolution (older presses) A part-revolution press has a bulge to accommodate the friction clutch next to the flywheel.

25 Full-revolution and part-revolution machines: (a) full revolution; (b) part revolution.

26 Press Classification Hydraulic or pneumatic Press brake (blades instead of ides) Mechanical power press Full revolution Part revolution

27 Point-of-operation Safeguarding Four categories ranked according to degree of security: 1. Prohibiting the operator from reaching into the danger zone altogether. 2. Prohibiting the operator from reaching into the danger zone any time the ram is in motion. 3. Prohibiting the operator from reaching into the danger zone only while the dies are closing. 4. Not prohibiting the operator from reaching into the danger zone but stop the ram before the operator can reach in. Categories 1 and 2 can be trusted for a full- revolution press. Category 3 is questionable for a full- revolution press. Category 4 is permitted only for part-revolution press.

28 Press Safeguarding Die enclosure, fixed barriers, adjustable barriers, and interlocked barriers are acceptable in mechanical power presses: Die enclosure is exclusively used on mechanical power presses. Fixed barrier is very popular for power presses that employ automatic feeding of coils of strip stock and automatic ejection of the finished parts. A guard or safeguarding device must be installed on every mechanical power press except if the full-open position of the ram results in a gap between mating dies of less than 1/4 inch (too small to be a hazard). Hand feeding is illegal. Safeguarding (in more general term) encompasses a variety of mechanical and electromechanical devices that protect the operator even when hand feeding is used.

29 Gates Look like a guard, but they open and close with every machine cycle. Unlike interlooked-barrier guards, gates can be used for manual feeding. There are two Types: Type "A" gate closes before the press stroke is initiated and stay close until all ram motion has ceased (safer than type "B") Type "B" gate closes before the press stroke is initiated and remain closed during the downward stroke then open (not recommended for presses with full-revolution clutches) Advantage of type B : can save labor time/costs.

30 Presence-Sensing Devices A bank of photoelectric cells to set up a light screen, the penetration of which will immediately stop the ram. Infrared frequencies is used rather than the visible-light spectrum Programmed crisscross scan may reduce the number of sensors and gives an effective protection. Electromagnetic field type: the tripping threshold is not sensitive, and it has to be tuned for a given operator and setup. Presence-sensing devices are permitted only for part-revolution presses and it is permissible to deactivate the sensing field on the upstroke (muting) to give the same production efficiency like type "B" gate.

31 Photoelectric presence-sensing screen

32 Programmed crisscross scan for sensing plane Electromagnetic field type of presence-sensing device.

33 Pullback devices for safeguarding the point of operation. Hold-outs or restraints for restraining the operator s hands from reaching into the danger zone at all times (compare with pullbacks).

34 Two-hand control.