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I am designing a hydrophone, from the piezo buzzer. To hold the circular piezo plate, I have designed a mechanical fixture. I need to use washer/o-ring to make it leak-proof. I am confused about which one to use.

Can you please explain the difference between washer and o-ring. What is the difference between them?

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An o-ring usually (but not always) has a round section and is made of a compressible material such as rubber, neoprene, silicon etc.

A washer tends to have a square or rectangular section and is usually made of a harder material brass, copper, aluminium, steel etc.

It is not about advantages or disadvantages, but which is used will depend on the characteristics of the purpose, so if it is a delicate device at low pressure then a softer material can be used especially if the units are designed with matching channels for the sealing component to "sit" in.

Many times a copper washer can be used - some car sump plugs have copper washers for sealing, others just use machined steel faces...

Just to muddy the waters as it were, there are o-rings called Wills rings which are a hollow metal torus filled with an inert gas, these are capable of sealing at high temperature and pressure. One use was as part of an engine head gasket sealing the head to the block around the combustion chamber...

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    $\begingroup$ Isn't one of the most significant differences is that o-rings act as a gasket, sealing the opening that contains the bolt, while washers spread the force over a larger area without necessarily sealing anything? E.g. a lock-washer will almost never provide a seal. $\endgroup$ – Ray Butterworth Aug 5 '19 at 12:25
  • $\begingroup$ @RayButterworth so a copper washer used with a sump plug is not a copper washer but a what? $\endgroup$ – Solar Mike Aug 5 '19 at 13:24
  • $\begingroup$ I've seen plenty of rubber 'washers', inside garden hose connectors being the primary example that comes to mind. $\endgroup$ – SoronelHaetir Aug 5 '19 at 16:03
  • $\begingroup$ There are also washers that have a built-in rubber "ring", and many washers are disks with a center hole. I've seen plastic washers as well as steel washers. I've even heard of silver washers, when electrical conductivity was critical. Typically, a washer is used to spread out the force of the bolt better or to cover/protect a hole the head of the bolt would otherwise not work with. There are also plenty of lock washers around, but that's getting a bit off the OP's question. $\endgroup$ – computercarguy Aug 5 '19 at 16:31
  • $\begingroup$ @SoronelHaetir: I think of washers as metal, with a hose washer being an exception. I think a hose washer would be better called a gasket, but usage disagrees. $\endgroup$ – Ross Millikan Aug 5 '19 at 18:21
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You can also use a gasket, which is typically soft sheet material, either rubber or paper. They are often used when the interface you are sealing is a more complicated shape than a circle. O-rings can do that too if you cut the groove properly. The point of all of them when you are sealing is to make up for unevenness in the metal parts that are going together.

My experience is that o-rings will take up much more unevenness than the other approaches, but they are more expensive to incorporate because they need a groove to sit in. There are documents to show what sort of groove is best with a given cross section and material of o-ring.

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  • $\begingroup$ O-rings will typically take less force from the bolt, so it can't be torqued down as hard. In fact, many soft 0-rings will tear to overly deform if they are torqued too hard. Washers will typically take more force. Even the "soft" ones like copper are designed for high torque to essentially smash them into the correct shape. These are often one time use washers, needing to be replaced if the bolt is ever removed. There are also times when a o-ring will be used with a washer, to help prevent the bolt head from tearing the softer o-ring. $\endgroup$ – computercarguy Aug 5 '19 at 16:36

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