Hi, 20t class machine. Custom made thumb and cylinder currently mounting. I’m about to weld on the cylinder top mount to the stick but not sure on the geometry constraints around the cylinder being fully retracted and the stress’s when opening from this position. What I mean is, I want to crowd the thumb up the stick as far as possible to keep it out of the way but ensure the cylinder will still function safely and not bend anything. I currently have it propped up with the centre of the pin point on the top mount sitting 100mm off the stick, the pin centre of the thumb mount is 200mm off the stick. Would like to crowd the thumb up further if possible. I’m hoping someone on here has fitted thumbs before and can advise on how far I can go? cheers
Mine lays down almost flat. it has two small u shaped cradles welded to the stick for the pin boss to stop on. maybe I can get a couple picktures. Use a 1 1/2 board and you will be close to what I have. Same style thumb.
A lot depends on the length of the hydraulic cylinder you mount on the dipper. If the stroke isn't long enough you will be limited on how far the thumb will extend. If the stroke is too long the rod will be bent from hitting the dipper when it is extended. The relief pressure in the thumb valve will (should) prevent damage to the cylinder. The thumb cylinder rod rod should be the same diameter as the bucket cylinder. That will help quite a bit when the bucket and thumb are clamped together at maximum force and NOT the bending the thumb cylinder rod. IMHO of course.
What i've done is pin the cylinder on and the plate, and pull the mounting plate up as high as it will go, while leaving around 1" from where the thumb will hit the stick, tack it then run the cylinder to full extension to make sure the cylinder won't hit the arm, i've never had it happen but better to check first. When you are first extending cylinder it won't be hard on it as it will never have any pressure on it only weight of the thumb. As far as cylinder sizes, the relief being set properly is most important. No matter what, I am not personally someone who will ever force the cylinder back, relief set properly or not. On my machines I always work one cylinder back as I extend the other. Unless it's something soft that I know will crush easily, I won't clamp it with all the force of the machine, only enough to grab it. Recently saw a 8 ton machine with the thumb cylinder bent into nearly a U. That had to take a lot.
The cylinder has plenty of length to it and 70mm diameter against the 80mm of the bucket. I’ve set reliefs to 1800psi to begin with. Was going to do as you suggest Suladas and leave an inch on the rod When retracted, may be overkill if I weld a cradle for the barrel and both ends of cylinder have cushioning. I think I’ve got the retract position sorted and said safety mechanisms just got to make sure I cover the full extend position as the cylinder at full length is 2625mm so quite long.
That cylinder looks really long, you may run into issues with it hitting the stick at full extension.
Here is the stop cradle on mine and where it sits when the thumb is in stored position. It was 0 deg here so i didn't start it to fold it up. I agree the cylinder looks pretty long. if it goes to far the rod will hit the boom when the bucket is rolled all the way back. You might have to tack weld it on and test run it to see where it ends up.
Extend the cylinder all the way out with the thumb being held by the bucket so that the thumb cylinder is parallel to the stick. Tack the base boss to the stick and use air to retract the thumb to the stowed position. You can adjust the base position from that point to your preference.