The contact breaker is a mechanism to make and break the primary circuit of the ignition system for obtaining high voltage current in the secondary circuit. It consists of two points, known as ignition points or contact breaker points, one is fixed and the other is movable.
The movable point is operated by the rotation of a cam by which the two points come in contact and separate, thus making and breaking the prim nary circuit. The contact breaker is placed in the distributor housing itself. The cam is fitted to the distributor spindle. Various types of contact breakers are shown in Figure.
The main parts of a contact breaker are cam, contact points, levers, rubbing block (heel), spring and bush. The heel and bush are generally made of plastic. The points are made of platinum, tungsten, or an alloy of platinum and iridium.
Requirements of Contact Breaker
A good contact breaker should have the following requirements:
- The contact points must open and close at the correct time.
- They should not produce noise in operation.
- They should not be oxidized or corroded.0
DUAL BREAKER POINTS
In some V-8 distributors, two sets of breaker points are used. They are connected in parallel so that either set can complete the primary circuit to ground. One set of points is called maker points and the other breaker points. The maker points will always close the primary circuit and the breaker points will always open it.
While the cam is in operation, the maker points will close before the breaker points open while the breaker points are still closed. The circuit will be opened only when the breaker points are opened, i.e when both sets of points are open, thus the primary current is made to flow for a larger period of time, giving the coil the building up of a large cam angle.
CAM ANGLE (or Dwell Angle)
The cam angle is the number of degrees traveled by the distributor cam while the ignition points are closed.
- A 4-cylinder engine has about 41 cam angle.
- A 6-cylinder engine has about a 36° cam angle.
- An 8-cylinder engine has about a 31° cam angle.
A 6-cylinder engine has 6 lobe cam for contact breaker and the cam angle is about 36°. It means during the 60° of cam rotation devoted to the firing of each cylinder, the points are closed for 36° and opened for 60°- 36°=24°.
The cam angle can be measured directly with a cam angle meter. The motor is connected to the ignition distributor during operation in the engine. Some meters are designed to measure the percent of dwell. A 36° cam angle is equal to 60% dwell, 360/60-60%.
The importance of the correct cam angle lies in the fact that the position of the points is closed, the coil building up so that when they are open, the proper amount of high tension current will be available at the spark plug.