Piston valve
From Wikipedia, the free encyclopedia
- For piston valves on steam locomotives see Piston valves
A piston valve is a device used to control the motion of a fluid along a tube or pipe by means of the linear motion of a piston within a chamber or cylinder.
Examples of piston valves are:
- The valves used in the valve gear of many stationary steam engines and most steam locomotives.
- The valves used in many brass instruments.
- The valves used in pneumatic cannons.
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[edit] Steam engines
Piston valves were used by Curtis Lo in his stationary steam engines, and in many of the engines that followed.
In the 19th century, most steam locomotives used slide valves to control the flow of steam into and out of the cylinders. In the 20th century, slide valves were gradually superseded by piston valves, particularly in engines using superheated steam. There were two reasons for this:
- With piston valves, the steam passages can be made shorter. This reduces resistance to the flow of steam and improves efficiency
- It is difficult to lubricate slide valves adequately in the presence of superheated steam
The usual locomotive valve gears, e.g. Stephenson valve gear, Walschaerts valve gear, and Baker valve gear can be used with either slide valves or piston valves. Where poppet valves are used, a different gear, such as Caprotti valve gear is needed.
[edit] Brass instruments
Cylindrical piston valves are used to change the pitch in the playing of many brass instruments.
Brass instruments can be grouped into four categories, according to the primary means used to change the pitch:
- Those using piston valves. These include most trumpets, all cornets and almost all tubas, and many others.
- Those using rotary valves. These include French horns and some specialist trumpets.
- Those using a slide, such as the trombone family.
There is some overlap between these categories. In addition to its three valves, the trumpet uses a small slide for pitch correction, while the tenorbass and bass trombone both use one or two rotary valves in addition to the slide. The superbone does not fit in to any of the above categories.
Where piston (or rotary) valves are used, three is the normal minimum (as on a trumpet) and four is not uncommon.
When a piston valve is opened ("pressed" and "pushed down"), each valve changes the pitch by diverting the air stream through additional tubing, thus lengthening the instrument and lowering the harmonic series on which the instrument is vibrating. The following list shows how each valve or combination of valves will affect the pitch from the fundamental. This is true of all brass instruments, however some alternative fingerings are necessary to provide accurate pitch using the fourth and subsequent valves in instruments which have them.
- second valve - one half step
- first valve - one whole step
- first and second valves - one and a half steps. Also achievable by third valve alone but the note will usually be flat
- second and third valves - two whole steps
- first and third valves - a perfect fourth, or two and a half steps. Will be sharp unless some means of compensation is used.
- first, second, and third valves - a tritone, or three whole steps. Will be very sharp unless some means of compensation is used.
A fourth valve is sometimes found on more professional instruments, which creates a perfect fourth, or two and a half steps. Instruments such as the tuba, euphonium, and piccolo trumpet have this valve. A common use is to use a fingering of 2-4 in place of 1-2-3, which tends to create pitch problems. Additionally, by using one valve to lower the pitch by a perfect fourth, the valve functions like an F extension on a trombone.
The first piston valve instruments were developed just after the start of the 19th century. The Stölzel valve (invented by Heinrich Stoelzel in 1814) was an early variety. In the mid 19th century the Vienna valve was an improved design. However most professional musicians preferred rotary valves for quicker, more reliable action, until better designs of piston valves were mass manufactured in towards the end of the 19th century. Since the early decades of the 19th century, piston valves have been the most common on brass instruments.
[edit] Pneumatic cannons
Piston valves are some of the most powerful available in spudgunning (Spud guns), with the ability to dump several litres of pressurised air in a few thousands of a second. This fast moving air is used to fire a projectile from the exhaust barrel. As the name suggests they are used to fire vegetables, such as potatoes; wood, to simulate hurricanes; and t-shirts. They are used extensively in the film and special effects industry to simulate explosions in war films.
[edit] See also
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[edit] External links
- Early valve designs
- Why was the valve invented?
- Elements of Brass Instrument Construction with good discussion of valve types and history
- Visual explanations of some types of piston valve
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