TRIPWIRE

Dino Busuladzic

The following experiments were intended to measure the effectiveness of the triggering ball in pulling tripwires. Furthermore, they were completed when comparison with a steel plate and grappling hook were conducted.

1. Subject of testing - Prototype of triggering ball


Diagram 1.

The parts of the prototype of the triggering ball (see Diagrams 1) manufactured from the following materials are:

  1. Body - cast iron.
  2. Ring - (rubber / ferro-magnetic material).
  3. Handle - steel.
  4. Axes - steel.

The handle is attached to two metres of steel rope, which is itself connected to a seventy metre standard rope. The construction of the prototype complies with the prerequisite for reliable testing, while keeping the cost of manufacture low. This low cost was achieved by stepping back from the original model of the triggering ball and making some modifications that did not influence the results of the experiment. The weight of the triggering ball is 4.2702 kg and the volume of its body is 451.61 cube centimeters.

2. Timing and location


Photo 2.

The test is conducted on Wednesday 22nd of April, 98 from 8.30 am to 4.00 pm.

The testing ground unit, located off Underwood Avenue, Nedlands, is the property of the University of Western Australia, Institute of Agriculture.

3. Test no. 1. - Effectiveness of triggering ball in the removal of tripwires

Description of the ground
A ground unit is leveled (plane) and sparsely covered with vegetation not higher than one metre. The dimensions of the ground unit on which the testing is conducted are 1.5x50 metres.

Preparation of the ground unit for testing


Diagram 3.

A steel or plastic tripwire was tightly fixed at one end to a wooden column of dimensions 25x25x300 mm, and at the other to a split pin. The split pin was placed into a hole at a desired height above the ground level, from where it could be removed with a force less than two kilograms. Finally, a board of approximate dimensions 5x120x120 mm with drilled holes for the split pins was nailed to the opposite wooden column with two nails.

3.1. Observation of movement


Diagram 4.

The triggering ball, in the course of being dragged back, took the position illustrated by Diagram 4. This position, let us call it the active stable position, is produced by the static equilibrium of the active forces and reactions on them. All of these active and reactive forces occur on the vertical plane, which is in the direction of the rope. Test no.1-A and no.1-B show that the triggering ball will lose its active stable position if these forces are not in static balance. Three cases found in the course of the test are:
  • The reaction or friction of the ground to the teeth will be inconsistent in the case of the triggering ball being caused to lift from the ground by a small stone or vegetation. Consequently, the triggering ball will lose its active stable position rolling sideways or backwards.
  • The pulling force of the rope in Test no.1-A was greater than required causing the triggering ball to bounce.
  • The triggering ball will lose its active stable position when pulling stops. It will return to this position in the next 10 to 20 cm when pulling is resumed. During this 10 to 20 cm it is unlikely tripwires will be removed.

Reducing the distance between the ground and the center of gravity could help avoid these occurrences. In the end, it is clear that a semi-ball shaped triggering ball would prevent it rolling sideways.

3.2. Efficiency of the triggering ball

  • Test no.1-A
    The number of removed tripwires is The efficiency of the triggering ball for the first tray was:

  • Test no.1-B
    The number of removed tripwires is The efficiency of the triggering ball for the second tray was:

In the end, the efficiency (E) of the triggering ball is:

E = 86%

3.3. Conclusion

The spherical shape of the triggering ball provides both,
a) an undisturbed pass through vegetation, and
b) the ability to scratches the ground with its teeth thus removing tripwires lying below the vegetation.Tests have shown these characteristics to be advantageous and are a good direction for further development of this equipment. The test has showed the following:
a) The effectiveness of the triggering ball in pulling tripwires is 86%.
b) The triggering ball maintains the direction of its movement in the course of being dragged back.
c) One man can pull the rope with no problems.
d) The pass of the triggering ball through scrub and bush can be characterized as satisfactory.
e) The triggering ball attains the best result if constantly dragged back at a speed not exceeding 0.5 m/s (effectiveness is 91%).

Any improvement or development should be directed toward increasing the stability of the triggering ball as it passes over different obstacles on its way. Test no.1 has found that maximum effectiveness of the triggering ball is while it is in the active stable position. One goal should be to find a design, which insures the triggering ball constantly maintains its active stable position regardless of surrounding disturbances. Furthermore, the rubber/magnetic ring (see Diagram 1) is not essential for the removal of tripwires; thus exclusion of this part could contribute to lowering the price of manufacture.

4. Test no. 2. - Influence of high vegetation on the motion of the triggering ball

Test no.2-A the triggering ball was thrown over the crown of a tree to land on the opposite side. Pulling the rope back, the triggering ball is pulled through/over the crown and retrieved on the same side from where it was thrown. On the ground unit chosen for Test no.2-B lay a fallen tree, a trunk (350 mm diameter) and numerous branches in different sizes and forms. The triggering ball was pulled over these obstacles for 8 metres.

4.1. Observation of movement

a) Vertical obstacles


Diagram 5.

The triggering ball, while in its stable active position when disturbed by a vertical obstacle, reacts as shown in Diagram 5. The mechanics and causes of this reaction are described in Test no.1. The movement is observed and characterised as the triggering ball making a satisfactory pass.

b) Horizontal obstacles

The triggering ball uses the principal of a lever in traversing horizontal obstacles. This event is explained in two steps:
1) When the rope is pulled over a horizontal obstacle the triggering ball will come to a position where its teeth come into contact with the obstacle (see Diagram 6).
2) With continuation of pulling, the face (the plane surface in the region of the handle) of the triggering ball presses onto the surface of the obstacle. At this moment, because of its lever effect the teeth are no longer in contact with the obstacle. The triggering ball slides over pulled by the force in the rope (see Diagram 7).

The movement is observed and characterised as the triggering ball making a satisfactory pass.

Diagram 6.


Diagram 7.

c) Parallel obstacles

Test no.2 found that the triggering ball passed this kind of obstacle with the most difficulty. The problem position occurred when the teeth on the opposite side of the rope pressed into the obstacle at the same time (see Diagram 8). In the course of this test force and repeated pulling of the rope were applied causing the triggering ball to move sideways and pass the obstacle. This is the design area that needs improvement.

Diagram 8.

The movement is observed and characterised as the triggering ball making a not satisfactory pass.

 

Dino Busuladzic - June, 1999.

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