In your May 25, 2006 article titled “For lack of a guard, a severed hand,” you hint at a design for a dust-collector guard that prevents a user from putting their hand in the machine, and increases machine efficiency. Would you be willing to explain the design?

Thank you for your time and service.

Tom Medill
Advanced Engineering Aide
Applied Research Lab
Penn State Univ.

I'm happy to help you.

The design of the guard is quite simple. It consists of two 90° plastic 4-in.-diameter plumbing angles and one straight 4-in.-diameter plastic plumbing pipe. Cost at Home Depot: under $10.00.

The 90° bends connect on either end of the straight piece. One 90° bend connects to the outlet of the dust collector. The collector's 4-in.-diameter flexible hose fastens to the other 90° bend.

I fully expected significant flow loss from the two 90° bends and the pipe's roughness factor which, in fact, did happen (approximately 30% loss). What I did not expect was that the guard, even with the flow loss, increased efficiency of the dust collector.

Five separate tests compared collector efficiency with and without the guard. It turns out the unit, sans guard, clogs with debris, but remains clear with the guard in place. My theory is that material picked up by the guard-equipped dust collector does not follow the air stream due to inertia, but instead carries to the edge of the first 90° turn, then goes to the bottom of the second 90° turn. This causes most of the material to enter the dust-collector body at or near the tips of the rotor and be thrown with much more force into the collection bag than would happen without the guard in place. Without the guard, material uniformly spreads across the diameter of the ducting system and slows at the rotor center. Just a small percentage of material enters the dust collector at high speed at the rotor tip.

Whatever the reason, the dust collector with the guard did not clog and carried much more material than the system without the guard. This again demonstrates that a theory is just a theory until it is confirmed by testing.

Lanny Berke is a registered professional engineer and Certified Safety Professional involved in forensic engineering since 1972. Got a question about safety? You can reach Lanny at lannyb@comcast.net