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Shim (engineering)

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. The term comes from a Kentish word of unknown origin, dating to at least as early as 1723, meaning "a slip of wood." Originally a piece of iron fitted to a plow for scraping soil; meaning "thin slip to fill up a space or raise a level" is from 1860. The verb meaning "to wedge up a surface by means of a shim" is attested from 1937.

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.

Shim: example usage

Examples of the usage of shim from different engineering disciplines are outlined below:

Aerospace engineering

Shims are widely used in Aerospace components to fill up extra spaces and it is used in different ways.

Automotive engineering

In automotive engineering shims are commonly used to adjust the clearance or space between two parts. For example, shims are inserted into or under bucket tappets to control valve clearances. Clearance is adjusted by changing the thickness of the shim.

Carpentry

In carpentry or joinery small pieces of wood may be used to align gaps between larger timbers.

Fencing

Thin pieces of metal called shims 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. Saber has a similar set of shims used to determine whether or not the blade is correct thickness. Foil tips were initially tested with shims as well, but this was abandoned as being useless.

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

Luthiery

In luthiery shims made of various materials are often used to adjust neck alignment.

Masonry

In masonry small stones may be used to align or fill gaps between larger bricks or slabs.

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.

Nuclear magnetic resonance / Magnetic resonance imaging

In NMR or MRI, "shimming" is used prior to the operation of the magnet to eliminate inhomogeneities in its field.

Initially magnetic field inside an 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 floor of the examination room get magnetized and disturb the field of the scanner.

There are two types of shimming: active, and passive. Active shimming is done using coils with adjustable current. Passive shimming involves pieces of steel with good magnetic qualities. The steel pieces are placed near the permanent or superconducting magnet. They get magnetized and produce their own magnetic field. Additional magnetic fields (produced by coils or steel) add to the magnetic field of the superconducting magnet in such a way that the total field becomes more homogeneous.

Plumbing

In plumbing, metal shims align pipes.

Tank (Vessel) levelling

When installing vessels over load cells, it is necessary to have a levelled surface. Thin metal plates (shims) are used for this purpose

Computer hardware

A thin layer between a central processing unit and a heat sink to protect it from physical damage.

Sources

  • Byrnes, Joe. "To the Point; A Brief History of the Shim." American Fencing Summer 2006: 16.
  • Gerald A. Pearson, SHIMMING AN NMR MAGNET, http://nmr.chem.uiowa.edu/manuals/Shimming-GAP-NMR-magnet.pdf

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