Reducing Press Machine Injuries
By Henry E. Tang, P.Eng
Famous baseball managers are often known to use the phrase "If you ain't got pitching, you ain't got nothing," Likewise, in press safety, "if you ain't got control reliability, you ain't got nothing."...
By Henry E. Tang, P.Eng
Famous baseball managers are often known to use the phrase “If you ain’t got pitching, you ain’t got nothing,” Likewise, in press safety, “if you ain’t got control reliability, you ain’t got nothing.”
Press machines — mechanical, hydraulic, pneumatic or the like — are extensively used and represent the workhorses for most metal-product manufacturers. They are indispensable machines; however, they are inherently dangerous for the people involved in operating or maintaining them. That’s one of the reasons the Canadian Standards Association has issued a new edition of its Z142 Power Press Standard.
Presses are known to be unforgiving, for they have the potential to inflict serious and often permanent injuries. If a company experiences injury incidents, it indicates that its operating or maintaining skill-sets alone may not be sufficient to avoid injuries.
A safe facility requires an added dimension for the overall safety of presses. This generally relates to the extent of design and engineering given to the active and passive supplemental safety devices in a press. In other words, it involves the level of control reliability.
Injuries, from an analytical sense, are preventable incidents. Control reliability is the best tool to prevent the likelihood of injury incidents.
Reliability is a measure of an outcome based on statistical data. The data is used to evaluate the probability of an event to which a reliability index can usually be formulated. It is commonly expressed in units of percentage, or over a scale of 0 to 1, “zero” being an absolute unlikely event and “one” being an absolute certainty event.
For example, the probability of getting a head or tail in a fair coin toss is expressed as 1/2 or 0.5. This means the probability of getting heads is 50% and of getting tails is 50%, since there are only two distinct outcomes (two sides of the coin). In this example, the reliability of a desired outcome is 0.5.
Reliability theory quickly made inroads in many engineering applications for there was tremendous value in knowing the probability of an outcome. Reliability engineering, in simple terms, is the probability of a component surviving a given time period when that component has a constant failure rate. Design engineers usually consider redundancies (two- or three-of-a kind) to assure or guarantee an outcome, if that outcome is significant or important — especially to support human life, such as with hospital life support systems.
Due to the inherent dangers from presses, designing and properly applying these redundancies have been and continue to be challenges — especially when faced with budgetary constraints.
Control reliability refers to a device, system or interface that is designed, engineered, constructed and installed in such a manner that a single fault or malfunction will not prevent the normal stopping action and will prevent successive operation. In presses specifically, this means stopping actions shall always be achieved and shall prevent a successive stroke of the press until the problem is corrected.
Control reliability, in essence, is the cornerstone of press safety and represents the main theme emphasized in the recently published CSA Z142-02 standard — Power Press Operation: Health, Safety and Guarding Requirements. The standard underscores the significance of control reliability in press safety. The objective of the standard is to reduce the risk of injury due to hazards from operation, maintenance and setup while working on, or adjacent to, a power press.
Arguments are easily made that control reliability is, or can be, an expensive proposition. Skill is needed to optimize costs to achieve a good measure of control reliability, while at the same time meeting safety compliance requirements. Achieving control reliability is therefore a science. It is a product of both safety engineering acumen and an in-depth comprehension of machine design, particularly with presses.
False sense of security
Some industries should be applauded for their initiatives to spend money acquiring and installing safety protection devices. However, poor engineering and installation techniques could render very little value to the overall safety component expenditures.
It is not uncommon to find that many installations have employed expensive electrical safety protection devices such as light curtains, scanners, etc., while the control reliability is weak or non-existent because of a poor utilization of expenditures. This situation unfortunately provides the machine operator with a false sense of security.
Control reliability in presses must recognize the safety integrity of both the mechanical and electrical infrastructure. A systematic approach must be developed to examine all control circuits (electrical, hydraulic, etc.) to assess whether they meet the intent of the Z142-02 standard.
The recipe to be compliant with the control reliability aspects of Z142-02 standard involves a systematic approach:
Study of the press/machine type and its current safety control components and protocols
Probabilities and assessment of an injury
Hazard identification and control
Selection of appropriate safeguards — what needs to done to be in compliance; define costs
Design and application.
Control reliability, according to the Z142-02 standard, specifies that safety circuits for electrical, hydraulic, pneumatic and muting shall be dual channel with monitoring, and designed, constructed and applied in such a way that any single component failure shall not prevent the stopping action of the press. Furthermore, the safety control systems shall be hardware-based and include automatic monitoring at the system level.
The Z142-02 standard affects all power presses that are fitted with a ram (plunger or slide) and dies for the purpose of blanking, cutting, trimming, drawing, punching, forming, stamping, assembling, or processing metal or other material. This new edition of the standard came into effect on Jan. 31, 2003.
Z142-02 is a National Standard of Canada, and is mandated in legislation in most provinces. The legislation requires that a schedule be developed and implemented that will ensure compliance with all applicable parts of this standard no later than two years from the date of publication.
The time to act is now. Formalizing what needs to be done to be compliant, and identifying the associated costs, should be the order of the day. Probable expenditures will warrant fresh money from new capital budgets, appropriately earmarked to support the undertaking.
Henry E. Tang, P.Eng., is with S.A.F.E. Engineering Inc., a company specializing in occupational health and safety compliance and control safety reliability. He can be reached at 416-447-9757. Copies of the CSA Z142-02 standard can be purchased from www.csa.ca for $95.