At present, suspended monorail systems constitute a very common means of transportation in the Polish hard coal mines.[1,2] The main advantages of the suspended monorail include the independence of the route from the working floor surface irregularities and the possibility to transport cargo of significant mass and size.
The masses and dimensions of machines and devices transported via monorail have increased considerably in recent times. This is particularly related to the transport of longwall system elements and road support elements. For the most part, the mass of the transported cargo loads the frame of the ŁP support, and in some cases, also the rock bolts that are used as support reinforcement elements. Fig. 1 presents an example view of a route of a suspended monorail with a transport of steel arches that load the ŁP yielding steel arch support frame.[3]
Fig. 2 presents the mine working diagram with suspended monorail. The dynamic influence of the vibrating transported cargo on the support elements is particularly visible during the sudden braking of the monorail. Experience shows that the vibrations of the transported cargo occur not only in the travel direction of the monorail, which results primarily in deformations of the monorail route,[4] but also in the direction of gravity.
The influence of loads on the support in the direction of gravity may affect the support elements in various ways, such as: support frame camber beam deformation,[5] significant yields in the joints, as well as the tension, bending and shearing of the rock bolts reinforcing the ŁP support camber beams. This is why the article focuses on the analysis of the operational characteristics of the support frame sliding joint and the yielding bolts subjected to static and dynamic loads, in order to present the various influence that loading may have on the support. Fig. 3 presents an example diagram of the loading of a sliding joint of an ŁP yielding support frame reinforced by means of rock bolts via short joists with a force
In Poland, the maximum speed of suspended monorail travel is 2 m/s. Due to the fact that preparations are currently underway to increase the maximum speed above 2 m/s, it is necessary to inspect what influence it will have on work safety and mining support stability.
Current operational experience and tests have shown that dynamic loads induced by suspended monorail transportation have a significant influence on the roadway support stability and working protection durability and on the monorail operators.[4,5] This is particularly true during the emergency braking of a suspended monorail by means of a braking trolley, where the overloads reach 3g.
Bench tests of selected steel arch and rock bolt support elements utilised in the Polish hard coal mines were conducted in order to determine the resistance of steel arch and rock bolt supports to static and dynamic loads.
The article presents the results of tests conducted on a steel arch support in the form of the sliding joints[6,7,8] of an ŁP/V29 (ŁP-type yielding steel arch support frame constructed from a V29 section), which is commonly employed in the Polish hard coal mines. Tests of elements of threaded bolts with trapezoidal threads over the entire rod length were conducted as well. The tests were conducted under static and dynamic loading.
Instrumentation of an existing GIG drop hammer (test method with the free fall of mass) facility for steel arch and rock bolt support tests[9,10,11] was carried out as well, together with the development of a methodology enabling the testing of bolt rods under dynamic bending and shearing loads.
Bolt rod tests were conducted according to the applicable Polish standards[12,13] in a tensile testing machine with digital force and displacement registration. The tests under static load were carried out using a ZD100Pu-type static testing machine. The force measurements were carried out via a strain gauge force sensor (accuracy class 0.5), while displacement measurements were carried out via a potentiometric sensor (accuracy class 1). The measuring sensors were connected to a measuring amplifier (accuracy class 0.03) coupled to a computer. The measurement data was recorded on the computer with a sampling frequency
The following elements of the bolt (Fig. 4) were tested:
bolt rod with a trapezoidal thread (13 mm thread pitch) over the entire 2 m length, 21.3 ÷ 21.8 mm minor diameter and 25.1 mm maximum trapezoidal thread outer diameter,
50 mm tall nut (13 mm thread pitch).
The result of the bolt rod test under static tensile loading is presented in Fig. 5.
The length of the studied bolt rod during testing was approx. 2000 mm. The maximum force registered during the test was
In order to determine the load capacity of the rod-and-nut assembly, tests were conducted according to Fig. 6.
Results of the three rod-and-nut assembly load capacity tests are presented in Fig. 7 and Table 1.
Results of the rod-and-nut assembly load capacity tests.
Test number | Maximum load value | Bolt elongation | Post-test inspection |
---|---|---|---|
1 | 247 | 56.9 | Bolt nut |
2 | 234 | 38.1 | thread shearing |
3 | 236 | 43.3 |
Nut thread shearing was observed during the tests, which was the main cause of bolt load capacity loss. At the same time, it was determined that the bolt rod thread exhibited slight superficial marks after the test, generated as a result of the shearing. This suggests that it may be possible to obtain greater bolt-and-nut assembly load capacity by using nuts with better mechanical properties and by fitting the rod-and-thread nut contact areas better (the rod coarse thread was produced by rolling).
The bolt tests were conducted at a drop hammer test facility, depicted as a diagram in Fig. 8.
The principle of the bolt tensile strength tests under dynamic loading is the free fall of the mass
The bolt impact velocity
where: g – gravitational acceleration 9.81 m/s2.
During bolt tests under tensile impact loads, the bolt dynamic resistance force
Each tested bolt is subjected to a series of dynamic loads, starting from a drop height
The bolt test results are presented in Fig. 9 and in Table 2.
The result of the bolt rod test under dynamic tensile loading.
Test number | Drop mass | Cross-bar mass | Drop height | Impact velocity | Maximum load value | Post-test inspection |
---|---|---|---|---|---|---|
1 | 4000 | 3300 | 0.01 | 0.44 | 110 | The rod and nut were not |
2 | 0.02 | 0.63 | 136 | destroyed | ||
3 | 0.03 | 0.77 | 164 | |||
4 | 0.04 | 0.89 | 200 | |||
5 | 0.05 | 0.99 | 204 | Bolt nut thread shearing and cracking |
A chart depicting the relation of the maximum force
Failure of the bolt nut as a result of the dynamic force’s shearing of the thread occurred at
Bolt rod shear tests under static and dynamic loads were conducted at a test stand presented as a diagram in Fig. 11. The tests used the same shearing instrument, which was in accordance with the applicable Polish standard.[13] The bolt samples for shear tests were 250 mm long. The shear load was exerted on the segment of the bolt by means of a punch with a diameter of 75 mm.
The principle of the bolt shear strength test under dynamic loading is the free fall of the mass
The bolt impact velocity
During bolt tests under shear impact loads, the bolt dynamic resistance force
The average maximum shearing force obtained during the shear tests under static load was
The result of the bolt rod test under dynamic shear loading is presented in Fig. 13, while a result compilation is presented in Table 3. They are comparable with the average bolt rod static shearing force.
The result of the bolt rod test under dynamic shear loading.
Test number | Drop mass | Cross-bar mass | Drop height | Impact velocity | Maximum load value | Post-test inspection |
---|---|---|---|---|---|---|
1 | 2500 | 0 | 0.07 | 1.17 | 194 | the rod was not shorn |
2 | 0.08 | 1.25 | 213 | the rod was shorn | ||
3 | 0.09 | 1.33 | 227 | |||
4 | 0.1 | 1.40 | 228 | |||
5 | 0.12 | 1.53 | 229 |
A chart depicting the relation of the maximum force
The rod was not shorn at impact velocities up to
Bolt rod bending strength tests under static load were conducted in a test stand presented in Fig. 15, while the stand for tests under dynamic load is presented in Fig. 16.
The tests used the same bending instrument. A 200 mm long bolt rod sample is propped in the testing device on the surfaces of the device, which produce an angle of 90°. Static and dynamic load is applied to the bolt until the rod bends by 90°. The main objective of the tests was to inspect whether the steel bar used to form the bolt rods would rupture.
The principle of the bolt bending strength test under dynamic load is the free fall of the mass
The bolt impact velocity
During bolt tests under bending impact loads, the bolt dynamic resistance force
The result of the bolt rod bending test under static load, with bending by 90°, is presented in Fig. 17.
The average maximum bending force obtained during the bending tests under static load was
The result of the bolt rod test under dynamic bending loading is presented in Fig. 18, while a result compilation is presented in Table 4. They are comparable with the average bolt rod static bending force. The differences occur only in the first peak for the dynamic load, the value of which is greater than the load corresponding to the yield stress at static load.
The result of the bolt rod test under dynamic bending loading.
Test number | Drop mass | Cross-bar mass | Drop height | Impact velocity | Maximum load value | Post-test inspection |
---|---|---|---|---|---|---|
1 | 2500 | 0 | 0.01 | 0.44 | 29.7 | The rod was not bent by 90° The sample did not rupture |
2 | 0.02 | 0.63 | 29.4 | The rod was bent by 90° | ||
3 | 0.03 | 0.77 | 32.1 | The samples did not rupture | ||
4 | 0.04 | 0.89 | 30.2 |
V29 sliding joint static load capacity tests were conducted according to the applicable standard[14] in a tensile testing machine for static testing where loading is applied by means of a hydraulic actuator. The strain gauge force sensor (accuracy class 1) and potentiometric displacement sensor (accuracy class 1) were connected to a measuring amplifier (accuracy class 0.03) coupled to a computer. The measurement data was recorded on the computer with a sampling frequency
The principle of the V29 sliding joint dynamic resistance test is the free fall of the drop mass (ram)
The test result for the joint constructed from V29 sections under static load is presented in Fig. 20.
After the first force peak of 213 kN, a systematic decrease in load capacity to approximately 160 kN occurs, which is an adverse effect signifying the loosening of shackle screws.
A similar effect occurs during testing under dynamic load. Results of the tests of joints constructed from V29 sections are presented in Fig. 21, while a result compilation is presented in Table 5.
The test result for the joint constructed from V29 sections under dynamic load.
Test number | Drop height | Impact velocity | Section | Number of shackles | Nut tightening torque, Nm | Maximum load capacity | Comments |
---|---|---|---|---|---|---|---|
1 | 0.1 | 1.4 | V29 | 2 | 350 | 197 | the joint stopped after |
approx. 10 cm | |||||||
2 | 0.2 | 2.8 | 206 | total joint yield | |||
3 | 0.3 | 4.2 | 197 |
The maximum impact velocity that does not produce total yield should not exceed
In the case of an impact mass
The conducted resistance tests of steel arch and rock bolt support elements under static and dynamic load have shown that dynamic load has decisive influence on the support’s retaining of its stability. Support element stability decreases along with the increase of the impact velocity. This concerns both the steel arch support and the rock bolt support.
The results of rock bolt tests under dynamic tensile loading with an impact velocity
At impact velocities of up to
The results of bolt rod tests under dynamic bending loads were comparable to the average static bending force exerted on the bolt rods. Differences occurred only at the first peak during dynamic loading, the value of which was greater than the load corresponding to the yield point under static loading. None of the bolt rod samples ruptured during testing.
The tests of a sliding joint under dynamic loading demonstrated that the beginning of the yield occurred at a force lower than that required under static loading. The maximum impact velocity not resulting in total yield should not exceed
In order to guarantee safety during suspended monorail travel at speeds exceeding the currently permitted
Further, resistance tests of steel arch and rock bolt support elements to static and dynamic loads are planned for the future, in order to determine their operational characteristics at various load rates.