Dynamic brakes

[bavli]

My understanding is that Diesel Locomotives in the UK do not have dynamic brakes. Did the Western Region's Diesel Hydraulics have hydrodynamic brakes? Do DMUs?

[ianmacmillan]

The railways in the UK do not have the long downgrades found in the US.
Trains can be controlled with the normal air brake and the shorter train length means that the loco can recharge the air after repeated applications of the brake.

The added complications of dynamic brakes are simply not worth the effort. There is also the problem of fitting the equipment into the smaller British loading guage.

Some electrics have regenerative or rheostatic brakes where the traction motors act like dynamos with the electrical resistance providing brake force.

[bgstrowger]

The Class 50s were originally fitted with rheostatic brakes, but this equipment was removed when they were refurbished.

[chriscooper]

Other than the 50s that have been mentioned, I don't think any Diesel Locos in the UK have had dynamic brakes, probably goes for europe too. Very few DMUs have either, the only exceptions are the 175s, 180s (hydrodynamic) and 22Xs (rheostatic). Dynamic braking on electrics is much more common though. All AC electric locos from 85s onwards have rheostatic brakes, and the 76s and 77s had regenerative brakes (far easier with DC, and taking advantage of the long falling gradients on the Woodhead route), not sure about the SR locos, although I'm sure the 73s have rheostatic. Electric Trams were fitted with rheostatic brakes from pretty much the start (useful since many old trams didn't have air brakes, just a handbrake), although London Underground only started fitting them in the 60s, and BR the 70s (the PEP was the first BR EMU with rheostatic brakes). Since then though, all EMUs have had dynamic brakes.

A problem with dynamic brakes is blending them in with the air brakes, and the technology to reliably do that was what held back widespread adoption. On older locos and trams it's manual, but on locos it was more designed for holding speed decending hills, not smooth stopping, and trams rheostatic brakes can pretty much stop them, the handbrake just taking over in the final few feet, plus the handbrake is smoother and more responsive, especially compared to early, none self lapping air brakes, so getting a smooth transition, whilst needing skill and practice, is not that hard. The 76s, 77s and 85s all had seperate dynamic brakes, which again were for holding speed on falling gradients, not regular stopping, but the 86s onwards, and all fitted EMUs, had automatically blended dynamic brakes. The early blended rheostatic brakes though were a little hit and miss, the London Underground C stock are supposed to be particularly bad, and it's not uncommon for them to be isolated.
Another problem with rheostatic brakes on diesels is that it requires extra equipment, particularly the resistor banks, which have to either be in good airflow (i.e roof mounted) or provided with cooling fans. UK loading gauge plays a part here, as there is less space to play with. Of cource, a DC electric can just use the starting resistors for braking, and whilst AC electrics do need resistor banks and fans, there is typically more room to play with.

[bavli]

Thank-you for the info. One further question. Is sand used on the various MUs? It is definitely not used here on our EMUs (Long Island, Metro North, NJ Transit or SEPTA (SE Pa Transit)-of course here a ALL axles are powered.

[chriscooper]

Use of sand on units has typically been limited until recent times, both for diesels and electrics. It's only really come into fasion in recent years due to the combination of higher powered units and disk brakes (which don't clean the wheels like tread brakes), and most units built this century have sanders, although some are only single shot emergency sanders for wheel slide. Sanders have also been retro fitted to many older disk braked units. Most locos have sanders though. London Underground doesn't have sanders on it's trains, but where adheasion is a problem (outer parts of the Metropolitan and Central) they have special "Sandite" trains which lay a special substance on the track to improve adheasion. Sandite is also used on some parts of the mainline network, especially the busy EMU lines south of London.

[bavli]

I don't think any Metro(subway) systems use sand, applied by the driver; I certainly haven't seen sanders in evidence anywhere. I don't know about special vehicles on this side,but according to the link below they are used here. Thanks for the answers.

http://en.wikipedia.org/wiki/Slippery_rail

[ralph0chadkirk]

Sandite makes the difference for the first 2 trains over. The rest are just playing ice skating.

[danielw2599]

Sand and Sandite are two different things that are applied differently.

[salopiangrowler]

Sandite is a gooey substance and not of good use after the first 2 trains have passed.