Steam locomotive valve gear

[TomDiehl]

QUOTE: Originally posted by trainjunky29

Just to nitpick, the 85% cutoff figure makes sense, but cutoff can go down to 0% when the reverser is in the center.

Not sure what steam theory you're working with, but based on my experience in the backshops at Steamtown, the mechanical practicality of the real world of steam locomotives, once the reversing quadrant reaches a certain point near the center, the valve no longer moves far enough to allow steam into the cylinder, hence the 15% figure.

Also, while it is true that back pressure slows down an engine, in any cutoff position, the force of the forcing steam will be much greater than that of the exhaust steam. Given an infinte amount of steam, maximum speed would be found with the reverser all the way forward or back. The only reason to bring the reverser back is to conserve on steam, since at high speed you would otherwise quickly lower your steam pressure.

Again, not sure of what theory you're quoting, but that "infinite amount of steam" needs to be contained and controlled through valve(s) and piston(s) to be converted into mechanical energy. The steam passages through the valve for steam admission or exhaust are about the same size, since the pistons are double ended, so the volume of the steam does become a factor. In the real world, the reverser being at either limit will severely limit your maximum speed, not allow maximum speed.

About the slide valves and superheating, there is no real problem (as far as I know) with slide valves on a superheated engine. It seems to me that this is the case simply because piston valves were widely adopted before superheating was.

The main problem was lubrication, as stated above. A slide valve was basically a plate with passages cast or machined into it, maiking it more difficult to lubricate than a piston valve

QUOTE: Originally posted by marknewton


The main difficulty was in properly lubricating the valves. The temperature of superheated steam was well above the flashpoint of the mineral oils in use at the time.

Um...oil can't burn when surrounded by steam, only in the presence of oxygen molecules, so the steam temperature doesn't really matter as far as the oil burning. Consider these two locomotives: Grizzly Flats Railroad (ex-Nevada Central) no. 2 and Ventura County Railway no. 2. GF no. 2 was built in 1881, Baldwin 5575, has slide valves and is not superheated. Ventura County Railway no. 2 was built in 1922, Baldwin 55415, and has piston vavles and is superheated. They both use Nathan hydrostatic lubricators, both using steam oil, also known as valve oil (as opposed to journal oil).

Sincerely and respectfully,
Daniel Parks

Two last points, the oil wouldn't necessarily burn in the common sense, it would vaporize. And vaporized oil would lose most of its lubricating properties. The other question to ask, is the Ventura County engine using the original type oil it was designed to use, ot is it using a newer type? As stated above this is a GENERAL rule.

[Anonymous]

Looking at the shortening of the cutoff question, trainjunky said that, given an infinite steam supply, maximum speed will be attained with the reverser in the corner.

This is not true. The steam is expanded during the power stroke; with full valve travel, the expansion is less than it would be with the travel reduced. Shortening the cutoff of the valve allows less steam to be admitted, but the steam utilizes its power and loses heat by expanding more. A smaller weight of steam does more work. If this is not true, why use steam? Compressed air would, under that thinking, do the same work.

Not only would full valve travel not produce higher speed because of the steam question, you'd have a much larger quantity of steam to exhaust during the time the port is open on the other end, causing more back pressure. If you are using less steam on the power stroke but allowing it to expand, you have less steam to get out of the cylinder before the next power stroke, during the port opening for exhaust. Your back pressure is going to be much less.

There were two reasons why slide valves went out of fashion. First was the difficulty of lubricating them and the fact that steam pressure had a mechanical effect on them, pressing them into their seats. The second was that port area was limited, because of clearances and the limited width available for the seats. The piston valve ports were all around the circumference of the valve. And steam had no mechanical effect on the operation of the valve.

Old Timer

[timz]

"cutoff can go down to 0% when the reverser is in the center."

Remember that Walschaerts (and Baker?) is a constant-lead gear-- so the valve is always open at the end of the piston stroke, and cutoff is never zero.

[TomDiehl]

The term "cutoff" in this example means that the steam flows for a certain percentage of the stroke, then the valve cuts it off. A zero cutoff would be a mechanical impossibility.

[M636C]

I have been told that Baker Gear, and Southern valve gear which was in some respects similar to Baker, required less maintenance effort than Walschearts gear.

Southern gear looked more clumsy, and wasn't used as much as Baker. I was told that the long hanging arms of the Southern gear could at times get an alarming swaying motion, although I don't know if this resulted in the gear impacting the connecting or coupling rods. That story related to the first test run of Queensland Railways experimental 2-6-2 locomotive, the only one on the system with Southern valve gear.

New South Wales had 120 2-8-0s fitted with Southern gear, the remaining large 2-8-0s having inside "Allan straight link" valve gear, a variation of Stephenson gear with a straight expansion link (which was much easier to machine).

A problem related by a senior engineer was that the Southern gear gave so little trouble that nobody ever looked at it, and the first sign of trouble was one of the major shafts failing and the valve gear falling off. This may be an exaggeration but the locomotives fitted with Southern gear kept it for their entire lives, even after Walschearts became common on later types.

The 2-8-0s with Southern gear could certainly run faster than those with inside gear, although this wasn't actually permitted. I'm sure that the outside gear got better attention than that inside, given the relative ease of access, even if it didn't need much.

Peter

[Anonymous]

The radius hangers and transmission yokes of Southern valve gear were designed with a wide enough base at the top (especially the transmission yoke - the forward of the two vertical members) that getting out of line laterally far enough to contact the main rod was not going to be a problem. The bell crank had two horizontal members, far enough apart(one inside and one outside of the valve gear hanger) to give the necessary stability to the transmission yoke.

The Southern gear required a long eccentric throw because the leverage inherent in the gear reduced the eccentric motion so much that even a 6-inch valve travel required a lot of movement to work with. That long throw gave the impression at speed that the stuff was going to fly off the engine. But it seldom, if ever, did . . .

In Southern steam excursion's early days, I worked on both the little Consolidations - the 630 and 722 - that were equipped with Southern gear. They ran just fine. The valves were square and stayed that way.

Old Timer