Unusual Valve Timing?

A special section just for steam engines and boilers, as without these you may as well fit a sail.
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Re: Unusual Valve Timing?

Post by dampfspieler » Mon Apr 02, 2018 4:02 pm

Hi Lionel,

thank you for "Don Ashton ..." i have looked for and found this website, it is very interesting.

Don Ashton's site devoted to steam locomotive valve gears and their design, kinematics and distribution problems, particularly for Walschaerts' and Stephenson's gears.

There are also available some spreadsheets for calculating stephensons and walshaerts valve gears.

Best Dietrich
Lionel Connell
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Re: Unusual Valve Timing?

Post by Lionel Connell » Tue Apr 03, 2018 1:32 am

Hi Dietrich,

Don's book is well worth buying, it contains more in depth information than the web site.


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Re: Unusual Valve Timing?

Post by steamboatjack » Tue Apr 03, 2018 8:55 am

First let me recommend that the standard conventions are used to avoid confusion as there are a lot of people unfamiliar with marine engineering terms.
“lead” should be considered as the amount the valve is open to steam at TDC/BDC and should be stated as such in inches or mm. NOT in degrees of crank angle as this is misleading, it varies with conn rod length etc. Etc.
Secondly the old chestnut referring to “ahead” guides on single guided engines like the 6A, the ahead rotation is clockwise when looking at the propeller from the stern. Assuming the engine is placed conventionally with the H.P. forward and the guide bars on the left of the same view.
Incidentally this means that if end suspended links are used this engine should have “crossed” rods.

I would adjust the amount of “lead” on a 6A to a quite small figure perhaps 0.010 to 0.020 inches. Why? If I were designing an engine of about this size I would expect it to run at about 1000 rev/min as a ball park figure (500 ft/min average piston speed, very modest). The usual speed for a launch engine as used by hobby steam boaters is less than 500rev/min and the reciprocating mass is quite low therefore the inertia loadings are very low.
The 6A does have a problem of high compression lifting off the valve from the face, another “cure” for this is to set the valve with even leads top/bottom and then adjust the valve up about 0.010 inch.

p.s. TDC/BDC should be accurately marked by the trammel method, a very small error here is significant to lead setting. This method has been discussed in earlier posts.
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