p.3 #1 · Sony 200-600mm failure of the foot attachment
I haven't pulled the screws out of my lens to check the size, but it doesn't take much cross sectional area to carry a lot of load (a 1/2" grade 5 bolt will support my Suburban with a factor of safety of 4). If it's a 2mm bolt and if that bolt has a tensile strength of 30,000 psi (which is extremely low for a bolt), then the approximate tensile load a single bolt can hold is 146 lbs. Given this, the screws themselves are plenty strong enough. I suspect what happened is that either one or more of the screws weren't tightened properly at the factory, or possibly one or more came loose over time. As clearance develops the bolts begin to be impact loaded every time the lens bounces up and down (while walking) which increases the amount of force. It also begins to load the bolt in bending (and not pure tension) which will tend to cause the head to pop off. One at a time the bolt heads pop off until only one is left. The one eventually fails, creating the tear in the aluminum around the foot mount. The fact that these are very popular lenses and this is the only one I've heard of which has experienced this failure mode further suggests the presence of a manufacturing defect.
What I plan to do with my lens is make sure the bolts aren't loose.
p.3 #2 · Sony 200-600mm failure of the foot attachment
It's very concerning that Sony didn't apply any thread locker as I can't see any residue on those screws. Screws without thread locker will 100% become loose under fatigue loadings, it's just a matter of time.
p.3 #3 · Sony 200-600mm failure of the foot attachment
Ltgk20 wrote:
I haven't pulled the screws out of my lens to check the size, but it doesn't take much cross sectional area to carry a lot of load (a 1/2" grade 5 bolt will support my Suburban with a factor of safety of 4). If it's a 2mm bolt and if that bolt has a tensile strength of 30,000 psi (which is extremely low for a bolt), then the approximate tensile load a single bolt can hold is 146 lbs. Given this, the screws themselves are plenty strong enough. I suspect what happened is that either one or more of the screws weren't tightened properly at the factory, or possibly one or more came loose over time. As clearance develops the bolts begin to be impact loaded every time the lens bounces up and down (while walking) which increases the amount of force. It also begins to load the bolt in bending (and not pure tension) which will tend to cause the head to pop off. One at a time the bolt heads pop off until only one is left. The one eventually fails, creating the tear in the aluminum around the foot mount. The fact that these are very popular lenses and this is the only one I've heard of which has experienced this failure mode further suggests the presence of a manufacturing defect.
What I plan to do with my lens is make sure the bolts aren't loose.
In my case, which I think was very similar to the OPs case, I think you are 100% correct.
It seems fairly obvious, once the screws are loose and there is some movement, disaster looms. After mine had loosened several times, the screws no longer had good threads to mate with. The early owners of the Canon lens did find screws were not properly tightened and tightened them themselves. It seems the employee responsible was given the Dunce's hat as a result and did not want to wear it again.
I checked my Sony 200-600 screws and all 4 could be tightened quite easily. Maybe 1/8th of a turn.
I found it does take a bit of care to check for movement. On my lens the ring does have a very small amount of play (when tightened). So I have to support the ring while checking for play in the foot. I will be using some Loctite in the next few days to avoid a repeat of my Canon experience.
p.3 #5 · Sony 200-600mm failure of the foot attachment
I noticed my foot was a wiggling a bit as I was packing it for an outing, the 2 front screws were loose. Reminder for everyone to check if there's any play with your foot, check the screws which holds the plate for your foot
