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Author Topic: Graham Farish S&CR N Class Chassis Problem  (Read 240 times)

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Offline icairns

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Graham Farish S&CR N Class Chassis Problem
« on: March 30, 2020, 12:13:10 AM »

Last year, I bought a Graham Farish S&CR N Class 2-6-0.  I modified the chassis to a 2-6-2 and used it for an ex-LNER V3 loco 3D print available from Atso-Cad.  I have been very pleased with this model.





However, the chassis has always had a slight problem.  The valve gear occasionally locks up with the hex-head bolt securing the front driving wheel (see photo below). 

The bolt should always be behind this piece of valve gear. 





Originally, this was no more than an occasional irritation.  But recently, this problem has been getting much more frequent.  The valve gear usually locks when going around my 2nd and 3rd radius Peco code 80 curves. 

I have tried:

1) Slightly tightening the hex-head bolt on the front driving wheel to make sure that it is not projecting out more than it should.

2) VERY gently trying to bend the valve gear rod outwards to create a little more clearance so that it always misses the hex-head bolt.  I am reluctant to do too much in this regard in case it damages the valve gear.


In normal operation, there is adequate clearance between the valve gear and the bolt and the loco can do 10-20 laps or more of my layout without a problem.  Then, without warning it will lock up.  It appears that there is too much “slop” on these components so that they miss each other most of the time but occasionally they get into a position where they hit each other and lock up the valve gear. 


a) Can anyone suggest what I might try next to fix the problem?

b) On a separate note, is it possible to buy a hex-head box spanner that will fit Graham Farish valve gear bolts?  I have the Dapol one but, of course, it does not fit GF bolts (it is too small).


Thanks, in anticipation.

Ian





Online chrism

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Re: Graham Farish S&CR N Class Chassis Problem
« Reply #1 on: March 30, 2020, 06:31:31 AM »
Could it be that the leading driving axle has excess sideways movement?
Maybe a shim to limit that play would help?

Offline Newportnobby

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Re: Graham Farish S&CR N Class Chassis Problem
« Reply #2 on: March 30, 2020, 09:56:15 AM »
A lovely looking model and I'm sure @Dr Al can help you

Offline Dr Al

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Re: Graham Farish S&CR N Class Chassis Problem
« Reply #3 on: March 30, 2020, 11:27:56 AM »
I'd check that the slide bar isn't bent inward slightly, and check the rods haven't been compressed by handling. The clearances are tight, but there's no inherent issue with this type of chassis, so the most likely that something is slightly bent or compressed.

Cheers,
Alan
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Offline Dorsetmike

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Re: Graham Farish S&CR N Class Chassis Problem
« Reply #4 on: March 30, 2020, 12:06:06 PM »
Sounds like I've just picked up a "spares or repair" one with the rod disconnected probably due to your locking up problem, I'm looking at £20 for a complete set of driving wheels with all the rods etc conneected
Cheers MIKE
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Offline icairns

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Re: Graham Farish S&CR N Class Chassis Problem
« Reply #5 on: March 31, 2020, 12:25:14 AM »
Thanks to everyone that responded to my original question problem. 

I think (touch wood) that I may have solved my own problem!

As Dr. Al pointed out, potential problems could have been that the slide bar and/or the connecting rod(s) could have been bent out of their correct position.  However, I had checked this before making my original post; the slide bar and the rod appeared in good shape and the same on both sides of the loco.

At 3.00 am this morning, trying to sleep, I wondered if it might be possible to replace the GF hex-head bolt with a Dapol version because I knew that the Dapol wheel pin was a slightly smaller diameter than the GF pin.  Amazingly, when I tried this later in the morning (after getting back to sleep), I found that the Dapol pin thread fitted the GF hole and the Dapol pin went in easily.  However, the problem was that although the Dapol pin has a very slightly smaller dimension across the flat sides, the head projects slightly more than the GF pin.  Unfortunately, the slightly smaller diameter Dapol pin was not enough to avoid hitting the adjacent rod and, because the Dapol pin has a bigger projection, it hit the rod and would not allow even one revolution of the wheel.

I then checked the back-to-back dimensions of the driving wheels to see it the front one was out of line but they were all exactly the same.

I then removed the keeper plate and checked the side play of the wheels.  They are all a bit sloppy but I am sure that they were designed that way and I did not want to mess around with anything in this area.

Then I noticed something.

If you look at the photo below, you can see there is a black plastic thingy (sorry, I don’t know the correct term for this) attached to the rear of the cylinder block.  I am not sure if this black plastic thingy is glued into the cylinder block or if it is all one molding.



On one side on the loco there was a very rigid connection between these two things (no play at all).  But on the other side of the loco, the black plastic thingy could be moved VERY slightly from side to side (towards and away from the driving wheel). 

I am not sure if this was because the black plastic thingy was not glued in properly or if there was a slight crack in the joint between these two things (but I could not see any crack even under magnification). 

So, the solution was easy.  By gently pushing the black plastic thingy away from the front driving wheel and applying a drop of superglue, the connection between the cylinder block and the black plastic thingy became rigid (just like the other side).

After allowing a suitable cure time, the chassis has run 50-plus laps in both directions on my layout without locking up.  The minute movement of the black plastic thingy must have been enough for the upper rod to occasionally get out of its correct position and hit the lower connecting rod bolt.

So, 67664 can now leave the works and get back to empty coaching stock duties.

Before I finish, I am still interested in finding out if there is a suitable hex-head box spanner that will fit Graham Farish valve gear bolts/pins? 
Does anyone have any information about this?

Regards,

Ian



Online andy911john

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Re: Graham Farish S&CR N Class Chassis Problem
« Reply #6 on: March 31, 2020, 03:01:41 AM »
Sorry,  don't have info on box spanner but, very glad that you were able to solve this problem.

 

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