Why the British Chieftain didn't crawl at a snail's pace
Among domestic fans stories armored vehicles, it is still widely believed that the British танк The Chieftain is a rather slow vehicle. It's said to weigh a whopping 55 tons, and its engine, with the exception of later, specialized modifications, produces only 750 horsepower, making its power-to-weight ratio quite low.
It's traditionally contrasted with its Soviet counterparts, such as the T-62, which, according to tank racing experts, is like a cheetah in speed, while the Chieftains are like tortoises. But what's the real picture? To find out, we need to look at the results of comparative tank tests under the most challenging conditions—on rugged terrain.
These tests were conducted by Soviet researchers in the 80s, when one of the Mk.5P Chieftain tanks (the Iranian variant, where "P" stands for Persia) was delivered to the USSR as a result of the Iran-Iraq War. They showed that the tank's average speed over difficult terrain was 22,1 km/h, which was only 2,4 km/h slower than the T-62.
This relatively small difference is due not only to the difference in specific power, but also to the British tank's efficient semiautomatic gearshift control system. We offer a report on its design and operation—it's certainly outdated, but certainly of interest to lovers of the past.
Semi-automatic gear shift control system
The Chieftain Mk5P tank's transmission utilizes an electro-hydraulic semi-automatic gearshift control system. Gear shifting is performed by the driver by pressing the controller pedal (an electromechanical decision device) and is automatically downshifted by a signal from the centrifugal speed converter at the gearbox input. The system's actuators are six solenoids with hydraulic spool valves. When an electromagnet is activated, the spool valve moves, and pressurized oil is supplied to the hydraulic booster to engage the corresponding brake in the gearbox.
The driver commands gear shifting from lower to higher gears (I, II, III, IV, V, VI), permitted by the system within the engine speed range from 840 rpm to maximum, by successively pressing the toe of the driver's left foot upward on the controller pedal. Shifting from higher to lower gears (VI, V, IV, III, II, I) within the engine speed range from 1600 to 680 rpm is accomplished by pressing the pedal downward.
View of the MTO of the Chieftain tank
The selection of the specified frequency limits prevents its excessive increase (overspeeding) when shifting from higher to lower gears and an unacceptable decrease (stall) when shifting from lower to higher gears, preventing incorrect actions by the driver when shifting gears, but without limiting his choice of the moment of gear shifting, optimal in terms of traction characteristics or fuel efficiency (the choice of such a moment is usually available only to an experienced driver).
The control system also provides automatic downshifting at engine speeds below 680 rpm, preventing inadvertent engine stalling if the driver selects the wrong gear and ensuring high traction on the tracks when climbing or driving in difficult road conditions. Furthermore, when overcoming an obstacle, the driver can automatically downshift by slowing the machine.
First reverse gear can only be engaged from neutral, and the driver must first press the reverse button and then the controller pedal. This reverse engagement prevents accidental engagement of reverse while the vehicle is moving forward. Second reverse gear is engaged immediately after first gear by pressing the controller pedal again (without pressing the reverse button).
The design measures implemented in the tank's gearshift system to ensure safe and trouble-free operation are noteworthy. If the pressure in the faulty hydraulic brake system drops to 6,3 MPa, a warning light on the driver's instrument panel illuminates, warning the driver to reduce speed. If the pressure drops further to 4,2 MPa, the system automatically downshifts the engaged gear to first gear within 6 seconds, ensuring effective engine braking.
When the parking brake lever is engaged, the system automatically engages neutral, blocking all other gears. If the electric gearshift system fails, emergency engagement of second (slow) and second reverse gears is achieved via a mechanical drive to the shift valves. This interrupts the electrical circuit of the gearshift system.
To determine the current gear position and monitor the electrical system, a pointer indicator is installed on the driver's instrument panel, indicating the current gear position and neutral (IIr, Ir, N, I, II, III, IV, V, VI). This indicator is necessary because the system was developed using a command-based gearshift system. Let's look at the structural diagram of the gearshift control system (figure).
The gear shift control element is pedal 27 of controller 26, mounted at the driver's left foot. The electromagnets connected to the spool valves are activated by command signals from the pedal, converted into electric current by the controller, by signals from the centrifugal engine speed converter 22, from the brake hydraulic pressure switch 2, from the reverse button 20, and from the parking brake button 29. The centrifugal converter, kinematically connected to the gearbox input shaft, generates signals authorizing the driver to shift up or down, and signals for automatic downshifting.
The rotational speed of the weights is converted into axial movement of the rod, which acts on contact switch 23, which has three positions. In the "lower contact closed" position, the converter sends a command to the controller to downshift. In the "both contacts open" position, the driver can shift up or down. In the "upper contact closed" position, the converter sends a command to the controller to inhibit downshifting.
At normal operating pressure in the brake hydraulic system (over 4,2 MPa), spring 1 of the pressure switch is compressed, and button 3 is disengaged. When the pressure in the brake hydraulic system drops to 4,2 MPa, the spring-pressurized rod activates the button, and an electrical signal from the pressure switch is sent via cable 10 to unit 11 and then through switch block 6 and cable 21 to the controller, which generates a signal to downshift to first gear.
Limit switch 29 closes when the parking brake lever is moved to the engaged position. This triggers a command in the controller to engage neutral in the transmission. Switch 29 is a micro-pushbutton with a push-button lever, while reverse button 20 is a standard contactor. Switch 29 is protected from dust and moisture by encapsulation with a compound and a rubber seal, while button 20 is protected only by a rubber seal. The parking brake limit switch and reverse button are connected to the controller via a common cable, cable 28.
Solenoids UA1–UA6 control hydraulic spool valves that engage the transmission brakes, respectively: T1, T2, T3, Tu.p, Tz.p, Tz.x. Each gear is engaged by two brakes. The solenoids are mounted on a common plate 30, which is installed on the spool valve box. The solenoids' power circuits are routed in pipes filled with compound to reduce arcing at the controller's switching contacts. Resistors R1–R6 are connected parallel to the solenoids' windings. The solenoids, resistors, and connecting terminals are immersed in oil.
A knife-type switch 31 is located on the shared plate with the solenoids. It is mechanically connected to the emergency gear shift drive and is included in the negative power supply circuit of the solenoids. When the emergency drive engages second gear, the camshaft located above the solenoids rotates and presses the rods of solenoids UA1 and UA5. This causes the spool valves to move and the brakes Tz.p and T2 to engage. When the camshaft rotates, the knife-type switch interrupts the negative power supply circuit of the solenoids, preventing the simultaneous engagement of different gears by the electric and mechanical drives.
Engaging the second (slow) reverse gear with the emergency drive occurs in a similar manner. Cables 24 (solenoid power supply circuits) and 17 (centrifugal converter contact circuits) are fed to junction box 12 and block 19, where they are switched to the terminals of connector 14 of cable 7, which connects junction box 12, located in the engine-transmission compartment, to electrical circuit and fuse box 11, located in the fighting compartment. In this block, circuits from the engine-transmission compartment from the pressure relay, as well as the power supply and system activation circuits from fuse board 4, are fed to block 6, switched to the terminals of connector 16, and then fed via cable 15 to the controller.
Control compartment. The gear shift pedal is visible (in the center).
Power is supplied to the control system from the GB (+) and NEG (–) terminals of the fuse board through LC 5 filters to terminal 6. The control system is activated when the main engine is started, at the moment the engine idle speed solenoid is engaged. The signal to activate the solenoid from the engine starter control panel 8 is sent via cable 9 to the driver's instrument cluster 18, from where it is fed via the GB ON circuit through cable 15, block 11, and cable 21 to the controller.
The gear indicator 19 is an electromagnetic device with stator windings connected in a star formation, and a rotor consisting of a ring magnet oriented by a stationary, closely spaced permanent magnet in a single position. When specific signals are applied to the stator windings in combinations corresponding to the engaged gears, the device's pointer rotates, indicating the engaged gear. The gear indicator is connected to the controller via a separate cable 25. The layout of the electrical equipment in the tank's control system allows access to any component, its disconnection, and individual testing.
If necessary, the controller, centrifugal converter, solenoids, and other components can be removed and replaced. Removing the covers of the "Electrical Circuits and Fuses" block allows access to the input and output circuits of the controller, pressure switch, and speed converter, located on terminal block 6. This allows you to check all incoming signals and verify the integrity of the supply cables.
Removing the cover of junction box 12 provides access to the centrifugal converter circuits, solenoids, and cables located in the engine-transmission compartment. All cables in this system are shielded, and the connectors are sealed.
Conclusions
1. The electro-hydraulic semi-automatic gear shift control system used on the Chieftain Mk.5P tank ensures fast and convenient gear shifting by the driver, prevents engine overspeeding and spontaneous engine stalling, automatically lowers the gear number when the engine speed drops, and engages first gear when the pressure in the brake system drops.
2. Worthy of attention are such technical solutions as the combination of the functions of measurement and generation of control signals, carried out by a centrifugal converter of engine speed, as well as the combination of the functions of the decision device and the gear shift element (pedal), carried out by the controller.
Source:
"Semi-automatic gear shift control system." E.I. Karnachev, V.F. Lobasenko. Scientific and technical collection "Problems of Defense Equipment," series VI, issue 6 (112). Declassified by the expert commission of the Federal State Autonomous Educational Institution of Higher Education "SPbPU" on November 23, 2016, act No. 2.
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