Shim
From Wikipedia, the free encyclopedia
In engineering, a shim is a thin and often tapered or wedged, piece of material, used to fill small gaps or spaces between objects. Shims are typically used in order to support, adjust for better fit, or provide a level surface. Shims may also be used as spacers to fill gaps between parts subject to wear.
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[edit] Shim materials
Many materials make suitable shim stock, or base material, depending on the context: wood, stone, plastic, metal, or even paper (e.g., when used under a table leg to level the table surface). High quality shim stock can be bought commercially, for example as laminated shims, but shims are often created ad hoc from whatever material is immediately available.
[edit] Shim: example usage
Examples of the usage of shim from different engineering disciplines are outlined below:
[edit] Automotive engineering
In automotive engineering shims are commonly used to adjust the clearance or space between two parts. For example, shims inserted into or under bucket tappets control valve clearances. Clearance is adjusted by changing the thickness of the shim.
[edit] Carpentry
In carpentry or joinery small pieces of wood may be used to align gaps between larger timbers.
[edit] Fencing
Thin pieces of metal called shims are used to test that epee tips conform to specifications. If the shim can be inserted into the narrow gap of the weapon tip, the gap is the correct size. Foil tips were initialy tested with shims as well, but this was abandoned as being useless.
[edit] Lock Picking
A small metal device used to quickly open a lock is called a shim. How to make a shim for picking pad locks
[edit] Luthiery
In luthiery shims made of various materials are often used to adjust neck alignment.
[edit] Masonry
In masonry small stones may be used to align or fill gaps between larger bricks or slabs.
[edit] Motorcycle maintenance
In the Robert M. Pirsig novel Zen and the Art of Motorcycle Maintenance one narrator discusses the use of a piece of beer can, as a shim, to tighten the handlebars.
[edit] Nuclear magnetic resonance / Magnetic resonance imaging
In NMR or MRI, "shimming" is used prior to the operation of the magnet to eliminate inhomogeneitites in its field.
Initially magnetic field inside a MR scanner is far from being homogeneous. It could be even 100 times worse with respect to its homogeneity than an "ideal" field of the scanner. This is a result of the production tolerances and magnetic field of the "environment" - iron constructions in walls and flour of the examanition room gets magnetized and disturb field of the scanner.
There are two types of shimming: active and passive one. The active shimming is done using coils with adjustable current. The passive shimming involves pieces of steel with good magnetic qualities. The steel is placed in the neighbourhood of the permanent of superconductive magnet. It gets magnetized and produces its own magnetic field. Additional magnetic field (produced by coils or steel) adds to the magnetic field of the magnet in such a way that total field is getting more homogeneous.
[edit] Plumbing
In plumbing shims of metal are used to align pipes.
[edit] Software engineering and computing
In web design a well known hack or kludge is to use a single-pixel transparent GIF as a shim to align spaces between tables in HTML to ensure a desired page layout. See spacer GIF.
In software engineering or hypermedia a piece of code (such as an API shim), placed between layers after the manner of a physical shim, in order to create a better fit or a smoother flow of code execution across layers.
In computer networking, a piece of extra code inserted between existing layers of the protocol stack to translate a communications protocol as the data is passed between the layers.
See also CPU shim for shims used to protect the core of CPUs from damage.
[edit] Sources
Byrnes, Joe. "To the Point; A Brief History of the Shim." American Fencing Summer 2006: 16.
