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Our small radio-control model cars have shims, spacers and washers. I am tasked with clarifying the manual and getting parts naming consistent.

We have these items from less than .5mm thin up to 1/8" thick. I'm thinking of calling everything under .5mm a shim, and everything .5mm and thicker that is available off the shelf a washer. And spacers will be anything .5mm and thicker that we have manufactured ourselves for specific needs on the car.

How do engineers define the difference between these terms?

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The three terms specify the function of the component, not its size.

A shim is a component used to fill up an unwanted gap. Shims may be flat or tapered, and are often specified as "select to fit" parts, rather than as a part with a predefined thickness.

A spacer or standoff is a fixed size component designed to separate two other parts by a known amount. It does not necessarily fill all the space between the components - for example the purpose might be to separate them for electrical insulation, or to allow cooling air to flow between them.

Shims are usually fairly thin, but spacers can be thicker (inches or even several feet thick, in large structures).

A washer is a part used to spread the load between the two components, e.g., between a bolt- or screw-head and whatever the bolt or screw is fixing.

Washers are not necessarily flat - for example, Belleville washers, split washers, star washers, wave washers, etc. The purpose of a non-flat washer is often to create a well-defined force between the parts on either side when it is installed by squashing it flat. That force may be used to resist unwanted rotation of a screw or bolt if the structure is vibrating, for example.

A particular part may have more than one of these functions - for example a shim or spacer may also act as a flat washer. In that case, the naming of a particular part is a bit arbitrary, or a pedantic name like "spacer-washer" might be used.

All this might lead to some problem situations. For example if you are selling a self-build kit, you might be including a packet of "20 washers" (all the same size and bought pre-packed from a hardware supplier) but in fact some are used in the kit as washers, some as spacers, and some as shims.

How you deal that might depend on what engineering knowledge you expect your customers to have. An experienced engineer might prefer to see the parts named "correctly" on an assembly drawing which follows international standard drawing conventions, while a beginner might prefer a more pictorial sketch which is mainly intended to show "what part goes where" when building the kit.

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  • $\begingroup$ Unfortunately, some parts are being used arbitrarily in a multi-use context. We have to give the part one name and it will be used in multiple places across multiple vehicles. I wondered if the usage I suggested would be appropriate. These are not engineers building the kits. $\endgroup$ – Steve May 31 '17 at 0:17
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    $\begingroup$ In the end you have two options: you change the current names, or you don't. You might consider the consequences of giving two different names to interchangeable (or identical) parts - especially if they are used in different vehicles - if you want to rationalize the names. Provided your company knows they are really the same part, does it matter if the customers think they are different? If those parts are unlikely to ever fail, and/or the customer could probably find a likely replacement in a local hardware store (or their junk box) instead of re-ordering from you, what are you losing? $\endgroup$ – alephzero May 31 '17 at 2:24
  • $\begingroup$ Alternatively you could follow the "Lego brick" method and name everything after its form, not its function. So you don't have "washers" or "spacers", you have parts like "4mm diameter disk with 2mm diameter hole, 0.5mm thick, steel, zinc plated". $\endgroup$ – alephzero May 31 '17 at 2:31
  • $\begingroup$ Form is exactly what I was going by. Shims, IDxODx.4mm, nylon. Spacers, IDxODx2mm, blue aluminum. Washers, IDxODx1mm, aluminum. $\endgroup$ – Steve May 31 '17 at 4:16
  • $\begingroup$ @Steve based on your last comment, this seems wrong to me : shims are usually thinnest the washers followed by spacers. However, washer an spacer can be interchangeable depending on the thickness... Only my opinion. $\endgroup$ – Solar Mike May 31 '17 at 5:02
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No real difference between a shim and a spacer. A shim is used to adjust the space/clearance between faying surfaces (where two pieces connect). Generally as a means of getting away from the cost of tight tolerances. Sometimes as a wear part that gets replaced to hold a particular clearance (e.g. the selectable shims that were used to adjust the clearance in my overhead cam on my old motorcycle). A washer is used to distribute the load from the head of a bolt or screw (or to keep your faucet from leaking). Also may perform a locking function. Nothing to keep you from using a washer, or stack of washers, as a shim - to adjust a clearance. I'd label the part by function. If you're using it to adjust a clearance call it a spacer. If you're using it under the head of a screw or bolt, call it a washer. In my opinion the function of the part in its application is more useful and relevant than an arbitrary name.

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