Railway Preservation News

Air Powered Turntable Throttle Levers and Valves
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Author:  joe6167 [ Sun Jul 23, 2017 11:16 am ]
Post subject:  Re: Air Powered Turntable Throttle Levers and Valves

softwerkslex wrote:
Can you jockey the table using the reverser?


Author:  Brian Norden [ Sun Jul 23, 2017 11:19 am ]
Post subject:  Re: Air Powered Turntable Throttle Levers and Valves

Just out of curiosity, where is this turntable?
I seem to have missed something in/about this thread.

Author:  Overmod [ Sun Jul 23, 2017 4:52 pm ]
Post subject:  Re: Air Powered Turntable Throttle Levers and Valves

To put a brief unexperienced word in here:

As I see it, a principal 'difficulty' is that you're using the wrong kind of valve. Some designs are optimized to minimize resistance to flow, or maximize throughput mass flow, when open -- this is one of those types. Often these are relatively ill-suited for use as throttle valves as there is little care taken to make them start to port with the kind of reliable positional precision as a steam edge on a locomotive valve, and difficult control both in opening and in rate of increase in flow at small degrees of opening. Add to this the small, double-sided actuating handle you have on that valve, and a sloppy fit on the stem, and you have a kind of perfect storm of disaster for fine control. I also suspect your air motor is like many industrial steam engines in having extremely rudimentary cutoff control, leaving much of the 'fine control' (as noted) to the wiles of the brake.

What you probably want is a valve explicitly designed as an air throttle or speed control, with the air-valve equivalent of trapezoidal ports to make the admission smooth and progressive over a wider range of arc. I suspect that fine control using multiple turns on a threaded stem globe valve is false precision in this particular application, but I'd also think you want a 'two-speed' control valve, one that cuts in to accelerate and move the table quickly, and a second for fine vernier control of angle and rate close to the stopping point. The coarse control can be a hand lever, the fine control a horizontal lever arc like an air-brake handle; the two should have separate block valves and relief cocks (as also noted) and I would recommend that you make them both with some form of reliable locking removable handle (in fact, I would recommend a standard air-brake handle) to give reasonable security with the table unattended but quick response by crews if it needs to be moved or stopped in emergency.

Why do you not have some system of range contacts set up, electrical for example, that tell you first when you're within a couple of degrees of center, and then in skate engagement range. The 'display' would be little different from some types of guitar tuner: say, two yellow lights either side of a green one. Sprung roller contact on the table meets two split sector plate contacts (for the respective 'in range' lights) and a narrow center one for the engagement. Not too much more difficult to rig up something that would indicate the stall selected, if it is difficult to see that from the control position. (Fallback, of course, is one of those cheap WiFi cameras mounted somewhere on the bridge and pointing toward the joint: pull up the feed on your smartphone and it's just like being there...)

Author:  joe6167 [ Sun Jul 23, 2017 6:01 pm ]
Post subject:  Re: Air Powered Turntable Throttle Levers and Valves

Thanks for the advice, I'll pass that on.

Yes, I cannot crack those valves open and get just a little bit of air, I have to crank the handles a few revolutions before the motors finally kick in (more than I need them too).

Funny thing is I did find a stash of air brake handles just the other day (leftovers from the railway).

And yes, there is no cut-off control on the motors, only a forward and a reverse notch in the reverse lever (and not even a neutral notch.)

As far as security goes, the turntable does have lockout valves on each end and I've recently started using them (with padlocks). Don't ask why this was never done, suffice to say we've only just gotten started on cleaning up the 10 years of mess that we've inherited and this business with the turntable is part of that process.

As for the electronics, I'll bet that once we can control the engine, those minor corrections won't be too much trouble.

Author:  joe6167 [ Sun Jul 23, 2017 6:39 pm ]
Post subject:  Re: Air Powered Turntable Throttle Levers and Valves

How about something like this?

https://www.acklandsgrainger.com/en/pro ... rchResults

Author:  Overmod [ Sun Jul 23, 2017 7:00 pm ]
Post subject:  Re: Air Powered Turntable Throttle Levers and Valves

See if they have one with the threaded ports on either side of the valve body, so it will mount in your existing line with just a couple of unions. You don't have any structure that you can 'bracket' the valve box to.

On the other hand, if you use a couple of risers and right-angle fittings you could adjust the handle-shaft axis height and align the handle vertically; this could be threaded onto one side of the existing piping with a union connection on the other side and have good integrity.

I take it there's no issue with the airflow having to fill the considerable dead volume of piping between the valve and the motor, and then overcoming any starting resistance in the motor -- no precision actually needed until 'short final' approach to the aligned position. You might need to practice a bit with the new valve on how you crack it open full (which in turn reduces some of the potential throttling wear on seats and clearances during unshrouding) and then bring it back to the right position either for steady state turn or as you approach final position.

Author:  crij [ Mon Jul 24, 2017 1:48 pm ]
Post subject:  Re: Air Powered Turntable Throttle Levers and Valves

With the alignment issue, why not do something as simple as an indicator arrow painted or installed on the edge of the platform and something mounted on the pit wall to indicate where the center of track is.

One easy to obtain indicator would be the nails for the Ramset guns (for nailing studs to concrete), as they have a 1" to 1.5" washer under the nail and can be driven directly into concrete if there is no tie in the area. Maybe even paint the stall/track number next to the washer to make it easier for the operator to find.


As far as the control valving, my gut says (based on my experience designing industrial processing systems), a total of 4 valves are going to be needed. Starting from the supply line, install a ball valve (isolation) and then a tee, after the tee on the straight side is another ball valve (Fast), then 2 more tees connected in series using the straight ports before going down to the motor, on the side branch of the first tee, install a reduced size modulating valve (Slow), this line connects to the 2nd tee, the 3rd ball valve (Drain) is installed on the 3rd tee.

WP_20170724_13_34_13_Pro.jpg [ 76.09 KiB | Viewed 925 times ]

Operation would be;
-Open track lock, set Direction, engage the Clutch, and stand on the Brake.
-Close Drain, open Isolation, use Fast to get you close.
-Use Slow to get you into position while riding the brake, stand on Brake when you are in position.
-Close Isolation, open Drain, open Fast (to relieve air pressure between Isolation and the control valves)
-Close Fast, disengage Clutch, Leave Drain open.
-Close track lock (if you have an assistant do it before 4th step, while you are still on the brake)

Alternately a pressure regulator could be installed after the Slow valve if a modulating valve can not be found. Install a screened plug on the Drain valve so it doesn't get plugged by Wasps. For ease of maintenance install a union next to the Isolation, Fast and Slow valves, ideally one on both sides of each valve.

Rich C.

PS: On a side note, have you tried partially opening the drain valve while opening the control valve on the current system, to do your fine adjustment? This would reduce the volume of the air running to the motor, as you can bleed off most of the flow by adjusting how far open the drain valve is.

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