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rotax 4-stroke - gearbox

912UL, 912A, 912F, 912ULS, 912S, 914UL and 914F Engines

All information provided here is correct at the time of publishing to the best of our knowledge, but Rotax recommendations may change at any time without our knowledge.  It is the engine owners responsibility to ensure that the engine is fit for flight at all times.  The information is provided here free of charge and with no obligation from us to the consumer.  To ensure that you have the correct and most up to date information, you should refer to the current Installation, Operators and Maintenance manuals for your engine.  The information provided here may not be valid for aircraft outside the UK.  Click here for ConAir SportsTerms& Conditions, including website usage policy.

slipper clutch / friction torque - to be carried out every 100 hours

Only engines fitted with a slipper clutch have a feature known as FRICTION TORQUE .  This is not the overload clutch slipping, it is a friction of the gear sliding between ramps on the “dog” (see section below).  The friction is determined by the preload on the disc springs in the gearbox.  There are two significantly different torques: Friction torque and Slipping torque - both a feature of the slipper clutch.

The friction torque should allow the prop to rotate through 30º (15º on some earlier models) with a torque of 45±15Nm (30-60Nm).  Engines without a slipper clutch will only move 1-2º.  To measure this the crankshaft must be locked.

To check the friction torque:

1) Turn the engine until No 1 cylinder is at top dead centre.  

2) Remove the bolt in the crank case (see photo) and install the crank locking pin 240880 provided in all tool kits supplied with new 912 / 914 series engines.

3) The prop can now be rotated in either direction between the ramps on the dog (there is no risk of rotating the engine backwards as the crank is locked).

4) The friction torque can be measured with a suitable strap around the prop and a spring balance: force x distance = torque e.g. 10kg (9.8 Newtons) x 500mm (0.5m) = 49Nm.

A more detailed explanation is given in the Line Maintenance Manuals.

loose gearbox

If the gearbox is allowed to lose its spring pressure, the engine will not want to start as the prop will be fighting the compression strokes.

Friction torque is an important part of maintaining the Rotax 912/914 series engines and it measures how “tight” the gearbox is.  A loose gearbox allows excessive movement between the parts and this leads to worn out parts.  A loose gearbox has a distinctive “rattle” at low rpm, if allowed to continue, the rattle will get worse and the internal parts will wear progressively faster.  In extreme cases, a resonance can build up which can cause serious damage to other engine components.  Fortunately the Rotax gearbox is easily removed from the engine in the event a repair is necessary.

GEARBOX - The gearbox part number should identify the gearbox specification such as reduction Ratio, gear set width, and

identify whether it has a slipper clutch or not..  The specification is occasionally altered and the part number is not amended, so

do not rely on the part number.  The serial number of the gearbox assembly is etched on the outside.  Early gearbox’s did not have any part numbers on them.

serial number position

serial number position

slipper clutch / friction torque

loose gearbox

what gear ratio do I have?

All 912S & ULS engines have a 2.43:1 gear ratio

912 engines may have a 2.27:1 or occasionally the optional 2.43:1 gear ratio

Serial production 914’s have a 2.43:1 ratio

You can calculate the ratio by using an optical tach and dividing its reading by the crankshaft speed.

what does the slipper clutch do?

The slipper, or overload, clutch protects the crankshaft in case of a propeller strike.  It does not slip during normal operation.  It is constructed of inner and outer plates that are forced together with spring pressure.  If the propeller suddenly stops these plates “slip”, isolating the shock from the crankshaft.  The method of testing the clutch is to measure the amount of torque required to “slip” these plates and is therefore called “slipping torque”.  The slipping torque is very high, about 700 Nm, and it would not be wise to attampt to measure in in the airplane.

gearbox service intervals

“dogs”

The Rotax gearbox is designed to deal with an incredible amount of “torsional damping”, as a result of the prop acting as a huge rotating flywheel.  The pistons are constantly producing torque pulses as they go through the firing order, and something has to absob this “argument” or parts will fail.  The Rotax torsional dampener consists of large triangular metal “dogs” that mesh with each other.  The dogs are angled in the contact area so they can ramp-up when they are twisted.  The ramping-up allows the movements necessary to prevent the piston pulses from reaching the prop.  The dogs are forced together with “disc springs” that serve to further dampen out the pulses.

“dogs”

what gear ratio do I have?

what does the slipper clutch do?

gearbox service intervals

Access point for crank locking pin

The friction torque should be measured every 100 hours.  If the friction torque is outside limits, the magnetic plug is contaminated or there are any other concerns over the gearbox, an inspection should be carried out, contact your local Service Centre.