The Inherent Risks of High Inertia Machines
As an educator that trains people to use power tools, machine tools, and CNC equipment, I am always working to identify potential safety risks. In a deep dive into the inherent dangers of high inertia tools, I was surprised to find that even manufacturing and industrial settings seem to overlook a significant, unnecessary, and easily-mitigated risk.
The Observed Problem
Industrial machines that contain rotating parts of significant mass coast long after they have been turned off. In the case of an industrial bandsaw, the saw blade will continue to move at a significant pace for more than two minutes after the power is switched off. Whether the machine is being turned off to be inspected, to receive maintenance, or for an operator to cycle a part, a human operator is expected to interact with the machine as soon as possible. This poses a risk of accidental contact with moving blades or parts of the machine, especially if the operator is under pressure to reduce cycle time. For example, look at the risks both during and after the cut on this vertical bandsaw video.
- For many industrial tools (i.e. lathes), OSHA requires protection from rotating parts and illustrates a brake as a potential solution.
- For other industrial tools (i.e. bandsaws), braking is listed as an additional safety measure.
Many Variable Frequency Drives (a common controller for large industrial machines) include a feature that lets an operator set automatic braking for VFD-controlled machines.
Some machines (many lathes, for example) include built-in manual braking via mechanical or electrical means.
Aftermarket suppliers offer electrician-installable DC Injection motor brakes.
Some AC motors take advantage of starting capacitors and creative switching to provide limited capacitive braking below a certain RPM.
Some universal motors (commonly chop saws, for example) employ built-in dynamic braking solutions.
Problems w/ Existing Solutions
There exists a large subset of industrial machines that are not VFD controlled and have no built-in braking options.
Many machines in use are decades old and built-in braking systems have not been maintained. In fact, anecdotal evidence suggests that many machine operators choose not to use built-in mechanical brakes in fear that they will wear them out, thereby making the brake inoperable in the case of a real emergency.
110VAC and 220VAC stationary power tools like table saws, radial arm saws, band saws, grinders, and sanders have been largely overlooked. Many contain no brake and can continue to operate for minutes after being shut off.
A Call to Action
Given the discussion above, here are some things to check out in your shop environment:
What are the coast-down times for machines in your shop?
Depending on the use case for the tool, this can be measured in two ways. First, it can be measured from the time the tool is switched off (or otherwise expected to be safe) to the time the tool is completely safe to touch. Or, secondly, you can measure from the conclusion of an operation (a cut, for example) to the time the tool is completely safe to touch.
How obvious is it that the machine is still unsafe to touch?
What visual or auditory clues are there that the machine is still moving? In the case of some band saws, their continued operation can be nearly silent and persistence of vision can cause the moving blade teeth to appear stationary or solid.
- What are common operator actions that immediately follow the machine action?
In the case of a band saw, the immediate action after the cut is to retrieve or measure the cut part. In the case of a lathe, the immediate action is to measure or retrieve the part, or to change tooling.
- Do any of the common actions described above put the operator in the proximity of, or at risk of contacting, a rotating part of the tool?
In all of the aforementioned examples, the operator’s immediate action would put them at risk of contacting the moving part, blade, or tool.
- How do operators currently navigate this dilemma?
Do your operators use the friction of their palm to manually brake the machine? Do they wait patiently until the machine comes to a complete stop? Do they perform another task while the machine coasts to a stop?
How does the current solution impact production?
If an operator is waiting for the machine to coast to a stop, is that impacting your cycle time?
Please contribute your thoughts and experiences in the comments for the betterment of the group.