McKENZIE & HOLLAND 1873 PATENT FRAMES
by Andrew Waugh
Prior to 1912 the standard interlocking frame on the Victorian Railways
(Australia) was the McKenzie and Holland No 5/5A/6/6A Pattern Frames. These
four varients, collectively known as the 1873 Patent frames, were all identical
in operation, and differed only in minor constructional details. From 1912 the
VR switched to a Cam and Tappet frame for new installations, but there was
never a policy to replace the older frames, nor were they relocked with tappet
locking. Seven are still in service, including the 196 lever Spencer Street No
The 1873 Patent frames appeared at a pivotal point in the development of
mechanical interlocking. Stevens had patented tappet locking in 1870, and the
other two main manufactuers had to come up with a workable alternative.
McKenzie and Holland refined their 1866 design to come up with the 1873 patent
design, just beating Saxby and Farmer who did not come up with a design that
wore well until 1874.
The operation of an 1873 Patent frame is hard to describe without
copious illustrations, but the recent abolition of Newport A gave the
opportunity to take a series of photos of the frame while it lay in the grass
The design is sometimes known as the `Hook, Cam & Soldier', which is
quite descriptive. The actual locking is carried out by large cast iron locks
which `hook' lock studs fitted to the levers. Operation of the lock is by means
of cams, rocking shafts, and soldiers.
The operation of the locking can be
followed from this transverse view. The locking is driven indirectly from the
movement of the lever (1) by means of a cam (17). Movement of the cam has two
phases. The first occurs during the initial movement of the lever. Attached to
the lever just below the floorplate is a cam stud (4) which operates in a slot
in the cam. As the lever is moved from the normal position, the cam stud forces
the cam upwards, rotating it clockwise around the cam shaft (16). During this
movement the tail of the cam moves forwards 1 1/8" which operates the
interlocking. The second movement of the cam occurs as the lever approaches the
first reverse notch. The cam stud again lifts the cam and the cam tail again
moves forward a second 1 1/8" to operate the locking for a second time. This
first movement of the cam locks any conflicting levers and the second movement
releases levers for subsequent operation. The lever can be placed in any of
three reverse notches. Further movement beyond the first reverse notch does not
affect the locking, but gives a greater travel to the lever tail.
The motion of the cam plate is transferred up and down the frame
by means of rocking shafts (13). Frames can contain two layers of rocking shaft
brackets, as shown in the diagram. Two cam links (14) connect the cam tail to a
soldier (15) clamped to a rocking shaft. As the cam is lifted by the lever, the
soldier rotates the rocking shaft.
The interlocking is performed by cast iron locks (7) which rotate
on a lock shaft (6) mounted on the rear of the frame. The tails of the locks
are connected by lock rods (8) to soldiers clamped to the rocking shafts.
As the rocking shaft rotates, the movement is transferred to the
cast iron locks which rotated on the lock shaft to lock or release levers by
means of a lock stud (5) mounted on the lever.
There are several different types of cast iron lock. The one
illustrated in the diagram is the main lock. This is used to lock a lever
normal when another lever is operated. As the Main Lock rotates around the lock
shaft, the heel of the lock drops in front of the lock stud preventing the
lever from being moved from the normal position.
A shot from the rear of the frame showing the
complete underfloor gear for one lever. Crossing the photo from left to right
can be seen the lock shaft (top), the cam shaft (middle), and the lever shaft
(bottom). A number of rocking shafts can be seen immediately above the lever
shaft. The lever shaft is turned down every 5 inches for a lever, but the cam
shaft and lock shafts are turned down twice every 5 inches. This allows two
cams or two locks between each lever. The cam tail and lock tails are in the
same vertical plane, so where there is a cam and a lock on the same side of the
shaft (as for this lever), the lock rod must be set to clear the cam tail. The
set can be clearly seen in this photo. Victorian frames were normally arranged
for a vertical leadout, and the lever tail projected out the back of the frame.
A variety of lengths of lever tails were used in Rocker frames. Short tails
were cheaper to fabricate, but gave limited travel and were mostly used for
points or FPLs.
A close up of the previous photo showing the
relationship between the levers, cams, and locks. At the top of the photo can
be seen a cam, with the two rises in the cam slot clearly visible, with the cam
stud in its normal position at the top of the first rise. The cam is prevented
from jumping off the cam stud by a large square washer and split pin. (The
horizontal pin bolted to the rear extension of the cam drove an electric lock
mounted above floor level.) Under the cam can be seen a Main lock, mounted on
the lock shaft which runs from right to left across the photo. The lock stud
can be seen between the heel of the main lock and the lock shaft. If the main
lock dropped down, the lock stud would be trapped behind the heel of the main
lock, holding the lever normal. On the other hand, if the lever was reversed
the lock stud would pass underneath the curved lower edge of the main lock and
prevent the main lock from rotating, hence locking the levers that drive the
A shot taken when the frame had been
partially dismantled showing a lever in the first reverse notch. The cam has
operated through the two rises, and it can be seen that in this position the
back quarter of the cam stud is supporting the cam, preventing it from
dropping. The lever could be worked to any of the three reverse notches, the
cam stud travelling in the final section of the cam slot but without moving the
A view looking down `through the floor'
onto the top rocking shafts. The soldiers can be seen clamped to the shafts,
with the cam links and lock rods visible running vertically towards the top of
the photo. The cams are connected via cam links (the closely spaced parallel
bars which can be seen in the middle and to the left of the photo) to soldiers
clamped upon the rockers shafts. Other soldiers on the shafts drive lock rods
(the thicker rods to the right) which operate the locks. One point to notice is
the relative sparsity of soldiers. This photo was taken near the middle of the
frame where most of the levers are point and FPL levers. In 1873 patent frames
the rocking shafts were generally driven by the point and FPL levers and the
locks were applied to the signal levers. The middle of the frame consequently
had few soldiers.
A detail shot of the soldiers clamped to
two rocking shafts. The lower shaft has the drive from a cam; the cam links are
the two flat parallel mild steel bars (1 ½" by 5/16"), one placed each
side of the soldier head. The upper rocking shaft has a drive from a cam at
left, and locks on the two adjacent levers. Lock rods are solid ¾"
wrought iron rods with an forged eye to connect to the soldier head. Each lock
rod is different, and must be made to order by a blacksmith.
closeup of a soldier and a slotted lock rod. A sloted lock rod was used when
two (or more) rocking shafts drove one lock. This was common. In this case, two
shafts drove a Main lock. When the soldier rotated (counter clockwise) the tail
moved away from the camera and operated the lockrod. When the other soldier
driving this lockrod rotated (not visible in this photo) the lockrod simply
slid over the tail of this soldier.
A general view of the frame from Newport A from the rear showing
the rocking shafts which transfer the motion of the levers up and down the
frame. The number of rocking shafts on an 1873 Patent frame is limited (by the
width of the box, if nothing else) and this was one of the major limitations to
the complexity of the locking that could be applied to these frames and
influenced interlocking practice. A variety of techniques were used to minimise
the number of shafts used on a frame. For example, to avoid having to have
opposing signals directly lock each other, FPL levers were normally used as
`direction' levers, releasing the signals applying over the points when the FPL
was in and the signals applying over the points in a trailing direction when
the FPL was out. Rocking shafts provided when the frame was new often extended
the full length of the frame, but rocking shafts provided during alterations
were often as short as possible. Note that the weight of the rocking shafts has
caused the frame to tip to the rear. In service it would be upright!
A view of the rocking shaft brackets from the left
hand end of the frame. Two tiers of rocking shafts can be applied to a frame
(known as the `top' and `bottom' rocking shafts). Thirty shafts could be fitted
to the Newport A frame; 15 in the top bracket (which has 16 holes, but the
first could not be used as it fouls the cam tails) and 15 in the bottom
Not from Newport A, this photo is
included to show a different type of cast iron lock, a branch lock. When the
branch lock is in its normal position (as shown here), the heel of the lock
locks the lever normal. When operated, the heel drops from in front of the lock
stud and releases the lever. When the lever is reversed, the curved tail of the
branch lock prevents it from being lifted again, and so the lever(s) operating
the branch lock are locked reverse. There were six basic types of lock, with
A square on view of the rear of the frame showing how locks and
cams were fitted between levers. Two locks could be fitted between each lever.
Thus, at most four different types of cast iron lock could be applied to one
lever (two on each side), but this limited the number of locks which could be
applied to adjacent levers. Four locks on a lever was very rare, a more typical
number would be two: a main lock (locking the lever normal) and a branch lock
(releasing the lever from normal). Two cams could also be applied between each
lever, and this can be seen between the third and fourth levers from the right.
The cast iron A frames supporting the interlocking frame also occupied space
between the levers and restricted the locking on the levers adjacent to the
Two bays of the frame from the rear. The basic support of the
frame were the A frames. Older interlocking frames had A frames every 7, 8, or
9 levers. The exact frame spacing depended on the length of the frame, and
there was some tendency to have a closer frame spacing in the middle of the
frame. A frames reduced the locking capacity of the frame and added weight.
Later frames appeared to standardise on 10 lever spacings, but never longer
than that. This photo also shows that these frames were not totally packed with
metal; although it usually seems like it when the frame is in the box.
This article is an edited version of an
article that first appeared in 'Somersault',
the magazine of the Victorian
Group of the SRS.
Comments about this article should be addressed to Andrew