ZWO AM3 AM5 dovetail saddle locking knobs loosen or slack off replacement recommended

[Elp]

Just did a quick search, didn't find someone with this issue at the moment.

I was testing a rig with my AM3 and was checking east/west heavy clearance of the tripod and going fully north/south. It's a good job I checked it indoors whilst I could see it because when it was pointing straight up, I nearly had a heart attack, I saw the dovetail of the rig shift slightly and quickly grabbed it. Sure enough it had worked loose despite the TWO saddle locking knobs having been tightened prior. I checked the knobs, one had slacked off a little, so tightened it hard again. A few seconds later it happened again.

So questions:

1. Anyone had this issue?

2. If so did you find any knobs/handles that you can replace them with that you can tighten more than a scrooge.

People lament some ioptron tiny locking pads and levers, but those lock tighter than this mounts knobs.

Looking at it a little, I suspect the (silicon/nylon?) washers between the locking knobs and the saddle are the cause, acting like sponges with minimal friction on the knobs which then slacken off.

As a preventive measure I've got the space, I'm going to put two bolts somewhere on the front side of the dovetail bar to act like physical stops, the rig itself has many physical stops designed in to prevent most of the equipment from coming loose and falling.

Also, I implore anyone with these mounts and any invested expense in their imaging trains to check check and recheck for this issue.

[Ratlet]

1 hour ago, Elp said:

Just did a quick search, didn't find someone with this issue at the moment.

I was testing a rig with my AM3 and was checking east/west heavy clearance of the tripod and going fully north/south. It's a good job I checked it indoors whilst I could see it because when it was pointing straight up, I nearly had a heart attack, I saw the dovetail of the rig shift slightly and quickly grabbed it. Sure enough it had worked loose despite the TWO saddle locking knobs having been tightened prior. I checked the knobs, one had slacked off a little, so tightened it hard again. A few seconds later it happened again.

So questions:

1. Anyone had this issue?

2. If so did you find any knobs/handles that you can replace them with that you can tighten more than a scrooge.

People lament some ioptron tiny locking pads and levers, but those lock tighter than this mounts knobs.

Looking at it a little, I suspect the (silicon/nylon?) washers between the locking knobs and the saddle are the cause, acting like sponges with minimal friction on the knobs which then slacken off.

As a preventive measure I've got the space, I'm going to put two bolts somewhere on the front side of the dovetail bar to act like physical stops, the rig itself has many physical stops designed in to prevent most of the equipment from coming loose and falling.

Also, I implore anyone with these mounts and any invested expense in their imaging trains to check check and recheck for this issue.

I've not had that behaviour but I do much the same as you where I can by fitting bolts either end of my dovetails so that if they come loose they won't slide all the way out.

[Elp]

Today I looked further into it, I took off one of the knobs. The assembly (didn't take a photo) is like this:

M8 cap head screw > plastic knob with M8 female thread > plastic/very hard rubber washer > sliding guide rail (locking wedge piece) > soft spring > into female M8 hole of am3 saddle. 

I didn't tighten onto a vixen dovetail but when moving the wedges into the zone where they would engage with a dovetail bar I noticed the knobs continued to turn/tighten but the the cap heads would be sticking out of them slightly because the knobs would continue to thread down the bolts but the bolt itself has nowhere to go (as it's not locked onto the knob). I've got a feeling they'd lock, but because the knobs are threaded and the bolts not holding them in place under tension, any force on the wedge form will push back onto the knob forcing it up the bolt thread slightly, hence taking some tension off the wedge form and subsequently off the dovetail.

In the meantime I've order some different knobs, ones that have the bolt head cast in so all of the locking force will be within the knob, and not a separate mating M8 bolt.

[Ratlet]

How long are the threads?  I've got a couple of M8 threaded knobs kicking about that I don't need.

[Elp]

22 minutes ago, Ratlet said:

How long are the threads?  I've got a couple of M8 threaded knobs kicking about that I don't need.

It's alright, I'm getting the knobs delivered tomorrow. The m8 bolts are 50mm underhead, but they leave 40mm useable thread I think when screwed into the existing knobs.

[Elp]

I replaced the factory knobs with these M8 x 30mm threads ones (Amazon). When you compare them side by side with the factory ones, the replacement ones are far better as you can grip and tighten them more. The knobs also are small enough not to infringe much past the top height of the saddle. Maybe a worthwhile upgrade considering how cheap they were:

[Ratlet]

They look proper swish right enough and like you can get some decent torque on them.