Turntable questions

[TomR]

I think I have a turntable like this one:

http://www.walthers.com/exec/productinfo/933-3171

The number on the base is 7003135\A

It has unreliable slip rings for power to the turntable track, and has a motor. It has plastic bearings, a large black plastic drive gear, a smaller pinion gear on the motor driving a shaft that drives a small straight cut gear that fits to the large black gear.

It appears to be a really old unit that is a little different mechanically from current units at that link.

Unfortunately the low voltage DC motor spins up real fast and then slows to a crawl a few times each second, making the turntable move in jerks. This appears to be a "feature" of the actual motor, not a defect. I say this because the motor, when removed from the gear drive system, spins up and crawls at a periodic rate like a step motor or eccentric gear motor might do.

This is NOT friction or anything, it is a behavior of the motor itself.

Anyway, I want to make this turntable work reliably and move naturally. I have a 1/2 RPM high torque 4-16 volt DC motor that I can use to directly drive the turntable. It has very high torque, and if I short the motor supply leads it acts like an electric brake. It pretty much instantly stops the motor without coasting down very much.

My thoughts are in two areas...

For track connections, I was going to add two beryllium copper spring contacts at each end, and put eyelets in the wall of the turntable base. My thought was to align the eyelets so the spring fingers snap into the holes to both make contact for power transfer and help position the turntable for a rail match. I would, in essence, make the turntable a giant switch with the eyelet being a locating detent.

I could either sense the track electrical connection between turntable rail and selected feeder/exit rail to stop the motor at a desired position, or use an optical device to cut motor power.

Anyone been through this, or is there something to buy that can automatically locate the correct stop position???

Thanks,

Tom

 

[flh80]

To get a turn table to index properly has always been a problem for modelers unless you have an elborate system, but it can be done. It just depends on your capabilites, most people can't or don't want it to complicated. Myself I ended up reworking a Heljan turn table, I have a geared 12v motor and run it on 3v, and I use a momentary button switch to control it, it's slow enough to line everything up. Also installed a reversing switch to change directions of the bridge and a reversing switch for track power (non DCC). An old trick I've seen, some people mount a mirror above the turn table angling towards them, so they get straight overhead shot and can line up the tracks that way.

 

[Selector]

Without a cycling rheostat to explain your DC motor's speeding and slowing cyclicly, I don't think the motor can be at fault. It is something to do with the wiring in your transformer. It heats and then cools, or some weird electronic thing. The motor spins rapidly with even a few volts applied to it, so if its cycling of speed and slowing is anything over half a second, you have a voltage regulation problem, not an internal problem with the motor. For the motor to cycle due to faulty innards would mean its cycle of speeding and slowing would be measured in a portion of a single rotation of the axle which happens inside of a 50th of a second, too soon for you to detect it.

I had one of those large black ring gear turntables and found every one of the problems in the design...bad lower main bearing for the bridge pivot, pit wall out of round, bad interface between pinion and drive gear, brass power rings not being reliably wiped by the copper wipers...which I was able to fix by fashioning styrene flanges just below them to keep the wipers at least mostly in contact with the brass rings.

I gave the thing away, and have happily enjoyed the $270 I spend five years ago on the new 90' built-up indexed version. Yay for Walthers.

 

[TomR]

My turntable is old, but it is round and runs true all the way around. No rubbing no binding and the gears are all good.

The power source is perfect, it is just that the motor revs and creeps at lower voltages with very light loads. At first I thought it was a special motor to index, but I took motor out and on my bench and at about 5 volts it winds way up and then slows to a crawl a few times a second.

I thought maybe the motor was intentionally designed that way, so I opened the case. Unfortunately, it is a regular old DC PM motor and everything looks great...it just has some odd behavior at low voltage. I wouldn't say it is bad because it is a 12 volt motor and runs smooth with the regulated supply cranked to 8 volts or higher.

The problem with 8 volts or more is the turntable rotates too fast. Looks like I have to do a different drive motor if I don't just get a turntable that works out of the box.

My other choice for motors is a gear motor I have that runs at 1/2 RPM. I'm thinking 2 minutes per turn is too slow though.

Maybe I'll look at another turntable or look around for a slower gear motor.

 

[Cjcrescent]

Most prototype turntables turned at 1rpm or just slightly faster. They couldn't go much faster due to the inertia created by the weight of the locos on the turntables.

While there are many reliable systems for indexing on the market, I personally still do it the way the prototype did it, which was using the ole Mark I Eyeball and experience.

My main TT is a direct drive using a 12V DC 1 rpm motor from Edmund Scientific. I will eventually have two more that will be driven similarly.

 

[TomR]

Most prototype turntables turned at 1rpm or just slightly faster. They couldn't go much faster due to the inertia created by the weight of the locos on the turntables.

While there are many reliable systems for indexing on the market, I personally still do it the way the prototype did it, which was using the ole Mark I Eyeball and experience.

My main TT is a direct drive using a 12V DC 1 rpm motor from Edmund Scientific. I will eventually have two more that will be driven similarly.

Hey Carey,

Thanks. I've been looking at this more and concluded the motor that came with this won't run reliable speeds when starved for voltage. I drilled out the center of the plastic pin that the bridge rotates on to 1/4 inch. I took a piece of 1/4 inch aluminum rod and bored a hole in the center and threaded it for a 6-32 thread.

I threaded the shaft of my 1/2 RPM gear motor. (I have limitless supply because we use them in a commercially manufactured product). It will run REALLY slow when starved for voltage, plus if the drive leads are shorted the motor acts like a brake and immediately stops the motor from over running and coasting down.

I mounted the motor below the housing of this "90 ft" turntable and it turns fast enough for me. At 18 volts it takes 45 seconds to turn 180 degrees. At 4 volts it just barely creeps but has lots of torque still.

This gave very solid slow speed rotation, virtually no play, and gobs of torque and only 45 seconds to do a 180 direction change.

I've decided to use wiping contacts to power the rails, but fix the way they did it by silver plating the commutator rings and using silver buttons out of relay contacts on the spring bars, instead of copper wiping what might be brass rings. I'll do as you suggest for now, and just nudge the rails into alignment. Maybe later I will add an optical sensor to do automatic alignment. That should not be hard at all.

The biggest thing was the rotation, and that looks to be solved now. I've tossed out all the gears and cheap poorly operating motor they used. For now this wound up simple. About two hours or less on the lathe and drilling machine to fix the drive, and I can use my free motors.

Thanks, Tom

 

[flh80]

What do you have for a bearing surface? The Heljan's came with a Torrington Roller brg, which does roll smooth, plastic races though, I don't lube, so not to attract dirt.

 

[TomR]

What do you have for a bearing surface? The Heljan's came with a Torrington Roller brg, which does roll smooth, plastic races though, I don't lube, so not to attract dirt.

I just use the stock plastic shoulder washer. I looked at using a bearing, but I can put a lot of weight on the bridge and it doesn't cause a problem.

The four little plastic wheels out at the edges that wide on the plastic rail gave me major problems. They were not perfectly round, so any pressure on them would make the turntable vibrate and the bridge jerk.

I turned down some 1/4 inch aluminum rod into 4 new aluminum wheels and put steel axles in the bridge where the old plastic wheels went. Now I can push down with my hand and rotate and the bridge runs smooth.

I modified the wipers by using point contacts and silver plated the rings, and have 100% contact now. The bridge has been rotating all night and no loss of connections.

Tom

 

[flh80]

I just use the stock plastic shoulder washer. I looked at using a bearing, but I can put a lot of weight on the bridge and it doesn't cause a problem.

The four little plastic wheels out at the edges that wide on the plastic rail gave me major problems. They were not perfectly round, so any pressure on them would make the turntable vibrate and the bridge jerk.

I turned down some 1/4 inch aluminum rod into 4 new aluminum wheels and put steel axles in the bridge where the old plastic wheels went. Now I can push down with my hand and rotate and the bridge runs smooth.

I modified the wipers by using point contacts and silver plated the rings, and have 100% contact now. The bridge has been rotating all night and no loss of connections.

Tom

That sounds good,
I still have the stock wipers on mine but made new contact rings from copper tubing, one 1/2" outer and 3/8" inside, sliced thin and super glued in place. I did coat them with Rear Window Defogger paint (repair kit type), it is real silver in a paint solution. I have a large jar from years ago, all you do is paint a line and it conducts current.

 

[RexHea]

Tom, I don't know if you are DCC, but installing a cheap mobile decoder in the power to your TT will allow you to very your speed smoothly and down to a crawl. Just hook the track-in to 12 volts and motor-out leads to motor; operate with any Throttle. Been using mine on a CMR TT for years.

 

[Dortoh1]

Tom, is the motor you used commercially available?

Doug

 

[TomR]

Tom, is the motor you used commercially available?

Doug

I suppose so. We still use them in production quantity. I'll get the vendor's info when I do a little writeup with pictures.

This might not be my final drive. I'm looking at stepper motors now.

 

[TomR]

Tom, I don't know if you are DCC, but installing a cheap mobile decoder in the power to your TT will allow you to very your speed smoothly and down to a crawl. Just hook the track-in to 12 volts and motor-out leads to motor; operate with any Throttle. Been using mine on a CMR TT for years.

I'll design a control panel that has a speed control.

Right now I patched in a manual controller I designed for an antenna tuning unit. It has both high and low speed forward and reverse momentary contact buttons, and runs off battery power or a wall wart.

I have a second automatic controller that can be adapted. It runs off 12 volts dc, has reversing, and will automatically stop at an input trigger. Because the motor goes past the stopping point this controller then automatically backs up very slow until it hits the stopping trigger signal again. If you can picture this...it would run at the push of a start left or start right button. It would run until the bridge sent back a trigger it was aligned, and it would coast down. It would then reverse and very slowly creep backwards until it hit the trigger point again and then short the motor to use it as a brake to stop immediately.

I did that because this particular motor needed to tune an antenna, and people don't want to wait all day. This way the motor can run really fast (about 1/2 RPM). That's too fast to stop precisely, so I just let it over run and then pulse the motor backwards back into the stop point it just passed. This is because the system never knows where the stop point will be, so it has to hunt it until it finds it and then stop precisely (within a half degree) on the stop point.


I'm having just a little trouble with slop in the drive, so I have to find a way around that. We spring load the thing we drive with this system so it always either leads the motor or pulls against the drive. I'm trying to figure a simple way to do that, although I can probably live with the backlash.

I'm thinking about backlash cures.

I'm non-DCC.

Tom