Andrew Briddon Locos

Andrew Briddon Locos

preserved railway vehicles

Why does an industrial loco overheat at speed?

Or why does an industrial loco with a Twin Disc torque converter overheat when I try to run it at full speed?

As we have explained in another FAQ, the efficiency curve of a Twin Disc converter is at its best when in the mid engine range (typically about 1400-1600rpm) and is then only 80%. In other words, if we apply full engine power in to the converter (say 300) 20% of the input power (300/5 or 60hp) is rejected as heat. This heat must be dissipated through the cooling system - the shaft seals on a Twin Disc are carbon and will fail if cooked, quite apart from any potential fire risk.

The converter has a charge pump, whose function is to maintain a base pressure in the converter, typically 40-60psi, though it wil generally work well provided there is some base pressure. The converter then ejects fluid to a cooler, and having passed through the cooler, the fluid returns to the converter. A temperature switch or switches are located on the converter output line. At the highest point of the cooler is a tapping, which bleeds a small amount of fluid out of the circuit and returns it to tank, but on the way passes through a restrictor known as an 'orifice' which looks at first like a peculiar pipe fitting. Strip it and you'll find the orifice has a protective gauze filter to prevent it becoming blocked. (Inspect this filter from time to time - particles of metal may indicate wear/damage in the converter.)

OK, that's the basic system. On most locos the cooler is linked to the engine coolant - the heat in the converter is transferred to the engine radiator and out through the main radiator/fan. There is generally more heat coming from the converter than from the engine! (We have seen mis-guided people fit much smaller lorry radiators in place of genuine loco radiators and wonder why it doesn't work!) On such systems, the converter and engine temperatures move up pretty-well in synch - they're bound to, because if the engine temp is higher than the converter temp, the engine will transfer heat back into the converter circuit.

So imagine, you are pushing your loco to run at about 20mph on level track. Your engine speed is at full (1800-2100), the converter rpm ditto, the efficiency of the converter is well down towards zero (i.e. most input hp is being rejected,) and over 50 gallons per minute of fluid is whizzing around the converter circuit, with frictional heat adding to the problem. Unfortunately the converter is already warm and as the heat rejection is now greater than the abilities of the cooler and radiator to cope, the converter temperature starts to rise. (The direction of travel may have an influence, too, is the rad fan blowing with or against the direction of air flow?) Actually, the worst case is when you are running downhill with a train behind you! The input hp is now coming from the train pushing, but you the driver have throttled back, slowing the engine fan and the water pump and so preventing the cooling system from removing as much heat as it could.

There are ways round this, and having had 30 years of extensive experience, Andrew's father is willing to assist - contact him via his website.

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