Eccentric sheave #27 for Stuart triple steam engine..

Hi folks, i have the Stuart triple steam engine and there is a part # 27 for middle pressure cylinder who is splitted in 2 parts and fastened togheter with 2 x 7 BA cheese head screws.

I need to know how do with the working method of the eccentric sheave.

Edited By JasonB on 08/09/2013 17:26:18

Jens

A split eccentric is made from two pieces of rectanglular steel section of identical width (a little wider than the maximum diameter) but different height that are screwed together. Their juction is the split face through which the crank shaft runs. You need to cut off sufficient lengths so you can hold it in a 4 jaw chuck and still have enough lenth to tun the cams and part them off.

Clamp the two sections together and mark out the final postion of the 7 BA screws in the top of the 'thick' section. Drill through with 7BA tapping drill then open up the thick section holes with a clearance drill and counter sink. Then screw the sections together (these screws will be machined so are sacrificial, put in a 4 jaw chuck and 'face off' so you can acuratly mark out the position of both the crank shaft bore, which must be EXACTLY on the split joint between the sections, and the (off-set) rotational centre. Set the steel to run on the rotational centre and turn the OD to size (it is best to rough off the corners with a file to reduce the shock loading while turning a two piece square section, then reset the chuck so that the (now offset) shaft bore is centred and drill and ream the bore then part off and finish the face and width. If you have been clever with your marking out and the drilling of the 7 BA screws you could counter-boore the holes so the screw heads are untouched - I had to modify my holes after turning and before I took them out of the chuck

I hope this makes sense!

Tony

Tony, the eccentric sheave is in whole oversized cast iron in the cast iron set of triple steam engine. Is it not better to drill/tapping the hole for the 7 BA screws first then splitting the eccenter sheave to make exact location when the splitted sheave is screwed togheter?

I has not problem to make the double eccenter, i used the jig with degree disc for lathe when i maked the double eccenter for Stuart D10 who has reversible gear.

Edited By Jens Eirik Skogstad on 11/09/2013 22:09:13

Edited By Jens Eirik Skogstad on 11/09/2013 22:09:51

Jens,

Yes you are correct, I guess you could make life easier by drilling and tapping through with 7 BA then you have more tolerance with you split the sheave. I would find that spitting job to be quite difficult using a thin saw in a vertical mill - but holding seems difficult.
You could open up the clearance holes in the larger section after turning. I have never made these things from cast iron and I found the 7 BA hole location to be absolutly critical and not easy to get right

Good luck!

Tony

Tony, thanks! Now i am more sure enough to begins with the splitted eccenter sheave.