Kiron made two different versions of this lens. The picture below shows three different lenses. The lens on the left is the Kiron branded 28 F2.0. The center is the Vivitar version of the same lens. These lenses are identical except for the branding. The filter does not rotate when the lens is focused.
The lens on the right is much different. The filter does rotate when focused, it is slightly larger and heavier, and the focus throw is much longer. The Kiron and the center lens both focus from infinity to close focus in about a third of a turn. The lens on the right takes about three quarters of a turn. All three focus to 0.3 Meters close focus.
The Vivitar lens in the center is like the one you have. I have completely disassembled the lens and the aperture mechanism to clean oil off of the blades. This job is not for the faint of heart. The most difficult part is that the focusing helices have to be put together in exactly the same orientation that they are disassembled from. This took a lot of time making measurements and notes to get it back together in exactly the same position. There are two different helixes that move in diffent directions at different rates - I think that this is called a floating lens design.
Anyway - I did not just unscrew the front of the lens - it is not that easy. I found a person posted how to disassemble this exact lens and I followed his directions.
CLA Kiron 28 f/2 part #4/4 on Flickr - Photo Sharing!
The only caution I would say is that removing the aperture mechanism, do not use the marking method he shows - it will not work. You must mark different places to be able to assemble in the same postion.