Authored by:
Steve Piacsek
Product Development Engineer
A & A Mfg. Co. Inc.
New Berlin, Wis.

Resources:
For information on networked safety and IEC safety standards, visit motionsystemdesign.com/mag/machine_ safety/index.html

Safety hazards associated with moving machinery can be reduced or eliminated with articulating physical barriers and torque limiters. Key to this undertaking are detailed analyses and engineered solutions.

Isolating pinch points
Scissor-lift mechanisms, tilt tables, and similar equipment represent potential sources of injury because pinch points can be created when these mechanisms move. One way to protect against these hazards is to enclose moving parts with bellows that deter personnel from reaching into hazardous areas — yet permit machinery motion. Other benefits are improved machinery aesthetics and protection against dust, debris, and foreign objects.

In one iteration, heavy-duty accordion- type skirting is made from rigid PVC coextruded with flexible high-cycle polymer hinges and fabricated corners. The resulting rigid, movable cover is suitable for lift tables and verticalmotion applications. Vents allow uniform airflow during operation, while tie strips guarantee proper expansion and retraction.

For applications necessitating lateral and linear motion, such as medical table bases, imaging tables, dental chairs, and amusement rides, bellows can be constructed using various materials and internal wires that maintain the necessary shape. Fabrication from myriad materials is possible, including some that meet UL burn standards or incorporate antimicrobial agents. Suitable for shear-type or lateral movement, the design’s compactness allows retraction into tight spaces.

Controlling machine motion with safety clutches
In the name of safety, motion designs such as CNC machine-tool doors and gear mechanisms can also be controlled with slip clutches. A continuous slip clutch is a cost-effective safety option that can provide surprisingly long life in myriad applications. A typical unit consists of a cartridge and a housing. The cartridge is set screwed or keyed to the input shaft. The housing is similarly attached to the output shaft. Elsewhere, the clutches connect via a bronze bearing to output gears or pulleys to allow their relative motion with the input shaft.

In short, torque is transmitted from the flats on the clutch hub to mating flats on inner plates through friction pads to the outer plates — then through torque pins to the housing and output gear or pulley. Torque level is controlled by compressing the springs with an adjusting nut. For a ‡ fixedtorque clutch, a collar is attached to the hub in a ‡ fixed position rather than the adjusting nut.

Either the input shaft or the housing can act as the input; the other member is then driven. Torque capabilities are from a few ounces-inch to 1,000 lb-in.

Automated access doors
Automatic-door actuators, which reverse automatically when objects are detected in their path, are available for retro‡ tting to existing automatic machining centers with single or double doors. For workers, these reduce injuries from repetitive door operation and crushing incidents.

One door design incorporates a preprogrammed control module allowing several input and output commands. These can be wired into the machine’s controller or a simple on-off circuit; key outputs and messages can also be fed to a controller. Distinct benefits are unlimited travel distances, self-calibration, electric (pneumatic-free) operation, and no need for limit switches, light curtains, pressure switches, or photo eyes. Integrated braking engages at all positions. Some large-equipment OEMs of large machinery report that operators required to open and close safety doors can experience shoulder pain, and door actuators reduces repetitive shoulder injuries by half or more.

Covering open pits and other hazardous openings
Large machine tools such as planer mills often are installed partially below „ floor level, resulting in potentially hazardous pits. Often, workers must remove pit grates or slats to service the machine. Here, metal roll-up, walk-on covers offer an alternative. Typically made of reinforced stainless steel, these covers allow partial pit access while preventing falls at other times or into other pit areas.

Case in point: One aircraft manufacturer uses large fiber-placement machines to build fuselages from composites. Adjustable tailstocks of the two-station machine travel on guideways located in a pit below floor level. Production workers must access workpieces as they’re produced, necessitating pit covers during production, yet also requiring a way to remove the covers for repairs and adjustments. Grating is time consuming to move, and slats require complex support structures that complicate maintenance tasks.

Instead, the aircraft manufacturer uses rugged walk-on metal pit covers with extruded aluminum stiffening ribs bonded to stainless-steel sheets; there are no hinges to collect debris. These roll up onto steel rollers equipped with take-up mechanisms.

Roll-up covers can also be used to enclose chemical tanks and other vessels to contain fumes or prevent injury. These covers are not designed to support the weight of workers, so they are made of lighter materials such as polypropylene to withstand corrosive fumes. Wiper strips can keep the cover under tension and remove debris before retraction; electric motors and gear drives are employed in the take-up mechanisms.

Protecting against welding flash and spatter
Portable screens protect workers’ eyesight against welding flash and UV radiation, and con‚fine weld spatter. Basic weld curtains on stands can be moved around the factory floor to provide shielding where needed. Freestanding curtain modules simplify work-cell assembly.

Motorized weld curtains take it a step further in custom-engineered frames — suitable for robotic-welding areas and automated equipment. Some incorporate gearless direct-drive motors that can be programmed to open to various widths at speeds to 2 m/sec.

Reducing equipment noise levels
The noise levels of some equipment, such as that on construction sites, are dif‚ cult to reduce. However, portable sound barriers can be set up to suppress sound. Some designs retract into compact wheeled cases for easy transport. Unlike traditional sound curtains consisting of basic frame-mounted material, these use multilayer materials incorporated into a pleated accordion design that increases sound mitigation and simpliŽfies setup. For excessively hot operations, screens can also double as heat shields when suitable materials are used.

Other equipment may be mufŒfled directly with specially designed jacketing. Pavement breakers, commonly called jackhammers, produce 120 to 125-dB sound levels at the ear of the operator and approximately 90 dB at a distance of 50 ft. A light jacket reduces these levels by around 9 dB, which cuts the breaker’s perceived loudness by half. The jacketing can reduce sound intensity, which leads to hearing loss, by 88% because most of the noise is produced by the breaker’s internal components.

The 2-lb jacket, made of three layers of material sewn together, is wrapped around the breaker and fastened with industrial Velcro. It covers the lower housing from just below the handle bar and encloses the opening at the bottom that allows the tool bit to travel. A suspender strap over the handlebar keeps it from blowing off due to air pressure.

In addition to reducing noise levels, the jacket protects the worker from the exhaust air of these pneumatic units and the oil that often spews onto the worker’s legs. Finally, reŒflective strips make it visible at night for improved operator safety.

 

© 2013 Penton Media, Inc.