How it Works: The Starter Motor

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Bosch Starter Motor
The self-starting, or cranking motor, was introduced in the 1920s when motoring began to grow in popularity and manufacturers were competing to make their cars more attractive to the would-be purchaser. Previously, turning the starting-handle or pushing the car, with the gears engaged were the only methods of starting the engine. Most motorists were happy to adopt the new device, although the starting-handle was retained as a ready stand-by for another thirty years or more.

The Breakaway Torque

The heaviest load on a starter motor occurs when it first turns the engine from rest; this is known as the 'breakaway' torque. After this first movement, the 'resisting' torque comes into effect. For an average-sized engine, the initial current required to overcome the breakaway torque is approximately 450 amperes for a 12-volt system, falling to about 250 amperes at an engine speed of 1000 rev/min. This means that, however large and powerful the starter, both the battery and its connecting cables must be adequate to supply this heavy starting current; and the starter motor must always be considered in relation to the battery and associated wiring.

The components of the breakaway torque consist of the inertia of the engine, which depends upon its size; the friction of the cylinder walls and bearings; and the viscosity of the lubricating oil, which depends partly on the grade of oil and partly on its temperature. A cranking speed of 70 to 100 rev/min is usually sufficient for a petrol engine to draw in an ignitable petrol/air mixture, to start firing and to run up on its own. Diesel engines require a higher cranking speed and a longer starter engagement period. Starter motors are invariably series wound: that is to say, the field coils and armature are continuous, the same current flowing through each. This type of motor delivers its highest torque at low speeds, so that it is ideal for overcoming the breakaway torque at its maximum efficiency.

The transmission of the turning moment of the motor to the engine is accomplished by mounting a ring gear on the outer edge of the engine flywheel, so that it can be engaged by a small pinion on the end of the starter-motor shaft. The gear reduction ratio is large, usually between 10: 1 and 15: 1. The leading edges of the pinion teeth are chamfered to facilitate their engagement. To keep the two gears permanently engaged would result in a waste of power and possible damage, so a device is incorporated that disengages the pinion when not actually performing its starting function.

This device operates as follows: with the engine stationary, the starter switch is operated and the motor revolves, accelerating rapidly. The pinion, turning on helical splines (coarse screw threads) on an extension of the motor armature shaft, spins away from the motor as the result of its own inertia to engage in the flywheel at the end of its travel. When the engine fires and runs up to a speed greater than the starter speed, the pinion is automatically thrown out of engagement with the flywheel ring gear and rapidly travels back along the helical splines. The shock of the first engagement of the two gears is considerable. To overcome this, a substantial torsion spring is arranged so that the starter-motor pinion is actually driven through the spring.

The Original American Bendix Drive

This design constitutes the original American Bendix drive and it and similar arrangements are 'outboard meshed'; that is, the pinion moves away from the motor towards the flywheel on engagement. A British variation is 'inboard meshed'. This starter is positioned so that the flywheel ring gear lies between the starter motor and the pinion. The starter motor rotates a sleeve splined on its inner surface mating with splines on the armature shaft. The outer surface of the sleeve has helical splines, similar to the Bendix, and the pinion rides on the sleeve. On switching on, the inertia of the pinion throws it into mesh with the flywheel ring gear, but the reverse thrust so generated forces the sleeve back against a compression spring, to absorb the considerable shock of engagement.

The advantage of this inboard meshed design is that the shaft between the motor and the pinion is very much shorter and more rigid, for a given diameter, than the outboard meshed type. Both the American and British variants are known as inertia engaged starters. There have been various developments from time to time, one of the most important being the Eclipse and barrel drives, in which the pinion is mounted on the closed end of a separate member. This enables the pinion size to be reduced and the starter-to-engine gear ratio increased lessening the enormous initial exertion of the starter. Other additions include an anti-drift spring to prevent the pinion working forward along the helical threads and into contact with the flywheel, and a spring to prevent the pinion from bouncing back when it is ejected by the flywheel.

Bendix Starter Motor
The most common type of starter for many years was the Bendix, which span the pinion into mesh with the flywheel ring gear as soon as the motor turned and out of mesh as soon as engine speed exceeded motor speed.

Premature Ejection

Inertia engaged starters were widely used up until the 1980s and gave good service, but had one inherent defect if the engine fired, but did not pick up properly, the engine speed was nevertheless greater than that of the starter motor and the pinion was thrown out of mesh. This state was known as premature ejection and was particularly troublesome with light flywheel engines. If the starter switch was immediately operated with the engine still turning, the motor could be heard whirring round while the starter pinion grated against the flywheel gear, unable to engage. To overcome this defect, the pre-engaged starter was developed; in this type, the pinion remained engaged until, with the engine running evenly, the starter switch was released.

Additional components were required for the pre-engaged starter; these were a solenoid, which actuated an engagement lever, and an overrunning roller clutch assembly. A feature of this type of motor was the face-type commutator, which had four brushes mounted on the motor end plate. This commutator had a lower rate of brush wear, greater reliability and was cheaper to manufacture than the barrel type. When the starter switch is operated, the solenoid was energised and the engagement lever moved the starter pinion along its shaft until it was fully meshed with the flywheel ring gear. At the completion of this movement, the lever operated a switch that closed the motor-battery circuit and the motor turned the pinion.

The pinion could be held in mesh until the engine had overcome any initial hesitation - so that premature ejection was prevented. If the engine over speeds in relation to the starter motor, the pinion was not thrown out of engagement, as with the inertia starter, but the clutch disengaged, acting as a freewheel device. To overcome the difficulty that could arise if the pinion teeth butt exactly against the teeth of the fly-wheel ring gear and are unable to enter into mesh, the engagement lever was spring loaded. This allowed the lever to over-reach and close the starter-motor contacts, thus turning the motor so that the pinion could slide into mesh.

The Spring Starter

In larger engines, a dry plate clutch was often employed instead of the roller type. Outboard meshing had to be used with this pre-engaged type of clutch; however, the long tension spring of the Bendix type was not required and the overall shaft length was no greater than that of the inboard meshing type. Mechanical methods of starting were occasionally employed. The spring starter consisted of a large 'clockwork' spring, wound up by a handle and ratchet device and released by a press button. This would deliver sufficient stored energy, through a gear train, to turn the engine over for several revolutions. In another type, a rope was used to draw a spring return rack across a pinion attached to the engine, or a ratchet wheel was employed in a similar manner.

Cold Weather Starting

There are three main causes for starting difficulties in very cold weather: Increased viscosity of the lubricating oil, hence higher breakaway and resisting torques. This problem is best overcome by the use of multi grade oils, the viscosity of which does not markedly increase at low temperatures. Reduced output from the battery, because chemical reactions within the battery are slowed down by low temperatures. This, coupled with the increased effort required to turn the engine over, means that less current is available to operate the ignition system so that only a weak, or no, spark appears at the spark points.

With a 6-volt battery, the voltage available to feed the ignition system under a heavy starting current drain diminishes almost to vanishing point; for this reason, amongst others, most manufacturers changed to 12-volt systems, which provide a greater margin under these conditions. The lower cranking speed means that less petrol/air mixture is drawn into the cylinder, and much of the petrol is not properly vaporized. The use of the carburettor choke overcomes the fuel deficiency, but not the lack of vaporization, hence the danger that too much use of the choke will tend to wash the lubricating oil film off the cylinder walls.

Starter motors rarely give trouble: one of the most common with the inertia engaged type is the jamming of the pinion in the fully meshed position. The starter motor is unable to turn because the torque required is too great - it has no opportunity to spin up from rest, as in normal starting. The remedy is either to turn the motor shaft with a spanner (the shaft has a square end, protruding from the motor case, for this purpose), or to switch off, engage gear and push the car to and fro. Both methods release the pinion and wind it back clear of the flywheel. Road dirt thrown up onto the sleeve is obviously undesirable; the best remedy is to fit a light gauge protective shield.

Subject to manufacturer's recommendations, the only lubrication required on older type starter motors is a little light oil on the sleeve. The motor armature shaft bearings are usually supported in sintered metal bushes and do not require lubrication. When a roller clutch is fitted, it should not be cleaned out with solvent as this would wash away the grease with which it is prepacked. If the battery is clearly failing, as evidenced by increasingly sluggish action on first starting up, the only remedy is to replace it.
Starter Motor Schematic
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