This is the most common type of governor. In the centrifugal type governors, rotation of the crank shaft is taken to a vertical spindle through suitable gears. It consists of two balls of equal mass, which are attached to the arms as shown in Fig. These balls are known as governor balls or fly balls. The upper arms are keyed to the spindle and the lower arms which are known as links are connected to sleeve. The sleeve surrounds the spindle as shown and can rotate as well as slide over the spindle. One end of a bell crank lever is attached to the sleeve and the other to a rod which actuates the fuel supply valve.
Principle of Working
The centrifugal governors are based on the balancing of centrifugal force on the rotating balls by an equal and opposite radial force. When the load on the engine decreases, the engine and the governor speed increases. This increases the centrifugal force acting on the balls and the balls move radially outwards. Therefore the sleeve rises upwards. This upward movement of the sleeve reduces the supply of the working fluid and hence the speed is decreased. Thus the engine speed falls and comes near about the mean speed.
Similarly, when the load increases, the speed of the engine and the governor decreases. This results in the decrease of centrifugal force on the balls. Hence the balls move inwards and the sleeve moves downwards. The downward movement of the sleeve increases the supply of the working fluid and hence the speed is increased. Thus the engine speed rises and comes near about the mean speed.
In inertia governors, the balls are so arranged that the inertia forces caused by an angular acceleration or retardation of the shaft tend to alter their positions. The amount of displacement of governor balls is controlled by suitable springs and through the governor mechanism, alters the fuel supply to the engine. This governor is more sensitive than the centrifugal, but it becomes very difficult to completely balance the revolving parts. For this reason centrifugal governors are not used more frequently.
Terminology Used in Governors
The important terms used in the study of a governor are presented in Table.
|1. Height of a governor : The vertical distance between the centre of the governor balls and the point of intersection between the upper arms (or arms produced) on the axis of the spindle, is known as the governor height. It is usually denoted by ‘h’.|
|2. Radius of rotation : The horizontal distance between the centre of the governor ball and the axis of rotation, is know as radius of rotation. It is usually denoted by ‘r’.|
|3. Equilibrium speed : The speed at which the governor balls, arms, sleeve, etc., are in complete equilibrium and there is no upward or downward movement of the sleeve on the spindle, is known as equilibrium speed.|
|4. Mean equilibrium speed : The speed at the mean position of the balls or the sleeve is known as mean equilibrium speed.|
|5. Maximum and minimum equilibrium speed : The speeds at the maximum and minimum radius of rotation of the balls without tending to move either way are know as maximum and minimum equilibrium speed respectively.|
|6. Sleeve lift : The vertical distance travelled by the sleeve on the spindle due to change in equilibrium speed is termed as sleeve lift. The vertical downward travel may be termed as negative lift.|
|7. Centrifugal force : The force exerted by the fly balls radially outwards is known as centrifugal force of a governor. It is given by
Fc = m ω² r
|8. Controlling force : An equal and opposite force to the centrifugal force, acting radially inwards (centripetal force) is termed as controlling force of a governor. In other words, the force acting radially upon rotating balls to counteract its centrifugal force is called the controlling force.|
|9. Governor effort : The mean force acting on the sleeve for a given percentage change of speed (or lift of the sleeve) is known as the governor effort.|
|10. Power of a governor : The power of a governor is the work done at the sleeve for a given percentage change of speed. It is the product of the mean value of the effort and the distance through which the sleeve moves. ∴ Power = Mean effort × Lift of sleeve|