Brass Technical Tips
Number Eight: Diagnosing and Curing Binds
Welcome back to Brass Tech Tips.
In this issue, we are going to tackle the problems of mechanical binds in locomotives.
A mechanical bind can be defined as an interference in the free motion of a moving part.
In a model locomotive, we are most often speaking of parts that move in a rotational
manner, or slide back and forth. A bind is also categorized by the extent of its
duration in a cycle of travel. For instance, a rotational bind usually manifests
itself at one particular segment of a rotation (e.g., from 5 O'clock to 7 O'clock),
and repeats itself at the same point during additional rotations.
It is this characteristic that allows us a shot at finding the cause of most
binds. Indeed, the discovery of many binds is simply the process of
‘informed observation', that is, having an awareness of what parts COULD contribute
to a bind, and then looking to see if one (or more) IS causing the bind.
Sounds simple, doesn't it?
Sometimes it actually is. There are those ‘other' times, though, when a well
hidden bind will make the finding of an electrical short seem like child's play.
With these joyous thoughts in mind, let us proceed.
To be able to correctly diagnose a mechanical bind, it is imperative
that all aspects of electrical operation of the model are in prime condition
(See Other Tips). If this is not the case, and there are electrical maladies,
attempting to troubleshoot a bind can be an exercise in futility. In fact,
it would be difficult to even state that a bind exists, under such conditions.
So, look to the electrical well-being of the locomotive first.
Whereas electrical maladies usually produce erratic and irregular operation,
most binds tend to be regular and repetitious (i.e., cyclical) in nature.
I say ‘most', because some binds may manifest themselves under very unique
and short-in-duration track and/or operational conditions (e.g., a left or
right hand curve, a turnout, etc.).
Check the simple things first. These include checking that all wheels,
especially driving wheels, are mounted securely on their axles, and that
wheel rims are mounted securely and squarely on driver centers. This is
especially worth checking on the drivers with the insulated rims
(normally on the fireman's side). Then, if all this checks out, check
the gauge of all wheels, especially drivers, with the appropriate NMRA
Standards Gauge. An out-of-gauge driver can cause an apparent bind
while going through certain kinds of trackwork, especially turnout
frogs and guardrails. Similarly, a driver or driver rim that is not
mounted square on an axle will wobble as it rotates, and can come in
unwanted contact with chassis/superstructure parts. This can produce a
drag or bind at the point of contact. Indeed, if it is an insulated driver rim,
it can also produce a short.
Do not assume that all drivers of all brand new locomotives are always gauged
correctly. Although out-of-gauge wheelsets on new models are rare,
they do crop up occasionally. Also, they can happen on both lower priced,
as well as on high priced models. A slightly more common occurrence,
on older models, is a loose driver rim on the insulated side. This
apparently happens as the insulating strip shrinks with age, allowing
the driver wheel rim to become loose and cause random out-of-gauge events.
These can cause all sorts of operational headaches, including binds,
shorts, wobbles, derailments, tantrums, etc. In these circumstances,
such symptoms may, in fact, manifest themselves equally between the model
and its owner.
If this happens to a driver, all is not lost. If the insulation
material (usually a red or white material) is intact and
otherwise in good shape, it can be the basis of an entirely adequate repair.
This is accomplished by applying a thin and uniform bead of Loctite
to the insulation strip. This will be drawn in to wheel rim/center joint.
This is done AFTER you make sure that the wheel rim is squarely on
the driver center, and that the wheels are properly in gauge.
Check the gauge at a minimum of six spots around the driver
(i.e., at 60 degree rotational intervals). This assumes, of course,
that the other side of the driver set is square to the axle.
Allow the Loctite to set up before operating the model. I suppose you could
also use ACC in the same manner, but ACC's effect
is almost immediate, and does not allow time for
slight position or gauge corrections.
If the insulation is torn or severely deteriorated, it must be replaced.
New insulation material must be of adequate and equivalent thickness to achieve
a tight press-fit assembly of driver rim to driver center.
The successful reassembly of these three components is,
in my estimation, a kind of art form, and requires good Karma and the
proper incantations (midnight at a crossroads may be appropriate.
I have not yet found one bad enough to require the participation of a
dead sacrificial animal).
Assuming that all of the above aspects of the model are peachy,
let us proceed further. Mechanical binds tend to manifest themselves
mostly at slow speeds. Increasing the speed increases the momentum of
the motor, gears and driver assemblies. This tends to mask all but the
most serious of binds, the most serious of which is known as
‘locked up solid'. So, what you want to do is run the model at extremely
slow speed, such that the model stops when the bind occurs
(i.e., at point of bind). Pick up the locomotive, carry it to your
well lit work area, and place it upside down in a foam cradle
or similar holder. Note the clock position of the crankpins
on each side (e.g., 5 o'clock, 9 o'clock or whatever).
Why? Because many mechanical binds will occur once per driver
revolution, and it helps to know exactly where the bind occurs
and that it occurs repetitively at the same spot.
Now, start delicately prodding each driver (a toothpick is
the perfect tool here) and everything connected to it. This includes
the side rods, main rods, crossheads crosshead guides and valve gear.
You are looking for the part that is ‘tight', that is,
the part that is in a ‘won't wiggle or move' condition. Most locomotives
have enough clearance in moving parts such that a non-binding side rod,
for instance, will move just a bit on its crankpin. Once you have
located the suspiciously tight part (as compared to other similar
and neighboring parts, you need to determine what is causing the tightness.
Some common causes are as follows:
A. Crankpin screw head rubbing on inside of siderod or mainrod.
B. Tip (end) of mainrod to crosshead screw rubbing on driver or
C. Main driver eccentric crank out of proper position. These normally
point toward the
driver counterweight. Usually either the eccentric
crank is loose on the main crankpin or the crankpin itself is loose.
Either case will produce a very nice bind down the attached valve gear linkage.
These can have the added attraction of occurring in a rotationally random manner,
as the eccentric crank randomly changes position. (Note: The eccentric crank
we are talking about here is not the author, though there are those
who would dispute this.)
D. Crosshead binding in crosshead guides, due to guide misalignment.
E. Piston rod binding in cylinder hole or hitting front of cylinder
This can occur if the cylinder assembly is loose or not square
to the chassis frame. There is usually a screw at the bottom
of this assembly to adjust or tighten its mounting. Others
are soldered to the frame: if the solder joint is broken,
it must be redone with the
cylinder assembly properly and squarely positioned. A canted cylinder
assembly can sometimes allow a ‘just long enough' piston rod
to come out of the hole. This usually results in a ‘locked up solid' condition.
F. Valve gear linkage out of position or orientation.
Compare it to the linkage
on the other side (careful, it could be wrong too). Warning!
Be aware that the valve gear linkage on some models will only be
positioned/oriented correctly when the locomotive is upright and
the valve gear linkage is hanging down. Running these in a cradle upside
down can allow parts of the linkage to flop out of position and
create a new bind. This adds immeasurably to the fun of finding the
original bind. I realize that I exposed you to this risk, by advising
you to examine the model upside down in the cradle, but this is
the only good way to do this kind of work, and hence the warning. The floppy valve
gear parts can usually be supported by the judicious placement of toothpicks.
Occurrences of the above kinds of binds are often the result of mishandling
steam locomotive models. The act of picking up the model by thumb and fingers
around the drivers will usually result in squeezing things below the running
boards and will pinch the valve gear, crossheads, and guides inward such that
their clearances from side and main rods are reduced, and parts start to
interfere with each other. My own pet way to pick up a steam locomotive
is to use two hands, one holding the cylinders and one holding the cab
or end of the cab roof, assuming you can uncouple the tender
while on the rails. If this is awkward, and you must pick up the
locomotive and tender together, try to hold the locomotive mostly by
the cylinders, running boards and cab, keeping fingers away from running gear.
Binds that occur mainly on curves, as opposed to tangent track:
These are usually the result of interference that occurs when one or more driver
sets move laterally in the frame. All steam locomotives have built in lateral
driver clearances. This means that each driver set has room to move
back and forth across the chassis, usually up to a couple of hundredths of an inch.
There is usually more lateral clearance in the front and rear driver sets
than in the middle driver set(s). This lateral clearance is exaggerated
in model locomotives, and is what allows them to navigate model railroad
radius curves successfully.
Typically, on a curve, the end drivers move laterally toward the
inside of the curve, while the inside drivers move laterally toward the
outside of the curve. If you draw yourself a sketch of four driver sets
on a very sharp curve, you will see why this has to happen. This is also
allowed by the clearances in side rods, especially jointed side rods. So,
if you have a bind that occurs on a curve, the initial approach is similar
to the one before; run the locomotive at a speed slow enough that it
will stop at point of bind. Again, make note of the clock position
of the crankpins. Now, when you pick it up (remember how), the drivers
will tend to return to their laterally centered positions and the
bind may cease to exist.
What to do?
If the bind has indeed disappeared when the model
is upside down in your cradle, you need to simulate the operation through the
curve. Do this by running it with test leads while moving each driver set
laterally with a toothpick, as they would move laterally through the
curve you just left. Do this to all driver sets. If nothing bad happens,
especially at the clock position previously noted, you may need to move
all of the drivers laterally at the same time, to absolutely mimic
operation through the curve. Talented fingers are helpful here. Use fingernails
to gently move drivers and minimize friction interference. You need enough
power to overcome this, but just enough to run slow and hopefully
observe the bind occurring. It could be due to any of the reason
listed above (A. to F.). Additionally, it could be due to a brake shoe
or hanger rubbing on a driver rim or flange.
Most mechanical binds can be found through these approaches:
Run the model slowly in the cradle and study the movements of
rotating and moving parts. Do this while inducing all lateral driver
position combinations. Observe closely; often a potentially binding
part may twitch or jump out of line at a potential bind point.
One other condition that is lost when the model is upside down in the cradle is the
locomotive weight on the drivers. This is usually not a large factor in Hon3 and HO models,
as factory springs are normally on the stiff side. It could be a factor if the springs are
light or a significant amount of weight has been added to the model. It could also be a
factor in S and O scale models. The weight situation can best be judged by closely
observing the locomotive as you put it on the rails. If there is any settling down of
the body, as you release your hand support, weight could be a factor, especially if all
the other possibilities have been exhausted to this point.
The weight factor can be simulated in the cradle, by inducing a slight amount of
downward pressure on the drivers. However, this can not be done directly to the wheels
while the model is running. This will cause way too much drag and mask any other problems.
It can be done by applying slight downward pressure to the journals (axle bearings) of a
driver set. This must be done to the journals on both sides of a driver set concurrently,
else you will cause the axle to bind in the journals. The access to do this with a set
of probes is very tight, between the frame and the driver itself, but it usually can be
done with a thin tool. A hobby knife with a No. 11 blade can do it (actually a pair of these).
What is almost impossible is to try to do this on all drivers at the same time,
without some kind of exotic jig. This is approaching a pretty esoteric level of
diagnosis here. If weight is suspect, it may indeed be easier to remove the weight
from the boiler and then see if the bind is gone. In any event, what you are looking
for in all this ‘weight' business is something that happens when one or more drivers
moves upwards (toward the superstructure).
Another cause of binds which needs to be mentioned here is old hardened grease,
especially on driver axles and bearings. This is actually a more frequent culprit
than you may imagine. It is prevalent in older models and seems to manifest
itself in some older PFM models.
Look for it in gearboxes as well. Incidently, a great cleaner for this is a
product known as ‘Goo-Gone'. One last approach, when all else has failed, is to
start removing drive train/running gear components, one at a time, until the bind
disappears. Start with the side rods at one end and work inward toward the powered
(geared) driver set. This brings us to a halt for now.
There are a couple of more topics to consider here on the subject of binds.
These are binds caused by an out of quarter driver (rarer than you may think)
and binds caused by gears and drive shaft problems. These warrant an issue of
their own and will thus be addressed next time.