Dangerous rollers

Rubber mills bring people into intimate contact with large, high-powered rollers and making them truly safe is a difficult task.

As with many large and dynamic machines, rubber mill operators found that Category 4 level safety control demanded a better solution than traditional hard-wired relays. Digital technology from Pilz offered triply redundant safety while adding greater flexibility to make changes as the plant grew.

The two large rollers of a rubber mill work like a mangle on an old-fashioned washing machine, revolving at about 22 rpm and forcing raw rubber through. An operator stands close to the rollers, cutting and massaging the ribbon of rubber by hand.

This work is rated at the highest risk level, requiring Category 4 safety control systems under Australian Standard 4024-1. The rubber mill is totally unforgiving and there is frequent exposure to the hazards, which are not easily avoidable.

Guarding the mills completely with a physical barrier is impossible. Instead, workers have traditionally relied on a belly bar and emergency stop buttons for their safety.

"It would have been very complicated and messyto shut the mill down properly, while keeping the power on to reverse the mill."

While Australian Standards are not very specific about the guarding of dual roll mills, European standard BSEN1417:1997 offers a detailed and comprehensive set of requirements, when used in combination with AS4204.1.

High among the requirements is that rolls must stop within 57.3 degrees of rotation when a safety mechanism is activated, before reversing within two seconds by between 57.3 and 90 degrees to free potentially trapped limbs.

The standard also mandates belly bars on each side of the mill, both of which need only 200 Nm of force to move the 10 mm or less that activates shutdown of the mill. Once triggered, there must be no opportunity for automatic reset. Switches and interlocks must also be redundant and monitored to satisfy Category 4.

Because it was so well defined, some Australian rubber mill owners used BSEN1417:1997 as the basis of an audit for a new mill sourced from Japan. Guarded only with a knee bar, a recent import fell far short of the standard.

The company engineers called on Sage Automation, which had already fitted a wide range of programmable safety systems within a number of plants.

A Sage spokesperson said the 15 year old machine had been refurbished in Japan, but was only equipped with a basic 'shoebox' PLC. Hard-wired safety devices alone were not viable.

"It would have been very complicated and messy to shut the mill down properly, while keeping the power on to reverse the mill, plus monitor the stopping distances," he said.

"The only real option was a digital safety system certified to Category 4 with dual channel redundancy and fault detection."

The facility already had extensive Pilz safety equipment and the flexibility of the Pilz SafetyBUSp remote modules was important. Pilz, the first to offer a Category 4 certified programmable safety system, was also preferred for its reputation and proven record with the technology.

At the core of the solution is a Pilz Programmable Safety System (PSS), coupled with a Steggmann programmable parallel output encoder. The encoder has several functions. Most importantly, it records the stopping distance of the rolls after an emergency stop or belly bar has been triggered.

The system also monitors and flags any deviation from set tolerances for the power drawn by the DC drive, bearing temperatures and water temperatures.

The result has been a dramatic improvement to safety.

Meanwhile, routine safety audits at another site had identified a 30-year-old rubber mill as a high priority area for safety upgrades.

The mill was already fitted with a belly bar, which company electricians recognised was "remarkably advanced for its time". An upgraded safety system ensured the old, but reliable, mill kept pace with world's best practice in safety standards.

While the upgrade would have demanded hard-wiring hundreds of relays, the Pilz installation called for just 49 safety inputs. The Pilz PSS more than halved the time that potentially could have been taken to upgrade the machine.

Installation and commissioning took two weeks over the Christmas shutdown and involved a total rewire of the field wiring, which engineers considered an investment in efficiency as well as safety.

The first stage was surprisingly low cost and planned work guarding the cutting knives will be simple programming changes, rather than wiring.

Another three mills and a Banbury mixer at the site are scheduled for similar upgrades.