Guarding power transmission equipment

Dec. 27, 2005
Injuries happen around power transmission equipment and they can be quite painful. Adopt these four key safety management practices and your plant can be safer.

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 standardsTwo 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.

In addition, there’s a table in OSHA standard 1910.217 -- the mechanical punch press guarding standard -– that applies to the point of operation on a punch press. This table (Table O-10) is similar to the ASME table and can be used to determine if effective safeguarding has been installed. Both tables are based on the size of the opening in a guard compared to the distance to the moving power transmission apparatus.

For example, let’s say that you plan to guard a belt drive using expanded metal. You pick a piece of expanded metal with 7/8-in. openings. Based on Table O-10, the guard would need to be at least 6.5 in. from the moving parts. In another example, the rotating shaft in Figure 1 should have a guard that meets the specifications of OSHA Table O-10 or ASME Table E1. If the guard is 3.5 in. above the rotating shaft, the guard should be within 5/8 in. of the motor housing. Figure 2 shows an example of a guard on a horizontal shaft having an additional strip of expanded metal added to conform to these two tables.

Numerous retrofit guards are available on the market. When you purchase a new guard, state in the purchase order that it must conform to OSHA 1910 Subpart O and ASME B15.1. If you hire a third party to build custom guards, ensure through design review and routine installation audits that the new guards meet or exceed the same requirements. Figure 3 shows an example of a well-guarded horizontal shaft.
In addition to physical guards, the ASME standard also recognizes guarding by motion sensors and safe distances.

Managing the guards
Power transmission apparatus can’t be installed and then forgotten. The first step to ensure effective guarding of existing power transmission apparatus is to conduct a risk assessment of each piece of apparatus at your location. Doing this gives you an inventory of power transmission apparatus to know where to inspect routinely and provide preventive maintenance. A simple risk assessment ensures that the barrier guard’s openings meet OSHA Table O-10 or ASME Table E1. A more thorough risk assessment would examine the strength of the guard, its physical condition, how the equipment is used and maintained and other relevant information. A good guide for a risk assessment is ANSI B11.TR3 – 2000.

Once the risk assessment is finished, prepare a prioritized schedule to upgrade any deficient guarding. Next, train those who work around power transmission apparatus to understand the hazards presented, the methods of safeguarding and any applicable safe work practices.

The fourth key management practice for power transmission apparatus is to inspect it routinely to look for damaged guards, missing guards, modified guards and other problems.

Injuries can be prevented
There should never be any injuries involving this type of equipment. We’ve known how to guard this equipment since 1927, when the first edition of ASME B15.1 was approved. This standard was originally written and kept current by knowledgeable professionals from industry who, over the years, have recognized the potential hazards and addressed them in a good standard. Too often, however, out-of-sight, out-of-mind keeps us from auditing, maintaining and managing this equipment. How bad can this equipment hurt you? It can kill you. I don’t want you to dwell on that, but I don’t want you to forget it, either.

David F. Coble, MS, CSP is president of CTJ Safety Associates in Cary, North Carolina. Contact him at [email protected] or (919) 466-7506.

The OSHA standard is available at Information about the ASME standard is available at

For copies of the ASME B15.1 standard, contact Global Engineering Documents at 800-854-7179.

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