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DMU Freewheeel Device Problem

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theoldman
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Joined: Mon Feb 08, 2016 2:39 pm
Location: Bacton on Sea, Norfolk UK

DMU Freewheeel Device Problem

Post by theoldman »

No, not a Daimler car, but Daimler transmission.

We had a Freewheel device fail on a DMU. My theory is below. Can anyone tell me whether I'm on the right track as the device supplier has indicated that it failed due to being subject to a torque a lot bigger than the engine (It's a Leyland 0680 - 150bhp) could produce which is around 325 lb.ft. at 850RPM.

Oddly, the device failed in the freewheel position.

Any help would be appreciated.

Thanks.


The DMU, in top gear at 1800 RPM will do a maximum of 70mph, ie, approximately 4mph per 100 RPM.

At our line speed of 25mph, that means, under power, it will theoretically be doing around 625 RPM, give or take a few RPM for flywheel slip if going uphill. (but of course you would never engage top at this sort of speed under power - would you????)

The idle speed of the engines is approximately 400 RPM

That means at idle speed, in top gear, the unit will be doing 16mph

It follows then, that at this speed and any less, rather than coast, the engines will be trying to exert power to try and increase speed, through the fluid flywheel and freewheel device against the resistance of the prop shaft, which will be trying from its end to decrease the revs of the engine! The freewheel device will only be as such if the speed of the outer annulus is greater than that of the inner and in the explanation just given this will obviously not be true.

This is putting undue strain on both the flywheel (which will heat up) and the freewheel device, which as we saw last week will, with regular treatment like this, just eventually give up....................

What i am suggesting is that below speeds of 16mph, even if the driver wishes to "coast", a lower gear must be selected and just a little power applied. If the unit is coming to a stop, at a station or signal for example,  then neutral must be selected below that speed. The manual contradicts itself stating (correctly) that the gearbox oil pump is driven by the input shaft and then states (incorrectly) that if in neutral - being a state where no brake bands are applied and the top gear clutches are disengaged - no drive will be applied to the oil pump, but being on the INPUT shaft this is clearly incorrect as the input shaft always revolves if the engine is running. If what the manual states is true, the gearbox would promptly seize up after about 5 minutes when the unit is stationary, which is clearly not the case!! In any case, when moving, the "splash" effect of the gear trains in the lubricant will keep everything smothered in oil.
Normal for Norfolk

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