Delft University of Technology
Faculty Mechanical, Maritime and Materials Engineering Transport Technology
R.M.A.F. Verschuren Oil flow responses to road disturbances in a modular transporter. Masters thesis, Report 99.3.VT.5151, Vehicle Engineering.
In this report, the responses of a modular transporter to road disturbances are determined. A modular transporter is a vehicle used for transporting exceptionally large and heavy loads. A modular transporter is a platform supported by twelve wheel assemblies. Hydraulic cylinders, mounted in the wheel assemblies, support the vehicle platform and load.
In driving operation the hydraulic cylinders of more wheel assemblies are connected to form groups of communicating vessels. The oil flows in between the cylinders enable the wheels to follow the road surface. To prevent the vehicle to capsize in case of a leakage in the hydraulic system, safety valves are mounted on all cylinders. The safety valves disconnect the cylinders from the rest of the hydraulic system in case of a leakage. The safety valves close when the oil flow exceeds a threshold of 200 l/min.
To prevent the safety valves to come into action on uneven roads unintentionally, it is necessary to determine the oil flows between cylinders as a function of driving speed and load condition on different types of road unevennesses.
A linear vehicle model is constructed in this report to calculate the oil flows between connected cylinders. The vehicle model describes the in-plane dynamic behaviour of the vehicle. This model contains three simplifications:
1. The horizontal accelerations of the vehicle components have not been considered. 2. The oil is assumed incompressible
3. A linear viscous friction model is used to describe pressure losses in piping.
This vehicle model is used to determine the oil flow responses to road disturbances as step input and also as a stochastic process. The stochastic process is modelled for road disturbances of three types of roads: asphalt, concrete and unpaved road surfaces. The vehicle's responses to stochastic road input are expressed by the variances (σQ of the oil flows. The variances are used to construct probability intervals. The probability interval [-3σQ,3σQ] contains 99.7% of the oil flows, while driving on a certain type of road.
The results of the analysis in the case of step input show that the responses of the oil flows are quite large. Even at moderate speeds there is a high probability of the safety valves coming into action. The largest oil flows occur in the cylinder of the third axle when that tyre hits the obstacle. The oil flows in this linear vehicle model are approximately proportional to the product of speed and height of the obstacle (see report, figure 5-10).
The mass of the vehicle body has a large effect on the oil flow responses to step input. Largest oil flows occur if the vehicle is empty. The damping in the hydraulic cylinders has a considerable effect on the oil flow responses (see report, figure 5-6). The effect of tyre damping on the contrary is almost negligible.
In contrast to step input, the-effect of the mass (and mass moment of inertia) is large in case of stochastic input. Within the range of vehicle masses investigated (32.103 kg, 210.103 kg) largest oil flows occur at low speeds if the total vehicle mass equals approximately 160.103 kg.
If this interval is applied to the situation of an unpaved road with vehicle mass of 160.103 kg, this will give maximum oil flows of 3.|σQ| = 200 l/min at a speed of approximately 3.5 m/s in the cylinders of third and sixth axles.
On an usually uneven asphalt or concrete road surface there is almost no danger of accidentally operation of the safety valves, at least for speeds up to 6 m/s. Because the vehicle's maximum speed is 20 km/hr (5.56 m/s) it can be driven on such roads without any problems.
The cylinder damping has again considerable effect on the results. The error due to linearization of the cylinder damping results in an error in the same order in the oil flows. Especially when the oil flows are low, the linearization error is large. The calculated oil flows will be too low for small oil flows. At an oil flow (variance) of 25 l/min the error is around 6%.
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