What is the key interchange rate?
The concept of key interchange rate applies to lock production, and there are corresponding national standards for this rate based on different lock applications. Depending on varying security requirements, the rate ranges from a few percent to a few thousandths, a few ten-thousandths, and even a much lower ratio.
The key interchange rate is defined as the percentage ratio of mutual opening among a specified quantity of locks in the same batch within a set time (calculated as the quotient of the number of successful opening attempts divided by the total number of test attempts, expressed as a percentage).
For anti-theft door locks during production, manufacturers calculate the maximum number of key combinations in advance according to machining precision and the number of pin tumblers. Mass production is carried out in a continuous cycle following this permutation and combination, which inevitably results in a certain key interchange rate.
The fewer locks a single key can open, the stronger the safety and reliability of the lock. Well-known lock brands boast strong technical capabilities, with their products achieving a key interchange rate of only one in tens of thousands. In contrast, products from unregulated and incompetent manufacturers may have an interchange rate of one in dozens, which means such manufacturers may only have a mere dozens of lock production molds.
At present, China classifies anti-theft door lock cylinders into three grades: Grade A, Grade B and Super Grade B. In accordance with the corresponding national standards, the key interchange rate of Grade A locks shall be ≤ 0.03% – meaning 3 out of 10,000 locks can be opened with the same key; that of Grade B locks ≤ 0.01%; and the rate of Super Grade B locks is even lower, approximately ≤ 0.0004%, which equates to only 4 out of 1,000,000 locks sharing an identical key.
How is the key interchange rate of a lock determined? Let's look at the structure of the most common pin tumbler lock:
The principle is actually simple. The lock shown in the figure has five pairs of pin tumblers (i.e., five top pins and five bottom pins), which can slide up and down in their respective pin chambers. When a correct key is inserted, the five top pins retract from the lock cylinder into the lock body and align perfectly with the interface between the lock cylinder and the lock body. At this point, the key drives the lock cylinder to rotate and unlock the lock. If the key is mismatched, the pin tumblers will get stuck at the joint of the two components, and the key will naturally fail to open the lock.
The number of key bittings for a single set of locks is determined by two factors: the number of pin chambers and the number of pin grades. We have already mentioned the number of pin chambers; the number of pin grades refers to the number of different lengths of pin tumblers (the sum of the lengths of a top pin and a bottom pin is fixed). Their lengths are not arbitrary but follow a specific grade difference. Thus, it is easy to calculate the total number of distinct key bittings available.
For example:
With 4 pin chambers and 5 pin grades, the number of key bittings is 5⁴ = 625; With 6 pin chambers and 5 pin grades, the number of key bittings is 5⁶ = 15,625.
(In actual production, the number of key bittings is less than the theoretical value because some special bittings are excluded for production safety.)
Naturally, the more key bittings there are, the lower the key interchange rate will be.