Description of the Hazard
A coasting (or “freewheeling”) hazard is when a machine continues to move after it has been switched off. This hazard is present on all machines to some extent but is most significant when one or both of the following are true:
- The machine has significant kinetic energy (see below)
- There is very little friction in the system
A freewheeling or coasting machine is a significant hazard because the machine keeps operating after the operator has turned it off. The operators expectation is typically that off = safe and this assumption is often untrue. Additionally, most machines do not provide clear auditory or visual clues to indicate that they are still in motion. This result is that operators come into contact with moving machinery or the point of operation and receive significant preventable injuries.
A quick note about hazardous energy …
Whether your talking about active chemical substances, heavy weights dangling overhead, or exposed electrical parts; it’s all about controlling hazardous energy. And just like controlling hazardous energy with lockout-tagout (electrical energy) or fall protection (gravitational potential energy), we must control errant hazardous kinetic energy in the form of coasting machinery.
Machine Shape & Speed is Important
When we think of dangerous machines or objects, most people will intuitively relate the danger level of the machine to the size of a motor or the weight of the moving parts. In the case of coasting machines, that is often wrong. The shape and speed of a machine part is MUCH more important than weight. How much more? Check out the infographic below.
Kinetic energy, similar to credit card interest, is often related to exponentials. Let’s take the case of a spinning disc. Mass (or weight) is part of the energy equation but if you double the mass, the energy only doubles. If you double the speed, the energy quadruples. If you double the diameter of a disc, the energy also quadruples. In other words – the diameter of spinning parts and the speed of rotation is a much more predictive indicator of coasting hazards.
E-Stop or E-Off?
The generic term e-stop (e.g. emergency stop) can be a bit misleading. The most common type of e-stop in the field is what’s technically called a ‘category 0’ or ‘uncontrolled stop’. Essentially it just means that power is removed from the machine motor. But because a machine has inertia it will still coast or freewheel for an unknown amount of time. To be succinct, nothing about a standard “e-stop” ensures that a machine is stopped.
In contrast, a ‘category 1’ or ‘controlled stop’ is when an engineering control is specifically designed to control the machine during the ‘stop’ process. The most common of these is an automatic brake. In these cases, they truly are ‘e-stops’ as they are configured to stop the machine in a hurry if there is an emergency.
Many EHS experts focus their risk assessment on limiting operator exposure to these hazards during a grinding operation but fail to consider the greater context of the operation. One must also consider the time before and after the intended operation. There are two significant exposures to consider:
Bench grinders are notorious for exceedingly long spin-down times of between 2 and 4 minutes. During this time, a well tuned grinder offers few if any auditory or visual cues that it is still spinning. All of the aforementioned hazards are still present during this coast-down time and the risk is multiplied because subsequent operators and other nearby employees are not conscious of the hazard and therefore at greater risk for injury. For this reason, a coasting grinder is considered a point-of-operation hazard under OSHA 1910.212(a)(1) and it must be mitigated with a braking system or additional guard(s).