Peculiar problem with running two Bachmann Class 20s coupled together.

[TX]

I have rather a strange problem which I hope some guru out there can throw light on. The outer loop on my layout is 16’ x 12’, with the longer sides more or less straight, and one side ascending at roughly 1 in 150  and the other descending likewise at 1 in 150. I have two Bachmann Class 20s coupled back-to-back hauling a seven-coach train. At high speed (say scale 80 mph), the train runs normally around the entire loop with some decrease in speed while ascending the slope.  When running at a slow speed, however, the two locomotives go through a fast and slow routine in that they slow down then speed up continually, each cycle over a distance of about 15”, but this only happens on the downward slope – on the upward slope they maintain a steady constant speed. If I run  each locomotive on its own there is no problem, each one maintains a constant speed. If I reverse the direction of the two locomotives, the same thing happens on the downward slope – a continual cycle of speeding up/slowing down but not quite as noticeable. Is anyone able to explain what is going on? Thanks in advance!

[Dagworth]

DCC or DC?

 

Andi

[TX]

DC, with Gaugemaster model Q 4-track controller

[Pandora]

5 hours ago, TX said:

DC, with Gaugemaster model Q 4-track controller

1)   Is the controller a design which senses  back-emf  from the track voltage?

2) 16 x 12 feet is long,  how many controller to track feeder wires  to the outer loop, and are they of good thick wire?

Edited February 28, 2022 by Pandora

[Phil Bullock]

Can you try a different controller? First thing to exclude….

[Miss Prism]

Each motor is getting choked (= confused) by the bemf from the other motor.

 

[dasatcopthorne]

Uncouple the coaches then separate the locos. Place 3 or 4 ins apart and then see how they behave. Is it just one of them?

 

 

Dave.

 

[Bill Radford]

I had a similar problem with Bachmann DMUs (running solo). It seemed as though there were phases of "free- wheeling" down the slope and then the motor slowing the unit down (as if in a lower gear). I solved the problem by inserting foam above the non-powered axles - as the 20s are powered on all axles I appreciate that this is not possible.

 

(Running DC using both Morley and Gaugemaster controllers.)

[fulton]

I had a similar problem descending a gradient in HO American, with some locos either singly or as a pair, was due in my case to slack in the drive train, under load going up hill was fine, fitting thrust washers would be a solution but I just relegated those locos to the flat sections or sold them.

[Theakerr]

Similar problem with a Hornby B17 (using a Guagemaster Q4 ) on the level especially after it had done a couple of circuits.  Replaced motor - now runs fine

[TX]

Thank you for all the replies, which prove what a great forum this is. I tried everything with regard to direction of travel, running with/without coaches, running both locos cab first etc. but dasatcopthorne's  idea of running the two locomotives uncoupled, a short distance apart revealed that one locomotive ran faster than the other (on the same track) both running steadily at their respective speeds. The speed up/slow down phenomenon only occurs when the two locomotives are coupled together. Conclusion from all of this? It must be the slight discrepancy in running speed of the two locomotives at low throttle values, combined with the forces created by the slope when running downhill, which create a push/pull effect. Thanks again for all replies and helpful  suggestions.

[Enterprisingwestern]

8 hours ago, TX said:

Thank you for all the replies, which prove what a great forum this is. I tried everything with regard to direction of travel, running with/without coaches, running both locos cab first etc. but dasatcopthorne's  idea of running the two locomotives uncoupled, a short distance apart revealed that one locomotive ran faster than the other (on the same track) both running steadily at their respective speeds. The speed up/slow down phenomenon only occurs when the two locomotives are coupled together. Conclusion from all of this? It must be the slight discrepancy in running speed of the two locomotives at low throttle values, combined with the forces created by the slope when running downhill, which create a push/pull effect. Thanks again for all replies and helpful  suggestions.

 

Whilst it's not relevant to yourself, that is one of the advantages of DCC, in that there is an ability to "tune" locomotives to avoid an imbalance.

 

Mike.

[JeffP]

Locos do have speed discrepancies.

Back in the 60's a mate and his brother both had Hornby Standard 4 80033 tanks.

We used to set up a large circle, place the locos diametrically opposite each other, and put the controller to full power.

We then watched as the faster loco, always the dame one,  caught up the other one.

Used to take over 60 circuits.

[wasdavetheroad]

1 hour ago, JeffP said:

Locos do have speed discrepancies.

Back in the 60's a mate and his brother both had Hornby Standard 4 80033 tanks.

We used to set up a large circle, place the locos diametrically opposite each other, and put the controller to full power.

We then watched as the faster loco, always the dame one,  caught up the other one.

Used to take over 60 circuits.

That's excellent, at least 120 complete circuits to catch up the slower loco which means it is 0.83% faster so no need to speed match at all.

 

this was discussed some time ago

 

Edited March 2, 2022 by wasdavetheroad

[RobinofLoxley]

I would be a bit concerned that two all wheel drive locos cant pull a train up a 1 in 150 without slowing down. Its only 3/4 coaches each

[kevinlms]

On 01/03/2022 at 09:35, TX said:

Thank you for all the replies, which prove what a great forum this is. I tried everything with regard to direction of travel, running with/without coaches, running both locos cab first etc. but dasatcopthorne's  idea of running the two locomotives uncoupled, a short distance apart revealed that one locomotive ran faster than the other (on the same track) both running steadily at their respective speeds. The speed up/slow down phenomenon only occurs when the two locomotives are coupled together. Conclusion from all of this? It must be the slight discrepancy in running speed of the two locomotives at low throttle values, combined with the forces created by the slope when running downhill, which create a push/pull effect. Thanks again for all replies and helpful  suggestions.

In which case, you want to put the faster loco at the downhill end of the grade. Not much help if you have gradients in both directions.

[proton]

I experience this with most of my Bachmann engines descending my 1 in 50 grades with heavy trains.  Seems to me the worm gear is alternating between 'driving' and 'being forced'.  I havent found a solution.  This is on analog DC and P4, single or triple heading.

 

John

[John Tomlinson]

On 03/03/2022 at 11:32, proton said:

I experience this with most of my Bachmann engines descending my 1 in 50 grades with heavy trains.  Seems to me the worm gear is alternating between 'driving' and 'being forced'.  I havent found a solution.  This is on analog DC and P4, single or triple heading.

 

John

 

I've had just this experience as well. Also with some Hornby locos.

 

Interesting that none of my Heljan locos do it however. Suggests to me there is some design feature in the motors that must be different. I've tried reducing the lengthways play in the motors with a little improvement, but not much. I probably ought to experiment with different controllers to see if that helps.

 

John.

[proton]

I think the problem is more to do with the gear train than the motors.  On my railway the surging is accompanied by a fairly loud chattering noise in the gear towers, so I think the weight of the train is trying to push the locomotive faster than it wants to go.

 

John

[RobinofLoxley]

2 hours ago, proton said:

I think the problem is more to do with the gear train than the motors.  On my railway the surging is accompanied by a fairly loud chattering noise in the gear towers, so I think the weight of the train is trying to push the locomotive faster than it wants to go.

 

John

Makes me think that the gear chain is jumping. So not too long before a tooth breaks off perhaps

[proton]

1 hour ago, RobinofLoxley said:

Makes me think that the gear chain is jumping. So not too long before a tooth breaks off perhaps

My locos have done this for quite a while (years), and gears in the bogies are quite substantial.  It doesn't sound like gears jumping, but nice idea.

 

John

[wasdavetheroad]

If one loco can pull the train uphill an extreme solution could be to have a dummy loco which ideally should be no heavier than one of the coaches. I did this with a pair of Lima class 20's, made easier by spare  non powered bogies being available.

[Nearholmer]

The voltage applied to the track sets the speed of the motor.

 

Going downhill the weight of a heavy, but free-rolling, train can attempt to push thr loco(s) faster than the motor is driving, and if there is a worm gear somewhere in the gear-train a fight will take place between the wheels and the motor, because most worms used in model trains are very inefficient, and  won’t “back transmit” motion from wheels to motor.

 

The natural motor speed at a given voltage will vary ever so slightly between locos, hence fighting between locos, as well as between loco and train. 

Solution? Add some drag to the train to prevent it overtaking the locos at low speeds. The idea is to ensure that the locos are always pulling, rather than being pushed.

 

Locos with clutches in the drive train, and locos with highly efficient gear-trains won’t exhibit the same problems.

 

PS: do the locos have rubber traction tyres? That will make things worse, because they can’t even skid with the wheels locked.

 

With respect to Miss Prism, and all those who agreed with her (him?), I don't think back EMF has anything to do with it, unless, and I think it very unlikely, the gear-train is very efficient. If the gear-train doesn't permt "back driving" of the motor from the wheels, the EMF produced by the motor will never exceed the line voltage at the loco, and when the two are equal, no current will flow via that motor, the only exception being if there is a flywheel, because that will cause the motor to act as a generator in the event of sudden interuption of the line supply. If the gear-train does permit back-driving, then yes, the motor speed could exceed that due to line voltage, and current will flow from it into the line if there is a receptive load, the motor on the other loco, assuming that has a slightly different response to the conditions.

 

 

 

 

 

 

Edited March 4, 2022 by Nearholmer

[RobinofLoxley]

14 hours ago, proton said:

My locos have done this for quite a while (years), and gears in the bogies are quite substantial.  It doesn't sound like gears jumping, but nice idea.

 

John

Yes I agree, the problme is what can cause an unusual noise. Possibly then the gears are only just engaging. Under load, as opposed to free running, if there is any force acting on one of the gears that can be relieved by the gear displacing sideways, it will. I have just dismantled an old Palitoy 0-6-2 tank and found the gears werent fixed to shafts they were only restrained by the sides of the 'box'. No wonder it sounded like a lawnmower. Im also thinking about Ringfield motor assemblies where the spur gear often comes loose on the shaft and this creates the effect you describe as the gear moves to the point of engaged/not engaged and sometimes drives, sometimes not. Personal experience.

[wasdavetheroad]

17 hours ago, Nearholmer said:

The voltage applied to the track sets the speed of the motor.

 

Going downhill the weight of a heavy, but free-rolling, train can attempt to push thr loco(s) faster than the motor is driving, and if there is a worm gear somewhere in the gear-train a fight will take place between the wheels and the motor, because most worms used in model trains are very inefficient, and  won’t “back transmit” motion from wheels to motor.

 

The natural motor speed at a given voltage will vary ever so slightly between locos, hence fighting between locos, as well as between loco and train. 

Solution? Add some drag to the train to prevent it overtaking the locos at low speeds. The idea is to ensure that the locos are always pulling, rather than being pushed.

 

Locos with clutches in the drive train, and locos with highly efficient gear-trains won’t exhibit the same problems.

 

PS: do the locos have rubber traction tyres? That will make things worse, because they can’t even skid with the wheels locked.

 

With respect to Miss Prism, and all those who agreed with her (him?), I don't think back EMF has anything to do with it, unless, and I think it very unlikely, the gear-train is very efficient. If the gear-train doesn't permt "back driving" of the motor from the wheels, the EMF produced by the motor will never exceed the line voltage at the loco, and when the two are equal, no current will flow via that motor, the only exception being if there is a flywheel, because that will cause the motor to act as a generator in the event of sudden interuption of the line supply. If the gear-train does permit back-driving, then yes, the motor speed could exceed that due to line voltage, and current will flow from it into the line if there is a receptive load, the motor on the other loco, assuming that has a slightly different response to the conditions.

 

 

 

 

 

 

Did they do something like that in the olden days of unfitted wagons, applied the wagon brakes and pulled the train down the hill?