CAM AND CAM FOLLOWER DWELL TIME IN AK 47

 

CAM AND CAM FOLLOWER DWELL TIME IN AK 47

In the AK 47, cam and cam follower dwell time refers to the brief period during which the bolt remains stationary, locked in place, as the bolt carrier mover slightly without unlocking the bolt. This dwell time is critical for managing chamber pressure and ensuring reliable cycling of the weapon.

Cam And Cam Follower Mechanism in the AK 47:

                                             Cam in the Ak 47 Bolt Carrier

                                    Cam Follower in the Ak 47 Bolt (Top Left Side Lug)

                                   Bolt and Bolt carrier group with operating rod (Piston)

·       The AK 47 uses a rotating bolt with cam and cam follower system to control the locking and unlocking the bolt. The bolt carrier has a cam groove that interacts with a cam lug on the bolt itself. This groove guides the bolts rotation, locking and unlocking it with precise timing during the firing cycle.

·   When the round is fired, the gas operated piston is driven rearward by gas pressure, moving the bolt carrier backward. However, during the initial rearward movement of the bolt carrier, the bolt remains locked also the bolt does not immediately rotate or unlock. The bolt carrier moves a short distance before engaging the cam lug to rotate the bolt after firing a round. This delay is the dwell time in the cam and cam follower system.

·       After this brief period, the cam lug in the bolt engages with the cam groove in the carrier, causing the bolt to rotate and disengage from the locking lugs in the trunnion. This controlled unlocking occurs only after the bolt carrier has already moved slightly rearward, allowing chamber pressure to safely reduce before extraction begins.

Purpose of Dwell Time in the AK 47 Cam System:

·   The dwell time ensures that the bolt does not unlock while chamber pressure is still dangerously high. By delaying the unlocking, the AK 47 design allows pressure to drop to safe levels, protecting the firearm and the shooter.

·      Proper timing helps prevent failures in the extraction of the casing, avoids excessive wear and reduces the chance of malfunctions that could occur if the bolt unlocked prematurely.

How Dwell Time is Achieved in the AK 47:

·    The shape and angle of the cam groove in the bolt carrier are specifically designed to delay the bolts unlocking. As the bolt carrier initially moves rearward, it travels a short distance without causing the bolt to rotate. This creates a dwell period before the bolt starts unlocking moving rearward.

·      The gas port and piston drive system are timed to ensure that, as gas drives the piston and bolt carrier rearward, the bullet has already exited the barrel, allowing chamber pressure to drop to a safe level before the bolt begins to unlock.

Duration of Cam Dwell Time in an AK 47:

·   The cam dwell time in the AK 47 is very brief, on the order of microseconds, yet it essential for reliable operation. During this dwell period, the bolt remains locked even as the carrier begins moving, which long enough to achieve pressure drop but short enough to enable the fast-cycling characteristic of the AK 47.

Factors Influencing Dwell Time:

·   Adjusting the angle and length of the cam groove affects how long the bolt remains stationary as the carrier moves.

·      High or lower pressure ammunition can impact the timing and duration of this dwell, as it affects how forcefully the piston drives the bolt carrier.

·      Over time, wear in the cam groove or follower can subtly alter dwell timing, although the AK 47 robust design is intended to minimize sensitivity to wear.

Formula for Estimating Dwell Time in the AK 47 Cam System:

·       Distance of Initial Bolt Carrier Travel (D): This is the distance the bolt carrier travels rearward before it begins to rotate and unlock the bolt.

·       Velocity of the Bolt Carrier (V): This is the rearward velocity of the bot carrier, which is affected by gas pressure, barrel length, gas dwell time, and ammunition used.

 

Formula = Delta t = D / V

Delta t= Dwell time in seconds.

D= distance of the initial rearward movement of the bolt carrier before bolt unlocking.

V= average velocity of the bolt carrier during this initial movement.

Steps To Determine Each Variable:

Distance of Initial Bolt Carrier Travel (D):

·     Measure or determine the length of the cam groove’s section that allows the bolt carrier to move without rotating the bolt. This is the section of the cam groove where no rotational force is applied to the bolt through the cam lug. This distance varies slightly among AK models but generally falls within a range of about 1-3 millimeters (0.001 to 0.003 meters).

Velocity of the Bolt Carrier (V):

·      The velocity of the bolt carrier can be challenging to determine precisely without specific instrumentation, as it is influenced by gas pressure, friction, and recoil spring tension. However, for estimation purposes, you might approximate the bolt carrier’s initial speed based on typical values for gas-operated firearms. In the AK-47, this velocity typically ranges from 5 to 10 meters per second at the start of its rearward travel. Testing with high-speed cameras and instrumentation could provide an exact measurement, but for an approximate calculation, you can use an average velocity (e.g., 7 m/s).

Example Calculation:

·       Distance of initial bolt carrier travel (D): 0.002 meters or 2 mm.

·       Average velocity of bolt carrier (v) = 7 m/s

·       Delta t = 0.002m / 7m/s = 0.0002857 seconds or 0.29 milliseconds.