In the formative years of tank design history, machine guns were often incorporated as a standalone weapon in an independent mount or in a turret, only occasionally supplementing a larger main gun. The role was usually filled by whatever medium machine gun was already currently in service. In the postwar era, very little had changed except that a machine gun coaxially paired to the tank's main gun was now universally regarded to be essential. Putting a machine gun in a tank sounds simple, but like many things in life, doing it right demands a great deal of attention to detail.
Since machine guns aren't air-tight, gunpowder gasses tend to accumulate in the crew compartment unless special measures are taken. As a rule, the crew compartment ventilator had to be turned on while firing at the very least but ideally the problem was addressed at its source. In the Leopard 1, Leopard 2, and Merkava series, an extractor fan was positioned next to the openings on the machine gun receiver. In the M2 Bradley, the coaxial M240C was mounted outside the turret. In the M1 Abrams, the front of its M240 was enclosed in a large-diameter pipe called a "smoke tube" or "smoke box" so that the gas system would vent outside the crew compartment.
The T-72's overpressure ventilation system kicked in when the gunner pressed the trigger for the coax, but the machine gun installation itself was otherwise totally basic. The issues of gas contamination were solved in its PKT machine gun itself with special modifications from the basic Kalashnikov PK.
Like the M240, the PK had a long-stroke gas piston mechanism that vented gas from its adjustable gas regulator once the piston received enough energy to cycle the action. The gas tube behind the gas regulator had vent holes so that gasses blowing past the labyrinth grooves on the piston head would not foul up the receiver. This was fine for infantry use, but the vent holes in the gas tube meant that even if its gas regulator was sealed off, the PK was simply not suitable for tanks without a something like the Abrams' smoke tube. The PKT largely solved this with a sealed gas regulator and gas tube. Air quality tests showed that one shot fired from a PK inside an inhabited enclosure released 83 mg of carbon monoxide while a PKT released just 23 mg.
This was well within tolerable limits, but it was still much higher than expected. Of the 83 mg of carbon monoxide the PK released, 75 mg was from its gas system. The remaining 8 mg escaped from its chamber. If the PKT gas tube was completely impermeable the chamber would indeed be the only route for gasses to escape, but the lack of vent holes meant that gasses blowing past the piston ended up in the crew compartment through the receiver. In 1988, a modification was made to the PKT gas regulator so that it would once again vent, but through a small duct leading outside the turret. In its final form the PKT solved the problems of gas venting and piston blow-by without needing fans, smoke tubes, or any other equipment that could take up space in the turret.
Of course, this solution was not perfect - the PKT could not use standard PK barrels, and the T-72 carried no spare barrels for quick-swaps. In T-72 - The Definitive guide to the Soviet Workhorse, you will not only read about the T-72, but also learn the nuances of its famous PKT and NSV machine guns, never revealed before even in books dedicated to these guns.
In T-72 - The Definitive Guide to the Soviet Workhorse, you will find an in-depth examination of the T-72's design and all the ideas, technologies, and design solutions that went into its creation. It is one thing to read a list of a tank's technical specifications, but another entirely to understand how they were achieved, and what consequences followed. The book is currently 20% OFF until 27 September exclusively on Lulu Press, additionally other books the Military History Group are reduced up to 10%: militaryhistorygroup.com/ It is also available on Amazon and Barnes & Noble.
Military History Visualized
Ryan A. Then: About Tank Machine Guns
In the formative years of tank design history, machine guns were often incorporated as a standalone weapon in an independent mount or in a turret, only occasionally supplementing a larger main gun. The role was usually filled by whatever medium machine gun was already currently in service. In the postwar era, very little had changed except that a machine gun coaxially paired to the tank's main gun was now universally regarded to be essential. Putting a machine gun in a tank sounds simple, but like many things in life, doing it right demands a great deal of attention to detail.
Since machine guns aren't air-tight, gunpowder gasses tend to accumulate in the crew compartment unless special measures are taken. As a rule, the crew compartment ventilator had to be turned on while firing at the very least but ideally the problem was addressed at its source. In the Leopard 1, Leopard 2, and Merkava series, an extractor fan was positioned next to the openings on the machine gun receiver. In the M2 Bradley, the coaxial M240C was mounted outside the turret. In the M1 Abrams, the front of its M240 was enclosed in a large-diameter pipe called a "smoke tube" or "smoke box" so that the gas system would vent outside the crew compartment.
The T-72's overpressure ventilation system kicked in when the gunner pressed the trigger for the coax, but the machine gun installation itself was otherwise totally basic. The issues of gas contamination were solved in its PKT machine gun itself with special modifications from the basic Kalashnikov PK.
Like the M240, the PK had a long-stroke gas piston mechanism that vented gas from its adjustable gas regulator once the piston received enough energy to cycle the action. The gas tube behind the gas regulator had vent holes so that gasses blowing past the labyrinth grooves on the piston head would not foul up the receiver. This was fine for infantry use, but the vent holes in the gas tube meant that even if its gas regulator was sealed off, the PK was simply not suitable for tanks without a something like the Abrams' smoke tube. The PKT largely solved this with a sealed gas regulator and gas tube. Air quality tests showed that one shot fired from a PK inside an inhabited enclosure released 83 mg of carbon monoxide while a PKT released just 23 mg.
This was well within tolerable limits, but it was still much higher than expected. Of the 83 mg of carbon monoxide the PK released, 75 mg was from its gas system. The remaining 8 mg escaped from its chamber. If the PKT gas tube was completely impermeable the chamber would indeed be the only route for gasses to escape, but the lack of vent holes meant that gasses blowing past the piston ended up in the crew compartment through the receiver. In 1988, a modification was made to the PKT gas regulator so that it would once again vent, but through a small duct leading outside the turret. In its final form the PKT solved the problems of gas venting and piston blow-by without needing fans, smoke tubes, or any other equipment that could take up space in the turret.
Of course, this solution was not perfect - the PKT could not use standard PK barrels, and the T-72 carried no spare barrels for quick-swaps. In T-72 - The Definitive guide to the Soviet Workhorse, you will not only read about the T-72, but also learn the nuances of its famous PKT and NSV machine guns, never revealed before even in books dedicated to these guns.
In T-72 - The Definitive Guide to the Soviet Workhorse, you will find an in-depth examination of the T-72's design and all the ideas, technologies, and design solutions that went into its creation. It is one thing to read a list of a tank's technical specifications, but another entirely to understand how they were achieved, and what consequences followed. The book is currently 20% OFF until 27 September exclusively on Lulu Press, additionally other books the Military History Group are reduced up to 10%: militaryhistorygroup.com/
It is also available on Amazon and Barnes & Noble.
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