Alternator brush spring tension

[John Roberts]

Please can anybody confirm that there should be one full turn of the curly spring holding the brushes against the slip rings on an '82 airhead 280W alternator. There's nothing about it in either the BMW workshop manual (which seems to be crap in a lot of things) nor in the Haynes manual. Having said that, it does look like one full turn and a bit from one of the photos. Can anyone tell me any different, please?

ACTUALLY, I'm fitting a 400Watt replacement from Motor Works, there are no instructions whatever included. I think, however, that an extra half turn would be excessive even though the current will be greater.

Sorry to be a pain with all these questions, I comfort myself with the thought that the answers might be useful to others some day.

Thanks yet yet again

John

[Grant Johnson]

one turn it is.

[John Roberts]

Great!

I've just come across a bit of general information regarding brush pressure since posting the thread from the Little Red Book published by Brook-Hansen, official title 'Installation & Maintenance of Electric Motors' If anyone is interested it's an excellent reference book.

It states that 'as a general rule the brush spring pressure should be 0.2Kg per square cm for speeds up to 2,500rpm and 0.25Kg per square cm at higher speeds.

So. Size of my brushes are 10mm by 5mm, that is, one cm. by a half cm. making the area one half of a square centimetre. As the speed is the same as the engine speed then it will be way over 2,500rpm so the pressure should be 0.25 x 0.5 = 0.125Kg i.e. 125gms or four and a half ounces.

Anyway, after all the calculations it wouldn't be easy to measure in my case. One whole turn will do me fine, Grant.

Many thanks

John