My attempts to get an MDC Shay to run well

Dear Sir:

No snickering here. There are logical reasons behind any noisy mechanism and I'd rather see somebody trying to make it right than yet another who says 'Go buy one ready made' which is another way of saying 'I am too lazy to figure this one out'. Hmf. This is a hobby. It's supposed to be stressful and aggravating. :)

I think it is possible that the driveshaft to the 3rd 'tender' truck is bent, or that a U-joint is out of whack. Unfortunately, I don't own this kit myself. I am just going by what happens when other driveshafts do what you seem to be describing. Perhaps you could straighten the shaft?

Is it possible to install the two U-joints in different positions? If so, you could take one off and turn it 90 degrees

i.e. if the two joints look like this: ]-----[ you might change to this: ]-----= or =-----[

U-joints generate vibration of their own, and this might help to cancel some of it out. If the U-joints are irredeemably bad, I think NWSL and hobbytown sell them.

As far as the noise, could you perhaps turn the machine over and run it slowly, then use a long, slender screwdriver, with the handle held to your ear, and the tip touched to various areas of the mechanism, to track down the source of the noise? This works really well on 1:1 scale car engines, and I don't see why it wouldn't here...

Cordially yours, Gerard P.

snipped-for-privacy@gannon.edu spake thus:

Thanks for the suggestion, but there's actually only 1 universal joint on the shaft to the 3rd truck: the driving part is on the worm shaft that sticks out of the 2nd (rear loco) truck, and the driven part is on the front of the 3rd truck worm shaft. Also, I don't think this is bent; however, I'll take it apart and make sure it isn't.

Another good idea; just wish I could do that with the loco actually running. It only makes the really bad hammering sound when the tender is hooked up. Maybe I could figure out how to run all 3 trucks in some kind of stationary setup.

A single U-joint allows ONLY angular misalignment, and then only if the center of angular rotation occurs EXCATLY at the center of the u-joint.

You need two u-joints to allow sideways displacement of the shafts or a rotation about some other point (the usual situation).

The tank of a Shay does most certainly NOT merely rotate, but must move from side to side and up and down to accomodate changes in track alignment. Thus a single u-joint cannot work properly in this situation, in principal.

Dan Mitchell ============

Daniel A. Mitchell spake thus:

OK, now we're getting somewhere. This all makes sense, and reduces my ignorance regarding universal joint operation somewhat.

What MDC gives you is in fact a single U-joint at this connection: a "horned ball" on the end of the loco driving shaft and a driven "cup" on the end of the tank.

So, referring to my trusty NWSL catalog, it looks as if what a guy would want to get are the parts to make a double universal: a cup on the end of each shaft with a cardan shaft between them, just like the ones between the bull gear and each loco truck. Is this correct?

Would this stand a chance of making this thing run better?

OK, now I understand how the NWSL universals work: they give you 2 cups and 2 horned balls, and you supply the shaft material to make up the length needed. Does this work OK? any problems with potential shaft misalignments? I'm guessing that so long as the shaft isn't bent, it should work well.

This is my first look at this thread so I may be repeating something already said.

- Not only does the cardan shaft need to have a flexible joint at each end, but the shaft also needs to be able to shorten and lengthen due to it's position to allow for the changing distance between the motor output and truck power input as the truck turns.

Probably the easiest way to do this is to use telescoping square tubings for the cardan shaft, and perhaps rod soldered inside each end to mate with the universals. Unless you use a spring inside the cardan shaft the universals parts are going to have to hold the cardan shaft halves firmly enough to cause it to extend to match the spacing.

Regards, Greg.P.

Yes, it sound like the NWSL double U-joints will help a lot here. They will not, of course, solve a bind in the gearbox or other location. However, IF the problem is binding due to the inadequacies of the single U-joint provided in the MDC kit, it may be your answer. In any event, it should improve the overall performance. The floating section between the u-joints need not be long .. for an HO Shay I would think 3/8" would be a fine starting point. Even less can be used, as long as the 'cups' don't touch. Grandt line makes a double U-joint with a hex on each shaft, and a hollow hexagonal tube as the floating section. It is a bit shorter than the typical ball and cup type, but rather larger in diameter.

Dan Mitchell ============

True in principal, but since the MDC driveline is on the loco centerline, and the coupling between the Shay and it's tank is VERY short, such a slip joint may not be needed here. The normal slop in the two U-joints is probably more than adequate.

The side "Shay" driveline, on the other hand, most certainly needs such joints as it must get longer and shorter every time a truck swivels. On the MDC Shay this is only for looks, but it still needs the variable length feature.

Dan Mitchell ============

Dear folks:

Let me add to the confustication some more, after looking at my MDC parts catalog's delightful exploded views, felt-tip pen edits and all.

The MDC single U-joint isn't as bad a design as it looks. Greg Procter's concern is legitimate, but the U-joint cup is slotted to allow for the small amount of end play required. In this case, too, you don't quite need 2 U-joints. The truck pivots to the water cart that is pivoted to the main frame, and the double-jointing is what takes care of the sideways alignment.

In order for this to work, however, the distance from the pivot point of either U-joint to its corresponding truck center has to be nearly equal. Any difference will cause some side pressure in the joint, and make noise. This would not show up on straight track...if your engine is equally noisy on tangents and on curves, your problem isn't the use of a single U-joint. If it does get noisier on curves, try measuring the distance I just mentioned, and adjust the ball and cup to correct it.

Here are some other things you might check: see if both drive shafts are straight and free from wobble, and make sure the cup and ball are driven squarely onto the shafts and not warped or bearing excessive flash. If the U-joint is impossibly cruddy, you might be able to replace it with a single NWSL joint.

This is actually making me want to buy a Shay just to tinker with it. I think I must be insane. Cordially yours, Gerard P.

Well, you've still got a problem.

The tank should NOT merely be hinged to the back of the loco. A proper drawbar should be used, with a pivot at each end. There is no way the rear truck can ever track properly if it's merely hinged to the main loco frame. If you have access to a Bachmann Shay, look and you'll see that it has a drawbar, short to be sure, but it's there. Most, or all, brass Shays also use a drawbar. The misalignment caused by the lack of a drawbar is slight, but becomes significant on sharp curves. Along with the drawbar, you need a two U-joint connection shaft as we've already discussed.

I suspect MDC has attempted to simplify things by eliminating the drawbar AND the second U-joint. The result is NOT good. It may work on larger radius curves, and slop in the truck mounting and various other joints may allow it to fuction to some extent, but the design is fundamentally flawed as you describe it.

A real Shay also has substantial truck side play due to a specially sprung bolster (not visible, or present, on HO models). This aids with tracking under sharp curve or rough track conditions.

Dan Mitchell ============

Dear Sir:

I thought the same as you at first, and wondered how even MDC expected it to work, until I noticed that the truck is itself pivoted to the water tank frame. This in effect makes the whole water tank frame a drawbar, hinged to the engine frame at one end, and hinged to the truck at the other. No tracking problems here, unless the curve is so sharp that the corners of water tank and engine collide...

There is a fundamental flaw, however, and I didn't notice it when I last posted. Trucks 2 and 3 are identical, and the pseudo-drawbar-water tank allows both to be perpendicular to the track center line. Therefore, if you adjust the U-joints so the distance between their hinge point to the truck center is equal for both trucks (triiiicky, I bet) the U-joint members should both displace the same amount off the track center, and everything is lovely. BUT if there is any fore-and-aft play in the funky water tank's two hinge points...and there will be...the ball will slide in the U-joint cup, making the just-mentioned distances unequal, and putting side pressure on the joint...and if there isn't enough slop, making noises. However, without the engine here I can't say if the slop is adequate or not. People are fond of overtolerancing stuff in model railroading and there isn't always that need...but you can't get away with everything...Arbour using a flexible rubber tube as a combination motor coupling and drawbar comes to mind...

You know, the double-U-joint would make this a non-issue, so maybe I should stop being so stubborn :).

Cordially yours, Gerard P.

Daniel A. Mitchell spake thus:

OK, so in addition to the double universal, I should add a drawbar, is that right? What would you suggest: drill & tap the frame (metal--Zamak?) and screw in a metal drawbar? I've eliminated the "hot chassis", so no problem there.

Is there any need to equalize the lengths of the U-joint, as another poster suggested?

By the way, I neglected to mention that my test track is a 48" dia. circle, so the loco was constantly running in a curve. (The world's dumbest "layout".) I figure if it runs well there, it should run perfectly on a straightaway.

Before you make that assumption, turn the loco around and run it the other way!

Dear Sir:

1. Try installing the double universal, but no drawbar. In this case you don't need one...the water tank unit ACTS as a drawbar, since it is hinged to both the main unit and the third truck. Try to visualize the action of an engine-tender drawbar and the third-truck unit and you will see what I mean.

1. My suggestion about equalizing the lengths isn't needed unless you want to keep the single-universal arrangement.

2. The circular test track would explain why the noise comes and goes...I bet it runs quiet on the straightaway...the double u-joint should fix that.

Cordially yours, Gerard P.

A drawbar would be the best solution, with the truck then mounted rigidly under the tank, and a double U-joint.

However, you say the truck is free to pivot under the tank. If so, then you don't NEED the drawbar, just the two u-joint driveshaft. The point is, BOTH the tank/gearbox/truck *AND* the driveshaft need TWO points of articulation for the tank to 'track' properly.

On wider radius, simple curves, you may sometimes get by with less flexibility, as you get mainly angular misalignment ... but when going through "S" curves and much switchwork (which usually includes "S" curves), you need TWO points of articulation since lateral (side to side) misalignment also occurs there. Without such flexibility the truck cannot accommodate properly to the track.

Dan Mitchell ============