The Old Timer of Royal Oak, Mich., was a regular contributor to H&P years before we ever even heard of the internet. But most of his advice is just as useful — and interesting — today.
So rather than leave his wisdom printed on pages archived in our storage room, I pulled out issues from the late 1980s and early 1990s and have been reproducing relevant entries in this blog. Here is my 23rd entry, which was originally published in the November 1989 issue:
A lesson in air valve center positions
Some years ago, our engineering department sent down a design for a light-duty transfer system using a 6-ft pneumatic cable cylinder operated by a 3-position directional control valve with blocked center for jogging. The design looked fine on paper but turned into a first-class disaster in production after assembly. A major contributor to our problem was the fact that our shop had standardized on valves with metal-to-metal seals — which were relatively new at that time — because our ancient pneumatic distribution system was full of rust, compressor oil, and other debris. These valves digested contaminants fairly well, but to do so, they inherently left some leakage paths open.
We had our electrical people interlock the solenoids so that the last one energized stayed on until the other was energized. This took away our jogging capability, but it also ensured that there was always air pressure in the cylinder to resist the next stroke. Things worked fine until lunch time, when the electricity was shut off for some minor adjustments. Springs centered the valve spool, and enough air leaked down from both ends through the blocked center so that when we restarted and energized solenoid a, the part on the transfer was launched about 40 ft across the plant floor, and another wreck occurred.
Since those early days, spool valve designs have improved dramatically, and we’ve learned a lot about air preparation. I guess the main lesson of this reminiscence is that you have to consider the system — rather than a single component — during both design and troubleshooting.