In order to steer a tracked vehicle, it is necessary to drive one track faster than the other, causing the vehicle to turn toward the slower track. This is called "skid steering" or "differential steering". While the theory is simple, itís execution is not.
A steering transmission must, in addition to steering the vehicle, be easy to use. Most fast track-layers are tanks: incredibly heavy, powerful, and expensive machines that are operated by teenage recruits with limited experience, at night without lights, over rough and unfamiliar ground, with extremely limited visionÖ not to mention under fire. Thus, whatever steering mechanism is used, it has to be fairly simple to operate.
It must also be efficient. Any inefficiency produces waste heat, usually from the friction in a slipping clutch or brake. Since a tracked vehicle implies a heavy vehicle, which in turn implies a powerful engine, inefficiency can produce problematic amounts of waste heat. For example, a 1500 hp engine running a 50% efficient transmission will produce 560,000 watts of waste heat. All this heat has to be extracted from the vehicle before it causes problems. Furthermore, all that energy is no longer available for driving the vehicle, meaning the engine ... and fuel tank ... could be made much smaller if the transmission were more efficient.
Both of these problems are significantly less for slow track-layers, such a bulldozers, but they still apply.
The simplest way to achieve this is to drive each track with a separate power source. The earliest tracked vehicles, including the Holt Tractor of the late 1800ís and the Whippet, a WW I British Light Tank, operated on this principle. However, it has its drawbacks.
First of all, it requires two engines. In the case of a steam powered tractor, this isnít such a problem, since the "engine" is only a small part of the whole, and two (or more) can, and often do, share a single boiler. In the case of internal combustion engines, however, it requires two complete engines, with all the attendant extra weight, complexity, and maintenance headaches. In this case, two engines does not result in twice the reliability, but half the reliability, as if either engine fails, the vehicle is effectively immobilized and capable only of spinning in circles.
A second problem is that it becomes difficult to drive in a straight line. Track speed is a function of power and ground drag ... while it is possible to coordinate two engines such that they produce the same amount of power, it is highly unlikely that each track will experience the same amount of drag, and as a result the tracks will turn at slightly different speeds. Moreover, the drag on each track will be constantly changing, making continual adjustments necessary.
However, driving each track with a separate power source has one advantage ... by reversing one engine, it is possible for the vehicle to turn in place, called a "pirouette" or a "neutral turn". The advantages in maneuverability are obvious.
The difficulty in traveling in a straight line is minor at low speed, so modern slow track-layers like bulldozers often use this system, though slightly modified ... with a single engine driving a hydraulic pump, and hydraulic motors driving each track. Many remote controlled tracked toys use separate electric motors, as this system is very cheap and simple to build.
Far less complicated (as it only requires one engine) is the Clutch-Brake system, where the output of a single power source drives both tracks directly. Since they are physically connected to each other, the tracks must turn at the same speed and the vehicle will travel in a straight line. To allow for turns, each track can be disconnected from the engine with a clutch, allowing that track to slow and the vehicle to turn fairly gently ... a "free turn". A brake allows the disengaged track to be slowed to tighten the turn, even to the point of stopping the track so the vehicle turns in a very tight radius ... a "braked turn".
This system is fairly simple and easy to drive, and most of the tanks of the First World War used this method of steering. However, it is not very efficient. Braking one track slows the vehicle, and wastes a large portion of the power produced by the engine to be converted into heat. While this is not a significant problem in a small vehicle, a large vehicle with a large engine can produce a tremendous amount of heat in a very short time. Braking one track also slows the vehicle down significantly, which is a consideration in military vehicles where speed is paramount.
It is also a bit unpredictable in steering. The braking force vs. yaw curve is basically flat, meaning that a tiny change in braking force can result in a huge change in the rate of turn. If the clutch to one track is disengaged while climbing a very steep slope, the turn it produces may be quite dramatic even without the application of the brake. While descending a very steep slope, the turn may even be in the wrong direction!
Lastly, the Clutch-Brake system does not allow a vehicle to execute a neutral turn.
One solution that solved both the inefficiency problem and the unpredictability in steering problem is that of Geared Steering. In this design, a single engine is connected to the tracks through separate transmissions, and the vehicle is steered by selecting different gears for each track. For example, driving the left track in 1st gear and the right track in 2nd gear would result in the left track turning more slowly and a left turn. If the gear ratios are not even multiples, a great number of turning radii can be achieved, and if each transmission includes a reverse gear, then a neutral turn can be accomplished.
This system requires great driver skill, but is efficient and not terribly complex. One minor drawback is that the vehicle cannot be steered while in 1st gear.
The Clutch-Brake Drive can be simplified somewhat by driving the two tracks through a differential and eliminating the clutches. The result is the Braked Differential, wherein as one track was slowed, the differential gears rotate and speed up the other track.
This system is even less efficient than the Clutch-Brake system, in that the brake not only dissipates the energy of the slowed track, but also part of the power of the engine. It also leads back to the problem of straight-line travel, and the system was even less predictable in steering. However, it is extremely simple, and can be based around a conventional automotive rear axle and brakes.
Again, the braked differential system does not allow a neutral turn.
The "Clectrac" system was introduced by the Cleveland Tractor Company in 1921. In this system, a differential is designed such that by applying a brake, the differential can be forced to rotate at a given rate in proportion to speed. This system produces a turn of a single fixed radius, but is quite efficient. By slipping the brake, turns of greater radius can be achieved, but the efficiency advantage is lost.
This system could, like the Clutch-Brake or Braked Differential systems, "self-steer". However, as the brake force vs. yaw curve was fairly direct, it was easy to control.
This system was widely used in military vehicles during WW II.
The Maybach system is a refinement of the Clectrac system, wherein instead of the differential gears precessing around a fixed gear, the differential gears precess around a gear driven at a speed proportional to engine speed. The result is a different radius turn for each forward gear.
This system is also capable of neutral turns, by leaving the main gearbox in neutral (hence the name "neutral turn"), and rotating the differential gears.
This system was used by the Panther, often considered the best tank of WW II. It had seven forward gears, and thus seven turning radii, not including a neutral turn.
A further refinement of the Maybach system, developed by Major Wilson of the UK in 1928. This involves adding a second transmission between the steering input of the transmission and the engine. This allows the differential gears to be driven at several different rates in proportion to engine speed, and thus provides a multitude of turning radii, each of which is in turn proportional to the selected forward gear.
Furthermore, it is not subtractive like the Maybach transmission, as while it slows one track, it speeds up the other track.
This is essentially the forerunner of all modern fast track-layer steering transmissions, as it has all the essential characteristics: it is efficient, will not self-steer, allows multiple turning radii, and allows a neutral turn.
Using a hydraulic motor or electric motor to drive the steering shaft, instead of a fixed ratio transmission, allows for any turning radius at all, including zero (a neutral turn). While this is an improvement in some ways, it is a loss in others, due to the modest efficiency of electric and hydraulic systems. This often leads designers to prefer a limited number of turning radii rather than having to design for the waste heat generated.
I've created an animation of this transmission (AVI format, 16 seconds, 7 meg). It shows the vehicle traveling straight (both output shafts turning at 3.6 degrees per frame), turning right (left 6.0°/f, right 1.2°/f), turning left (same but opposite), and performing a left neutral turn (left -2.4°/f, right +2.4°/f). Watch the input and output shafts carefully. This design uses normal differentials instead of the planetary differentials of the diagram above.
Adding a third differential produces a transmission that is, in a sense, a double differential transmission with a braked differential transmission for steering.
Virtually all modern fast track-layers use triple differential steering.
Here is an animation of this transmission (AVI format, 8 seconds, 4 meg) which shows the vehicle traveling straight (both output shafts rotating at 2.8°/f) and turning left (left 2.0°/f, right 3.6°/f). Again, it uses normal differentials instead of the planetary differentials used in the diagram.
I am not, however, sure I fully understand this transmission. My research indicates that both input shafts are driven by the engine, but I can't for the life of me see how it can perform a neutral turn unless the input shafts are driven independantly. I cannot conceive that modern tanks would forgo the advantage of being able to perform a neutral turn.
Allow me to introduce an idea of my own. (Patent Pending!) This is basically a differential with the two output shafts, one reversed by an idler gear, geared together through a toroidal Continuously Variable Transmission (CVT). Changing the ratio of the CVT will steer the vehicle.
When the ratio of the CVT is 1:1, the two output shafts will rotate at the same speed and the vehicle will travel in a straight line. If the ratio of the CVT deviates from 1:1, the shafts will travel at different speeds, resulting in a turn. The CVT itself will only have to vary between 1:1 and 2:Ĺ (or 4:1 if you prefer) to produce a turn that is very close to a braked turn. The ratio of the CVT will, in fact, determine the radius of the turn, by determining the ratio of the radii of the paths of the two tracks. At 2:Ĺ, the inner track will describe a turn radii of one-quarter the radii of the outer track; or, the radii of the turn (measured at the inner track) will be a third of the width of the vehicle. Close enough to a braked turn.
Unfortunately, the system does not allow for neutral turns.
It takes considerable power to steer a tracked vehicle. As the vehicle turns, the leading and trailing ends of the footprint, or contact patch, skid sideways, perpendicular to the direction the tracks roll. Hence the name "skid steering".
The worst-case scenario is the neutral turn, which may well require as much power to execute as to travel at full speed. Turns of greater radii will require less power, as the energy required to overcome friction is spread out over a longer period of time.
In this diagram, the arrows indicate the direction in which the contact patch will move during a right (clockwise) neutral turn. As you can see, the further toward the ends, the more the track will move in a direction other than the direction in which it would normally move, i.e. forward, or upward in this diagram.
The second diagram shows the magnitude of the frictional forces that must be overcome in order to make the vehicle turn about its vertical axis. These are simply the horizontal component of the direction that each point of the contact patch will move as the vehicle rotates. As you can see, the friction at any point is proportional to the distance forward of the vertical axis. From this it follows that the total force required is proportional to the length of the contact patch, the weight of the vehicle, and the inverse of the radius of the turn.
Longer, narrower vehicles resist turning more than shorter wider vehicles. To an extent, the drag can be minimized by concentrating the weight on the middle of the contact patch, where the sliding movement is smallest.
© 2003 W. E. Johns