Torque transmission

One-way clutch

All types of pre-engaged starter motors transmit torque to the engine flywheel via an overrunning or one-way clutch. The main function of this mechanism is to ensure that torque is transmitted in one direction only, i.e. from the starter motor to the engine. This prevents the engine from driving the starter when it fires (when engine speed will be greater than starter-motor speed). If the drive connection between the starter and engine were not disconnected under these conditions, the starter motor armature would be driven to excessively high speed and this would cause fatal damage to the motor.

Figure 5.15 Pre-engaged starting motor (Lucas M50)

112 Starting-motor systems Fundamentals of Motor Vehicle Technology: Book 3

In certain designs of starter motor (commercial vehicle types) this clutch performs an additional function of torque limitation to prevent overloading of the starter motor itself. There are two main types of overrunning clutch in use:

● Roller-type clutch: this is the most common type

encountered on light-vehicle starting systems. The central component is the clutch shell with a ramp-type roller race. The clutch shell forms the driving member and is connected to the armature shaft via a helix spline. The frictional link between the pinion shaft (i.e. driven member) and the clutch shell is formed by cylindrical rollers that are able to move within the roller race. When the mechanism is at rest the springs force the rollers into constricted space between the roller race in the clutch shell and the pinion shaft. The rollers lock in place (due to the ramp constriction), and via friction they transmit force from the clutch shell to the pinion shaft when torque is applied from the armature (see Figure 5.17).

When the torque transmission is reversed, i.e. the engine tries to drive the motor, the friction between the pinion shaft and the rollers pushes the rollers into the wider part of the roller-race ramp. This disconnects

to frictional force at the roller surfaces between clutch shell and pinion shaft and allows the coupling to slip, thus preventing drive from engine to starter motor.

● Multiplate clutch: most common in larger starter motors found on commercial vehicles. This clutch type contains a multiplate clutch pack which is preloaded via a disc spring. In order to transmit torque the clutch pack is compressed under load via a helix arrangement through which torque from the armature to the engine is transmitted. This tends to compress the clutch pack and driving torque is thus transmitted via the friction faces of the clutch plates. This arrangement has an inbuilt limit stop to prevent compression of the clutch pack beyond a design limit. Therefore, at a designed maximum torque limit, the clutch will slip thus limiting torque transmission and preventing damage to the motor itself. Under reverse torque/overrun, the clutch pack disengages due to ‘unwinding’ of the helix. This releases the friction forces between the clutch plates and hence disconnects the drive completely (see Figure 5.18).

Figure 5.16 Pre-engaged starter – drive engagement

a Resting position b Extended position,

shown at end of lever travel

11 Overall pinion travel

12 Solenoid-armature travel 13 Helical travel 14 Free travel 1 Cap 2 Pinion 3 Driver and clutch shell 4 Roller race 5 Cylindrical roller 6 Pinion shaft 7 Springs a Direction of rotation

Figure 5.17 Roller-type clutch

1 Driver flange 2 Disc spring 3 Laminated core 4 Clutch race 5 Stop ring

6 Stop collar on output shaft 7 Helical spline on output shaft

Reduction gear

In a conventional starter-motor arrangement, the pinion rotates at the same speed as the motor armature. Normally the pinion/clutch assembly is mounted directly on the armature shaft and in order to generate sufficient torque, the motor must be relatively large, and therefore heavy. If reduction gearing is utilised, a motor of the same power rating but with higher speed/lower torque characteristics can be employed, and due to this the motor can be 30–40% lighter. In addition a smaller, lighter starter motor allows designers a higher degree of freedom when designing the engine compartment and placing other equipment in it.

The reduction gear starter also has a higher apparent inertia (note that the base inertia of the armature is multiplied by the gear ratio squared) and this provides a considerable ‘flywheel effect’ which damps out instantaneous speed variations during cranking and helps to carry the engine through the cylinder top dead- centre position smoothly. As a result, the engine speed is consistent during cranking when the fuel is injected and this has a positive effect on the fuel injection pattern, which aids the starting process. For engines with fewer cylinders, this ‘flywheel effect’ ensures that the high peak torques of each cylinder can be overcome with a relatively low amount of starting power.

Reduction gear starters commonly use planetary gears or a spur type gearbox:

● Planetary-type gears: planetary gear systems are favoured in starter motors manufactured by European manufacturers. Planetary gears are compact and can offer low gear ratios (see Figure 5.20). The gear geometry offers high torque generation with minimal noise. The planetary gearing has a fixed internally toothed ring gear (the ‘annulus’). The drive input is via the ‘sun’ gear which is attached to the armature shaft. The ‘planet’ gears (normally three of them) are in engagement with the sun and annulus. As the planet gears ‘orbit’ around the sun gear, their bearing shaft journals drive the output shaft which carries the helix/pinion/clutch assembly. The arrangement is shown in Figure 5.19.

The transmission ratio can be calculated as follows: Transmission ratio = 1 + Number of teeth on annulus

Number of teeth on sun In standard designs, the sun and planet wheels

are steel and the annulus is a composite plastic. In more demanding applications, the annulus is also made of steel. Generally the transmission ratio is in the region from 3:1 to 6:1 and this allows optimisations of the starter motor to match the engine characteristics. Higher transmission ratios allow increased speed for better warm-starting performance whereas lower ratios allow more torque for improved cold-start capability.

● Spur-gear type: an alternative to the planetary gear arrangement is the spur-gear type. This

1 Planet gear 2 Sun gear 3 Internal gear

Figure 5.19 Epicyclic gear set schematic

Figure 5.20 Starter motor reduction gear set

Types and characteristics of starter motors 113

design of starter motor is preferred by far-eastern manufacturers and involves a different general construction and arrangement of the components (although the components are the same). The general design is shown in Figure 5.21.

114 Starting-motor systems Fundamentals of Motor Vehicle Technology: Book 3

Note that the motor and solenoid are effectively reversed with respect to their relative positions when compared with the standard design. The motor armature is effectively in the piggy-back position and drives the pinion/clutch via reduction gears which give an overall transmission ration of approximately 3:1. The solenoid sits directly behind the pinion/ clutch and acts directly upon it to move it in/out of mesh with the flywheel. The motor used is a standard four-pole machine but runs at high speed to generate the necessary power and this is converted to torque at lower speed via the gears. The armature and reduction gears are mounted in ball or roller bearings due to the high rotating speed of these parts. These units are well proven and are commonly used in the application range of 1–1.5 kW for light-vehicle starting systems.

5.3

ELECTRICAL CIRCUITS