Return Crank setting

[Runner]

Hi all,

it states here http://www.modeng.johnbaguley.info/Valve Gears/valvegear1.htm fourth paragraph, that for the Walschaert's valve gear, the return crank can be set using a simple jig and that the lengthy set up that specified by LBSC, et al, is unnecessary. However, it doesn't show how the jig is made and indicates that the return crank is set to a position relative to the valve gear design and therefore doesn't take into account any build up of tolerances as LBSC's method does. Also the latter (LDSC's) requires that the eccentric rod length be determined from this set up method.

Can anyone tell me how such a jig is made?

Thanks in advance.

Regards,

Brian

 

[kvom]

For my Kozo A3, the return crank length is set and it's the eccentric rod whose length is determined with a jig. However, I think the same idea would work.

Make two pieces of flat bar stock such that one has slots so that they can be bolted together and have the length adjusted by sliding the mounting screws in the slots. A hole in the end of each bar allows attachment to the crank pin and eccentric strap.

To use the jig set the crank pin at back dead center, and the valve such that it's just before opening. Now position the eccentric rod/jig junction so that it's directly over the centerline of the axle. The correct return crank length is one where the radius rod can be moved up and down without moving the eccentric rod. When that position is achieved the jig screws can be tightened, and the distance between it's mounting holes measured.

Kozo's return crank is 1.149 * the crank pin offset from the axle. So I have no reason to believe that ratio would not work elsewhere.

 

[Niceonetidy]

Setting Top and bottom dead centre is important with anysteam valve events. I use a Dial TestIndicator each end, and measure the last few thou of travel, mark the flywheelor wheel with the same distance travel relative to a fixed point. And TDC ishalf way between the two marks. Then adjustthe angle of the Expansion link to a position that does not move the valve rodwhen you stroke the reverser from full forward to reverse. This is where you put the test jig on to theengine to set and measure the centres.



Happy days,

 

[Runner]

Thanks Kvom and Niceonetidy.

At first reading Kvom I thought you were proposing making an adjustable eccentric rod, however this jig goes between the return crank and eccentric rod. This has me flumixed, since the eccentric rod is connected between the return crank and the expansion link. The object of the exercise is to determine the return crank offset and the length of the eccentric rod. The return crank length is already known. I am sorry that I am unable to grasp your explanation. Is there a URL reference that shows the use of this jig on a locomotive?

Niceonetidy is your reference to top and bottom dead centres interchangeable with front and back dead centres, which are important setting points for locomotives?

Regards,

Brian

 

[kvom]

I corrected the typo. Jig is to determine the length of the eccentric rod. You're just making a temporary eccentric rod whose length can be varied. The jig is two pieces attached together with screws. One piece has slots for the screws so that the two pieces can be adjusted.

As said before, the return crank is positioned so that at back dead center the end of the crank is directly over the axle centerline.

 

[Runner]

As said before, the return crank is positioned so that at back dead center the end of the crank is directly over the axle centerline.[/QUOTE]

Is this true for all configurations of Walschaerts valve gear? I looked at the GA drawing for my locomotive and the postion of the return crank doesn't appear to fit this criterion. It was drawn at bottom dead centre so I rotated it 90 degs to be at back dead centre. As you can see the crank pin is not at the axle centreline but slightly forward. It maybe due to drawing error.

 

[kvom]

Here's the diagram from Kozo's book, showing the procedure and jig I used. Kozo uses a jig block to position the crank pin at back dead center; I used some gauge blocks and verified with a height gauge.

 

[Runner]

Thanks kvom, so tantalising close to a solution. The essential criteria is making sure that the crankpin is set at back dead centre and the positioning of the crank (offset) is correct. Both aspects provided by the jig presumably covered by fig 17-36.

Regards

Brian

 

[robmort]

It is a pity this thread, like so many on this subject, ignores two important aspects.
Using the eccentric rod jig to determine its length depends, among other things, on firstly:
1) setting the return crank position accurately so that the pin is accurately on a line at 90deg to the line between main crank pin and wheel centre.
2) ensuring that the driving wheel is set vertically in the middle of it suspension travel, or at the "normal" ride height.