Rotax gear box preload, how to properly preload a Rotax A gear box, Rotax B gear box.

Incident report:
An ultralight pilot is cruising along in his plane when all of a sudden his engine revs up and then abruptly stops.

He dead sticks his plane safely down into a farmers field.

A quick inspection reveals that he can not turn the engine over, it has locked up. He removes the engine, and takes it to his local Rotax ultralight aircraft engine repair station.

Upon disassembly it is found that the gear box ring halves (30) have shattered, the angular retaining ring (29) has been damaged and the groove on the prop shaft that the ring halves fit into has been damaged.

Further inspection reveals that the spring washers (25), are worn badly on the edges and at the center. The service center decides to reassemble the gear box to check for gear box preload. To get the proper preload a number of shims had to be added. For diagnosing proper preload see below.*

The improper preload was allowing the springs to compress and decompress. This was putting a "hammering" action on the retaining ring and ring halves, pounding the halves against the prop shaft groove, until they failed.

Suggestions:

If you are flying on a Rotax A or B gear box it is suggested that at 150 hours and at engine rebuild 300 hours the gear box be inspected and the proper preload on the spring washers be set.

Ring half 827 860 (1 set) Angular ring 827 840 (1) Disk Springs (12) 939 020 Shims 944 474 .01 944 472-0.5 944 470-0.2 944 473-1 944 471-0.3

1. Place gear box on a suitable support under hand press.
2. Place the mounting yoke, part no. 876 880 on the dog gear (20).
3. Place angular ring (22 fig 021) upside down (with "L" shape down- wards (see ill. fig 021) on prop shaft to facilitate reading of the distance A.
4. Apply a load of 16 kN (3600 lbs fig 020) on the dog gear via the mounting yoke. With this load, disk springs will be completely compressed.

ATTENTION: Do not exceed maximum load of 16 kN (3600 lbs), otherwise cover or dog gear will be damaged.

With springs in compressed state, measure A, the distance from top side of angular ring to lower edge of groove (see illustration). Relieve pressure, remove all items from prop shaft and compensate distance A, by placing appropriate shims (16 fig 022) under distance ring (17 fig 021).
Shims are available as spare parts in the sizes 0.1 / 0.2 / 0.3 / 0.5 and 1.0 mm.