Page 186 - The Tribology Handbook

Many machine systems require clutches which can pick up the drive to a machine that is already rotating or alternatively
can release the drive when another driver takes over. For high power applications friction clutches tend to be large and
generate heat due to slipping before synchronisation. Simple free wheels or sprag clutches are unsuitable for high torques
because of their small driving contact areas.
High power overrunning clutches need to provide positive engagement and achieve a large driving surface area by
transmitting torque through concentric internal and external teeth, generally gear teeth. When the clutch is disengaged
these concentric teeth are separated axially.
A synchronising self-shifting clutch is shown in Figure Oil lubrication is required for the clutch teeth and pawls
5.1. This has a pawl and ratchet mechanism to sense to prevent wear and corrosion. Lower power and speed
synchronism between the input and output shafts and to (e.g. 1 MW at 3000 rev/min) clutches are usually seif-
align the teeth which are then shifted into mesh by the contained units with an integral oil system. Higher powers
small torque applied through the pawls to helical splines. and speeds require a force fed oil supply, this is normally
Conversely reverse torque on the splines causes the teeth arranged to be common to the other machines in the
to disengage. system.
Powers of up to 300 MW have been transmitted by When the clutch is disengaged with the output side at
these clutches and the limiting factor in their design is high speed there will be a drag torque on the input shaft
centrifugal stresses in the outer geared rings. due to oil viscosity effects that may tend to keep the input
High speeds in both the overrunning and engaged machine rotating continuously at low speed. This can
modes can be achieved (up to 15000 rev/min) because the usually be accepted, if not a brake can be fitted to the
driving teeth of the clutch are separated axially when the clutch casing.
clutch is disengaged and because the pawl and ratchet When the clutch teeth are engaged there is a strong
mechanism can be designed for such high relative speeds centring effect between the input and output parts. If the
without wear. input and output parts are always in good alignment (e.g.
Clutches can be mounted directly between two shafts, in in a gearbox) this centring effect is acceptable, a semi-rigid
a separate casing or in a gearbox. clutch of this type is shown in Figure 5.2.