There are a number of different types of bowling machine available to cricket coaches, each quite different in the ways they achieve the required delivery, though most allow the use of remote control, so that a coach can be closer to a batsman when the stroke is played.
The main mechanism of the machine consists of two heavy wheels, between 30 and 50 cm in diameter, fitted with solid or pneumatic rubber tyres, each driven by its own electric motor. These are mounted in a frame such that the wheels are in the same plane, about 7 cm apart (slightly less than the diameter of a cricket ball). A ball joint allows the machine a wide range of movement. The whole assembly is mounted on a sturdy tripod or other frame so that the plane of the wheels is roughly at the height that a typical bowler would release the ball. A chute delivers the ball between the wheels, protecting the coach’s hands.
The motors are typically powered by a car battery, and turn in opposite directions. A controller allows variation of the speed of each wheel, allowing the machine to be slowed down for less experienced batsmen, or when the motors are not running at the same speed, swing or spin bowling can be simulated.
These machines will work with any ball of roughly the right size and weight, such as normal cricket balls or tennis balls. However, they usually work best with their own balls, bowling machine balls which are made of hard plastic, and are covered in dimples. These dimples are to help with the swinging characteristics when this type of delivery is desired.
This is achieved by setting both wheels to the same speed, as fast as the batsman is able to deal with, although the machine itself would have to be tipped horizontally. The coach can move the machine around slightly to vary the line and length of each ball.
The plane of the wheels is flat, and the motors are set to run at slightly different speeds. This means the ball will spin about an axis perpendicular to the ground, causing it to swing due to the Magnus effect. If the ball is spinning anticlockwise (looking from above) the ball will swing from right to left; clockwise spin results in a swing from left to right. This means that if the coach slows down the wheel on one side, ball will swing in that direction. The swing produced in this way is different from normal swing bowling, however – it is much more like a curling ball in football.
To achieve spin, the machine should be tilted to one side, and the wheels set to different speeds as per swing bowling. It will also probably be necessary to make the machine bowl significantly more slowly. The combined action of the spin imparted by the wheels, and the non-perpendicular axis of that spin will cause the ball to bounce up at an angle. While spinning always see the ball when it has been started to deliver the ball towards the wicket
This is difficult to achieve since the operator cannot control exactly how the ball will roll into the machine and between the wheels. However, by the law of averages, a ball will occasionally come out with its seam at the correct angle, and bounce unpredictably as a result.
To get balls that bounce unnaturally high or low, the plane of the machine must be set so that it is vertical. Not all machines can do this, simply because their ball joint doesn’t give the required range of movement. To get balls that bounce higher than normal, the lower wheel should spin slightly faster than the upper. Deliveries that bounce much lower than normal are less common and are usually the result of the ball striking a broken-up patch of ground. These balls can still be simulated, however, by increasing the speed of the upper wheel.