I too had never heard of it but guessed it was something to do with machinery. I then found the following using Google:
http://freepages.history.rootsweb.ances ... 32.html#m1Malcolm Bull's Calderdale Companion
Keep t'band in't nick
Maintain a relationship.
I sent her a birthday card, just to keep t' band in t' nick
The expression comes from the mill-worker's task of ensuring that the yarn did not jump out of the guiding mechanism, and the driving rope or belt did not come off the wheel.
See Mill band
Mill band
The rope or belt which drove a machine in the mill.
On bonfire night, this was a popular means of lighting fireworks, because the oil-soaked rope could be lit and would burn slowly during the celebrations.
Keep t'band in t'nick