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Jun 15, 2020

Optimizing the performance of railcar dumper positioner systems

Nick Smith
Nick Smith
Global Product Specialist
Cargo and bulk materials on rail cars are unloaded using a coordinated array of equipment commonly known as car dumpers. Do not be fooled by the relatively simple name! Even the professional international engineers at Metso agree—car dumper systems are very complex, and optimizing their performance requires careful considerations.

In addition to the primary car dumper component, car dumper systems typically consist of train positioning and holding equipment. In many cases, multiple pieces of equipment are used in positioning and holding the train with additional important roles performed by ancillary components such as apron feeders, takeaway conveyors, dust suppression or extraction equipment, automatic lubrication devices, and other parts.

The following diagram shows a typical rail car dumper system:


Operating Sequence

Each component functions independently and operates on its own time schedule; however, as parts of an even larger coordinated system, all components must also perform together in a precisely orchestrated manner. The operating sequence and duration of each component in a typical system time cycle is shown in the following chart:


Analysis of the system time cycle can identify the critical path and highlight improvement areas. Examples where improvements can be made often occur during periods of time when:

  • no system activity was taking place (these periods could easily be cut)
  • multiple pieces of equipment could work simultaneously to shorten the cycle
  • equipment was idle or waiting for other equipment to complete an operation.

These scenarios represent potential areas where specific equipment time cycles could be minimized, often by making speed improvements, by performing upgrades, or by installing new equipment.

Common Problems

When comparing the operating sequence times in the chart above, inexperienced engineers often assume that the best place to focus their process optimization efforts is on reducing the positioner forward index car time since it is the longest individual operation. However, the solution is not always as simple as speeding up the rate of travel of the positioner. Unfortunately, simple changes in velocity, acceleration, and even deceleration can increase the arm load on the positioner, resulting in one or more of the following common problems:

  • Increased pinion tooth load and failure of positioner rack and pinions
  • Increased rope tension and failure of ropes or sheaves
  • Insufficient installed power on the positioner and failure to achieve target speeds
  • Excessive arm load and premature arm failure
  • Increased overhauling forces and train holding device failure

Recommended Approach

A much better solution uses a train positioner simulation program to calculate the theoretical positioner loads during the indexing cycle. The program models the train as a series of distributed masses connected by non-linear spring couplers. A set of differential equations for motion are solved to predict the positioner arm load including overhauling forces. The program requires a range of inputs including time cycle data—for example,  indexing speeds, acceleration, and deceleration ramps—and information on the composition of the train such as the number and gross mass of cars, gross mass of the locomotive engines, and axle friction factors. Track details (grades and curvature) and component mechanical data (coupler draft gear data and haulage drive component inertias) can also be included. The theoretical data can be calibrated to match the actual field conditions by adjusting these inputs.

The chart below shows an example: 


Motor RPM and torque are measured on-site throughout a range of time cycles and the corresponding arm loads are calculated. The arm load can also be directly measured through strain gauges. The rolling friction factor, curve friction factor, and the draft gear coefficients are then adjusted in the program to obtain a strong correlation between the field and calculated results. The program can then be used to simulate the load conditions generated when operating under different input conditions such as running the positioner at faster speeds.

The following chart for a typical corrected simulation shows the results for three different runs. The machine parameters in this case are for a 160-car rake with a positioner forward index time of 51 seconds at a velocity of 0.46 m/s. The maximum arm load is approximately 100 metric tons and overhauling load is approximately 60 metric tons.


The simulation is run for every car throughout the rake; however, it is customary to show only a limited subset of results identifying the positions in the train where the highest hauling and overhauling loads are reached. (Note that the highest overhauling force is for 120 full cars and 80 empties) and typically the first index.

In a second simulation for the same train shown below, velocity increases from 0.46 m/s to 0.6 m/s resulting in a corresponding decrease in positioner forward index time to 40 seconds. The maximum hauling load has increased from 100 to 135 tons and the overhauling load from 60 to 90 tons. Maximum loading now occurs at a point further down the train. About 86 cars have been tipped as compared to 78 in the first example.



By investigating different operating scenarios and performing the relevant simulations, you can establish a comprehensive understanding of the system and can truly optimize the positioner operation. This could include increasing the positioner speed and acceleration/deceleration ramps, operating at different speeds throughout the train, or determining if and when train holding devices need to operate.

You can now determine the bottlenecks in the operation and can work towards your objectives such as achieving maximum throughput within current system parameters or achieving an increase in capacity through equipment upgrades (not limited to the positioner) with minimal disruption and capital expenditure.

However, these activities should not be done in isolation. As an engineer, at minimum, verify that the positioner drives can achieve and maintain the required torques and that the train holding devices can handle the overhaul forces without being overrun.

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