When producing anti-theft door locks, manufacturers pre-determine the maximum number of key combinations based on machining precision and the number of pins. These combinations are then produced in a continuous cycle, inherently resulting in an interchange rate. The interchange rate refers to the percentage of locks within the same batch that can be opened by the same key within a specified timeframe. It is calculated as the ratio of the number of successful openings to the total number of attempts.

The fewer locks that a single key can open, the more secure and reliable the lock is. Leading lock brands, with advanced capabilities, achieve an interchange rate as low as one in tens of thousands. In contrast, locks produced by less reputable manufacturers may have an interchange rate as high as one in a few dozen, implying that these manufacturers may only have a limited number of molds for lock production.

 

Currently, the security lock cylinders for anti-theft doors are classified into three levels in China: Grade A, Grade B, and Grade C. According to relevant national standards:

•The interchange rate for Grade A locks should be ≤0.03%, meaning 3 out of 10,000 locks are identical.

•For Grade B locks, the interchange rate is ≤0.01%.

•Grade C locks have an even lower interchange rate of approximately ≤0.0004%, meaning only 4 out of 1,000,000 locks are the same.

How is the Interchange Rate Determined?

To understand this, let's take a look at the structure of the most common pin tumbler locks:

The principle is quite straightforward. The lock in the diagram consists of five pairs of pins (five upper pins and five lower pins) that can slide up and down within corresponding pin holes. When the correct key is inserted, the five upper pins retract into the lock body, aligning perfectly with the shear line between the plug and the lock body. At this point, the key can rotate the plug to unlock it. If the key does not match, the pins will remain stuck at the shear line, preventing the lock from opening.

The number of key cuts in a lock is determined by two factors: the number of pin holes and the number of pin levels. The number of pin levels refers to the different possible lengths of pins (the sum of the lengths of the upper and lower pins is fixed). These lengths are not arbitrary but follow a specific increment. Therefore, it is easy to calculate the number of key cut combinations.

For example:

•If there are 4 pin holes and 5 pin levels, the number of key cuts is 5⁴ = 625 combinations.

•If there are 6 pin holes and 5 pin levels, the number of key cuts is 5⁶ = 15,625 combinations.

(Note: In actual production, the number of key cuts is slightly less than the theoretical value, as certain special cuts are excluded.)

The greater the number of key cuts, the lower the interchange rate.