Mixed Traffic Loco - Variable Gearing

[GrimShady]

I was wondering if there is a such a thing as varibale traction motor gearing out there available for mixed traffic locos?

Such a system would enable a loco to have high top speeds for passenger work and low gearing for freight work. Something along the lines of having the traction motors in the body attached to the gear system and driving the axles via carden shafts. Maybe there's enough space in a bogie Class 91 style for such a system?

 

[big all]

not an option for day to day running as its a change off mechanical gearing a physical effortlike a drewry shunter or a dmmu
re gearing like a deltic from 90 to 105 mph hst from 125 to 143 epbs from 75-90mph[suburban and express ratios]
electrical transmission is very powerful efficient and flexible resistance series parallel and weak field often in many stages fantasic control avoiding mechanical gear changing

 

[MarkyT]

I recall some old French electrics, possibly from the 1950s onwards, had the ability to change the physical gear ratio, but not while in motion. They could instead be adjusted when stationary for different duties. Modern electrical drives don't really need this feature, as big all describes. Extra gear sets are heavy and bulky to carry around and no doubt increase unsprung weight on the driving axle, which is considered a bad thing for ride and wheel/rail wear, especially at high speed.

 

[GrimShady]

not an option for day to day running as its a change off mechanical gearing a physical effortlike a drewry shunter or a dmmu
re gearing like a deltic from 90 to 105 mph hst from 125 to 143 epbs from 75-90mph[suburban and express ratios]
electrical transmission is very powerful efficient and flexible resistance series parallel and weak field often in many stages fantasic control avoiding mechanical gear changing

I recall some old French electrics, possibly from the 1950s onwards, had the ability to change the physical gear ratio, but not while in motion. They could instead be adjusted when stationary for different duties. Modern electrical drives don't really need this feature, as big all describes. Extra gear sets are heavy and bulky to carry around and no doubt increase unsprung weight on the driving axle, which is considered a bad thing for ride and wheel/rail wear, especially at high speed.

If the gear box was at the motor end of the carden shaft then wouldn't that reduce the unsprung weight? Obviously the total weight of the loco would be increased but this would have the additional effect of better adhesion.

If I'm understanding both of you correctly, does that mean the days of dedicated freight and pax locos are over and that any new loco design would be equally at home with both cargo types unless it was designed specifically for one type?

 

[furnessvale]

With modern traction motors and control systems, isn't a gearbox surplus to requirements?

The torque available at all motor speeds means a single gear ratio can cope with all reasonable expectations.

 

[big all]

i am not saying it cant be done it can but there is cost and weight involved
if a loco spends 90% off its time on heavy freight then a 75mph gearing will give it the greatest efficiency overall
if its then say a 70/30 split then perhaps a 90mph gearing this would reduce pulling power by around 15% as a compromise for higher speed
then a say 25/75 split and 110mph you the weight you can pull will be reduced by around 30%
traction motors will always have gearing as the motor spins far far faster than needed

 

[Pigeon]

Both the last two replies are right

Traditional traction motor speeds are limited by commutator performance and by the effect of centrifugal force on the armature windings. Induction motors don't have either of these items, and with variable-frequency drives can provide useful torque over a wider speed range than a traditional motor, so you can use "freight" gearing to up the tractive effort but still run at "passenger" speeds without overspeeding the motor.

Nevertheless it remains true that lower gearing gets you the same tractive effort from a smaller and lighter motor, and higher gearing gets you a higher top speed without exceeding the motor's limits. It's just that you have rather more room before these tradeoffs start to become significant.

Putting the motor in the body driving through a cardan shaft means you're a lot less restricted in the size and weight of the motor, so while it does make it easier to add a changeable-ratio gearbox it also means you have even less need for one.

With motors on the bogie, unsprung weight is already a major concern for fast operation, so increasing it even more with the weight of a changeable gearbox means you haven't solved the "dual use" problem after all; you may now be able to change the gearing from "freight" to "passenger", but you still can't do "passenger" speeds because of the extra unsprung weight. It is mechanically simple, and I've half an idea that it's one of those things where the apparent simplicity has led people to try it out, only to discover it's not worth the candle for just the reason I've described.

 

[MarkyT]

Both the last two replies are right

Traditional traction motor speeds are limited by commutator performance and by the effect of centrifugal force on the armature windings. Induction motors don't have either of these items, and with variable-frequency drives can provide useful torque over a wider speed range than a traditional motor, so you can use "freight" gearing to up the tractive effort but still run at "passenger" speeds without overspeeding the motor.

Nevertheless it remains true that lower gearing gets you the same tractive effort from a smaller and lighter motor, and higher gearing gets you a higher top speed without exceeding the motor's limits. It's just that you have rather more room before these tradeoffs start to become significant.

Putting the motor in the body driving through a cardan shaft means you're a lot less restricted in the size and weight of the motor, so while it does make it easier to add a changeable-ratio gearbox it also means you have even less need for one.

With motors on the bogie, unsprung weight is already a major concern for fast operation, so increasing it even more with the weight of a changeable gearbox means you haven't solved the "dual use" problem after all; you may now be able to change the gearing from "freight" to "passenger", but you still can't do "passenger" speeds because of the extra unsprung weight. It is mechanically simple, and I've half an idea that it's one of those things where the apparent simplicity has led people to try it out, only to discover it's not worth the candle for just the reason I've described.

Thanks for a great explanation of the trade-offs.

 

[GrimShady]

Both the last two replies are right

Traditional traction motor speeds are limited by commutator performance and by the effect of centrifugal force on the armature windings. Induction motors don't have either of these items, and with variable-frequency drives can provide useful torque over a wider speed range than a traditional motor, so you can use "freight" gearing to up the tractive effort but still run at "passenger" speeds without overspeeding the motor.

Nevertheless it remains true that lower gearing gets you the same tractive effort from a smaller and lighter motor, and higher gearing gets you a higher top speed without exceeding the motor's limits. It's just that you have rather more room before these tradeoffs start to become significant.

Putting the motor in the body driving through a cardan shaft means you're a lot less restricted in the size and weight of the motor, so while it does make it easier to add a changeable-ratio gearbox it also means you have even less need for one.

With motors on the bogie, unsprung weight is already a major concern for fast operation, so increasing it even more with the weight of a changeable gearbox means you haven't solved the "dual use" problem after all; you may now be able to change the gearing from "freight" to "passenger", but you still can't do "passenger" speeds because of the extra unsprung weight. It is mechanically simple, and I've half an idea that it's one of those things where the apparent simplicity has led people to try it out, only to discover it's not worth the candle for just the reason I've described.

Interesting post. Thanks to all for the contributions.

 

[edwin_m]

As far as I'm aware the following classes are/were identical or nearly so, except for the gearing:

• 08 (15mph) and 09 (29mph)
• 310 (75mph) and 312 (90mph)
• 66/6 (60mph) and all other 66 sub-classes (75mph)
• The single Class 86/5 was re-geared - not sure if this ever happened to the various other sub-classes of 86, some of which were given lower top speeds.

I think also the 458s and 460s were re-geared when the fleets were merged converted to 5-car units - something to do with the cooling fans being geared off the motors and not providing enough cooling in the original arrangement.

 

[Shimbleshanks]

As far as I'm aware the following classes are/were identical or nearly so, except for the gearing:

• 08 (15mph) and 09 (29mph)
• 310 (75mph) and 312 (90mph)
• 66/6 (60mph) and all other 66 sub-classes (75mph)
• The single Class 86/5 was re-geared - not sure if this ever happened to the various other sub-classes of 86, some of which were given lower top speeds.

I think also the 458s and 460s were re-geared when the fleets were merged converted to 5-car units - something to do with the cooling fans being geared off the motors and not providing enough cooling in the original arrangement.

I think the 08's top speed is 20mph and the 09 is 27mph. Also, the 312s came in two variants; the West Midlands ones were 75mph and the Eastern Region ones 90mph.

 

[big all]

I think the 08's top speed is 20mph and the 09 is 27mph. Also, the 312s came in two variants; the West Midlands ones were 75mph and the Eastern Region ones 90mph.

the reason for 15mph was no overspeed but at least the southern ones where marked a 20 but had overspeed protection that would go off at 20mph and 27.5 with 09sthe overspeed relay was in the cab and the usual practice was to remove the brass cap and position your small leather strapped bag over the cap thus stopping it operating as it was an emerency brake application as 350s[400hp] speedo needles would often wave at you fluctuate by say plus 3 and minus 3mph so would go off as low as say 24mph so by holding it down you wouldnt actually go any faster than allowed but stop the learning game where you had to keep below around 23mph to save it going off having said that you get a loud click when it goes off so know to take action
not worth risking above 29mph as you can bend the rods