PUBLISHED ON August 17, 2022  Mechanical Design

Why pantograph is used in train?

Pantograph : An overview

Have you ever seen a z shaped body over the roof of a train? This device is called a pantograph. On many occasions, such as under-bridge track or a hill-slope of ground, it is difficult to maintain a parallel distance between the train and OH(Overhead) line as shown in Fig 1. The line height always varies. That's why we need a pantograph.

Its a mechanism which can adjust to these height changes automatically. While doing so it also makes sure that a sufficient contact force is present between the current collector and the overhead line (refer fig 1).

Let's go through the short summary of its design and modification from the beginning.

Current collector's various designs

Trolly pole collector

Let’s assume a simple current collector design - a simple copper rod with a compressed spring arrangement (refer fig 2a). The current can flow to the motor and return back via ground using this current collector arrangement (refer fig 2b).

This design would be a complete failure with the increase in speed because it will become unstable and can easily fall due to the lack of lateral stability.

Let's lean the rod to make it stable. Here, the spring tension can bring the rod vertical again, which can break the overhead line. So to counterbalance this spring force and to position the rod, a rope is tied. For precise alignment, we can attach a grooved copper head as shown in (refer fig 2c). This current collector design idea is called trolley pole collector invented by Frank J. Sprague.

Why are overhead lines zig-zag?

Due to continuous running of the train, the regular friction at one single point can break this groove head after some time. Therefore, engineers had to do two things. First they increased the length of the head and kept it perpendicular. Secondly you can observe in the image below how the OHE line’s zig-zag pattern reduces the wear and tear on the collector.

To minimise wear and tear further, a carbon strip is used above the collector head. This modified design can run smoothly at increased speeds of up to 35 to 100 kilometres per hour. This design, with additional side horns, is called a bow collector as shown in fig 4a. I will explain the importance of these horns in the next article. As the speed increases, one major drawback with this design is the air resistance and vibrations. You can see the air flow and air vortices in fig 4b. This will lead the collector to downward force and a loose contact with OHE.

To overcome these issues, John Q. Brown in 1903 patented a current-collector design called the pantograph.

Invention of diamond pantograph

The output motion of the pneumatic pistons is connected to the mechanism in such a way that when the driver increases the air pressure, the height of the pantograph increases. The springs as you can see in fig 5a, in the middle of the mechanism, are compressed initially. When spring is relaxed it will reduce the height of the pantograph.

The driver ensures that the pantograph is always in close contact with the overhead line. This symmetrical teardrop design solves the vibration issue and reduces the air drag considerably. This symmetrical design ensures that the collector head is always horizontal.

However, this design of pantograph is bulky and heavy, it requires significant power to raise and lower. To address this problem, Mr. Louis Faiveley introduced a new technology—the single-armed pantograph. This advanced version of the diamond pantograph called single arm pantograph is stable and compact (refer fig 5b.)

How are pantographs raised and lowered?

Does the lower portion of this pantograph seem familiar to you? (fig 6a.)

Observe fig 6b. If the green bar rotates clockwise, what happens to the small, yellow bar? It will also get an angular variation right! Let’s extend this yellow bar and attach the collector head at the tip of the yellow bar. Now, when we rotate the green bar, the height of the collector head changes. Which is exactly what we needed. This is the way a pantograph is raised and lowered using a four-bar arrangement(refer fig 6c.)

I hope you have acquired the knowledge of different designs of current collectors. We will see the rest in detail in the next article.

Thanks for reading!

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