Modifications To Rob Roy Drawings (0-6-0 Tank engine by Martin Evans)

From: Michael Jones 
Subject: Suggestions for corrections to Rob Roy


Suggested Corrections to the Rob Roy Drawings by Martin Evans

The Rob Roy locomotive in 3.5" gauge is often suggested as a beginners 
locomotive. Therefore, it's a bit disconcerting when one realises that 
after so many have been built, no corrections to the plans seem to have 
occurred, leaving each new builder to find the errors on their own. This
list is presented in the hope it will save builders time by pointing out 
certain pitfalls in advance.

These suggestions and corrections are meant as guidance only. They are 
not to be taken as the final word on how you build your loco. Most 
people, for example, don't fit brakes to this.

Some of the ideas presented here as opinions are just that. You probably 
don't have to make the changes, but they're presented for you to 
consider, even if stated as fact. 

The members of the Whitchurch (Cardiff) & District Model Engineers and 
particularly Ken Stoat were very helpful in the compliation of this list.

General design changes (which effect several drawings)

Lubrication can be changed to allow displacement lubrication. This is 
done by fitting a tank and the associated pipe-work in the space between 
the frames behind the rear axle and the firebox. The displacement 
sightglass fits nicely on the left side of the cab. This change will also 
require an additional outlet on the steam turret on the boiler.

Drawing No. 1.

Brakes
If using laser-cut frames, holes may be spotted for brake gear hangers. 
The "mystery" holes behind the rear hornblock cutout are for the brake (or 
bellcrank) hanger whihch is not specified on the original plans, and only 
shows in the book "Building the Rob Roy and William" by Martin Evans 
(hereafter "the Book"). Unfortunately, no size for the holes is indicated.

Plans may be available from Nexus (Argus Plans) which provide more 
complete details of the shape and size of the brake bracket. In lieu of 
that, it is suggested either ignoring these spotted holes (and not 
fitting any brakes) or spotting through with a 7BA tapping drill. If 
there is even a remote possibility of fitting the brakes, drill these 
holes before assembling the frames. 

Additionally, there is a hole shown on the prints above the dump pin for 
the ashpan. This is for a brake block hanger and should *not* be drilled 
5/32", but *threaded* as mentioned in the book. 

Frame Stretcher
An additional frame stretcher should be fitted forward on the frames above
the cylinders and in front of the front hornblocks. It is felt that
without this, the cylinders and pipe work are acting as frame stretchers
which isn't such a good idea on a rough track. The front buffer beam 
isn't enough to support the frames in this area.

Drawing No. 2

Coupling Rods
Bearing flanges are needed on the inside of the coupling rods. Allow some 
extra length on the leading and trailing crankpins.

Valve Spindles
VITAL!! Change the pitch of the thread on one end of the valve spindle! 
This allows the setting of the valves to be more easily modified. Others 
have made carriers for the valve spindle which ease adjustment of the 
valves. 

Fit thin locknuts to both valve and piston glands.

Drawing No. 3.

Valve Crosshead Alignment
Valve crosshead interferes with leading hornblocks and axleboxes. Also 
reported as the valve link brackets don't line up with the valve 
crosshead or valve spindle.

One solution is to increase the centre line dimension of the valves from 
1/2" to 9/16". This will require planning vefore machining the cylinders 
to leave enough on the valve face to permit this modification. Otherwise, 
the hornblocks and possibly the axleboxes will have to be filed to 
accomodate the valve crosshead.

If the centre line distance is modified as recommended, the suspension 
lever bearing will have to be lengthened a similar amount, but all other 
parts can be repositioned by sliding the eccentrics inwards and moving 
the lifting arms inward along the weighshaft. (see the drawings if this 
doesn't make sense).

Drawing No. 4.

Crosshead pump
The bottom section of the crosshead pump is too close to the cylinders. 
Modifications of the pump location are necessary.

Drawing No. 5.

Blast Pipe
The blast pipe is in the wrong position. Draughting is all wrong. Several
solutions exist, but increasing the height of the blast pipe by about 3/8"
to 9/16" seems to improve draughting. Make an extra top (or two) from
7/16"  hex with approriate dimensions wouldn't be a bad thing just to have
on your first steaming day. It may be necessary to change your blower tube
location, too. 

Experimentation will be necessary. But the best draughting seems to be 
when the pipe's top is about even with the centreline of the boiler.

Also see comments for drawing no. 6.

Boiler Crown Stays (over the top of the fire box) 
The crown stays should be made into a "T" section and alternated into the
space between the two flat stays. Several members have noted that the
crown sheet on their boilers has deflected after just one steaming.
Instead of an "L" joint on the crown sheet, it is thought that a "T" joint
would be better. 

One member riveted an extra piece of copper in an "L"  along the bottom of
the crown stays (inside the 7/8"  seperation" to make the "T" joint. 
Alternatively, making tabs from the bottom of the stay and bending them
alternately into the center of the crown sheet might work, too. 

Drawing No. 6.

Boiler Dome Bushing
IMPORTANT!! The dome bushing on the boiler should be 3/4" diameter bore!

Regulator Spindle
Making the regulator spindle 3/16" with a 3/8 x 32T bushing is recommended.

Handpump
Increase the handpump ram size to increase it's volumetric capacity. 

Draughting suggestions, Part II. 
To further improve draughting, it is suggested that several holes about 
3/8" diameter be added to the ashpan. 

Whistle and Turret Valve
Although no specific suggestions have been put forward, it is felt that
the cramped conditions of the cab warrant that the turret valve could be
of a better design (and would have to be modified anyway, if displacement
lubrication is fitted). Short of this, members have suggested splitting 
the spectacle plate in half to allow easier access to the workings of the 
boiler backhead.

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