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One of the chief characteristics of a tram
is that it runs on rails. These serve two purposes: guidance and
returning the electric current from the motors back to the power
The very earliest tramways had ordinary waggon wheels on the vehicles and guidance flanges on the rails. This 'plateway' system allowed the vehicles, usually coal or mineral tubs, to be manoevered and run away from the rails on ordinary flat road surfaces.
The system allowed some degree of tolearance in the positioning of the plates - and cross ties would have tripped-up the horses - so stone blocks were set in the ground to hold the plates in place . (An oak peg was driven into holes in the blocks and each end of the plate was nailed down to a peg.)
|A disadvantage of the system was the tendency of stones to lodge on the running surface of the track, but at low speed, this did not pose much of a danger. As horses were supplanted by mechanical power and speeds rose, derailment from stones became more of a hazard, and rails, which had their running surfaces raised above their surroundings, offered a safer alternative to plates.|
| Unlike plateways, these
railways did not have guidance flanges on the rails, so flanges
had to be provided on the vehicle wheels instead.
A certain amount of clearance had to be allowed between the flanges and the rails, so that any slight misalignment did not cause the flanges to 'bind-up' tight, but if there were too much clearance, the wheel could slip down between the rails. The rails needed to be held accurately to an agreed width, the 'gauge' of the track, and cross-ties were installed to hold it in place. (If horses were used as the motive power, the gaps between the sleepers had to be built up level with the sleeper tops to provide a smooth pathway for the horse)
|As speeds began to increase, it became apparent that a system of guidance based on flanges allowing lateral movement and jolting alternately from one side to the other as the flanges contacted the rails was unsatisfactory. The system of cone guidance was developed as a satisfactory alternative.|
The part of a railway wheel which takes the weight is called the 'tread', if this tread is made cone-shaped, it assists the guidance of the vehicle.
The wheels of a railway vehicle are fixed to a single axle and rotate together. If one were bigger than the other, the bigger wheel would go further at each revolution and would tend to overtake the smaller wheel. This would have the effect of making the vehicle run in a circle, to the right or to the left., depending which wheel was larger.
Obviously the wheels cannot be made to change size in order to steer the vehicle, but it is possible to make different parts of the same wheel differ in diameter by machining the tread to a cone shape.
When the wheel comes to a curve in the track it will tend to go straight on, throwing the wheels towards the outside of the rails. If the wheels are correctly coned, the one at the outside of the curve will find itself running on a larger diameter and the one on the inside will be on a smaller diameter, this will then steer the vehicle around the curve
If the vehicle is travelling on a straight track, the wheels should run on the middles of their treads and any slight tendency to wander to one side or the other will be counteracted by the coning of the wheels. There should be no need for the flanges to contact the rails at all.
CONE & FLANGE GUIDANCE IN TRAMS
Cone guidance is limited by the differences between the maximum and minimum diameters of the coning, if the curve is too sharp, there will be insufficient steering effect for cone guidance to operate effectively.
In the case of street-running tramways, many of the track curves have to fit within the available road space and are much sharper than cone guidance allows. Under these circumstances, the tram wheel flanges make contact with the rails and the vehicle becomes flange guided. On straight sections, however, cone guidance will still operate and prevent flange rubbing.
Modern trams, which are usually very quiet, can generate surprising amounts of noise if they are forced to operate around very sharp curves. Whenever possible, sharp curves should not be installed in residential areas.
Automatic greasers can be fitted to the track so as to apply a small amount of non-creeping lubricant to the wheel flanges of a passing tram on the approach to a sharp curve. This is helpful in reducing noise and track and wheel wear. (In the past, water was used for lubrication, being applied by a junior employee stationed at the trackside with a watering can)
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