Posted by Ian mcColl (203.27.231.2) on February 14, 2003 at 07:43:56:
In Reply to: Hiatt darby lock - different types of keys - problem posted by Andy on February 10, 2003 at 05:50:37:
Hi Andy, Darby handcuffs have what is called Bastard thread sizes. These are non standard thread sizes. There are about 12 different thread sizes to suit Hiatts cuffs that I have found and made taps for. Apart from the thread size which is the pitch (shape) and TPI ( threads per inch) there is also the diameter of the lock bolt.
The thread doesn't go all the way up the locking bolt shaft, because if it did, the thread would get caught on the square hole inside the lock tube and not allow the bolt to slide freely.
The problem is that the first few threads are worn down ( the first section of thread get the key screwed over it many more times that the threads at the other end as the key is wound in and then out again.
The length of the key from its shoulder to its tip should be about two or three thread helix' longer than the shoulder stop on the lock tube to the tip of the bolt you can see inside the tube.
If the key doesn't grab the thread within the first few turns, both the key and the bolt are worn.
As you say, the spring is very strong and this causes the wear over the years. Like any old cuff, it will be worn and some more than others. Just because the darby mechansim is simple it suffers the same as any other machine.
Hope this helps.
Ian