Bowling machine

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.

Most of the space in the machine is a hopper that contains the balls. At the bottom of the hopper, near the front, is a rotor with space for six balls. The balls slot into the spaces on the rotor by gravity, which then takes them into the innards of the mechanism. The rotor passes over a trap-door, which the ball opens with its weight, falling into a small chamber.

A pump is used to provide a flow of air into the chamber that the ball drops into. The airflow pushes the ball along the chamber to rubber ring, or gasket, known as a "restrictor". The restrictor has a hole through it that is sightly smaller than the ball, which the ball presses up against. This seals the hole, so air pressure builds up in the chamber. When the pressure is so great that the restrictor can no longer hold it, the ball bursts through, out of the main body of the machine and into an external tube, which guides it upwards and releases it at the height of a bowler's arm.

A system exists that allows air to escape from the barrel shortly after it has passed the restrictor. This reduces the pressure on the ball, reducing its muzzle velocity. Closing this increases the speed to its maximum; its maximum speed is 160+ km/h (100+ mph).

To do this a curved piece of tubing is attached to the end of the barrel. The ball rolls around this causing it to spin and swing.

The tubing used for swing bowling can also be used for spin bowling, by pointing the curve somewhere between vertical and horizontal.

Seam bowling is impossible on this machine as it only accepts tennis balls and cricket training balls such as Slazenger's "Slazball" (a hard, small, low-bounce, heavy ball, similar in appearance to a tennis ball but usually red in colour). Neither of these types of ball has a seam.

The tubing can again be used, this time to simulate high and low bounce by mounting the tube vertically.