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Monochromelody

Tank neutral steer capability

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The neutral steer, or pivot steer, means the tank drive one track forward while the other track backward, to perform a turn-on-spot. 

The very first tank with neutral steer, believe it or not, is the German A7V in WWI. 

A7V_6.jpg

Here is the transmission of A7V. 

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The transmission incorporate two independent gearbox, each connected to a 100-hp engine. The gearbox use clash gears to select speed, and bevel gear to select forward or reverse. It's easy to perform a neutral steer. 

 

Those tanks with electric transmission(St.Chamond, Porsche Tiger, etc.) can easily perform neutral steer, for their twin driving electric motors can rotate in opposite direction. And tanks with twin driving hydraulic motors(Panzer IV mit Hydrostatischem Antrieb) can neutral steer in a similar manner. 

 

As mentioned above, the simplest way to neutral steer is to have seperated gearboxes or sub-transmissions driving both tracks. US airborne tank M551 Sherridan and Ukraine T-84 Oplot are modern approach of this manner. Althought they neutral steer with tracks driving in different speed, thus they cannot pivot steer precisely on the central spot. 

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The neutral steer capability is more common on dual-flow transmission. 

A dual-flow transmission, or “双流传动” in Chinese, means the power flow from engine into the transmission splitted into driving power flow and steering power flow, then they join together with mesh gears or planetaries. Control these power flows allows the driver to change speed and steer left or right. 

 

The dual-flow principle itself came up even earlier than the tank. The 1899 Vedovelli Priestley electric taxi equipped with double differential steering system, is capable of neutral steering. 

Industrie_electrique_1899_cab_electrique

And the first tank with dual-flow transmission is the Schneider-Renault SRB, then evolved into the Char B1 heavy tank. 

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The Char B1 tank's dual-flow principle is quite simple. The power splitted into two: the driving power flow into gearbox and drive the main differential; the steering power flow into the appareil Naëder, a hydrostatic pump-motor assembly, can rotate on different speed and direction. 

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Char B1 can use appareil Naëder to control its hull howitzer precisly, this principle inspired the Swedish Strv 103 tank. And the US M1 Abrams tanks use a HSU(Hydrostatic Steering Unit) based on the same principle of appareil Naëder. 

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(To be continued)

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A lesser known fact is that certain soviet AFVs can also neutral steer, for example the MTLB and all its derivatives, and everything based on the GM-123 chassis, like SP Artillery or certain elements of the 2K11 Krug SAM system.

Here a beautifully restored 2S3M, at 1:55

 

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2 hours ago, heretic88 said:

A lesser known fact is that certain soviet AFVs can also neutral steer, for example the MTLB and all its derivatives, and everything based on the GM-123 chassis, like SP Artillery or certain elements of the 2K11 Krug SAM system.

Here a beautifully restored 2S3M, at 1:55

 

Yes, Soviet tanks started to have dual-flow transmission after WWII, like Объект 770 heavy tank, Объект 906 amphibious tank and so on. 

The МТ-ЛБ transmission is similar to АТ-Л artillery tractor's transmission, which is also dual-flow transmission. 

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13 hours ago, Collimatrix said:

Interesting that these tracked AFVs have double or triple differential steering systems, but none of the Russian MBTs do.

T-14 Armata uses double differential steering systems and is able to neutral steer. 

Some Russian prototype MBTs can also perform neutral steer, like T-80UM-1 Bars and T-80UM-2 Black Eagle. 

The first Russian tank with neutral steering is Object 770 experimental heavy tank.

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Another approach to neutral steering is the triple differential steering system. 

Henry Edward Merritt, an engineer and gearbox designer, is famous for his unique invention, Merritt triple differential transmission. The first British mass production tank with triple differential transmission is the Churchill tank. 

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The Churchill tanks have a Merritt-Brown H4 gearbox, 4 stands for four gears forward, H stands for horizontal, because the two rows of gears in the gearbox are horizontally located. 

A great advantage of Merritt's design is compactness. The transmission incorporate the range gears and steering mechanism into a single box, thus reduce the longitude size. The H4 transmission can provide four gears forward, one gear reverse, and a neutral position for neutral steer.

The basic design itself is simple, but how to explain it could be a little bit difficult. Three differentials are arranged like this: 

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The left and right differentials are used for combining the driving power flow and steering power flow, and the steering differential is used for creating different speed of the steering gears. The bevel pinion gears are not easy to produce, so the design transformed into cylindrical gears. 

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↑The Z5 gearbox on Cromwell cruiser tank

When driving forward, the sun wheel and ring gear of planetary F1 and F2 always counter rotate. The carrier of F1 and F2 rotate on an average speed of the ring gear(forward) and the sun wheel(reverse), and because the ring gear rotates faster, the carrier rotates on the same direction with the ring gear, so the vehicle goes forward. 

When steering brake J1 actuated, the sun wheel of F1 is stopped, now the carrier of F1 accelerate. The steering differential C then drives the sun wheel of F2 back rotate even faster(double time), so the carrier of F2 decelerate.

When in a Cromwell/Centurion/Chieftain/Scorpion, the driver pulls right steering lever, which actuated the left steering brake, makes the track on left side go faster, the track on right side go slower, and the vehicle turns right. 

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Another important design feature of the H4/Z5 gearbox is the reverse gear. When reverse gear is selected, the main shaft(E) of the gearbox is locked and stopped. The main shaft is connected to the F1/F2 ring gear, so the ring gear is brought to a stop. F1/F2 sun wheel going reverse, driving the planetary carrier reverse. This design has advantages and disadvanteges. The advantage is that the gearbox doesn't need to insert a idler wheel between range gears to change the driving direction, thus ease the operation of gear changing. The disadvantage is that the reverse gear is very slow, due to the large reduction ratio. The reverse gear ratio of Z5 gearbox is approximately 22.894, makes the tank's reverse speed very slow. 

 

When the vehicle is stopped on a solid plain terrain, and neutral position is selected, the planetary carrier stay still.  The sun wheels go reverse, bring the ring gear and main shaft rotates forward. Pull left steering lever, right sun wheel is stopped and forward-rotating ring gear bring the planetary carrier slowly forward, so the right track goes forward. The left sun wheel reverse double-time and faster than forward-rotating ring gear, bring the planetary carrier slowly reverse, thus the vehicle pivot in place. 

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The US CD-850 is another instance for triple differential steering system. On the British Merritt-Brown, when the vehicle goes forward, the sun wheel and ring gear of the planetary always counter rotate. On CD-850, when the vehicle goes forward, the sun wheel and ring gear rotate toward the same direction. The neutral steering of CD-850 is similar to the Merritt-Brown system. 

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11 hours ago, Monochromelody said:

T-14 Armata uses double differential steering systems and is able to neutral steer. 

Some Russian prototype MBTs can also perform neutral steer, like T-80UM-1 Bars and T-80UM-2 Black Eagle. 

The first Russian tank with neutral steering is Object 770 experimental heavy tank.

 

IS-4 could neutral steer.

 

 

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On 10/25/2018 at 12:19 AM, Collimatrix said:

 

IS-4 could neutral steer.

 

 

Nicholas Noran a.k.a. "Chieftain" mentioned about this in his program "Chieftain's hatch", but it was a mistake. 

The ИС-4s use a ЗК-steering mechanism, named after its designer.

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Г. И. Зайчик(1905-1977)

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М. К. Кристи(1890-1965)

According to the ИС-4 heavy tank operator's manual(Руководство по эксплоатации тяжёлого танка ИС-4), published by Soviet Red Army in 1949, the tank could pivot on one track when in neutral position. 

Pivot on spot is different from pivot on one track. 

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R=B/2 is pivot on spot, R=B is pivot on one track. 

Some historical document films also shows ИС-4 pivot on one track. 

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