Guarding power transmission equipment

Getting any body part caught in a belt and pulley, chain and sprocket, gears, flywheels, shafts and couplings and other mechanical power transmission apparatus can hurt.

I’ve been a member of the American Society of Mechanical Engineers (ASME) B15.1 Committee on Safeguarding of Mechanical Power Transmission Apparatus for the past 18 years and became chair of that committee two years ago. We occasionally get letters and reports from individuals describing how someone was injured by being pulled into a belt drive, or how a finger was lost when a ring caught on a burr on a pulley, or about a scalping when someone’s hair was wrapped around a rotating shaft. During my 12 years with North Carolina OSHA, I investigated several accidents involving power transmission apparatus, including one fatality at a sawmill where an unguarded smooth rotating horizontal shaft about waist-high grabbed a worker’s loose shirt tail and wrapped him around the shaft like limp spaghetti.
As a consultant for the past 20 years, I’ve run across about 100 cases involving missing fingers and an occasional missing hand snatched away by some inadequately guarded power transmission equipment. Practically every workplace has power transmission apparatus, and the potential to encounter these moving traps is high when they’re not guarded adequately.

The hazards
Mechanical power transmission apparatus is any device involved with transferring power from a motor to a machine. Each such apparatus presents several hazards, the chief among them being nip points. These are defined as moving parts that come together to produce a point at which fingers, hair, clothing and the like can be pulled in and held.

A belt running on a moving pulley (Figure 1), a chain running onto a sprocket, or two gears that come together close enough to grab fingers, hair or clothing are examples of nip points. It’s not a rare occasion that someone reaches blindly into a machine part, not realizing there’s a moving belt or chain waiting to capture a finger or two.

Figure 1

Another hazard of transmission apparatus is that as it rotates, sometimes at great speeds, various parts can break and be thrown at workers. I’ve seen the evidence. An unguarded pulley about 18 feet above the floor on the lifting motor of an overhead garage door broke apart. Pieces flew 75 feet across the shop and lodged in a plaster wall. No one was injured in that case, but it got everyone’s attention about the importance of maintenance and barrier guards on these devices.

A third serious hazard is burns and abrasions from body parts rubbing against rotating parts. It doesn’t take long to rub off more than just a few layers of skin. I can recall two separate cases I’ve examined in which air currents caused loose, dangling ropes to wrap around exposed rotating shafts. Those ropes then pulled hands into the shafts, with the rope literally tying a hand to the rotating shaft and rubbing the skin off of fingers in no time. A similar event crushed three fingers.

National standards
Two national standards address mechanical power transmission apparatus: ASME B15.1 and OSHA standard 29 CFR 1910.219. While similar, there are major differences between the two. The biggest difference is that the OSHA standard is based on the 1958 ASME B15.1 standard, adopted when it was still quite common for manufacturing plants to have overhead belt-driven line shafts and oilers -- workers whose job was to go from line shaft to line shaft and oil the moving parts.

Another difference between the two standards is that there are a number of guarding exceptions in the OSHA standard that are no longer in the ASME standard. Examples include exceptions for small, slow moving belt drives; for openings in guards used for oiling; for attendants in engine rooms and a limited exception for the textile industry because of the cotton lint fire hazard.

As technology has improved, and the use of oilers and engine rooms have declined drastically, these exceptions have been removed from the current ASME standard (2000) because there should be no exceptions when safety is an issue. The current ASME standard requires power transmission apparatus on stationary equipment located within eight feet of a working surface to be safeguarded -- without exception. The OSHA standard, on the other hand, requires safeguarding up to seven feet. In the mid-1990s, the B15.1 committee recognized that people are getting taller and raised the guarding requirements to eight feet to meet the most recent anthropometric data.

Another difference between the two standards is that the OSHA standard -- at 1910.219(p)(1) -- requires power transmission apparatus to be inspected every 60 days. Guards routinely become damaged, worn out, modified improperly by operations, or removed and not replaced during maintenance operations. The intent of this inspection requirement is to examine these guards routinely to ensure they’re still functional and in place. This inspection can be part of the preventive maintenance process or, as in many plants, it’s part of the routine safety inspection process.

Definition of adequate safeguarding
Probably the most controversial issue of either standard is defining what constitutes an acceptable guard. ASME B15.1 is quite clear on the subject because of the anthropometric tables and sketches it includes (Figure E12 and Table E1 in the B15.1 standard). On the other hand, the OSHA standard -- at 1910.219 -- has no real definition of adequate guarding. The OSHA standard simply uses the words “shall be guarded.” However, the intent of the OSHA standard is that guards meet the requirements of ASME B15.1.