Anatomy Of A Gear-Stripped Lens

Some of us with old screw-driven Sigmas, the Sony 18-70mm F3.5-5.6 DT kit lens, and even my Tamron 200-400mm F5.6 LD 75DM, have experienced gear stripping – that is, the internal gear track of the lens has broken, usually a few gear teeth on a plastic track. What does this mean?

I’ll open my gear-stripped Tamron 200-400mm to show you! I’ve sent this to repair before, but they said they were out of parts (this is the first generation of this lens, and the second-generation 200-400mm was then made obsolete with the Tamron 200-500mm F5-6.3.

The symptom of a gear-stripped lens is simple – mount the lens on camera, turn it on, and the camera will focus the lens to infinity… and it will start vibrating and making a loud whirring sound there. The reason is that the gear cannot catch onto the teeth because it is broken. You can then change your camera to manual focus, and then turn it towards close focus. Auto focus may work, but if you miss and the camera hunts, it will get stuck at infinity and vibrate again.

Note that this does not only happen to Alpha-mount lenses – Nikon F-mount lenses can get it too. Also, this was a good reason for Sony to dump the 18-70mm for the Sony 18-55mm F3.5-5.6 DT SAM which has a in-lens motor and would not face this problem. Likewise for the Sigma lenses with HSM – they would not face this problem.

Anyway, the A-mount is rather simple from the back – there is a black shield piece (in between the metal mount and the rear optics.) You need to remove this first.

Note that the information chip gets in the way – so you gotta pull it aside without breaking it. Here, the black shielding piece is removed. The black-colored metal piece with two silver screws close together, is the aperture lever, and the other black-colored metal piece with two widely spaced silver screws, opens and closes the aperture blades by transferring motion to the aperture pin.

After removing the metal mount, I could remove the gears – here, it changes the gear ratio to a much slower one. Note the large gear in the bottom-right – this is in contact with the gear track. The screw drive of the camera connects to the shaft in the top-right. All metal.

I then removed the rear lens collar bit, and viola! Here’s the broken gear track. The broken teeth are in the top-right corner. The hole is where the aforementioned large gear transfers rotation to the lens.

In theory, I could drive a nail down where I have marked it in red, to prevent the lens from getting to infinity (and thus getting stuck on the broken gear teeth.) However, my later pictures will show you why it can’t be done.

The screw drive shaft, and the aperture uh… pin.

I then removed the outer part, with the focal lengths marked. As it turns out, the gear track is on that big long plastic tube in the top-left!

If I had embedded a nail, it would have stopped the big long plastic tube from rotating. 🙁

I removed the big long plastic tube with the gear track. This is almost as long as the lens, when retracted! No wonder the gear track is not made of metal – if it was, it would make the lens a lot heavier, and the focusing would be slower due to the weight.

Slightly out of focus, and to the right, is a rod that grabs onto a pin that pulls the focusing group nearer or further, to focus.

My thumb is holding that pin. Here it is at minimum focus distance. If I let go, it will roll down, and focus on infinity.

Fully extended. The gold-plated track tells the chip what the focal length is.

I also peeled off the label on the focus ring, but I could not unscrew the screws (they were too tight) so I couldn’t open it any further. 🙁

My initial plan was to stop the lens from going to infinity, but seeing how it wasn’t possible (the focus ring would’ve been my last hope) I ended up reassembling the lens but leaving the screw drive shaft out. So there is a hole where the screw drive used to be, and the camera will try to spin its screw in there if in AF. Focus confirm, autoexposure and SteadyShot work fine. I just have to manually focus, but at least it doesn’t have the annoying gear-stripped symptoms. Also, since there is no screw drive, manual focus is very, very smooth!

2 thoughts on “Anatomy Of A Gear-Stripped Lens

  1. dreamingArtemis Post author

    Hmm, I think if you really want, there is another way to fix it. Simply make a small mold cast of a small section of the thread from undamaged section, then whip a batch of either fibre plastic compound, or use some epoxy compound, then affix the mold cast made earlier to the section where the threads have been damaged, pour in the compound and let it harden.

    That should fix this problem though you need to do some filing or sandpapering then followed by some shillet surface coating on it.

    Sorry, the engineer in me is really a big nerd when it comes to making DIY repairs. XD


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