When trying to find ways to fix my Tamron SP 90 f2.5 aperture, I had to disassemble the whole lens. This tutorial cover the rest of the lens.
After removing the 3 screws (A), unscrew the cover (C). Then remove the 3 screws (B) to remove the metal part.
This is how the lens lok after removing part (C) in th step 1. Note the aperture detent ball bearing & spring. Remove the spring for safe keeping.
Now you could remove the focus scale. The adhesive tape that hold the 2 plastic rings determine the infinity focus adjustment.
Remove the 2 screws and plastic housing on each side of the lens. After this you could take out the plastic shell. Note I took this picture before removing the focus scale.
On installation, do the infinity focus adjustment as the last step, just before attaching the rubber ring.
Attach the lens to a camera. The idea is to make the focus scale shows the infinity mark when the lens reach infinity focus. Focus the lens to infinity, aim at an object 300 meters away.Then carefully move the focus scale so it point at infinity. Stick a short (~1 cm) adhesive tape so these tube won’t move against each other. Double check the focus. If everything is fine, then wrap the longer adhesive tape around the tube.
Moving to the front. Remove the front ring with a friction tool.
The screws (A) hold the front lens group. remove these and then remove the front lens group. Be careful, if these screws are removed, nothing will hold the front group. The position of the group will affect the aperture size. On installation, set the aperture at f2.5, then adjust the position so it just at the largest opening.
Removing screws (B) will enable you to access the helicoids. There are 3 of them. I wouldn’t recommend messing with these unless absolutely necessary.
I took this picture when I need to readjust the aperture size. If you need to access the lens elements or aperture blade, remove part B. The lens elements are all in A, attached with rings. Be careful when removing the middle elements, do it slowly. Mine got stuck in the tube, and I have to use heat to dislodge it.
If you need to access the helicoids, remove the 2 screws on each side. Note the position. On reassembly, you have to position the helicoids into this position again ie. they all reach the minimum position at the same time. Mark each release point, and count how many turns needed for each helicoid.
These helicoids should be assembled so they reach their shortest length at the same time.
Reassembly of these part is a spiritual experience.
These are the front groups. The grease on both my copies are still in good condition.
When installing, arrange the rearmost helicoid so the slot for the aperture lever are opposite the ball bearing position.