These are very popular INVACARE powerchair 8 X 2" Caster Wheels.  It has a 2 piece black plastic wheel that is connected with 4 bolts and nuts and a gray/black urethane tire.  The hub width is about 2 1/2" and comes with 7/16" precision bearings pre-installed.  Price is for one pair.  

Compatibility:

Invacare 3G Storm Series Arrow, Invacare 3G Storm Series Ranger X, Invacare Storm Series Torque SP, Invacare Excel FWD 250, Invacare Nutron R32

Invacare Nutron R32LX, Invacare Nutron R50LX, Invacare Nutron R51/ R51LX/ R51LXP, Invacare Power Tiger, Invacare Pronto R2, Invacare Ranger II MWD

And many other wheelchairs with 7/16" Bearings