Pulley Woes

[Corset]

I have been happily planing and thicknessing all day and was almost finished when I hear a flapping noise.
Looking at the back of my planer thicknesser I discover the belt is going mental and flapping about.
I took the back off and had a look, the belt is 3.4 inch wide and has 6 grooves or so that match with the pulley on the motor and the cutter block. I inititailly thought the pulley had become loose and so tighten the motor but when firing it up the belt seems to jump 1/2 on 1/2 half off the lower motor pulley.
So is the pulley out of alignment and how do i know what is correct alignment?I am a bit beyond my knowledge base so would be grateful for any info or pointers.
The planer is a moretens h410.
Owen

 

[tjwoodms]

You could use a level too establish the correct alignment between top and bottom pully it dosnt need to be exact but as close as you can get it. It would also be worth checking that the motor hasnt twisted on its mounting.
Wilst your at it check that the pulleys are tight on their shafts and havnt moved on planer thicknessers the cutterblock pully can become loose overtime if that is the case you may need to change the pully as when they bocome loose they vibrate and wear on the shaft (a new pully can be matched by and good bearing suppier at a fraction of the cost of the machine supplier.

 

[Corset]

I have checked with a straight edge that the pulleys are in alignment now. No play in bearing or block and the motor does not appear to have any twist.
Whe I tensioned up the belt the problem seems to be that there is an initial flap on startup and a little bit off flap on running but the belt is not jumping off the pulley any more. Does this sound like a worn belt?
Owen

 

[tjwoodms]

yes it does sound like the belt is worn/stretched try increasing the belt tension but not to much or you could wreck the bearings on a flat ribed belt you should have even tension accross the belt i.e simular flex in the middle as the outside edges