Daily Archives:February 20th, 2013

A most sinister thread…

One of the things we try to do here at Status-Q Labs is to reduce the amount of frustration experienced by our fellow men and women in their daily lives.

Take, for example, the case of reverse threads. This is not, as you may suppose, an advanced mode of needlework, but rather the practice of using screw threads which turn clockwise to undo or loosen, and anti-clockwise to tighten, something that observant readers will detect as being contrary to the natural order of things. There are good reasons for using these — also known as left-hand threads — in situations where the normal use of the the device would tend to cause it to unscrew of its own accord.

However, there are few things more frustrating than not knowing that the thing you're trying to unscrew is, in fact, tightening up, especially if it's old or rusted or damaged and you expect it to be somewhat tricky anyway. Here are a few situations where I've encountered reverse threads in normal life: remembering these may improve the level of your future happiness and avoid some skinned knuckles and unwarranted expletives.

  • Left-hand bicycle pedals. These are screwed into the crank with a reverse thread. Which is a little strange, if you think about it, because their rotation on their spindle is clockwise, but there are other effects at work. More information here.
  • Gas cylinder valves. This is for safety, rather than mechanical reasons. Combustible gases, such as the propane or butane you might connect to your caravan or gas barbecue, use left-handed threads, so you can't accidentally connect them to things expecting an inert gas. This is a good idea, but I can't help wondering how often frustration has caused people to start hitting spanners attached to explosive cylinders with heavy objects…
  • Drill chucks. I had to replace the chuck on my hammer drill recently because the jaws had seized up. It screws onto the main spindle of the drill with a standard thread, but is held in place there by a bolt which goes the opposite way. (I guess you only need this on a reversible drill!) Incidentally, even when you know which way the threads go, chucks tend to be fairly firmly fixed, and I might not have managed it if my friendly local hardware store hadn't shown me the allen key trick.

So there you go. Lodge those in your little grey cells and one day, I promise you, you'll thank me!

Anyone know any other situations where the unsuspecting might encounter left-handed threads in normal life?

Update: Lyndsay Williams pointed me at this Wikipedia page, which lists under 'Handedness' a few other places where left-handed threads are used. My favourite is that lightbulbs on the NYC Subway used to have reverse threads, so you couldn't steal them and use them anywhere else!



Short, sharp and to the point

Beginners in photography can (understandably) get confused by the fact that big numbers mean small apertures, or that shooting 'wide open' implies a small depth-of-focus. I liked this mnemonic from Chris Orwig on a recent This Week in Photo podcast:

Imagine you're shooting a line of people. If you want one person in focus, use f/1. If you want all 16 in focus, use f/16.

Nice and simple.


© Copyright Quentin Stafford-Fraser