Brit Progress: Setting the Return Crank

[Deleted]

I wasn't looking forward to this job as I've read so much about the potential difficulties! However, good old LBSC's method is simplicity itself.
For those who may be interested, set the crank at forward TDC: I used a clock gauge against an extended crankpin to obtain this because a small error makes a huge difference to the position of the return crankpin. Disconnect the lifting link from the radius rod and position (waggle) the expansion link so that the die block can travel full deflection without moving the piston valve rod. I put some packing between the lifting link and expansion link and used a toolmaker's clamp to clamp it firmly in place. A long dummy pin was put in the hole in the Ex.link, and the return crank positioned about 7/16" behind the main crankpin. Measure the gap, turn crank to BDC and measure again. Repeat and keep adjusting until the distance is the same. I got within 2 thou after the third go so I hope that's OK! Subtract the pin diameters and you have the centre distance of the eccentric link.

I lightly centre-popped the join between the return crank and shaft, and again used a toolmaker's cramp to hold everything while the two holes were drilled and reamed. The journal is harder than the return crank so be careful not to open up the outer holes! I now have to repeat the exercise on the other side.
JB

[garethp]

Looks good, keep up the good work! Your pics provide some much needed inspiration for me to make some more parts.

[Deleted]

Gareth, it's nice to know that my pictures have reached the parts that you haven't yet reached!
Now where did I put that can of Stella Artois......
JB

[drjohn]

It's looking good JB.

But why do you start a new thread for each job you're doing on Brit - it would be much more useful to have all your endeavours (the Britannia ones - not ALL your endeavours ;D) in one thread.

DJ

[Deleted]

DJ. I'd thought about the continuous thread approach, but I don't know how to change the title of an added section of thread to reflect the current progress. Enlighten me! If it can be done I'll move the lot and delete the dross!
JB

[drjohn]

You can't change the title of the thread, but you can put a heading on each post in bold, and the date of posting will show there is a new post.

It's just that the speed at which this site adds threads makes it impossible to keep going back to find your previous postings - at least with my internet service depriver, that is!

I'm sure if you talked to the moderators, they could change the thread to merely "Brit progress" - in fact, as we have sections devoted to Simplex and other stuff, maybe you could get the moderators to move all your stuff into a section dedicated to Britannias.

DJ

P.S. Then again, you could always start your own website ;D

[Shawki Shlemon]

Hi DJ
To quickly find your thread like you suggesting to JB , say you are on general chat and start a thread and want to keep track of it , make a not of the page number ( eg 1 of 76 ) then later on you want to go back to your thread and the number of pages has increased to 86 , your thread will be on page 11 or 12( depending when the page changes from 1 to 2 ) . Just add the difference between 86 & 76 .

[Deleted]

Help!
Having now set the crank for the other side, I find
the length of the eccentric link is some 0.050" shorter than the other side. This return crank is also displaced more than the recommended (approx) distance of 7/16" from the driving wheel centre. The expansion link is also describing a greater arc than its counterpart resulting in the link fouling the motion bracket! the first link was shorter than expected by 0.070", although LBSC says take the measurement from the job. The motion brackets are positioned spot on to the drawing and there is no significant slackness in any of the gear joints. I've just watched the youtube video by trainman and he talked about huge differences on each side of a locomotive. Should I be worrying about it?
Help!
JB

[baggo]

Hi JB,

that does seem a big difference between the two sides. I wouldn't expect more than a few thou if all the components are identical and in the same position. Your getting excessive swing on the expansion link because the return crank pin is too far from the axle centre and it's describing too big a circle. The problem then is that the length of the return crank is not correct for the diameter it's describing. Are both expansion link pivots at the same height above the motion centre line? They no doubt are but just an idea.

John

[baggo]

Hi JB,

Had a quick doodle in CAD and drawn out the valve gear . By my reckoning the eccentric rods should be approx. 3.76" long assuming the return crank is set to give a throw of 7/16". There's no guarantee that the valve gear is accurate though as Old Curly used to round the measurements to the nearest 1/64" or 1/32" to make the figures easy for his 'Tyros'

John

[drjohn]

Hi JB.

I would have thought that the eccentricity of the return cranks should be identical despite anything else, and the differences in overall geometry get sorted out by the legnth of the eccentric rod.

Your return cranks are both the same length? or has one shrunk in the boiling?

DJ

[redmog]

JB.
Looks like doing a lot of measuring today!
Sounds like there is an error somewhere.
I think it's a process of elimination in order to detect the error.
First stage -
I would first remove the radius rod and check the return crank and expansion link movement by making up a dummy eccentric rod of about the required length.
The return crank position needs to be close to the design dimension in order to impose the correct arc distance movement on the expansion link.
Now measure the distance the expansion link moves.
If they are equal - the expansion link centres and position are OK.
Second stage - I'm still thinking about it!
I hope I'm giving you food for thought!
Chris

[Deleted]

Thanks John (spot on with the CAD) and Chris (the right approach to validate the thing). I'm fairly confident that the position of the RH motion bracket is incorrect. I lined up the assembly to give me a perfect right angle for the reverser rod across the frame, and free running in the bushes. I forgot that the frame isn't a surface plate and the bush may be a bit high.
In retrospect I should have put a pair of holes thru the frames where the trunnion bushes should line up. I shall do that today after I've checked out the positions!
JB

[drjohn]

The other thing JB that's quite important when you're doing all this measuring, is the wheels need to be jacked up to running position ie about half the spring travel.

This is actually more significant with inclined cylinders than with the all square setup as in simplex.

DJ

To expand on my previous post, Don Young suggests that the return crank position is done even before you put the wheels on the axle by making a jig which fits in the hole in the wheel with the appropriate offset for the hole in the crankpin - if you get Don's / my drift.

[Deleted]

Thanks for all the advice. Problem solved. I had made a serious positioning error with the motion brackets, so I drilled a hole each side to line up the trunnion bushes. redrilled and bolted the brackets, worked from there!
I even took some advice and did a little DrJohn jig to position the return crank in the right place. It works! (I await the smart comments on that one!)
To determine the length of the eccentric rod I took the average of the distances between the eccentric link at rest, and the crank pin at TDC and BDC. I hope that is the right way to do it. I got 3.78", not the 3.76" predicted by Baggo, but nobody's perfect!
JB

[baggo]

Apr 17, 2009 17:00:10 GMT simplyloco said:

I got 3.78", not the 3.76" predicted by Baggo, but nobody's perfect!

Hey, what's a few thou between friends! ;D

Glad you've sorted it

John

[spamcanman]

This was a common error on the Winson Britannia motion bracket it was 1/16th out so could not be timed properly maybe that's why some people said they never run!