I wanted to install a garage door opener and thoughtfully made measurements before purchasing the door opener. I was short by three inches the minimum amount perpendicular from the inside of the door back across the ceiling -- a heating duct was already there.
Figuring I could make it work anyhow, I purchased the unit and began installation. My job was to shorten the guide rail by three inches so that the head unit would butt against the heating duct. I encountered no problems as I assembled the head unit, rail, and the sprocket that mounts directly above the door.
In those days -- the mid-1970s -- you received a full chain drive, not part chain or part wire cable. The chain came in a nicely sealed plastic bag as it was heavily oiled. When I cut the bag, the chain came out as one big piece and one small piece -- an obvious error. It should have been one long piece.
However, once I looked closely, I noticed the smaller piece consisted of 8 .750-inch links, making it 6.0 inches overall. This is exactly the amount I would have had to shorten the chain to fit the shortened rail. Sorry Murphy and Ohms, you didn't get me this time. The fact that it was in two pieces worked perfectly, and, the manufacturer of the door opener never received an angry complaint from this customer.
I had the idea that the chain length would be correct with the standard rail. This is to be expected. Shortening the rail by three inches means the chain needs to be shortened by six inches because the chain wraps around the sprocket at the other end. The sprocket simply acts as a pulley to allow the chain to circulate. If anyone else had picked up that box, they would have had to buy another master link to connect the short and long pieces, plus they might have had to remove one of the links as the chain would have been three quarters of an inch too long.
The only way to really check the chain length would have been to do a trial assembly. This could have been done on the garage floor, but I only wanted to handle the oily chain once. Before purchasing the unit, I made very careful measurements that included the height of the door opening and the luggage rack on top of the station wagon.
Three inches less travel of the door puller meant that the bottom of the door would stop a little lower when opened. In general, this is basically what engineering is all about: Doing the design on paper or CAD and doing the calculations so that the first one to be built will properly function.
What makes my story fairly unique is that a goof by the manufacturer was 100 percent acceptable for my situation. It actually saved me the trouble of having to shorten the chain. That meant another purchaser didn't get stuck with a problem, and the manufacturer didn't receive a complaint he would have had to correct. I wonder if any other readers have experiences where an initial mistake actually turned out to be the right thing in practice.
This entry was submitted by Bob Salter and edited by Rob Spiegel.
Bob Salter is a mechanical engineer who works for American Roller Bearing Company. He is a dedicated do-it-yourselfer. He has done many custom installations on cars and homes that were unconventional. In this instance, he had to install something that the manual said wouldn't fit and would not be acceptable to the average installer.
Tell us your experience in solving a knotty engineering problem. Send to Rob Spiegel for Sherlock Ohms.