ISSA 640 Motor – Sparks from Brushes/Commutator

Hi all

I have just been and fitted some new brushes to an ISSA 640 Commercial vacuum cleaner but have a problem which I cant work out the answer to.

I have cleaned the commutator and blown out all the dust and other dirt from the motor, I have also checked for and roughness by placing my finger against it and spinning it manually as well as put the meter across all the segments to check that there are no dead ones, all of this looks good and I have taken great care in making sure that the brushes are a snug fit to the commutator – the motor now spins but I now have inch long sparks flying from where the brushes meet the commutator which can’t be right. The brushes were supplied by the customer and looked to be the correct ones – they had replaced them a couple of months ago after the original ones failed after 18 months (used 7 days a week for about 7 hours a day).

Why would there be sparks flying and what can I do to stop it?
Why is eating the bushes for breakfast?
Are the two related?

The only answer that I can come up with is that the replacement brushes have a softer carbon than the originals but I thought that one carbon brush was pretty much the same as any other – am I wrong and is there more to it than the size/fit?

Thank you

Jem