Mystery of the missing ‘teeth’
QWhen checking the alternator belt on our boat I noticed many of the ‘teeth’ were missing. The belt had been on for 12 months and is a good make. I adjust it each time before we set off (10mm free play). The belt is 1025x10: do I need a thicker one? Is it normal to have to adjust the belt every four to five hours? MAGGIE ALLEN, via email
ATONY REPLIES… You should not have to adjust the belt every four or five hours once it has bedded in. The ‘10’ figure is the width of the belt. On a 10mm belt I would not want to try driving anything much over 70 amps (with 90 amps as an absolute maximum).
Have a look here: scribd.com/doc/52177682/V-belt-selection. You will see they list two different belts with a 10mm top width, Z and SPZ, but they have different bottom widths which means the belt face angles are different. This must be matched to the pulleys in use, otherwise you will get slipping and excess wear. Take an unworn belt, twist it inside out and push it into each pulley in turn. There must be an air gap between the belt (otherwise the belt is too thin) and the bottom of the pulley and the pulley sides and belts faces must be exactly parallel (otherwise the angle is wrong).
Use a straight edge on the front of the crankshaft pulley to ensure that the belt runs from that pulley are parallel with it. If not, use washers to pack the alternator front bracket until they are. ( The back bracket often has a sliding bush that will adjust itself for the new length).