Otis Gen2 belt slip

Hi Guys

Just changed some Otis belts on a 2000Kg Gen2, all went well. returned to service and tested to all floors was just about to leave and filling out log card when we noticed the lift had moved slightly.
On looking further we found that the belts are creeping over the sheave and the lift is slowly moving up, seems a lot when you look at the video but in relation to car its about 10mm every 15mins.
Checked tensions and all springs are within 1mm, belts are original manufacturer, no changes have been made to affect the balance, there is no coating on the belts that requires removing or cleaning and its purely slip no drum/sheave movement.

Any ideas appreciated, posted a video here

Otis Gen2 New Belt Slip

Did you check to see if the brakes need tightening? I couldn't tell if the pulley was in motion w/the belts or not from the video.

My bad. I missed the part where you said the pulley is not moving. Did you check the pulleys and belts for oils that shouldn't be present.

Not sure what unitec recommends for cleaning on those but I'd start by thoroughly cleaning them. The coated ropes I've dealt with have recommended isopropyl alcohol. Definitely looks like something got on the sheave or belts.

Where did you get those belts? Brugg hasn’t made those in over a decade.

Going back tomorrow to clean belts and drum but they do look ok, got the belts from Sodica France European Otis parts distributer, checked with them and they have said there is no coating on the belts.
Managed to get hold of an official Otis belt cleaning kit so we will see how that goes tomorrow, if not next would be a half load balance test i suppose check it was balanced on installation although there was never an issue before with the old belts.

Worse thing is belts were fine until an insurance man buffered the CWT, not sure how he managed to do it as we couldn't replicate it but it appears the lift car top shoe impacted one of the belt monitor clamps and damaged the monitor and put a small rip in one of the belts....

Might be worth spraying some belt grip spray?

All sorted thanks for your input guys, sheave and belts cleaned and no slip, i don't know what shocked me most the fact that cleaning them worked as there didn't look any different or the lack of traction/friction these actually run with.
I think the main culprit was the sheave/drum pulled belt aside and gave it a good scrub.

It's hard to tell how they got slippery. Alot of lubricants or things not intended to be on your belts can dry into a clear film or powder that would be hard to tell is on there.

Regular wire ropes don't have this problem...

This is why Schindler 31/3300/5500 and TK Evolution belts have more robust grooves in them.

one day I acrossed that car was hitting limit switch while waiting on top floor. cause car pulley was slipping from belt. that was a 3300AP.

seems like AAA717W1 belts