1972 stock ''click''

[1972stock3567]

<<Thread moved from Bakerloo line - goldenarrow>>

What is that ''click'' sound when the 72 stock departs. I don't mean the camshaft but I mean the click when the train sets off.

Example: the brakes release and then it ''clicks'' with the camshaft coming with those gears to make the train accelerate.

[t697]

If it's the first click before the train moves and the train moves immediately afterwards it's probably the linebreakers closing in the traction equipment applying power to the traction motors. The camshaft then notches as the train accelerates, cutting out resistors keeping the motor current sensibly constant for the initial accelerating phase.

[1972stock3567]

I see! Makes sense however I always thought it was something knocking the brakes to release them to make the train move.

[t697]

Yes, the older trains don't have thyristors or IGBT's or anything like that so the line contactors have to open every time you don't want to motor! I guess that seems a bit quaint these days with only Bakerloo and Piccadilly trains still having this type of equipment.

[1972stock3567]

yeah but sometimes very faintly I can hear the camshaft on 1973 stock but not all the time. I think it is on the older units.

[t697]

All 1973TS (Piccadilly) have camshaft equipment. There are 4 motor cars on the 6 car train, M-T-M + M-T-M so the motored cars are always 1,3,4 and 6. The equipment is reasonably audible if you are in the middle of the motor car. The now withdrawn District D78 stock were quietest for camshaft sound in the saloon. It was virtually inaudible as there was a good equipment case and quite a space between that and the saloon floor.

[towerman]

It’s the linebreakers closing to give power to the motor circuit.

[metman]

I find the D stock and 1973 stock seem to engage quicker than the older stocks like the A C and 67/72. There seems a lot more ceremony with them but I guess it’s just a newer version on the 1973 stock. The 1972 has a camshaft bank for each pair of motors I think which is a legacy from the automatic operation on the former Victoria Line 1967 stock?

[t697]

Not quite correct on 72TS. There are two camshaft machines but one does the Series notching and the other does the Parallel notching. The motor grouping and Series/Parallel changeover is all done by separate contactors. After Series/Parallel transition you can hear the Series camshaft notch all the way back quite quickly as the Parallel camshaft remains notching under Notching Relay control. At Full Parallel another pair of contactors close and the parallel camshaft runs back to be ready for next use.
In rheo static braking, the brake circuits are set up by contacors and resistance notched out by the Series Camshaft then the Parallel camshaft. At last notch both camshafts are fully 'up' and runback when rheostatic current falls and 'de-proves'.

On 1973TS and D78 stock the single camshaft machine has more power contacts on it. It notches up towards Full Series and back for parallel notching similar to a 38TS, 59TS etc scheme. Again grouping and transition are done by separate contactors. There is a Series Diode in place of a traditional contactor which make the Series/Parallel transition easier.
In rheostatic braking, again the circuit is set up by separate contactors. The camshaft notches out resistance notching'up' then other resistors on the return rotation as grouped by other contactors.

Not sure what you mean by 'engage quicker'. The acceleration on notches is higher on 1973TS than 1972TS. Mostly this is because the 72TS train is heavier. The number of motors and the notching settings produce almost the same total tractive effort on these two stocks. First notch on 73TS does have a higher tractive effort than first notch on 72TS.

'A' stock had a much higher motor characteristic than 72TS and 73TS. So higher top speed and to keep the accelerating currents reasonable, lower initial acceleration.

[metman]

Makes perfect sense thank you. You can hear it all going on too.

What I meant by engaging was the point where the resistances might have finished cutting out. For example on the 72s the time taken to cut of the resistances seems to take longer than the later stock 73/D78 but that might be perception due to the noise. You can certainly hear the series camshaft followed by the parallel on the Bakerloo. I love it and will always ride a motored car if possible.

Cheers.

[t697]

Yes 72TS will take slightly longer to get to Full parallel. As mentioned the total tractive effort is about the same as 73TS but the 7 car 72TS train is heavier. Also there is a dip in tractive effort at first notch parallel in normal Rate 2 acceleration. This allows a smooth transition when driven in the lower 'Rate 1' acceleration.
On 73TS the Series Diode smooths the dip out as series motoring continues until the diode becomes reverse biased as a notch or two is taken in Parallel Rate 2.

Prevailing traction voltage also makes a difference to how long the notching sequence takes. The higher the voltage the higher the speed before Full Parallel is reached. However the accelerating current and hence tractive effort and acceleration remains sensibly constant while still under notching relay control.

Neither of these fleets has load weighed adjustment of accelerating current (73TS has the facility but not in use and never has been in motoring). So they don't accelerate as fast when heavily loaded.