The cricket sight screens are part of the equipment used to play cricket. Traditionally the ball used in this sport is dark red, therefore it might not be easy for a batsman to see it. The purpose of the sight screen is to create a background colour contrast in order to aid the batsman see the ball being pitched at him. In night time games and whenever a red ball is used the screen is white. During time some rules of this game have changed, so a white ball has been introduced for some specific matches. In this case a white screen would not contrast properly with the ball, therefore the white screen is substituted by a black one.
Because of the dynamics of the game, these screens are not kept in the same position for the whole time. According to the light and the action performed, they need to be moved in a prearranged space during the match. In order not to interfere with the game itself, the screen has to be moved rapidly. Once placed in position it does not have to move, so that the players do not lose their concentration. Usually there are two sight screens in each cricket ground and they are placed at the edges of the oval. Most of these sight screens are on wheels and are manually pushed into position by several people.
Because of their weight and dimensions, to ease and speed up this job in the major cricket grounds the screens are now placed on tracks and moved automatically.
For the evolution that the cricket game has had during the years, the length of a match may vary widely. According to the type of match played it might last up to 5 days. In any case each day the game lasts for several hours. In Australia one of the most important trophies is “The Ashes” trophy. It is a Test match between Australia and England. This type of game is usually played in group of matches called “series”. A series is made up of 3 to 5 matches, and each match might last up to 5 days. In an important oval in Australia the number of games played per year is around 12, so cricket is played for roughly 50 days per year.
This unit was installed in December 2013 at Bellerive oval in Tasmania for the Ashes test series between Australia and England. Because of its purpose, the size of the screen is fairly big. In this specific case, its dimensions are about 6 per 6 meters. It is placed on a trolley which runs on a steel track. The total weight of the screen and the trolley is 5,000kg. The trolley moves on the steel track with rubber wheels which have a diameter of 370 mm.
It has to be kept in mind that Tasmania is a very windy place for its geographic position. It is very easy to get strong winds which can reach a speed of 70 km/h and more. For the safety of the players and the spectators it is essential that the screen maintains its position. The maximum travelling distance of the screen varies between 10 and 15 meters. In order not to slow down the game, the screen has to be moved rapidly compared to its dimensions so the travelling time has to be in the range of 15-25 seconds. Once placed in its correct position it has to be kept there for a minimum of 60 seconds up to a maximum of 6 minutes. After that time, it is moved in a different position. For safety reasons, and for the game purposes, it is fundamental that once placed in position the screen does not move. As previously touched on, a cricket game might last for several days and each day the match is played for about 8 hours. In an oval such as the Tasmanian one, cricket is played for maximum 50 days per year. During each of these days the screen is in use for an average of 8 hours and it is moved around approximately between 20 and 100 times per day.
In order to chose the correct geared motor for the application it is necessary to consider all the specifications previously described. First of all we have to find out the speed of the track wheels, where the output shaft of the gearbox will be mounted. Being the travelling distance of the screen between 10 and 15 meters, knowing that it has to move in the range between 15 and 25 seconds and that the diameter of each wheel is 370 mm, we find an average speed of 35 rpm. Therefore the ratio of the gearbox would be roughly i=40, considering a 4 pole three-phase asynchronous electric motor running at about 1,400rpm.
The required torque on the output shaft of the gearbox has to be greater than the sum of two components: one is due to the weight of the screen and trolley and the other is due to the worst case of wind force which causes the sail effect on the screen. From the specifications we have, we find out that the gearbox needs a minimum output torque of M2 = 1,000 Nm. From the output torque and output speed values we can also calculate the necessary required output power, which is about 5 kW.
To chose the service factor for this application we can assume that the load type is heavy because of the wind and because of the shape and dimensions of the screen, that it can work for maximum 8 hours per day and it is moved maximum 60 times per hour.
The best solution for this application is to use a geared brake motor.
First of all we have to calculate the service factor according to the duty characteristics described above; from the diagram present in the B catalogue we find a minimum service factor of 1.6.
The gearbox we have chosen is a bevel helical gearbox B103UC with ratio i=41.1. This gearbox is matched to a 4 pole brake motor size 100 which can provide 3 kW of power. The selected geared motor can guarantee:
- an output speed of 34 rpm
- an output torque of 757 Nm with a service factor of 2.2
- the achievement of the required output power without exceeding the thermal power, the maximum torque and the maximum radial loads.
In order to reach the required output power, we need to install two units of the selected geared brake motor. As a redundancy factor, the installation of two separate units with such a high service factor rather than a single bigger unit, assures that in case one unit fails during the game the second one can take over and move the screen without interrupting the game.
The reason why B series gearboxes are used in this application is for the effi ciency of the gearing. In fact, this type of gearbox guarantees a minimum effi ciency of 90% which can go up to 95% in the best cases. These gearboxes are matched to brake motors because of the dynamics of the application; the brakes make sure that the screen is kept in its position during a specifi c action. At the same time in case of power failure on both electric motors, the brakes can be manually released and the screen can be moved by hand.
The finished package included a 7.5 kW variable speed drive complete with a remote pendant control, not Motovario supply. The drive controls both the geared brake motors at the same time. This device is used to move the screen forward or backwards into position when required. The remote control simplifi es the operation for the groundsman who can operate on the screen not on-site.