A shrink fit

[martin perman 1Participant @martinperman1]

Gentlemen,

I've just had a Magneto completely overhauled, including a coil rewind, and the friend who does this for me informed me last night that the 16 tooth chain sprocket that fits onto the magneto taper shaft has three cracks in the boss. I intend shrinking a ring onto the boss to close up the cracks but I'm sure what tolerance I should bore the ring to to allow it to end up an interference fit, could anybody advise please.

Martin P

[martin perman 1Participant @martinperman1]

[Bob RodgersonParticipant @bobrodgerson97362]

Hi Martin,

I can probably make you a new sprocket on the inc mill. Here is a link to one I made for somebody for a 1933 Sunbeam.

<iframe width="430" height="242" src="https://www.youtube.com/embed/flxh0OkzurQ&quot; frameborder="0" allowfullscreen></iframe>

[martin perman 1Participant @martinperman1]

Bob,

Thank you very much for your kind offer, on this occasion I will decline as the actual sprocket is in good order and I thought it would give me a chance to make a shrink fit sleeve as an exercise, once the sprocket is fitted there is a chain guard and it will not be seen.

Thanks again,

Martin P

[Bob RodgersonParticipant @bobrodgerson97362]

No Problem Martin.

[EmgeeParticipant @emgee]

Hi Martin

Is there sufficient working clearance when assembled to use a thicker split ring which could remain in place when operating. ?

If so you could use a single closing screw with 1 slit or depending on location of the cracks may have to use 2 slits and 2 screws to apply even pressure.

Emgee

[Clive FosterParticipant @clivefoster55965]

A thou per inch of diameter should be fine which will give you around 3 or 4 thou of room to get it on at red heat before it cools off and sticks. Sort of ring you are talking about cools very fast. If it does stick you will be able to finish the job by cold pressing. Book says more like 2 or even 2.5 thou per inch but it that jams it will stay jammed!

One time I did something similar I gave it half a thou, polished everything up for a good finish and pressed it on cold. Probably 1 1/4 " or so diameter.

Clive.

[martin perman 1Participant @martinperman1]

Gentlemen,

Thank you for your advice, I'm collecting the magneto next Wednesday so a little project for next weekend.

Martin P

[pgk pgkParticipant @pgkpgk17461]

Could i just ask if there might be a benefit in using material of a higher thermal expansion so you can get it on but shrink it tighter?

[duncan webster 1Participant @duncanwebster1]

as Clive says, a ring with little mass cools quickly. Try putting the sprocket in the freezer for a bit to shrink it as well as heating the ring, gives more chance of getting it all together before it sticks. In industry they often use liquid nitrogen and cool the inner bit rather tha heating the outer bit, especially when the outer bit has been heat treated. The exact interfrenec depends on the thickness of the outer ring and hub. Clive's suggestion of 1 thou per inch is a good rule of thumb.

[Clive FosterParticipant @clivefoster55965]

pga pga

Probably no benefit to using a material of higher thermal expansion to get a tighter shrink. Generally higher expansion goes with lower elastic stress limit .  So with a thin ring you stand a good chance of it not being strong enough to carry the hoop stress without permanent deformation stretching it to a larger diameter.  sfter stretching it wll be a larger diameter resulting in less shrink forces than you would have got by starting with a smaller interference fit. It gets complicated fast. There are on-line calculators for this sort of thing but, unless you really understand this sort of stuff, they are best thought of as quick sanity checks to verify that what you are doing is safely in the well behaved range.

Rules of thumb and KISS are good enough for folk like us. Best not to go where the map says "here be dragons". As mentioned previously I did a similar job by pushing on cold after taking advice from folk who knew more than I. Consensus was that shrinking would give more hold if done right but pressing cold would give enough grip and be much easier.

Clive.

Edited By Clive Foster on 13/03/2016 17:25:37

[Involute CurveParticipant @involutecurve]

The cracked boss is probably due to the taper fit being done up too tight in the first place that caused the cracks, I would think a shrunk on sleeve unless it has substantial thickness, will either stretch or simply crack also, personally I would make another and do a proper job or take up Bobs kind offer, if you don't have the facilities to carry out the work yourself.
I get the wanting to do it yourself bit, but sometimes its simply not the best solution.

Shaun

I just the read the above post, a good solution.

[martin perman 1Participant @martinperman1]

Shaun,

The reason for repairing the sprocket, yes it had been overtightened by someone, is that when I restore my engines in this case a Lister D, I like to keep all of its parts together where possible, I restore to working order but in its original clothes, no painting unless no choice etc, there is no real load or rpm on the sprocket and as far as the public are concerned it wont be seen because of a chain guard but I will know so a simple repair keeps me happy. My friend was dissapointed that he would have to rewind the coil as it and the condenser were original from 1937 but unfortunately the coil was tracking from within so needed rewinding.

As to access to machinery between my brother and I we have two lathes and three mills and as time served engineers are more than capable should the need arise 

 

Martin P

Edited By martin perman on 13/03/2016 18:28:39

Edited By martin perman on 13/03/2016 18:29:15

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