Return crank jig

[runner42]

I was aware in the past looking at certain posts that the return crank can be set using a simple jig, this was only referred to without details of how the jig is constructed, so that its position for setting and permanently fixing can be achieved early in the construction without waiting for the valve gear to be completed and assembled. I have come across this picture in isolation without any description on how it is constructed in terms of obtaining the required 90 degs angle. It appears to be using the triangular method, the jig providing one side of a right angle triangle together with the throw providing the other and the return crank along the hypotenuse. Here is the picture. 

What is the simple trigonometry values for determining the length between the centres of the jig. I assume that this applicable to all configurations ie in line and angled cylinders.

Brian 

return crank gauge by Brian Leach, on Flickr

PS it appears looking at it that it's rather obvious, ie the jig centres are = the square root of the (return crank squared) - (throw squared) 

[Lisa]

Ideally you'd want to adjust the length of the return crank to get the correct throw at 90° to the crankpin, rather than adjusting the throw to suit a certain length of return crank. Thus add the squares of the crankpin throw and eccentric throw together, and the square root of that number is the length of the return crank.

There's likely a pointy bit (technical term) in the other jig hole to fit into the centre drilled axle end, to keep it all locked together.

[rrmrd66]

Hi Brian

"What is the simple trigonometry values for determining the length between the centres of the jig?"

Do you mean the ratio of 3,4 and 5? (9+16=25). This will give you a 90 degree angle.

Regards

Malcolm

[simplyloco]

I've got some waffle from my past here: it might be of some use!
John

modeleng.proboards.com/thread/3816/brit-progress-setting-return-crank

[Roger]

Just to ask a stupid question... Isn't the angle that looks like 90 degrees in fact not 90? On my valve gear it isn't. Don Ashton's line drawing shows a radius for the return crank and then a length to the centre of the supporting crank pin and its radius.

If you have access to a CAD system, I'd just draw it an measure the angle rather than calculate it.

[92220]

This is interesting. I was proposing to set my motion up manually, and not looking forward to it one bit! I note that you are quoting the angle as 90 degrees (pythagorus). That prompted me to get the BR motion drawings out for the 9f. I've just drawn it up to their dimensions and the angle for the 9f actually works out at 91.5 degrees. Is this an error on BR's part in designing the motion? Should the angle be 90 degrees to be theoretically correct? I've not found anywhere else that quotes an angle at all. If 90 degrees is theoretically the best, then that is easy to achieve without going through the hastle of setting the valve gear manually.

Bob.

[stevep]

You are right, Roger. The 90 degree angle is only that if the valve gear is laid out "all square". Some American locomotives I've seen are like that, but in most (all?) British locos, the valve gear is set up slightly from the horizontal. The 90 degrees needs to be to that angle, rather than the horizontal.

[don9f]

Hi, I agree that the angle isn’t always exactly 90 degrees....it depends on the particular design of the valvegear. I’m sure I’ve read about this in one of Don Ashton’s descriptions of Walschaerts valvegear somewhere.

Edit: it’s mentioned elsewhere in the document as well, but see the last question/answer in the FAQ section, which follows on from what stevep said above

Here

To reassure you Bob, I think the Full Size 9F value of 91.5 degrees is correct as I made a jig very similar to the one in the photo for my own 9F and my calculations from the dimensions given on the drawing (diameter of the circles described by the main crankpin, the return crank, plus its length between centres) showed the angle to be 92 degrees.

Cheers Don

[runner42]

Thanks for the replies, some I understand and some I don't. I don't think any replies either support the feasibility of a jig or state that it is not practical. I accept that it is not necessarily going to be 90 degrees, and that the all square configuration is not applicable to the Black 5. However the picture shows that angle can be determined beforehand and it's used for fixing the return crank before the wheels are affixed to the axles. That must be an advantage. I would like to pursue this and hope that someone can me how to calculate the distance between centres of the jig.

Brian

[Roger]

Hi Brian,
If you don't have a CAD system to draw out the triangle represented by the three points, you can use one of the online calculators to work it out for you.

[92220]

Nov 11, 2018 21:05:10 GMT don9f said:

Hi, I agree that the angle isn’t always exactly 90 degrees....it depends on the particular design of the valvegear. I’m sure I’ve read about this in one of Don Ashton’s descriptions of Walschaerts valvegear somewhere.

Edit: it’s mentioned elsewhere in the document as well, but see the last question/answer in the FAQ section, which follows on from what stevep said above

Here

To reassure you Bob, I think the Full Size 9F value of 91.5 degrees is correct as I made a jig very similar to the one in the photo for my own 9F and my calculations from the dimensions given on the drawing (diameter of the circles described by the main crankpin, the return crank, plus its length between centres) showed the angle to be 92 degrees.

Cheers Don

Thanks for confirming that Don. I'll go ahead and make up a jig to set the return crank.

Bob.

Hi Brian.

If you can PM me the details, I'll have a look at drawing it up for you on CAD.

Bob.

[silverfox]

How does one know if the motion is 'horizontal'?

[stevep]

If the valve gear is laid out "all square", the tail of the expansion link will lie on the line through the centre of the driving axles.

[houstonceng]

Nov 12, 2018 14:57:45 GMT stevep said:

If the valve gear is laid out "all square", the tail of the expansion link will lie on the line through the centre of the driving axles.

I think you also need to add that this will be the line joining the centre of the driving axis and the centre of the little end. Else the cylinders may be angled and the return crank would not be at 90 degrees to the crank pin.

[don9f]

Hi Brian, that Triangle Calculator Roger linked to is used here to demonstrate the points raised earlier about angles etc.

Using the dimensions on the Les Warnett 9F drawings:-

Diameter of the circle described by the Return Crank = 1.307 inch....therefore its radius = 0.6535 inch
Main Crankpin throw = 1.25 inch
Length between centres of Return Crank = 1.429 inch

Inputting these as the three sides of the triangle shows that the angle we have been talking about = 91.841 degrees. (the calculator rounded 0.6535 to 3 decimal places)



I originally calculated this ("long hand" when I built the model in the 1980's) purely for interest.....but as others have said, you don't actually need to know it!

The jig I made to set my Return cranks was very like the one in the photo and the distance between centres of the axle and the end of the Return Crank was the 0.6535 dimension referred to above. Assuming the Wheels and Return Cranks are correctly made, they are then set as the designer intended.

Do your Black 5 drawings show this info in the same way?

Hope this helps

Cheers   Don

[silverfox]

Don

Thats great ,but when you dont know what the Diameter described by the return crank is You are up 5h1t creek without the paddle

Ron

( and yes it is nowhere on the plans!!!)

[Jim]

I thought Don Ashton covered all this in his book 'Walschaert's Gears for Model Engineers. I certainly found it straightforward and helpful when building the Britannia's valve motion.

Jim

[stevep]

Nov 12, 2018 16:41:08 GMT houstonceng said:

Nov 12, 2018 14:57:45 GMT stevep said:

If the valve gear is laid out "all square", the tail of the expansion link will lie on the line through the centre of the driving axles.

I think you also need to add that this will be the line joining the centre of the driving axis and the centre of the little end. Else the cylinders may be angled and the return crank would not be at 90 degrees to the crank pin.

Andy,

Sorry, I was making the (wrong) assumption that the cylinders would also be horizontal. You are quite right.

[runner42]

Hi Don,

The Main Crankpin throw is 1.25" and the Length between centres of Return Crank is 1.460" but I don't know the Diameter of the circle described by the Return Crank. Where is this specified on the drawings?

Brian

[stevep]

One of the initial parameters, when designing valve gear, is to decide the angular swing of the expansion links. Based on the distance between the hole in the tail (on Walshaerts gear) and the expansion link pivot point, determines the distance the hole moves back and forth to produce this desired swing.

As this hole is connected to the return crank pin by the eccentric rod, this in turn determines the diameter of the circle that the return crank pin describes. However, this will be altered slightly by two factors: the angle of the eccentric rod when in mid stroke, and the back-set of the hole in the tail of the expansion link (the back-set aims to produce equal swing in the expansion link).

I think you need to model this in one of the valve gear simulators if you want to determine the setting before making it all. Otherwise, you have to make an adjustable eccentric rod, and keep adjusting its length, and the angle of the return crank, halving errors each time, until you get the desired results.

I realise, this doesn't help you to make the return crank jig that was your aim.