Bruce Boldner describes the building of a 60-lever MODRATEC Interlocking Lever Frame for his Bath Queen Square P4 scale layout.

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Building a MODRATEC 60 Lever Mechanical Interlocking Frame - Bruce Boldner

© The content of this article copyright 2007 Bruce Boldner. Send email for Bruce Boldner via MODRATEC.


I'm currently in the midst of building an L shaped P4 layout, with one baseboard 4.8 metres in length, the other 3 metres. Both being 80cm wide.

The longer baseboard, on which all track is now complete, features a terminal station, loosely based on Bath Green Park, although I will be using the pre 1951 name Bath Queen Square, as my period is based in the pre-grouping era.

Photo 1 - the completed frame for Bath Queen Square mounted on its purpose-built table.

The term 'loosely' will gain greater significance when I say that of the 17 signals I have constructed to date, (from Model Signal Engineering components in various configurations: single, double, triple doll, etc) all are to Midland Railway design, rather than to that of the Somerset and Dorset, as my primary interest is in the former railway company.

My intention had been to plant a few signals (not necessarily operating) around the layout here and there, mainly to help impart a Midland 'look'. However, my friend Rex Crouch, who is a retired Signal Electrician from Victorian Railways here in Australia suggested strongly that he would not like to see yet another layout where signals, if installed at all, were there merely for decoration and whose intended purpose was usually totally ignored.

My trackage includes 32 turnouts, there also being a busy goods yard next to the station. With the number of signals required, I realised that unless they were interlocked with the turnouts they purported to control, I would soon become lazy and just run my trains wherever I wanted, without bothering to set the signals to reflect the intended route.

It was then that someone mentioned MODRATEC on the P4 email chat group. This is an Australian based business, which has an ingenious computer program titled SigScribe4, which the prospective purchaser can use to draw a signalling plan based on his own layout design. This program can be downloaded without charge from the MODRATEC website: www.modratec.com

After mastering the Sigscribe4 software and with invaluable assistance from Rex in configuring the signals, I emailed our first track design to MODRATEC proprietor, Harold. This was the first of 11 revisions sent to Harold! Upon receipt of each revised plan, Harold would either incorporate interlocking into the plan, which we hadn't the expertise to do, or modify one of our configurations to function more efficiently. Harold was always extremely patient, helpful and above all knew his signalling! On the few occasions when I phoned and he wasn't there, he rang back within the hour. And the longest he took to reconfigure the track plan was overnight. More usually he would email it back within a couple of hours.

I was shocked to discover that a minimum of 41 signals would be required to correctly signal my layout. Rex reckoned that we could comfortably have used 70 levers! However, at that time 60 levers was the maximum available in one frame from MODRATEC, although Harold has since advised that this number may be increased (I'm not sure by how many more) by special order.

We finally agreed that Modification No. 11 incorporated all the features we wanted with the most compact interlocking possible! Harold explained that his software uses an algorithm designed to calculate the most compact and efficient interlocking for a given configuration.

With the track plan finalised, I had to be very sure that I would not want to make any later alterations, as once the final plan was submitted to Harold and he was given the go ahead to proceed with the manufacture of a kit, that kit would be specific to my last submitted layout plan and would be virtually useless for any other! A comprehensive instruction manual would also be generated. But all specifications within this manual would of course only refer to that one track plan. Harold explained that it was sometimes possible to make small modifications to a lever frame at a later date, but that significant changes could prove a difficult and perhaps expensive exercise. [Note that most components a reusable and that a different interlocking configuration may be achieved, provided that the number of levers and slots does not increase, by replacing and/or reworking the locking bars and tappets. ED.]

The kit was generated to my specifications and a large, heavy plywood box arrived in the post. This contained various sizes of nuts and bolts, brass tube spacers, right angle strips of brass cut to length to form the frame, 60 tappets, 31 locking bars, 60 levers, piano wire to link the levers to the tappets, 3 sheets of clear acrylic (to cover the track diagram and tappets) and sundry other items. Also included were a number of pre-drilled jigs made from both brass and aluminium right angle section. These jigs were used by placing them over the section of brass to be drilled and the appropriate sized drill was centred through the indicated hole in the jig. Hundreds of holes had to be drilled and many of them tapped. But provided the instructions were very carefully followed, accuracy was ensured. The jigs were ingenious in that they were often used for the drilling of two or more different items, using different holes on an alternate face of the jig.

Photo 2 - assembly of the locking bed complete.

An excellent construction article by Stewart McSporran, including photos taken throughout the building of his 18 lever MODRATEC frame is available for viewing on the MODRATEC website and was a great help to me in visualizing each stage of construction.

The lever frame can be used to operate mechanical linkages to turnouts and signals, or alternatively electrical operation can be achieved by fixing electrical switches beneath each lever, to be activated when the base of the lever moves against the switch button. MODRATEC supply switches, complete with PCB printed circuit boards and materials to attach a bank of the switches beneath the levers at the correct height.

However, I decided not to use the MODRATEC switches, as I wanted a switch with a higher amperage rating, thereby ensuring I wouldn't have to add relays to run accessories such as Tortoise motors. I opted for the mini micro switches recommended by the Shropshire & Herefordshire Area Group for their excellent lever frame (available through the Scalefour Society Stores). This switch is rated at 3 amps and in the UK is listed as Squires Cat. No. MSR500 (according to SHAG), whilst in Australia it is available from Dick Smith Electronics as Cat. No. P7802. These switches incorporate an actuating lever which can be bent into a curve if necessary, making it easier to achieve the optimum clearance between the micro switch and the base of the MODRATEC lever above it.

Photo 3 - levers, lower tappets and locking bars in place.

Once completed, my 60 lever frame was quite heavy (measuring 650mm x 267mm) so I made up a table with cross braced legs for it to slot into. This table will of course be placed next to the layout. I plan to incorporate the wiring into multi-pin socket plugs, to enable the frame to be conveniently unplugged and moved if necessary.

I read with great interest Phil Taylor's article on adapting his MODRATEC interlocking to be connected to an MSE lever frame. I think this is an excellent idea, as to my mind, the one slight disappointment with the MODRATEC product is that the levers supplied are rather short looking (at approx. 7cm from fulcrum to top, they look especially short against the width of my 60 lever frame) and apart from the turned handle on top, are of square rather than rectangular section. They also do not have drop boxes and the associated trigger levers and lifting rods. There is also no slotted quadrant plate on the MODRATEC frame, the levers instead being separated by tubular brass spacers. However, the MODRATEC levers do have a nice, smooth, solid feeling action, with a sprung ball bearing seated in each lever pushing against the adjacent brass spacer, creating a total absence of slop. (I should mention here that the oval number plates on the first 50 of my levers are spare plates from my SHAG levers! MODRATEC does not provide brass number plates and instead suggest the lever numbering is printed onto the track diagram, which is secured under the clear acrylic sheet over the tappets. Does anyone know who might be prepared to etch me some plates for Nos. 51 to 60?!)

Harold's response to the issue of building realistic-looking levers was to point out the dilemma of deciding which pattern of lever should be adopted. There are dozens, perhaps hundreds of lever styles used in real railways, some with catch handles, some with stirrups, some with no catch handles at all, some for operating double-wire pulls. Then there are miniature levers with electro-pneumatic interlocking. On top of all this was the consideration of strength, hence the (relatively) short lever length. The longer they are, the more torque that can be exerted by an abusive operator - hence snapped levers. He also advised that economics dictated that off-the-shelf brass sections are used.

I acknowledge the validity of these arguments, especially where they assist in keeping the prices of the MODRATEC lever frames to what I consider to be very reasonable levels.

However, I do love the look of Phil Taylor's use of MSE levers with his 18 lever MODRATEC frame and will certainly consider doing the same if I build another MODRATEC lever frame for the second baseboard of my layout. That is, provided that the frame is of considerably less than 60 levers! I note from Phil's article that the wires from levers to tappets have to be increasingly offset where they enter each tappet, due to the difference in lever spacing between the MSE levers and the MODRATEC levers.

If Phil had to accommodate increasing offset in his 18 lever frame, it may well become too much to incorporate into a 60 lever frame. I've built up 35 of the SHAG levers and was thinking of utilizing these to operate a subsequent MODRATEC interlocking unit. But SHAG levers come in groups of 5, MSE levers come in groups of 7 and MODRATEC levers come in groups of 6, all with different spacing between the levers! Perhaps a series of 18 lever frames would be best.

Anyway, despite my mild desire for more 'realistic' looking levers, I am entirely happy with my MODRATEC lever frame. It looks great in gleaming brass and acrylic and it's been a pleasure to construct something which has been so ingeniously and precisely engineered. It has a very sturdy feel to it and I am sure will give many years of service, undaunted by the hands of the odd ham fisted operator.

Photo 4 - the pulling list [Click for larger image]

Above all it has required me to research, discover and develop an interest in the science of railway signalling, an absolutely essential and inseparable part of railway engineering which I will never again take for granted or ignore.

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My Modratec lever frame for Dachsbauten station has been finished today. There had been absolutely no problems while building it. The Construction Manual had been of great help.... (Christoph)

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