Piston valve

Encyclopedia : P : PI : PIS : Piston valve

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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.

Steam engines

Piston valves were used by James Watt in his stationary steam engines, and in many of the engines that followed.

With the exception of those fitted with Caprotti valve gear, almost all steam locomotives use piston valves to control the flow of steam into and out of the cylinders. This includes early locomotives using Stephenson valve gear, and later ones using its descendents such as Baker valve gear and the famous Walschaert valve gear.

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.

• Those using keys, such as the serpent, the cornett and the keyed bugle.

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. Also, on rare instruments, there is a fifth valve, which creates a lower octave of the note, or six tones. These valves are found mostly on tubas and other low brass instruments. More valves than five is ultra rare, but they have been seen, such as on the six-valved cimbasso.

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.

See also

• Valve

External links

• [Early valve designs]
• [Why was the valve invented?]
• [Elements of Brass Instrument Construction] with good discussion of valve types and history
• [Piston valves resources]

 

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