Notes on differential steering
‘Everything you always wanted to know of differential steering but were afraid to ask.’
I would like to share the following wisdom with you, just as Mike Tull and Trevor Ford of the Bovington tank museum shared with me.
Rolls Royce was involved in developing
a tank transmission during WWII, using their aircraft engines. What follows is
an abstract of the paper they did on the subject;
Resistance to motion. A tank differs from wheeled road vehicles in that most of the
resistance to motion is in the form of tractive resistance, which unlike
windage (aerodynamic) losses, does not vary greatly with speed. The tractive
resistance does, however vary greatly with the nature of the ground. Thus, a
tank which achieves maximum speed in top gear on the road, may only achieve perhaps 80% of this speed
on grassland but in a lower gear.
Steering. The accepted method of steering a tank is to somehow make the inner track run more slowly than the outer track. The surprising thing is the large magnatude of the force required to do this. In order to persuade the tank to deviate from a straight course, it is necessary for the tracks to exert a force, in opposite directions, of up to three times that of normal top gear tractive effort. Turns are therefore almost always executed at full or near full engine power.
There are various methods of steering tanks and the following is a brief description of the characteristics of these systems.
1 Single differential. To steer the tank, the brake on the inner track is applied to produce the desired resistance to motion of that track. This brake has to absorb half the engine output in addition to the energy from the inner track. Although the single differential is the simplest form of steering mechanism it is also by far the most wasteful of power. If the engine cannot provide the extra power, the tank will slow dramatically and can even stall the engine. Differential When the brake is applied to make a turn it dissipates (wastes) the energy from the inside track as well as half of the engine power. However, if the brake is used to lock the inside track, then all of the engine power is transferred to the outside track and the tank rotates about the inside track. The losses will reduce to levels similar to that of clutch/ brake steering i.e. a 25% reduction in wasted power. Steering by locking one track can obviously only be used at low speeds.
2 Clutch and brake. This method differs from the first in that the inner track is disconnected from the drive during the turns and it's brake only has to absorb the work done in slowing the inside track. All the engine power is directed to the outer track. It is estimated that the Clutch/ Brake method is 25% less wasteful of energy than the single differential. Power is only applied to one track so the vehicle also slows significantly during the turn.
3 Double Gearbox, geared differential (Cletec). These types drive each track with a different gear ratio. Turns are generally fixed radius. It is estimated that the double gearbox/ geared differential method is 50% less wasteful of energy than the single differential.
4 Double, triple, controlled differential. All these are similar in principle, all have a separate propulsion power and steering power inputs to the transmission. All are fully regenerative, which means that the power required to slow the inside track is transferred to speed up the outer track and is not wasted in a brake. The steering power may be derived from the main engine or from an independent source.
In consequence the power wasted in this class of controlled differential is very small and is estimated to by more than 75% less wasteful of energy than the single differential.
know that to steer a tank, the two tracks need to be driven at slightly
different speeds. We also know that considerable extra power (up to
three times) is required to skid the tracks during a turn. This extra power can
be given to or can be extracted from the individual tracks.
We know that all braked steering methods waste the power available from from the inside track , it is absorbed as heat by the track brake. These inefficient braked systems therefore need far more engine power to compensate. We also know that the simple braked differential is even more inefficient and wasteful of power, not only is the power from the inside track wasted in the brake, but the brake must also absorb half of the applied engine power. A braked differential system requires even more extra power than the simple braked system and so is not really a viable solution.
An efficient steering system (the double differential system) can been designed which extracts (the term is regenerates) the power from the slower moving inside track and applies it to the faster moving outside track. Only a small amount of extra power (10%) is required to provide the steering input. In other words, a double differential transmission system requires the least amount of engine power of all the candidate systems, while providing excellent proportional steering control. This is why this form of steering is today the most commonly used method of steering a tracked vehicle. Furthermore, in a model tank, the extra steering power can be provided by a small variable speed electric motor.
Royce went on to build a dedicated factory to build the Meteor tank engine. The
used lots of cast iron to replace aluminium (precious metal in wartime) used
2nd quality Merlin aircraft parts, recycled parts from early marks of Merlin
engines. Whatever it took to get them out of the door.
Meantime Rover were getting deeply involved with Frank Whittle and the gas turbine (at the Governments insistence) and had also built dedicated factories for development and mass production. Rover did not like working with Frank Whittle (few did) and did not like the added quality/ reliability problems of aircraft engines. RR for there part thought Tank engines were beneath them and really wanted to get into the jet age big-time. So they simply swapped their factories and products. RR were prepared to put all their engineering talent and resources to invest into the jets. Rover were quite happy screwing together the Meteor tank engines from what ever they could find. RR simply ignored Frank Whittle and did it their way.
When Hives, the boss of RR, was told the gas turbine was basically a very simple engine: he replied that he had a design team who would quickly design the 'simple' out of the engine and make it produce some real power.