DIY HO locomotive test stand

A locomotive test stand is an expensive piece of equipment that allows a locomotive to run while it is stationary. Fortunately, there are methods to build it yourself, and here I present a six-axle locomotive test stand buiding method that is based in part on this (now defunct) spanish-language website.

There are two versions of the build. The first, newer method is much simpler but requires 3D printing of the bearing holder. In the second, older and more complicated method, the bearing holder is constructed from elements bought at the hardware shop.

The essential element of the build is the terminal block. A terminal block consists of insulation block and 12 metal tubes that can be extracted out of it. Each metal tube has two screw holes perpendicular to the central hole. You can buy the bearings and the optional copper sheet on Ebay and AliExpress, while you can buy the other parts at a hardware store.

Method using 3D-printed bearing holder

The required resources are:

terminal block
24 MR84 bearings (4.0 mm × 8.0 mm × 2.0 mm): AliExpress
round metal extrusion profile (brass, 4.0 mm diameter)
6 3D-printed bearing holders (picture): STL file
0.2 mm thick copper sheet (optional): AliExpress

The total cost of the required resources is about €15 plus the cost of 3D printing, which is about €2 in terms of filament.

The pictures below show the final assembly of one wheel support and the finished test stand.

Method using constructed bearing holder

The required resources are:

terminal block
24 MR84 bearings (picture: 4.0 mm × 8.0 mm × 2.0 mm): AliExpress
round metal extrusion profile (picture: brass, 4.0 mm diameter)
flat plastic extrusion profile (picture: 23.5 mm × 2.0 mm)
L-shaped metal extrusion profile (picture: aluminium 10.0 mm × 10.0 mm, thickness 1.0 mm)
24 washers (picture: brass 3.2 mm × 6.0 mm × 0.5 mm)
0.2 mm thick copper sheet (optional): AliExpress

The total cost of the required resources is about €20.

The pictures below show the final assembly of one wheel support and the finished test stand.

Choice of parts and the complications of building

As already mentioned, the design of a test stand must begin with the choice of terminal block, as they come in different sizes. The variations are limited, because even with different manufacturers, all the essential dimensions of the various products are usually the same. Below I show the most important dimensions and their values for three typical products.

type	M	H	D	N
A	M3.0	3.5 mm	8.0 mm	4.0 mm
B	M3.5	4.0 mm	10.0 mm	5.5 mm
C	M4.0	5.0 mm	11.0 mm	6.5 mm

The points to be considered are:

The minimum distance between the axles of the locomotives is equal to 2D because each wheel of the locomotive is supported by two bearings. Larger hole spacing means a larger minimum distance between the locomotive axles. In addition, the locomotive wheel sinks too deep. This rules out the metal tube type C.
With a small hole spacing, small bearings must be used, which makes the clearance between the screw and the wheel too small. This rules out metal tube type A.
The combined thickness of the bearing and the bearing holder (L-shaped profile plus washer) should be N. This rules out metal tube type A in the case of a thicker 3D-printed bearing holder because the screw is too short. This also means that thin bearings without shields must be used. In the case of the thinner construced bearing holder, the screws of the metal tube may be too long and need to be shortened.
The diameter of the available metal rods (round metal extrusion profiles) limits the value H. It is generally very difficult to obtain rods with a half-value millimeter diameter, which makes the use of tube type A difficult.
The inner diameter of the bearing is limited by value M. It is generally very difficult to obtain bearings with a half-value millimeter diameter, which makes the use of tube type B difficult.

There are also two special considerations:

The outer ring of the bearing should not come into contact with the bearing holder so that it can rotate freely. To achieve this, the 3D-printed bearing holder has a bulge and the constructed holder uses a washer. The outer diameter of the bulge or the washer should be approximately the average between the inner and outer diameters of the bearing.
Material of the metal rod (round metal extrusion profiles). The metal rod must obviously be conductive. However, this excludes the use of anodised aluminium, as an anodised surface does not conduct electricity.

I chose the metal tube type B, and cheap 4 mm × 8 mm × 2 mm bearings which means that the diameter of the inner bearing ring (4.0 mm) does not fit perfectly to the screw (3.5 mm). Normally this is not a problem, but in order to get as precise build as possible, I took a 0.2 mm thick copper sheet, made small rings and put them in between. As for the washers, the only reliable solution I found was to take washers for M3 screws and increase the inner diameter from 3.2 mm to 3.5 mm by drilling.

Introduction

Method using 3D-printed bearing holder

Method using constructed bearing holder

Choice of parts and the complications of building