Can any one explain how to model a gear in Solid works?

or

Is there any modelled Gear in the Solid works library

If you have SolidWorks Office then you can turn on the Toolbox addin and there are gears in there under Power Transmission -> Gears. You can open one of these and see how it was modeled or just find the one you need and use it.
If you don't have Toolbox you can go to ww.3dcontentcentral.com, login to the user section and there are some configurable gears that you can download there.
Depending on whose gear it is you may be able to find a solid model of it on their website.
You didn't say what you need it for, so this may not be an option, but If none of those suggestions work and you only need it for an assembly to make sure it will fit. I would just make a circle with a simple tooth cut and pattern it. No point in wasting computer power with a perfect tooth profile if you don't need it.

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Todd Bryant
Ransohoff
www.ransohoff.com