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In the previous article of pantograph we have studied about the need of pantograph and its preceding designs. In this article we will study in detail how the pantograph raises and lowers, how its collector head stays perfectly horizontal during this up and down motion. And advanced design concepts such as working of input operating systems, use of insulators and why there are two pantographs on one single train! Let’s go.
Observe the below image. This green bar in the image below is the input to the pantograph motions - Raising and keeping the collector head perfectly horizontal. It is nothing but the lower arm of the pantograph. First let me revise how the raising and owering mechanism of a pantograph works? Observe the fig 1. Let’s provide different colours to each bar as shown in fig 1.
If the green bar rotates clockwise as shown, can you predict what happens to the small, yellow bar? It will also get an anticlockwise angular variation. Right! Just as shown on the fig 2a. 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 attached to the extended yellow bar changes. Perfect! Now our pantographs raises and lowers just by operating the green bar. The system to operate the green bar we will explore later in this article.
Let’s revisit the raising of the pantograph, observe the collector head this time(refer fig 2b). As I raise the pantograph, the angle of the yellow bar changes, due to which the collector head’s angle with respect to OH line changes too. This is the big issue. Such tilt of the collector head will break the contact between the overhead line and collector head which will lead to sparking (refer fig 2c).
To resist the rotation of the collector head, I present to you an innovative solution engineers came up with - a balancing rod (refer fig 2d). This balancing rod is hinged with the lower arm and a small extension of the collector head as shown in image on the fig 2e. When you rotate the green bar, the distance between the collector head and lower arm hinge point decreases. The balancing rod cannot decrease its length, which is why you see this additional distance between the extended part of the collector head and the balancing rod’s end. However as we hing these two points, the balancing rod pushes the collector head in the opposite direction. This opposite push exactly cancels the collector head’s tilt. The collector head now achieved a perfect horizontal position throughout its height using a balance rod.
This shows how the pantograph uses a beautiful combination of two four-bar mechanisms. We have developed a technology that consistently keeps the collector head horizontal without any sensors or electronic controls.
This lower arm’s (green bar of reference figure 2a) rotation is achieved using pneumatic piston arrangement. (refer fig 3a and fig 3b).
In the first article we promised you to explain about the pantograph's horns. The horns are quite useful when the train switches tracks. During track switching, the pantograph also needs to switch the overhead lines(refer fig 4a). The horns help in the smooth switching between two or more overhead lines, otherwise the line could have stuck below the collector head.
The voltage across OHE is approximately 25000 Volts which is connected with the pantograph. So to isolate the metallic body of the train from this high voltage insulators with high electrical resistance are used (refer fig 4b). These insulators are also helpful to provide good mechanical support to the pantograph.
You might have seen that most of the time the pantograph is generally connected at the rear of the engine. This simple change will reduce the aerodynamic drag considerably. The reason is the boundary layer effect. Below is the velocity distribution around the train body. If the pantograph is connected to the front of the engine it has to move against high speed air as shown in fig 5b.
However, you can see the pantograph at the rear of the engine faces much lower air velocity since it is immersed inside the boundary layer. This definitely reduces drag.
Have you ever thought about why two pantographs are installed on top of a train engine? We have already learnt in the above section that the pantographs always need to be in the rear of the engine. Thus using two pantographs may be the way for it. Just check out the images below (refer fig 6a and fig 6b).
That’s all in this article. I hope you might have found both article 1 and article 2 of Pantograph interesting.
Thanks for reading!
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