How does a stadium open and close its roof?

1. It is a sunny day and the stadium roof is wide open. Then dark clouds blow in and rain begins to fall on the field. Time to close the roof!
2. The roof is built in big pieces called panels. Here there are two halves. Each one rests on rails that run along the top of the stadium walls.
3. Under each end of a panel are carriages with steel wheels, like the wheels of a train. Electric motors turn them through gears. The gears make the wheels turn slowly but with great force.
4. Both ends of a panel must move at the same speed. If one end gets ahead, the panel twists across its rails and sticks. Sensors watch each end, and a computer slows whichever one is in front.
5. The two halves roll toward each other and meet in the middle. Soft seals squeeze together along the join, so the rain runs off the roof instead of onto the field.
6. When the weather clears, the motors run the other way and the halves roll back to the ends. Opening or closing takes about a quarter of an hour.

The stadium has been cut through, and the round lens shows one wheel carriage close up. Tap each part to see what it does.

• Roof panel. Each roof panel is a frame of steel beams with a light covering on top. It has to be strong enough to span the whole stadium, but light enough to move.
• Rail. A steel rail runs along the top of each long wall, just like a railway track. The whole weight of the roof presses down on these two rails.
• Wheel carriage. A wheel carriage, called a bogie, has steel wheels that roll on the rail. Each panel stands on several of them, sharing the weight.
• Motor and gears. An electric motor on the carriage turns the wheels through a set of gears. A small gear turning a big gear makes the wheel go slowly but push very hard.
• Position sensor. A sensor on each end of the panel measures exactly how far it has travelled along the rail, and tells the control room.
• Control room. An operator starts the roof from the control room. A computer then drives all the motors, keeps every carriage in step, and stops everything if something looks wrong.
• Seal. Along the edges where the panels meet are soft rubber seals. When the roof closes, the seals squeeze together and keep out rain and wind.

You are looking down on the stadium. Press Close the roof and watch what happens when the south motors are slower. Drag the yellow handles to move each end yourself.

Experiment: The south motors run a bit slower than the north motors, and there is no computer link. What happens when the roof starts to close?

1. Leave the south motors a bit slower and press Close the roof. Watch the twist.
2. Wait while the crew lines the panel up. Then switch on the computer link.
3. Press Close the roof again. Does it reach the middle this time?

What we found: When one end travels faster, the panel turns slightly and wedges across its rails, like a drawer pushed at one corner. With the computer link on, the faster end is held back so both ends arrive together. The roof closes safely, just a little more slowly.

Turn on JavaScript to try it yourself.

The computer link is switched off for repair, and a storm is coming. Press Start closing, then keep the panels straight by boosting whichever end falls behind.

Some stadiums can do something amazing: on a fine day the roof is open to the sky, and when it rains the roof slides shut. The moving part is built in giant pieces called panels. Each panel is a frame of steel beams with a light covering, wide enough to stretch right across the stadium.

The panels do not hang from above. They stand on steel wheels that roll along rails on top of the two long walls, much like a train on its track. Electric motors turn the wheels through gears. The gears make the wheels turn slowly but with enormous force, so the roof creeps along more slowly than you walk.

The clever part is keeping everything together. If one end of a panel moved faster than the other, the panel would twist and jam on its rails. So sensors measure how far each end has gone, and a computer speeds up or slows down the motors to keep them level.

It works like a sliding closet door on its track, but with one big difference. You can feel a door start to stick and stop pushing. A roof panel is far too big for anyone to feel, so sensors and a computer have to do the feeling. When the halves meet, rubber seals squeeze together to keep the rain out.

Key words

Retractable roof

A roof that can be opened and closed by moving parts of it out of the way.

Bogie

A carriage with wheels that carries part of the roof along the rail.

Truss

A strong, light frame made of beams joined in triangles.

Gearbox

A set of gears that makes a motor's fast, weak turning into slow, powerful turning.

Skew

The twist when one end of a panel gets ahead of the other.

Synchronization

Making separate motors move exactly together, by measuring each one and correcting it.

Check your thinking

1. What does a stadium's sliding roof travel on?

   Show the answer

   Steel wheels running on rails. The panels stand on wheeled carriages that roll along rails on top of the walls, like a train.

2. Why must both ends of a roof panel move at the same speed?

   Show the answer

   Otherwise the panel twists across its rails and jams. A panel is stiff. If one end gets ahead, the whole panel turns a little and wedges against the rails.

3. How does the computer know that one end of a panel is getting ahead?

   Show the answer

   Sensors measure how far each end has travelled. A sensor at each end reports its position, and the computer compares the two numbers.

4. Why do the motors turn the wheels through gears?

   Show the answer

   To make the wheels turn slowly but push very hard. A small gear driving a big gear swaps speed for strength, which is just what a heavy roof needs.

5. A linked panel has north motors that could do 6 meters per minute and south motors that can only manage 4. How fast does the panel move?

   Show the answer

   4 meters per minute. The controller holds the faster end back to stay level, so the panel travels at the speed of its slowest end.

6. Why is the roof not moved when the wind is very strong?

   Show the answer

   A moving panel catches the wind like a giant sail. A roof panel is enormous. Strong wind pushing on it while it is moving could shove it out of line.