MECHANICAL CHARACTERISTICS OF TANKS
The following very brief account of the mechanical characteristics of
tanks, it is hoped, will prove sufficiently simple and complete to give
to the non-mechanical reader some idea of the tank as a machine.
THE MARK I TANK (see Frontispiece)
The first British tank made, and to be used, was the heavy machine,
already described in the previous chapter, the Mark I tank, the general
outline of which remained the standard design for the hulls of all
British heavy machines up to the end of the war. As will be shown
later, many mechanical improvements, making for higher efficiency and
greater simplicity of control, were introduced from time to time, but
the fact remains that the profile of the Mark V tank of 1918 was to
all intents and purposes that of the Mark I of 1916, and surely this
is a striking tribute to the genius of the designers who, without much
previous data upon which to base their work, produced the parent weapon.
It is not proposed here to enter upon the general arrangement of the
Mark I tank, but reference to two important points in design is of
interest. The first is that this machine was fitted with a “tail,”
consisting of a pair of heavy large-diameter wheels, mounted at the
rear of the machine upon a carriage, which was pivoted to the hull in
such a manner that the wheels were free to follow the varying contours
of the ground. A number of strong springs normally kept the wheels
bearing heavily upon the ground, whilst a hydraulic ram, operated by an
oil pump driven from the engine, was intended to enable the carriage to
be rocked upon its axis, in order to raise the wheels well clear of
the ground on occasions when it was necessary to “swing” the tank.
The object of this “tail” device was to provide means of steering the
machine and, to this end, the driver was provided with a steering
wheel which, operating a wire cable over a bobbin or drum, altered the
path of the “tail” wheels, and allowed the tank to be steered, under
favourable conditions, through a circle having a diameter of about
sixty yards. The disadvantages of this fitting far outweighed any
virtues it may have possessed. Countless troubles were experienced with
the ram and its pump; the wire steering cable was constantly stretching
or slipping through the bobbin, thus affecting the “lock” of the tail
wheels; the driver was subjected to great physical strain in overcoming
the tremendous resistance offered by the road wheels; the whole device
was very prone to be damaged by shell-fire in action. Against these
indictments should be recorded the fact that the possession of a
“tail” enabled the Mark I tank successfully to span and cross a wider
trench than the later “tailless” machines of the same dimensions could
negotiate, owing to the fact that as the wheels supported the rear
of the tank over the point of balance, the risk of “tail dive” was
considerably reduced. However, at the close of the operations of 1916,
all tanks were shorn of their tails, and no subsequent models were
fitted with them.
The second point of interest regarding this early machine relates to
its sponsons. These, on both the male and female machines (armed with
full length 6-pounder and Vickers machine-guns respectively) were
planted upon and bolted to the walls of the hull and, for entraining
purposes, these had to be removed and carried upon special trolleys
which could be towed behind the tanks. It will readily be seen that
this arrangement involved a considerable amount of labour, and rendered
the process of entraining an extremely lengthy one; this led to an
improved form of sponson being produced for the Mark IV machine.
The chief outstanding weaknesses of the Mark I machine, disclosed
during the first appearance in action, were:
That the engine was provided with no silencer, consequently the noise,
sparks, and even flames, which proceeded from the open exhaust pipes,
passing through the roof of the tank, constituted a grave danger during
the latter stages of an approach march. Many ingenious tank crews
fitted to their machines crude types of silencers made out of oil
drums, or adopted the plan of damping out the sparks by using wet sacks
in relays, or covering the exhaust pipe with clay and mud.
That the observation from the inside of the machine was bad, and
efficient fire control was, therefore, impossible.
That the means provided for entering and leaving the tank were
unsatisfactory, and, in the case of the female machine, speedy
evacuation in the event of fire was difficult.
That the whole of the petrol supply was carried inside the machine, and
in a vulnerable position--a circumstance which added to the risk of
fire in the event of a hit in the petrol tank by armour-piercing bullet
or shell. Furthermore, gravity was the only means for transferring
petrol from the main petrol tanks in the front of the machine to
the carburetter, and, therefore, it frequently happened that when a
tank “ditched” nose downwards, the petrol supply was cut off, and
consequently the dangerous practice of “hand-feeding” had to be
resorted to.
THE MARK II AND MARK III TANKS
These machines were produced in small numbers, and their difference
from Mark I lay in various minor improvements, none of a radical nature.
THE MARK IV TANK (Plate I--see p. 26)
In 1917 this tank became the standard fighting machine of the Tank
Corps, and it was used in battle throughout this year and the
following. As already stated, in outline it corresponded so closely
with the Mark I machine that a study here of the main features of this
tank will serve generally as an illustration of what had taken place in
tank development up to this date.
The machine was 26 ft. 5 in. long over all, whilst the width of the
female over its sponsons was 10 ft. 6 in., and of the male, 13 ft.
6 in. The height of the machine was 8 ft. 2 in., and its weight,
equipped, was 28 tons. The armament consisted, in the case of the male,
of two 6-pounder guns and four machine-guns, and in that of the female
of six machine-guns; it was fitted with a 105 h.p. Daimler 6-cylinder
sleeve-valve engine which, at a later date, was replaced in a limited
number by one of 125 h.p. This increased power was obtained by the use
of aluminium pistons, twin carburetters, and by speeding up the engine.
Generally speaking, these engines gave very little trouble, although
somewhat under-powered for the work they had to perform. They were, it
may be added, particularly suitable from the maintenance point of view,
owing to their “fool-proof” nature, due chiefly to the absence of the
usual poppet-valve gear, with its attendant risk of maladjustment.
Power was transmitted from the engine flywheel, through a cone-type
clutch and a flexible coupling, to a two-speed and reverse gear-box,
known as the primary gear, this being under the direct control of the
driver, who could thus obtain first and second speeds, or reverse,
without other assistance.
The tail-shaft from the gear-box carried a worm which drove the
crown wheel of a large reduction gear, this gear also serving as a
differential to enable the track driving wheels to rotate at different
speeds, as when steering the tank on its track brakes. A device was
provided, under the driver’s control, for locking the differential
when it was desired to steer a dead-ahead course, or when negotiating
a trench or other obstacle. With the differential locked, the gear
became, so to speak, “solid” and obviated the risk of one of the tracks
slipping in bad ground, a condition very apt to cause a tank to slip
sideways into a trench and become ditched.
Some trouble was caused through breakages of this locking muff in the
earlier days, but latterly the arrangement was considerably improved
and strengthened.
The gear-box tail-shaft terminated in a brake drum, the band of which
was operated by means of a pedal at the driver’s foot. It may be of
interest to point out here that the whole of the items so far referred
to, _i.e._ engine, gear-box, and differential, formed the standard
power unit of the pre-war Foster-Daimler tractor, and thus provided a
known quantity around which the rest of the detail was designed. This
greatly facilitated production.
On either side of the differential case projected cross-shafts, the
outer ends of which were supported in bearings mounted upon the
outside wall of the tank, and, between the inner and outer walls of
the hull, two sliding pinions were carried on a splined portion of the
cross-shaft, one pair of pinions on each of the right and left hand
sides of the tank.
In describing the remainder of the transmission, it will suffice to
deal only with one side of the machine, the detail on either side being
identical.
The sliding pinions, already alluded to, were operated by means of
short levers by two gearsmen, whose sole duty it was to assist the
driver, who signalled to them his requirements from his seat in the
front of the tank, the two gearsmen being accommodated towards the rear
of the machine on seats placed over the primary gear-box. The sliding
pinions were of two sizes, known as the high-speed and low-speed
pinions, and immediately in their rear was mounted another pinion
assembly, also carrying two gear-wheels of different dimensions, with
which the sliding wheels could be engaged at will--in other words,
on each side of the tank there existed what were known as secondary
gear-boxes, each offering a choice of two speeds.
Thus it will be seen that the whole arrangement provided a range of
four speeds. Assuming the secondary gears to be at “low,” the driver
had the option of using either first or second speed by manipulating
the control to the primary gear-box, whilst in order to obtain third
or fourth (top) speed it was necessary for him to signal the gearsmen
to alter their gears to “high,” and to assist them in the process by
a great deal of intelligent clutch work. It need hardly be pointed
out that this arrangement was exceedingly clumsy, and often involved
much loss of time and temper. It might also be mentioned here that the
reverse gear, already alluded to, was considerably higher than the
lowest forward speed, so that there was little possibility of driving
backwards, clear of any obstruction which might have ditched the tank.
Hand-operated brakes, under the control of the tank commander in the
front of the tank, alongside the driver, were incorporated with the
secondary gear-box. These brakes, by checking one or other track,
enabled the tank to be steered in some measure with the differential
unlocked, whilst, by locking the differential and placing, say, the
right-hand secondary gear in “neutral,” the machine could be swung to
the right, practically upon its own axis, by applying the right-hand
brake. To swing to the left, the right-hand secondary gear was engaged,
the left-hand being placed in “neutral,” the differential locked and
the left-hand brake applied.
From the secondary gear-box a Coventry chain transmitted the power to
an assembly, at the rear of the hull, which carried, on either side of
the chain sprocket, two heavy pinion wheels, in constant mesh with the
final sprocket wheels, which in turn, engaging with the links of the
track plates, drove the hull along the track.
Each track was composed, normally, of ninety plates or road shoes,
the separate plates being coupled together by means of links (two per
plate) and link pins, the links themselves being recessed so as to
engage with the driving wheels as shown above.
The weight of the machine was carried upon the track by means of
rollers, whilst the track was supported on the top of the hull by skids
or rails.
Adjustment of track was effected by the movement of an “idler” wheel,
which guided the track over the nose of the hull.
Refinements to the transmission were introduced in the shape of guards
to protect the driving chains from mud, and also means were provided
to lubricate the secondary gear-wheels with oil. It is recalled that,
prior to the introduction of the chain-guard, the inside deck of the
tanks was often covered with a layer of liquid mud, several inches
deep, carried in by the chains, and delivered through the secondary
gears.
Petrol was supplied to the engine in the earlier days of the Mark IV
machine by a pressure-fed system which gave a great deal of trouble,
and, being also considered dangerous, was finally discarded in favour
of the Autovac system, which sucked the fuel from the main supply in
a tank outside of the machine and delivered it to the carburetter by
gravity.
Cooling of the engine was primarily effected by a copper envelope
radiator, which gave some trouble and was finally superseded by a
tubular type.
An efficient silencer, with a long exhaust pipe carried right to the
rear of the machine, considerably reduced engine noise and rendered the
approach march a far less hazardous undertaking than was the case with
the earlier models.
Sponsons were designed to collapse into the interior of the machine
when necessary, and the cumbersome practice of detaching them from the
hull came to an end. Short 6-pounder guns were introduced to render
this change possible.
Detachable “spuds,” to provide a grip for the tracks on difficult soil,
were first introduced for this machine, as also was a highly efficient
unditching gear. The latter consisted of a beam, rather longer than the
overall width of the tank hull, which was fastened by clips and chains
to each track, and, in passing round under the machine, actually took a
purchase from the obstruction under the belly of the tank.
Detail improvements to give easier entrance and more rapid egress in
case of emergency, as well as better and safer vision and fire control,
were also introduced.
THE MARK V TANK (Plate V--see p. 204)
With the introduction of the Mark V tank, which represents the standard
British heavy tank of to-day, great progress was made in all-round
speed, ease of manœuvre, radius of action, simplicity of control and
feasibility of observation.
The dimensions and weight of this tank were approximately the same
as those of the Mark IV, whilst the design of the hull still closely
followed the lines of the original Mark I. Equipped with the 150 h.p.
Ricardo 6-cylinder poppet-valved engine, specially designed for tank
work, the advent of the Mark V machine called for the introduction of
new courses of instruction for the personnel of the Corps, very few of
either officers or other ranks having, at this time, any experience of
the care and adjustment of the valve gear.
This Ricardo engine, of somewhat unorthodox design, was highly
efficient and, with proper care and attention, gave very little
trouble. From the engine, power was transmitted through a plate clutch
in the flywheel to a four-speed gear-box, immediately in rear of
which was the reverse gear, providing “reverse” _on all speeds_. The
cross-shaft, incorporated with the reverse gear, carried at either end
(in the same relative position as the secondary gears, explained in
dealing with the Mark IV machine) an epicyclic gear. It is not within
the scope of this chapter to describe this gear in detail, but it may
be regarded as serving the double purpose of a reduction gear and
clutch, combined in one unit.
From these epicyclic gears, the transmission of the drive through to
the tracks followed the principle of the Mark IV machine, except that
there was no second-line pinion assembly as in the secondary gear of
the earlier tank, the Coventry chain on the Mark V passing direct from
the single-unit epicyclic gear to the pinion assembly operating the
track driving wheels.
All the items enumerated above were under the direct control of the
driver, who was therefore enabled to perform, single-handed, all the
operations which previously required the work of four men. Hand levers
controlled the epicyclic gears, primary gear-box, and reverse gear,
whilst the clutch and gear were foot-operated.
To steer the tank at any speed, the driver had merely to raise the
epicyclic gear lever on the side on which he wished to turn. This had
the effect of interrupting the drive to that track, so that, being
driven by the remaining track, the machine would turn upon the “idle”
side.
Where a sharp “swing” was necessary, application of the foot brake
would automatically check the “idle” track, this being allowed for by
means of a single compensating link motion with which the controls were
interconnected.
The engine was petrol-fed by the autovac system, as fitted to the
later Mark IV machines. Cooling of the engine was effected by means
of a tubular type radiator, the water therein being itself cooled by
air drawn from outside the tank, through louvres in the left-hand
wall of the hull, and finally expelled through similar louvres in the
right-hand wall.
Further, the engine was completely enclosed in a sheet-iron casing,
from which the hot foul air was exhausted through the roof of the tank
by means of a Keith fan.
The Mark V armament corresponded with that of the Mark IV, whilst the
sponsons were of similar design to those fitted to the latter type.
The absence of the large differential gear, as fitted on the earlier
models, gave accommodation for a machine-gun in the rear wall of the
tank, and also allowed for large entrance doors in the back portion of
the roof.
A greatly improved type of rear cab was fitted, and thus provided
excellent all-round vision, and also rendered possible the fitting of
the unditching beam to the tracks from the inside of the machine. This
was accomplished through the side flaps of the rear cab of the Mark V,
whereas on previous models it had been necessary for members of the
crew to expose themselves to hostile fire, in the event of the tank
becoming ditched in action, as the beam could only be attached and
detached from outside.
CHARACTERISTICS OF BRITISH TANKS USED DURING THE GREAT WAR
--------------------+-------------------+-------------------+
| Mark I. | Mark IV. |
Characteristics. +---------+---------+---------+---------+
| Male. | Female. | Male. | Female. |
--------------------+---------+---------+---------+---------+
Length with Tail | 32′ 6″ | 32′ 6″ | | |
| | | | |
Length without Tail | 26′ 5″ | 26′ 5″ | 26′5″ | 26′5″ |
| | | | |
Width | 13′ 9″ | 13′ 9″ | 13′6″ | 10′6″ |
| | | | |
Height | 8′05″ | 8′05″ | 8′2″ | 8′2″ |
| | | | |
Weight, equipped | 28 tons | 27 tons | 28 tons | 27 tons |
| | | | |
Crew |1 officer|1 officer|1 officer|1 officer|
| 7 O.R. | 7 O.R. | 7 O.R. | 7 O.R. |
| | | | |
Armament |2 6-pdrs.| 5 M.G.s |2 6-pdrs.| 6 Lewis |
| and 4 | and 1 | and 4 | guns |
|Hotchkiss|Hotchkiss| Lewis | |
| guns | gun | guns | |
| | | | |
Engine |105 h.p. |105 h.p. |105 h.p. |105 h.p. |
| Daimler | Daimler | Daimler | Daimler |
| | | | |
| | | | |
Maximum Speed | 3.7 | 3.7 | 3.7 | 3.7 |
| m.p.h. | m.p.h. | m.p.h. | m.p.h. |
| | | | |
Average Speed | 2.0 | 2.0 | 2.0 | 2.0 |
| m.p.h. | m.p.h. | m.p.h. | m.p.h. |
| | | | |
Radius of Action | Hours | Hours | Hours | Hours |
| 6.2, | 6.2, | 7.5, | 7.5, |
|miles 12 |miles 12 |miles 15 |miles 15 |
| | | | |
Spanning Power | 11′6″ | 11′6″ | 10′0″ | 10′0″ |
--------------------+---------+---------+---------+---------+
--------------------+-------------------+-------------------+
| Mark V. | Mark V star. |
Characteristics. +---------+---------+---------+---------+
| Male. | Female. | Male. | Female. |
--------------------+---------+---------+---------+---------+
Length with Tail | | | | |
| | | | |
Length without Tail | 26′5″ | 26′5″ | 32′5″ | 32′5″ |
| | | | |
Width | 13′6″ | 10′6″ | 13′6″ | 10′6″ |
| | | | |
Height | 8′8″ | 8′8″ | 8′8″ | 8′8″ |
| | | | |
Weight, equipped | 29 tons | 28 tons | 33 tons | 32 tons |
| | | | |
Crew |1 officer|1 officer|1 officer|1 officer|
| 7 O.R. | 7 O.R. | 7 O.R. | 7 O.R. |
| | | | |
Armament |2 6-pdrs.| 6 |2 6-pdrs.| 6 |
| and 4 |Hotchkiss| and 4 |Hotchkiss|
|Hotchkiss| guns |Hotchkiss| guns |
| guns | | guns | |
| | | | |
Engine |150 h.p. |150 h.p. |150 h.p. |150 h.p. |
| Ricardo | Ricardo | Ricardo | Ricardo |
| | | | |
| | | | |
Maximum Speed | 4.6 | 4.8 | 4.0 | 4.0 |
| m.p.h. | m.p.h. | m.p.h. | m.p.h. |
| | | | |
Average Speed | 3.0 | 3.0 | 2.5 | 2.5 |
| m.p.h. | m.p.h. | m.p.h. | m.p.h. |
| | | | |
Radius of Action | Hours | Hours | Hours | Hours |
| 9.0, | 9.0, | 7.5, | 7.5, |
|miles 25 |miles 25 |miles 18 |miles 18 |
| | | | |
Spanning Power | 10′0″ | 10′0″ | 14′0″ | 14′0″ |
--------------------+---------+---------+---------+---------+
--------------------+---------+-----------
| Medium |
Characteristics. | Mark A. |Guncarrier.
| |
--------------------+---------+-----------
Length with Tail | | 43′0″
| |
Length without Tail | 20′0″ | 30′0″
| |
Width | 8′7″ | 11′0″
| |
Height | 9′0″ | 9′4″
| |
Weight, equipped | 14 tons | 34 tons
| |
Crew |1 officer|1 officer
| 2 O.R. | 3 O.R.
| |
Armament | 4 | 1
|Hotchkiss|Lewis
| guns | gun
| |
| |
Engine |2 Tyler, |105 h.p.
| 45 h.p. | Daimler
| each |
| |
Maximum Speed | 8.3 | 3.0
| m.p.h. | m.p.h.
| |
Average Speed | 5.0 | 1.75
| m.p.h. | m.p.h.
| |
Radius of Action | Hours | Hours
| 10.0, | 7.5,
|miles 40 |miles 15
| |
Spanning Power | 7′0″ | 11′6″
--------------------+---------+-----------
NOTE.--(i) The Mark V star tank could carry twenty men in addition to
its crew.
(ii) The Gun-carrier could carry 10 tons weight of stores.
(iii) Radii of Action are only approximate; they depend on the
nature of the ground, efficiency of the crew, etc.
THE MARK V ONE STAR TANK (Plate VII--see p. 220)
The Mark V star machine was 6 ft. longer than the Mark V, and the
weight of the male, equipped, was approximately 33 tons. There was no
change in the nature of the armament, or in the number of the crew,
which consisted of eight all told. In addition to the crew, the machine
was capable of carrying twenty to twenty-five other troops and would
cross a 14 ft. trench, as against 10 ft. for the Mark V.
The general mechanical arrangement of this tank corresponded with that
of the Mark V, the same engine and transmission system being adopted,
with the addition of a Cardan shaft between the flywheel and gear-box,
which was rendered necessary by the additional length of the machine.
The Mark V star was relatively slow to manœuvre, owing chiefly to the
amount of track-bearing surface on the ground.
THE MEDIUM MARK A OR “WHIPPET” TANK
(Plate III--see p. 176)
The Medium A tank, known also as the “Chaser” and “Whippet,” was the
British standard light-type machine, and it differed altogether from
its heavier relatives. Its weight, equipped, was about 14 tons, whilst
it was 20 ft. long, 9 ft. high, and 8 ft. 7 in. wide, carrying a crew
of three. It could attain a maximum speed of about 8·3 m.p.h., and
could span a trench approximately 7 ft. in width.
On this machine the tracks were not carried “overhead” as in the
case of the heavy tanks, but the two trackways existed as such only,
and formed the road members of what may be described as the chassis
upon which the engine-room and fighting cab were mounted. There were
no sponsons, and the tank was driven and fought from the cab at the
rear of the machine, provision being allowed for an armament of three
machine-guns.
Each machine was fitted with two 45 h.p. 4-cylinder Tyler engines with
an autovac petrol feed, and cooled by means of a tubular radiator
provided with two fans driven by chains from each crankshaft.
The power of each engine was transmitted through separate cone
clutches, leather flexible couplings, and four-speed and reverse
gear-boxes, to a casing, at the rear of the machine, containing two
worm gears.
The two worm-wheel shafts of these gears were in line, with their inner
ends nearly touching, and each carrying the keyed-on half of a jawed
coupling, one of which could be slid along at will, to engage with the
other, thus locking the two shafts together.
One of the shafts carried a friction-clutch arrangement, designed to
limit the power transmitted from one shaft to the other to about 12 h.p.
It will be seen, therefore, that either worm-wheel shaft could be
driven independently by its own engine, or the two could be locked
together so as to rotate at the same speed, driving the tank straight
ahead, provided that there was not more than 12 h.p. difference between
the developed powers of the engines. Extensions of each worm shaft
carried a band brake, as well as a fan for forcing air into the cab of
the machine.
Returning to the details of the transmission system, each cross-shaft
from the worm case terminated in a “driving chain pinion shaft,” the
outer ends of which were supported by ball bearings mounted upon
the sides of the track frames. The chain pinion carried by this
shaft transmitted the drive, through a roller chain, to the final
track-driving wheels, which, engaging with the slots in the track
links, drove the tank along the track. Each track consisted normally of
sixty-seven plates or shoes, and rollers served to support the weight
of the tank upon the track, as well as to carry the track over the top
of the trackway. Adjustment of the track was effected by movement of
the front “idler” wheel as in the case of the heavier machines.
The Whippet tank called for particular skill in driving, and a great
deal of practice was usually necessary to produce a really efficient
driver. “Stalling” of one or both engines was a common occurrence
during the earlier stages of training. Steering was effected by varying
the speed of either engine, and the radius of movement was proportional
to the difference in the speed of the two engines, this difference
being controlled by means of a steering wheel connected to the two
carburetter throttles, movement of the wheel producing acceleration of
one engine and deceleration of the other simultaneously.
THE GUN-CARRYING TANK (Plate VII--see p. 220)
Originally designed for carrying a 60-pounder gun or 6-in. howitzer and
ammunition into action, these machines during 1918 were chiefly used
for the transport of supplies across country. The engine, a 6-cylinder
105 h.p. Daimler, was placed right at the rear of the machine, and the
general lay-out of the transmission corresponded with that of the Mark
IV modified to suit the engine position, the primary and secondary
gears, etc., being mounted forward of the engine in the case of this
G.C. tank. The final drive to the track was at the rear, and exactly
followed the Mark IV practice, whilst the track itself was carried on
track frames, in this respect somewhat resembling the Medium A machine.
Four men were required to control the G.C. tank, the driver and
brakesman being separately housed in two small independent cabs mounted
one over each track towards the front of the machine, whilst the
secondary gearsmen travelled in the body of the machine.
A system of signalling by signs from driver to other members of the
crew was adopted.
Situated between the inner walls of the hull at the front of the tank
was a “skid” or platform which could be drawn out, and its front
lowered to the ground, forming an inclined runway up which the gun was
hauled, by means of a winding gear operated from the engine, to its
travelling position on the machine.
Drums for carrying ammunition for the guns were supported on platforms
over the tracks immediately in rear of the two control cabs.
The first G.C. tanks were fitted with “tails,” similar to those on the
Mark I machines, but these were later on discarded.
The above includes the brief mechanical summary of the various types of
British tanks used during the Great War, and though, undoubtedly, the
future will bring with it many improvements and may radically change
the whole form of the present-day tank, it is doubted if ever, in the
whole history of mechanics--let alone warfare, a novel machine has been
produced which has proved so efficient on first use and required in the
long run of two years of war so few changes.