Calidus prerotator problem

[twistair]

Recently we got a failure on a Calidus while training its owner: on a prerotation I felt jerks on a mast, stopped prerotation and found that prerotator gearbox has certain problems inside it. This machine has about 40 TT, ca. 60-70 prerotations only, mostly 240 RRPM, sometimes 250-260.
I e-mailed manufacturer about this problem, got a reply that POH limits prerotation under 220 RRPM though earlier Calidus designer e-mailed me that I can "easily prerotate up to 260 RRPM". Since they refused to replace damaged gearbox we ordered a new gearbox directly from manufacturer http://maedler.de/Product/1643/1331/...1339/1341.aspx
When it came I found that it needs some upgrade to fit Calidus which was made in few hours. During the story I contacted some other local Calidus owners and they all told they also had same failure. After reading technical info on this gearbox it came clear that this gearbox is definitely too weak for the purpose:
manufacturer of this gearbox limits its rpm to 1400 while in Calidus it sits directly on engine crankshaft and reaches 3200-3500 rpm when rotor reaches 200-220 rpm. Max torque on prerotation is also at least twice higher than manufacturer 's limit.
I disassembled damaged gearbox and found that everything is OK inside it less pinions which were trashed. Today we installed new gearbox and will try to see how long it serves if turned not higher than 220 rrpm on prerotation. My guess is no more than 100-120 prerotating cycles
I wonder if other owners got same problem and how you fixed this?
This failure was a disappointment while I must tell that the design of the whole assembly (pneumo clutch+gearbox) is very elegant and works exciting: one single touch of the activation button engages clutch soft and effective.
Now I wonder how to make gearbox more durable...