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The mesmerizing London Tower bridge still works perfectly, opening and lowering the bridge almost 800 times in a year. No pedestrians are using these high-level walkways. Let me remove them, Oh no! To understand what is going on here we must first learn about the secret technologies hidden inside these engine rooms.
Let’s see the most simple bridge design which can open - an electric motor directly rotating the bridge leaves. However, due to the huge imbalance of the bridge weight, the electric motor struggles to raise and lower the leaf.
To solve the issue of weight imbalance, the engineers behind the project attached a heavy counterweight to the other end of the bridge. Oh no, now the bridge is stuck! The engineers had to modify the foundation block to accommodate the movement of the counterweight. They built huge chambers in the foundation block. As a result, the bridge could be raised and lowered with ease. The movable section of the bridge is known as a bascule. The bascules are pivoted on these huge shafts and secured to the ground via these bearings. We will explore these special bearings in greater detail later.
In fact, no electric motor in the world would be able to produce the torque required to lift the bridge. They would struggle to lift the massive bascules, each of which weighs more than 1200 tons. You know the solution! Just introduce a transmission in between the motor and bridge. The engineers found an even more straightforward solution. Convert the motor’s rotational power to hydraulic energy using an axial piston pump. This pressurized fluid can be transmitted easily and, most importantly, the force value can easily be amplified. This hydraulic experiment we all are familiar with demonstrates how 1 Newton of force at the inlet can be amplified to 10 N at the other end.
Now comes the main hero of this project: the hydraulic motor. Just as electric motors work on electricity, hydraulic motors work thanks to high pressure liquid. These spring loaded pistons are kept inside this central block. Please note unlike the normal pistons these pistons are free to swivel slightly. The incoming high pressure liquid moves these pistons outward by compressing the spring and becomes low pressure. Please watch the animation again. The central block slightly rotates due to the piston movement. When the piston movement is repeated the central block completes a full circle. At the outlet of the motor, the liquid with low pressure is collected. This hydraulic motor is specifically known as a radial piston hydraulic motor. One specialty of them is that they rotate very slowly, between 100 to 200 rpm. The electric motor has an rpm of 2000. When the speed has become almost 1/20th at the hydraulic motor side, according to the energy conservation the torque has amplified 20 times. That’s a huge amount of torque multiplication.
However, the torque multiplication is not over yet. From the hydraulic motor engineers extended a drive shaft. The pinion gear which is having a diameter of one meter is the next crucial component of the mechanism. The engineers fitted the pinion gear to the end of the drive shaft. The pinion gear now drives a gigantic gear - a rack. The rack is directly fitted to the bascule. This further reduces speed and increases torque multiplication. The highly amplified torque will now be able to rotate the bascule.
Let’s finally discuss these unique, gigantic bearings known as trunnion bearings. They are used to support the main shaft of the bascules. The London tower bridge still uses the same bearings that were installed 130 years ago. How is this possible that they’ve held up for so long considering that the bearings of heavy load carrying equipment are usually easily damaged?
To understand the genius of 19th century engineers, let’s observe a detailed engineering model of the bascule mechanism. As the pinion gear spins counter-clockwise, the bascule gracefully descends, transferring its entire weight onto the trunnion bearing.
However, the story doesn't end there. As the bascule reaches its final position and gently touches the resting block, a crucial question arises: what happens if the pinion gear continues to rotate?
Pause the video and take a moment to predict the outcome yourself. Continuing the rotation wouldn't force the bascule down further, as the resting block acts as a barrier. The only remaining option for the bascule is to slightly lift, by making the resting block as the new pivot point.
Let's rewind and observe the motion study again. Notice the formation of a clear gap between the bearing shaft and the bearing itself. This also means that the load on the bearing is completely relieved. This is the reason why the original trunnion bearings are still working fine without any issue.
Let’s see the most simple bridge design which can open - an electric motor directly rotating the bridge leaves. However, due to the huge imbalance of the bridge weight, the electric motor struggles to raise and lower the leaf.
To solve the issue of weight imbalance, the engineers behind the project attached a heavy counterweight to the other end of the bridge. Oh no, now the bridge is stuck! The engineers had to modify the foundation block to accommodate the movement of the counterweight. They built huge chambers in the foundation block. As a result, the bridge could be raised and lowered with ease. The movable section of the bridge is known as a bascule. The bascules are pivoted on these huge shafts and secured to the ground via these bearings. We will explore these special bearings in greater detail later.
In fact, no electric motor in the world would be able to produce the torque required to lift the bridge. They would struggle to lift the massive bascules, each of which weighs more than 1200 tons. You know the solution! Just introduce a transmission in between the motor and bridge. The engineers found an even more straightforward solution. Convert the motor’s rotational power to hydraulic energy using an axial piston pump. This pressurized fluid can be transmitted easily and, most importantly, the force value can easily be amplified. This hydraulic experiment we all are familiar with demonstrates how 1 Newton of force at the inlet can be amplified to 10 N at the other end.
Now comes the main hero of this project: the hydraulic motor. Just as electric motors work on electricity, hydraulic motors work thanks to high pressure liquid. These spring loaded pistons are kept inside this central block. Please note unlike the normal pistons these pistons are free to swivel slightly. The incoming high pressure liquid moves these pistons outward by compressing the spring and becomes low pressure. Please watch the animation again. The central block slightly rotates due to the piston movement. When the piston movement is repeated the central block completes a full circle. At the outlet of the motor, the liquid with low pressure is collected. This hydraulic motor is specifically known as a radial piston hydraulic motor. One specialty of them is that they rotate very slowly, between 100 to 200 rpm. The electric motor has an rpm of 2000. When the speed has become almost 1/20th at the hydraulic motor side, according to the energy conservation the torque has amplified 20 times. That’s a huge amount of torque multiplication.
However, the torque multiplication is not over yet. From the hydraulic motor engineers extended a drive shaft. The pinion gear which is having a diameter of one meter is the next crucial component of the mechanism. The engineers fitted the pinion gear to the end of the drive shaft. The pinion gear now drives a gigantic gear - a rack. The rack is directly fitted to the bascule. This further reduces speed and increases torque multiplication. The highly amplified torque will now be able to rotate the bascule.
Let’s finally discuss these unique, gigantic bearings known as trunnion bearings. They are used to support the main shaft of the bascules. The London tower bridge still uses the same bearings that were installed 130 years ago. How is this possible that they’ve held up for so long considering that the bearings of heavy load carrying equipment are usually easily damaged?
To understand the genius of 19th century engineers, let’s observe a detailed engineering model of the bascule mechanism. As the pinion gear spins counter-clockwise, the bascule gracefully descends, transferring its entire weight onto the trunnion bearing.
However, the story doesn't end there. As the bascule reaches its final position and gently touches the resting block, a crucial question arises: what happens if the pinion gear continues to rotate?
Pause the video and take a moment to predict the outcome yourself. Continuing the rotation wouldn't force the bascule down further, as the resting block acts as a barrier. The only remaining option for the bascule is to slightly lift, by making the resting block as the new pivot point.
Let's rewind and observe the motion study again. Notice the formation of a clear gap between the bearing shaft and the bearing itself. This also means that the load on the bearing is completely relieved. This is the reason why the original trunnion bearings are still working fine without any issue.
Trunnion bearings are renowned for their ability to handle immense static and dynamic loads, making them ideal for applications like bascule bridges.
It’s almost impossible to believe how the engineers of 100 years back thought about overdriving the pinion gear and relieving the load on the trunnion bearing.
In short, at the normal bridge condition, the bascules are not resting on the bearing, but on the pawls, resting block, and at the support of the nose of the other bascule. The bearing carries the load of the road only during the lifting and lowering. This very clever engineering design is the reason why the original bearings installed over a century ago are still in use.
We 3D Printed all the Solidworks files and built a mini London tower bridge. Let me lower the bascule. The Bascule is gently touching the resting block. In this situation, if I continue to rotate the pinion gear the magic happens. The shaft gets lifted off from the bearing.
Once both the bascules are aligned horizontally, the nose bolts are activated. They are hydraulically activated and interlock with the next bascule. After this, the back end of the bascule is locked with the help of a few pawls. The pawls are also hydraulically operated. The vehicle traffic can resume now.
If we pan the camera downward, you will be able to see a secret chamber. This is the bascule chamber. They are huge! When the bridge opens, the counterweight lowers into this region. This is the engine room of the Tower bridge. The engine room used in our explanation was looking simple because we avoided the control valves between hydraulic pump and motor and all other control devices for the sake of simplicity.
The tower bridge is in fact a combination of suspension and bascule type. On both ends of the bridge you can observe a beautiful suspension bridge. Out of the total 240 meter length, 164 meters is carried by the suspension bridges. These stout retail chains carry the load of the suspension part of the bridge via the suspension rods.
The engineers behind this bridge made one more smart design choice. They didn’t want the flow of pedestrians to stop while the bridge was operating. That’s why they built these high-level walkways. These allow pedestrians to climb the tower via these stairs and easily cross the bridge while the bascules are still opening and closing. In reality, the bridge was able to open and lower in less than 5 minutes, so most pedestrians elected to wait for 5 minutes rather than climb the towers. I got a chance to interview Professor Geofrey Hartwell who has been involved in the engineering aspects of the tower bridge since 1974.
Professor Hartwell, could you please explain to our viewers why the high level walkways of the Tower Bridge are important?
The walkway actually contributes to the structure. So, when that bridge is down it’s a conventional cantilever bridge but when it’s actually up, it becomes a suspension structure. The towers are held up by tension in the walkway. Okay and then it’s anchored on either side.
What Professor Hartwell shared is fascinating. The high-level walkways are not only there for pedestrian convenience. They also play a major role in balancing the force acting on the towers. To balance the horizontal component of the main cable’s tension, the walkway structure should develop a tensile stress. Without the walkway, this part of the tower would have bent and this portion would have been in tensile stress, causing cracks to form within only a few years of operation.
To understand the details of the tower bridge operation, let’s get into the control room. The first operation for the bridge opening is to start the hydraulic pump and pressurize the liquid. The operator goes for a public announcement next. Now, he turns on the red traffic signal. When he presses the 5th and 6th buttons together the gates close automatically. Next, he presses the nose bolt button and you know what happens. This joystick controls the opening of the bridge. Once the ship is fully crossed, the operator closes the bridge. The commuters may feel this structure like any other road. Just by looking at the details of asphalt and structural steel you will be able to appreciate the intricacies of engineering they did more than 100 years ago. Thank you!
It’s almost impossible to believe how the engineers of 100 years back thought about overdriving the pinion gear and relieving the load on the trunnion bearing.
In short, at the normal bridge condition, the bascules are not resting on the bearing, but on the pawls, resting block, and at the support of the nose of the other bascule. The bearing carries the load of the road only during the lifting and lowering. This very clever engineering design is the reason why the original bearings installed over a century ago are still in use.
We 3D Printed all the Solidworks files and built a mini London tower bridge. Let me lower the bascule. The Bascule is gently touching the resting block. In this situation, if I continue to rotate the pinion gear the magic happens. The shaft gets lifted off from the bearing.
Once both the bascules are aligned horizontally, the nose bolts are activated. They are hydraulically activated and interlock with the next bascule. After this, the back end of the bascule is locked with the help of a few pawls. The pawls are also hydraulically operated. The vehicle traffic can resume now.
If we pan the camera downward, you will be able to see a secret chamber. This is the bascule chamber. They are huge! When the bridge opens, the counterweight lowers into this region. This is the engine room of the Tower bridge. The engine room used in our explanation was looking simple because we avoided the control valves between hydraulic pump and motor and all other control devices for the sake of simplicity.
The tower bridge is in fact a combination of suspension and bascule type. On both ends of the bridge you can observe a beautiful suspension bridge. Out of the total 240 meter length, 164 meters is carried by the suspension bridges. These stout retail chains carry the load of the suspension part of the bridge via the suspension rods.
The engineers behind this bridge made one more smart design choice. They didn’t want the flow of pedestrians to stop while the bridge was operating. That’s why they built these high-level walkways. These allow pedestrians to climb the tower via these stairs and easily cross the bridge while the bascules are still opening and closing. In reality, the bridge was able to open and lower in less than 5 minutes, so most pedestrians elected to wait for 5 minutes rather than climb the towers. I got a chance to interview Professor Geofrey Hartwell who has been involved in the engineering aspects of the tower bridge since 1974.
Professor Hartwell, could you please explain to our viewers why the high level walkways of the Tower Bridge are important?
The walkway actually contributes to the structure. So, when that bridge is down it’s a conventional cantilever bridge but when it’s actually up, it becomes a suspension structure. The towers are held up by tension in the walkway. Okay and then it’s anchored on either side.
What Professor Hartwell shared is fascinating. The high-level walkways are not only there for pedestrian convenience. They also play a major role in balancing the force acting on the towers. To balance the horizontal component of the main cable’s tension, the walkway structure should develop a tensile stress. Without the walkway, this part of the tower would have bent and this portion would have been in tensile stress, causing cracks to form within only a few years of operation.
To understand the details of the tower bridge operation, let’s get into the control room. The first operation for the bridge opening is to start the hydraulic pump and pressurize the liquid. The operator goes for a public announcement next. Now, he turns on the red traffic signal. When he presses the 5th and 6th buttons together the gates close automatically. Next, he presses the nose bolt button and you know what happens. This joystick controls the opening of the bridge. Once the ship is fully crossed, the operator closes the bridge. The commuters may feel this structure like any other road. Just by looking at the details of asphalt and structural steel you will be able to appreciate the intricacies of engineering they did more than 100 years ago. Thank you!