Combidrive
The Right Clutch For The Job
The process of selecting an appropriate clutch arrangement for a dual drive set up will need to have taken into account relative shaft speeds during each condition, prime mover types, (e.g. electric/diesel) and whether it is acceptable to 'motor over' one of the drives when not required. Comparing the following features and drawbacks of each clutch type with the particular application will help establish a satisfactory set-up.
- Motor
- Overrun clutch
- Centrifugal clutch
- Electromagnetic clutch
- Driven machine e.g.. pump
Centrifugal clutches
- Engage progressively with increase in speed (good for high inertia starts)
- Not cost effective for low speed riving (e.g. below 1400 rpm
- When 'freewheeling' wear is limited to only one or two ball races idling
- Speed point of engagement can be adjusted
- Will not release or freewheel when complete clutch is at drive speed
- Transmissible torque increases with speed
- Will drive clockwise or anticlockwise
Overrun clutches (one way, sprag or freewheel type)
- Engage immediately in one direction only, running free in the other direction
- Torque capacity does not depend on speed
- Preferable to centrifugal clutch for low speed operation
- When idling (freewheeling), limitations on speed must be taken into account *
- Not suitable for reversible drives
- Will release or freewheel when the output from the clutch exceeds the speed of the input
* whilst friction of component parts limits idling speed of many designs, alternatives are available with sprag elements which disengage (lift-off) or rollers which disengage hydro dynamically thereby allowing higher idling speeds
Electromagnetic clutches
- Not automatic, requiring an 'external' electrical supply to engage
- Torque capacity does not depend on speed
- Input and output parts can run free in either direction when released
- When released friction is only confined to associated bearings
- Can be released or engaged whilst stationary or running
- Are not dependant on speed to engage (see centrifugal type)
- Are not dependant on relative speed to engage (see freewheeling type)
Clearly mistakes can be made in haste resulting in a transmission which may be more expensive than need be, give too short a service life or will not work at all! One such case where our customer was adamant that freewheeling clutches would do the job for the reversible conveyor (see 'reversible drive' diagram above) resulted in a bottle of whisky over the difference of opinion - Cheers!