Reports: Sinkage and Angular Movement of Track Links of A.F.V.s

Figure 2 - Challenger

 

Figure 3 - A. 41 (pilot model)

 

Figure 4 - Sherman ( T331567 )

 

Figure 5 - Churchill ( T341333 )

 

Figure 6 - Black Prince

 

Figure 7 - Panther (with turret removed)

 

Figure 8 - Universal Carrier ( ? 272872 )

 

Figure 9 - T.16 Carrier (with Skid Rail) ( T93933 )

 

Details of Tests
The load distribution was investigated by means of two piezo-electric pressure gauges set 20 feet apart at a nominal depth of 9-in. below the surface of the ground in the path of one of the tracks. The amplified response of these gauges as a vehicle was driven over them was recorded photographically by means of a cathode ray oscillograph unit fitted with a rotating drum camera. A more detailed account of this apparatus is given in R.R.L. Note No. M.O.S./427 in which earlier tests were described.

In the present tests, a circuit actuated by a clock mechanism was used to superimpose timing marks on the records by producing momentary interruptions in the recorded trace at half-second intervals, so that the speed of the vehicle on each run could be deduced from the time taken for it to traverse the measured distance between the two gauges.

The rut depth was measured before and after each run and in addition measurements were made which enabled the position of the gauges with respect to the centre line of the rut to be deduced. After the conclusion of the tests on each vehicle the gauges were dug out and their final positions noted.

The site was that used for earlier tests; when the present series of tests was made, the surface soil was much drier and firmer than was the case in the earlier tests.

Moisture content determinations were carried out during the tests. The dry density of the soil was also measured and in addition unconfined compression strength measurements were made on 1 in. cylindrical specimens of soil taken from the ground at a depth of about 9-in. below the surface, in the vicinity of the gauges. The results of the soil tests are given in Table X.

During the tests an attempt was made to obtain some information on the effect of track tension on the load distribution beneath some of the vehicles by recording the response of the gauges beneath the SHERMAN and the CHURCHILL when these vehicles were being accelerated as rapidly as possible, it being hoped that under these conditions the track tension would be higher than normal. It was found, however, that this condition was not realised because the vehicles had attained their maximum speed before passing over the gauges. Some measurements were however made beneath the UNIVERSAL CARRIER with its tracks adjusted to be as slack or as tight as possible.

 

Results
The results obtained with the various vehicles are summarised in Tables II to IX, typical records are shown in Figs. 10 - 14. In the tables the ration of the various maxima and minima which occur to the mean value for each individual record has been given as a reasonable assessment of the degree of uniformity of load distribution, i.e. it is assumed that these ratios would be unity for a good load distribution. For each vehicle the results obtained from the gauges in the two positions have been arranged in order of increasing vehicle speed. Details of the corresponding rut depth and position of the gauge with respect to the centre line of the rut are also given.

In addition the value of the air pressure which would have to be applied to the gauge to produce a deflection equivalent to that of the mean deflection for each record, is quoted in the tables so that the relative magnitude of the deflections for successive runs over the same gauge in each series of tests may be compared. As was pointed out in Note No. M.O.S./427, the response of pressure-sensitive gauges embedded in the earth is liable to be affected by a number of factors whose influent has not yet been explored, and consequently it must not necessarily be assumed that the figures given are equal to the actual mean pressure in the ground at the appropriate gauge depth. Further, since some of the factors which influence the gauge response, for example, the state of the soil where it has been disturbed by inserting the gauge, may vary with each series of tests, the figures quoted should not be regarded as indicating with certainty the relative values of the real pressures in the ground beneath the various vehicles.

The chief points of interest in the results obtained in the tests are as follows:-

1. CROMWELL, CHALLENGER and A 41

These three vehicles all have wheels of large diameter and a comparatively large ratio of wheel spacing to track pitch (6.2 - 8.2). The same design of track was fitted to the CROMWELL and CHALLENGER whilst that on A 41 was wider and had a longer pitch.

Representative records obtained beneath these vehicles are shown in []Fig. 10[]; it will be seen that they are similar, to those obtained in previous tests on softer ground with the CRUSADER (a vehicle of similar characteristics) which are described in Note. No. M.O.S./427.

The results indicate that the load distribution beneath the A 41 was more uniform than that under the other two vehicles which had rather similar characteristics.

From Tables II, III and IV it will be seen that the values of the ratios of maximum to mean deflection in the case of
the CROMWELL lay between 1.8 and 4.9
for the CHALLENGER the corresponding values were between 1.0 and 4.4.;
for the A 41 they were between 1.3. and 3.5.

Between the wheels of the CROMWELL the minima recorded were usually zero but on a few occasions reached a value of 0.1 times the mean; in the case of
the CHALLENGER the ratios were usually about 01 and sometimes were as high as 0.3;
but in the case of the A 41 with its longer track pitch the corresponding values were never less than 0.2 times the mean and sometimes as high as 0.5.


2. SHERMAN

The chief features of the design of this vehicle are shown in Fig. 4. It will be seen that it differs considerably from the other vehicles, included in the tests. Of particular interest are the type of suspension, the comparatively small diameter wheels and the difference in wheel spacing between wheels on the same bogie and adjacent wheels on successive bogies, which accounts for the two values of the ration of wheel spacing to track pitch given in Table I. (4.0 and 5.5).

The results of the tests on this tank are given in Table V and typical records are shown in Fig. 11. It will be seen that the records obtained at position I and position II showed quite distinct differences in the relative magnitude of the maxima beneath the first and last wheels. At position I, the records were very similar to those obtained in previous tests with the same tank on softer ground (Note No. M.O.S./439) and gave values for the ration of maximum to mean deflections of between 1.6 and 2.7. Further, as in the previous tests, it was found that the ration of the minimum deflections between the closely-spaced 2nd and 3rd, and 4th and 5th wheels lay between 0.5 and 0.8, which was much higher than that between the more widely-separated wheels where the corresponding ration was never greater than 0.1. At position II, the load was distributed less evenly, the deflections under the first wheel being distinctly larger than those under the sixth whee in nearly every case, whilst ratios of minima to mean deflection between the closely-spaced wheels lay between 0.3 and 0.5. The cause of this difference in load distribution at the two positions which was the only case of the kind noticed in the tests, is not known, but it was observed that the surface of the ground was not level at position II and it seems probable that on a fairly hard soil such as was used for the tests, unevenness in the ground might have a marked influence upon the load distribution beneath a vehicle with a rather stiff suspension.

In the light of these and previous result with the same vehicle, it appears that on soft and even ground the load distribute beneath the SHERMAN was rather better than beneath vehicles of the CROMWELL type but the present results suggest that on hard uneven ground, the load distribution may be little better than that under tanks of the CROMWELL type.


3. CHURCHILL and BLACK PRINCE

From Figs. 5 and 6 and Table I it will be seen that these vehicles are similar in design except that the BLACK PRINCE has a wider track of longer pitch and wheels of larger diameter than the CHURCHILL. The ratios of wheel spacing to track pitch are low (2.3 and 2.2. respectively).

In the case of the CHURCHILL, as can be seen in Fig. 5 and in the records reproduced in Fig. 12, the sinkage under the ground conditions prevailing during the tests was so small that the track below the first load-carrying wheel was not in contact with the ground and only eight wheels were effective in distributing the load. With this difference, the records obtained beneath the CHURCHILL were very similar to those obtained in previous tests on softer ground (See Note No. M.O.S./427). The load-distribution is fairly uniform with regular superimposed fluctuations, and on many of the records it was not easy to pick out the loading due to individual wheels. Values for the ratio of maximum to mean deflection lay between 0.4 and 2.6 whilst the corresponding limits for the minima are 0.3 to 1.0. The loading due to individual wheels (see Fig. 12) was more clearly discernible in the case of the BLACK PRINCE than the CHURCHILL. These records show evidence of vibration and sharp peak occur as the loading under each wheel attains its maximum value. Partly on account of these peaks, the ratios of maximum to mean deflection recorded in the case of the BLACK PRINCE tend to be rather higher than those measured under the CHURCHILL and lie between 0.4 and 3.2 times the mean whilst between the wheels the ratios for the minimum deflections range from 0.2 to 0.9.

Thus the results obtained indicate that the load distribution beneath the BLACK PRINCE was not quite as uniform as that below the CHUCHILL.


4. PANTHER

This vehicle illustrated in Fig. 7 with its large-diameter leaved wheels and fairly long track pitch combines some of the features of the first group of vehicles considered, with a close wheel spacing and low ratio of wheel spacing to track pitch approaching that of the CHURCHILL and BLACK PRINCE.

As can be seen in Fig. 7 and in the typical records reproduced in Fig. 13, the small sinkage caused very little loading of the soil under the first and last wheels. In Table VIII the small deflections corresponding to these wheels have not been included but they can just be detected at the beginning and end of the records shown in Fig. 13. The remaining 6 wheels gave a load distribution which compared quite favourably with that of the CHURCHILL and BLACK PRINCE, the ratios of maximum and minimum deflections to the mean lying between 0.4 and 2.5, and 0.5 and 0.9 respectively.

On the PANTHER, the ration of wheel spacing to track pitch is 3.7; this value is only slightly smaller than the value of 4 corresponding to the most closely spaced wheels on the SHERMAN; moreover, both tanks have a 6 in. track pitch. A comparison of that portion of the first SHERMAN record shown in Fig. 11 corresponding to the load distribution beneath the second and third, and the fourth and fit wheels, with the first PANTHER record shown in Fig. 13 indicates a striking similarity in form. Thus the records for a similar ratio of wheel spacing to track pitch appear to be independent of the size of the wheels. This suggests that a vehicle having suitably spaced wheels of the same diameter as those of the SHERMAN could give as good a load distribution as one with the more complicated arrangement of interleaved wheels employed on the PANTHER.


5. UNIVERSAL and T16 CARRIERS

Tests on these vehicles were carried out at the same time, the vehicles being driven over the gauges in turn in the same path and it is therefore possible to use the values of pressure equivalent to the mean deflation as a measure of the relative pressures in the ground beneath the two vehicles.

Typical records are shown in Fig. 14 and the main features of the results are summarised in Table IX.

It will be seen that the first two wheels of the UNIVERSAL CARRIER gave ratios of maximum to mean deflection lying between 1.4 and 3.1 whilst between the wheels the ration of minimum to mean deflection was very small and only reached 0.2 on one occasion. Deflections were invariably higher beneath the single rear wheel where the ratios of maximum to mean deflection lay between 4.0 and 5.2.

Tests 6 and 8 were carried out with a slack and tight track respectively (see Table IX), but no marked differences in load distribution were observed.

A typical record of the T-16 carrier with skid rails is shown in Fig. 14. A tendency to higher loading towards the rear end of the skid may be noted. Ratios of the maximum to mean deflection beneath this vehicle ranged from 2.2. to 3.5 (Table IXb). In view of the rigidity of the skid which would prevent it from conforming to the contours of the soil on the hard ground on which the tests were made, it seems probable that an even better load distribution would be given by this vehicle on a more deformable soil.


Remarks

During tests on the CROMWELL it was thought that the vehicle was not keeping accurately to the same ruts and, as can be seen in Table II, there was a final relative movement of 4 1/2 in. between the centre line of the rut and the gauge at position I. In all the later tests the relative position of gauge and rut was checked after each test but as can be seen from the figures given in the various tables of results, there was no apparent correlation between the value of the pressure equivalent to the mean gauge response and the distance of the gauge from the centre line of the rut (which was always less than half the track width).

A comparison of the results of tests made at different speeds on the various vehicles showed no evidence of any marked effect of vehicle speed on the load distribution beneath the tracks.


Conclusions

The following are the main conclusions suggested by the results of these tests:-

(1) The load-distributions at a depth of 9 in. below the surface of the ground were comparatively uniform beneath the CHURCHILL, BLACK PRINCE, PANTHER, and T-16 CARRIER with Skid Rails. In all these vehicles the distance between the wheels is less than four times their track pitch.

(2) The SHERMAN, with a ratio of wheel spacing to track pitch of 4 and 5.5, gave a rather less uniform load distribution than the vehicles mentioned above but was better than the other vehicles employed in the tests. The results suggest however, that the load distribution may not be as good on hard uneven ground.

(3) The A 41, with its ration of wheel spacing to track pitch of 6.2 produced the most uniform load distribution of the remaining vehicles whilst the CHALLENGER and CROMWELL gave fairly comparable results which were rather better than those obtained beneath the UNIVERSAL CARRIER.

(4) Altering the track tension on the UNIVERSAL CARRIER did not produce any noticeable difference in load distribution.

(5) In the range of speeds (5 - 15 m.p.h.) in which the tests were carried out no marked effect of speed on load distribution could be detected.

 

Table I - Vehicle Characteristics

 

Table II - Cromwell Results

 

Table III - Challenger Results

 

Table IV - A. 41 Results

 

Table V - Sherman Results

 

Table VI - Churchill Results

 

Table VII - Black Prince Results

 

Table X - Results of Soil Tests

 

Figure 10. Typical Records obtained beneath CROMWELL, CHALLENGER and A-41 - Nominal Gauge Depth 9 in.

 

Figures 11 & 12