As John said, the part you are referring to is called a tuning machine, a tuning peg, or sometimes (confusingly) a tuner.
If you have the Schaller machines that are the most common on hurdy-gurdies, you can tighten the screw on the top. You can try finger-tightening it, but you may need a tool to get it tight enough to hold the string under tension.
Alden
I have banjo heads on my gurdy and it seems one of them is slipping/needs tightening. Do I do this using the screw in the centre of the head? Many thanks Jacqueline
