UPDATE: Making a class 87/90 chassis separator

Success: (Afternoon 7 March 2016): After further testing, I am happy to report that the reproduced chassis separators have reached their performance requirements. The requirement was for the new separators to not introduce extra load (current) on the motor. I am pleased to say that this has been achieved. Apart from swapping the old and new chassis separators, everything else was kept equal (I used the same chassis, motor, magnet, and so on). Further reaming of the central hole was required. This is a very simple process and only requires a needle file.

If you would like to purchase these separators then please follow the link to shapeways below.

Update (Morning 7 March 2016): The chassis separators detailed in the article below are now available on shapeways here:

http://www.shapeways.com/product/MCKUN7DNV/chassis-spacer-for-graham-farish-class-87-90

Some reaming/smoothing of the central drive shaft hole may be required. I plan to finish my investigation soon into how to get these functioning as per the original component (at present I draw a slightly higher current with the replacements) but they offer a good replacement to the originals, which are no longer available. Credit goes to David & Russell Hobbs at 3DR for this project.

The class 87 and 90 shared a common chassis, with the class 90 first appearing in 1995 and the class 87 appearing in 1999. In their design, they were a departure from the traditional method of drive transmission, and instead of a pinion gear on a drive shaft, as in most Farish diesel designs, they utilized “drive coils” connected to each end of the armature and meshing with a 25 tooth gear in the bogie tower. This method, utilizing a drive coil, was borrowed from the class 158 and 159 DMU’s, released in 1992 and 1995 respectively. The class 87/90 also had a split chassis design, a first for Graham Farish.

Note: The class 158/159 utilize a different length drive coil to the class 87/90.

Below you can see a picture of a class 87 body and chassis. Notice how it is a split chassis design and the motor has a flywheel – the first on a Graham Farish locomotive. This is the type IV armature. You can see the drive coils extending at either end of the chassis.
ResizeClass8790 body chassis

Given that these locomotives came relatively late in the day for Graham Farish, and hence were not in production for long (the takeover was at the end of 2000), an issue has arisen with the availability of spares. Notably, these are the bogie frame and the chassis separator, both made from plastic, both of which have a weak point and tend to break easily. In this article I am focusing only on the chassis separator and how a replacement was derived.

Below you can see a picture showing one half of the chassis, exposing the chassis separator blocks in black, and also an assembled chassis with the chassis separator blocks visible between the two chassis halves, acting as a divider between the two halves to keep them electrically separated.

ResizeSeparator halves

Two screws thread into the chassis separator (one each side) in order to hold the chassis halves in place and together. The force of these screws on the plastic chassis separators tended to be an issue and it was common for the chassis separators to crack / snap. Once this had happened, the chassis separator no longer did the job of holding the chassis tightly together. Whilst gluing the separator back together might be an option, it might prohibit future disassembly of the chassis and so the task was set to make a replacement piece. Additionally, secondhand chassis sometimes become available without the broken separator.

The best option for making a replacement separator would be by 3D printing. In order to do this an AutoCAD image would have to be produced, with measurements taken from an original separator. Russell of 3DR designs took a digital vernier to an original chassis separator and sketched the image below in PowerPoint (note, measurements removed).

After confirming the accuracy of the measurements the PowerPoint sketch was sent to David (also of 3DR designs) in order for it to be turned into an AutoCAD file ready for 3D printing. Below are the results of his work.

 The end product is a chassis separator which replaces an original which is no longer available. The material is sintered nylon, printed on a selective laser sintering (SLS) machine.

Both Russell and I are in the process of testing the separators to determine if any fine tuning needs to be carried out. In my case I needed to ream the hole through which the drive coil passes, since it was a fraction too tight. Whilst the separators are usable at present, I am experiencing an increase in current draw from the locomotives of approximately 20-30mA and the aim is to get this down to a zero increase.

This article shall be updated once the product is suitable for use. Should you be interested in further information about Russell and David’s 3D printing endeavours then you can visit their shapeways site here;

http://www.shapeways.com/shops/3dr_designs_for_n_gauge

The printing of the component was carried out by 3D Print UK, who can be found via the link below;

https://www.3dprint-uk.co.uk

If you are interested in acquiring one if these separators then please use the contact form under “Contact Us” to register your interest and we shall endeavour to communicate with you once a finished component is available.

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