Placing an Unbalance Mass on Hollow Shaft

[iboby]

I am modelling a rotating shaft by SOLID 186 with an unbalance mass locating on mid span of the shaft. How can I model the unbalance mass(force) applying to the hollow shaft? I tried to use CONTACT/TARGET element to locate the mass(MASS 21) on the shaft axis but the results were erroneous. In my model center of the rigid disk should be on the outer radius of the shaft.