Fit­ting tight shoes

Is a spe­cial tool needed to fit a dy­namo field coil?

Classic Bike (UK) - - Workshop -

Andy Reynolds asks if he needs the ex­pand­ing jack tool shown in old books to fit a Lu­cas dy­namo field coil or if sim­ply tight­en­ing the re­tain­ing screw with an im­pact driver will do the job.

Ro­tat­ing a coil of wire within a mag­netic field cre­ates a cur­rent in the coil, a stronger mag­net giv­ing more cur­rent. Sim­i­larly, cur­rent put through a coil of wire wrapped around a block of iron will turn it into a mag­net, and the greater the cur­rent the stronger the mag­netism up to the ma­te­rial’s limit or ‘sat­u­ra­tion’ point.

Mo­tor­cy­cle dy­namos em­ploy both of these fea­tures. A metal shoe in­side the body is en­er­gised by the field coil to make an elec­tro mag­net that pro­vides a mag­netic field that pro­duces charg­ing cur­rent in the ro­tat­ing coil (ar­ma­ture). Vary­ing the field coil cur­rent al­ters the mag­netism and thus the dy­namo out­put. The shoe squeezes the coil into place and runs very close to the ro­tat­ing ar­ma­ture so when re­new­ing the coil it is vi­tal to make sure the shoe set­tles fully home so it doesn’t foul the ar­ma­ture.

The Lu­cas tool was rather like a small car scis­sor jack that ex­pands to press the shoe home. I made my own ver­sion (above) but in prac­tice you just need to make sure you wran­gle the shoe into the coil so that it will not get pinched any­where be­fore fit­ting and that the shoe is set­tled com­fort­ably with its curve seated to match the bore of the dy­namo body. That achieved, you only re­ally need to tighten the screw nor­mally, al­though I would rec­om­mend a dab of Loc­tite. You don’t want this screw com­ing un­done…

Rick made a tool to fit a field coil, but reck­ons it isn’t re­ally needed

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