Pro tolerances and bad flex

[Maxwel Fisher]

Spent the better part of six hours at Greg's shop trying to sort out a peculiar issue with my Pro Cinema HD J box and DBIII. 

Until recently, I noticed that my sled doesn't maintain static balance once tilted either forward or backward. When returned upright it would seek a position way out of balance. Having recently adopted a longer drop time, I wonder if the problem always existed, but went unnoticed.
 

We started with my sled. Pro Cinema HD. DBIII. Gen 3 Battery. 2" XCS Post. XCS Gimbal. XCS Monitor Bracket. 4 second drop time. 

We then tested the gimbal and the battery rack with an Ultimate top stage (pro mount bottom, xcs bayonet mount top) and weight plate. The sled would seek it's original balanced position after each tilt test so we ruled out the gimbal and the battery rack (although there is a great deal of flex in the AB mounts since it's held on by one thumb screw and two banana plugs). Doing the same test as we did initially with my sled at the same drop time did not yield any irregularities.

We measured the conical pins in my top J box mount and found one was out of tolerance, though not terribly (the right rear suggesting that might be the culprit). Trying Greg's XCS/Pro mount with four pins was better. When tilted back the sled did a better job of finding its initial position, but when tilting forward the sled would still seek a position way out of balance. We also discovered that the mount he made took some extra machining to fit my J box. Based on the specifications he used to design it, it seems not all J boxes are made equally. For reference, it has S/N 093 stamped on the side.

We also tested the flex from the camera plate on the weight cage and found that at its worse, there was 56 thousandths of an inch in flex. Not the worst he's seen from Pro, but not the best. While 56 thousandths is not something you can necessarily see, it is by all means not great. By the end of it all, it seems that the issue might lie somewhere in the mating of the donkey box to the junction box. That flex seems to be what causes the sled to not seek its original static balance. One expects some flex as there are a number of components that are stressed when you put 30+ lbs of weight on something that has such a small footprint holding it all together to the post. What we saw was the flex when tilted would not release when returned upright, essentially sticking and then the sled finding itself out of balance.

I'm planning to send the Junction Box and Donkey Box off to Pro to be serviced in the coming weeks, but am curious how many other people have experienced this? When Pro redesigned and released the Cine Live J Box, did they make it more robust or has the problem persisted?

Overall, this isn't meant to lambast. I love my sled and all its quirks. It has served me well and I know it will continue to do so. It's just frustrating.

[Blaine Dunkley]

Currently having a similar issue but with an MKV sled and XCS gimbal. Could just be a knocked about gimbal issue so I'm sending in to Greg to see if it needs shimming. If that doesn't do the trick, then I'll have to consider a different sled if I'm ever to attain proper balance when panning. Greg had mentioned that the flex in the CF post could likely be the culprit, which makes sense since it might change any time we adjust a kip handle on the post, flexing and torquing the entire rig. I'll let you know what happens and look forward to any updates you have on your end.

[Maxwel Fisher]

Blaine,

Apologies for the delayed reply. Was talking to Greg the other day and he mentioned your sled arriving. Not sure what the issue will ultimately be. I've personally never heard of the XCS gimbal needing to be shimmed, but I do know that the way a carbon fiber post is wound can impact its rigidity. Please keep me updated on Greg's prognosis. 

As for my sled, I ultimately worked with Greg on designing and machining a mount that mated with the Pro CinemaHD upper J-box and connected to the XCS bayonet mount (https://steadicamforum.com/index.php?app=forums&module=forums&controller=topic&id=33641). This gave a wider and more solid mounting platform, both on the post side, but also on the junction box side. In eliminating the three conical pins, it also eliminated any chance for a "three legged stool" scenario which I believe was part of my problem. If one of those pins rounds out or seats differently than the others, you end up with one side shorter than the other and that can cause movement in the top stage. It's why Greg designed his Pro/XCS hybrid mount with four pins.

Since changing out the post mount, I've not had any issues with statically balancing the sled. I had also sent my Donkey Box to be serviced at Pro just to be on the safe side.