Construction of Class M7
1981 “BRUSH” diesel engine of Sri Lanka.
MECHANICAL POWER DRIVE SYSTEM
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This page shows the proposed mechanical drive system of the M7 project.
Precision machining of parts with close attention to clearances and tolerances becomes a must. Parts should fit into and mesh without grinding (or being too loose) to work properly. Parts should be interchangeable if very strict sizes are maintained during manufacture but quality does take more time to produce.
Each bogie will carry a motor plus the gear driver system directly onto the axles, the support members will have to be precision made and jig drilled in order to ensure that they all mesh correctly.
These have to perform faultlessly under arduous conditions as may be imposed by variable track conditions.
To determine what speeds are possible some simple mechanical calculations are required, you need to know the speed of the motor in rpm, multiply this by the driver gear teeth divided by the driven gear teeth. This will give you the axle speed in rpm, you then need the wheel tread diameter to be able to work out the distance travelled in one revolution, after that it is a few more simple calculations. The circumference is found by knowing the wheel diameter and multiplying it by 3.142, it is then possible to determine what mph can be obtained from a given gearing set as fitted to the drive system.
A set of 8, CNC turned wheels type, WH522SS have been obtained that are considered an almost exact match in size for those needed to fit this engine. A minor amount of machining should give a finished profile that is near enough compatible to that required with an average tread diameter of 119mm this model.
The set of spur gears to be used is of type G1.5 using 20t driving a 40t and a 20t driving a 50t gearing set up onto the axle.
The Bosch motor is rated at 3300 rpm, so should deliver 660rpm at the axle. This in turn should give the engine a maximum speed of about 9.2 mph on a flat track pulling 3 carriages, no-load.
Once trials have been done then any need for adjustment (or redesign) can be considered. In essence, the general operating speed whilst carrying public passengers is about 7.5mph. At this speed the driver should have adequate braking control of the train.
The ability to stop safely must always be the overriding criteria.
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 bogies mechanical drive
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 top gear motor – bottom on axle
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 gear drive plate assembly being made
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 drive gear wheel mounted on one axle
Note:
The front and rear axles of each bogie will be connected by a heavy duty chain, a 17t cog wheel will be fitted onto each axle. This will ensure that the best possible traction and adhesion is obtained at the rail.
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SLM7 – Mech. drive – stage. i