I have a question. Here's the short version... If you put a load on a fully retracted hydraulic cylinder, that is, physical force pushing it in, maybe or maybe not with the system fluid additionally pressing it in at full pressure (1300psi), even though it's already in all the way), will you damage it, or is it designed to happily deal with that? Is that welcome/unwelcome metal on metal, or is there a rubber bumper in there? Here's the long version of the question. I have a recreational motion simulator. It's a big cabin (box) that 12 people sit in, with a movie screen at the front. The movie may for eg. be of a roller coaster ride, and the box pitches and rolls synced to the video to make the passengers feel like the experience is more real. The cabin moves via three hydraulic cylinders which connect it's bottom to the underlying support frame (at front center, rear left & rear right). The cabin is also connected to the supporting frame with a mechanical arrangement that keeps it perfectly centered and straight (free of yaw) no matter how it pitches, rolls and vertically thrusts. When the machine docks, it comes down and rests on 3 landing points where the underside of the cabin and the support frame each have 3 corresponding 3"x3" landing pads (front center, rear left & rear right). The machine was designed so that when the cylinders retract, the cabin rests on it's landing pads just before the cylinders bottom out (say with 5% of their travel left). At these 3 dock points, the cabin has 3"x3" metal flat surfaces, and the support frame has 3"x3" rubber pads sitting on 3"x3" metal surfaces. The problem I am having is this... Let's say there is a sequence during a roller coaster ride where we want the cabin at roughly maximum roll to the right. So we command the front cylinder to 50%, the left to 100% (top), and the right to 10% (safely away from the rubber landing pad). But the digital control system commanding the cylinder movements is not as fast as it should be, and so often what happens is that the right cylinder will try to go right down to 0% before it realizes where it is, and then may perhaps go back to 10%. Over time this sort of behaviour damages the rubber pads, eventually rips them completely off, and goes on to wear out ruts in the metal pads on the cabin and the support frame that the safety inspectors don't like to see. Obviously improving the control system is the proper way to correct this behaviour. That is on the agenda. However it is a costly and/or time consuming process, so I am hoping to come up with an easy immediate solution. For now, I'm getting it welded up pretty and will insert the rubber pads only for inspections and during transport. Doing this regulary would perhaps be good enough, but a nuisance, a messy solution, still have metal on metal wear. yuck. Not ideal. What if I remounted the hydraulic cylinders 1/2" higher at their base, so that the cabin NEVER touches the landing pads ever. I don't know how cylinders are designed, so I don't know if this is a big problem or no problem at all. If a front end loader has a bucket full of fill, and it's bouncing down the road with its elbow cylinders fully retracted so the weight of the load is coming down between the bottom of the pistons and the bottom of the tubes... is that business as usual, or is that damage being done? If I may complicate the issue a bit further... back to my simulator... At the front pad, the mechanics are designed such that the front pad always comes down perfectly flat, so there is no problem at all at the front. Rubber gets squeezed, but it doesn't mind. In the rear, if the left and right come down together, they are both flat and no damage occurs. Rubber pads get squeezed... they don't mind. Even when only one rear cylinder comes down, if the other is at 100%, there is no contact, and no problem is created: because the rams are positioned outside (further apart) from the landing points, if you can picture the geometry, when right is at 100% and left is at 0%, there is NO CONTACT at the dock pads. The only behaviour that causes a problem is for eg: when the machine attempts to move from a position of left 100%/right0% to left5%/right5% - when the second rear cylinder comes down, the cabin pivots on the first dock point as the first cylinder gets pulled from 0% up to 5%, and metal rubber pads shear/scrape laterally. Yikes. But here's the interesting bit I pondered... At the point where I have left 100% and right 0%, this is like the situation I described when the loader is bouncing down the road... The right ram is fully retracted and the weight of the cabin plus the 1300psi fluid is still pressing it down even though it's position is at 0%... This makes me ask... is there any damage being done right there? It's been doing this for 20 years without cylinder malfunction... so I'm guessing not, and so I thought it shouldn't cause a problem if I remount my rear cylinders so that they simply sit at bottom more often than they have been. Like the front end loader would do. Instead of scraping metal against metal. But it never hurts to get expert advice... I wouldn't want to make a huge misteak ;-) Thank you for your time and interest... much appreciated. Ryan.
Most front end loaders have metal pads in the bucket linkage that stops the bucket cylinders from bottoming out.
some excavator cylds. have hyd cushioning built in them but it still not good to bottom them out. all it takes is one small metal flake to ruin the whole system
Ryan, Since this is a contraption that rides 12 people, you should do that which is necessary to fix it properly. I'll bet your insurance company would agree. Since this is an electronically controlled system, and you admit the controller is not behaving properly, I wonder if it could malfunction such that it sent cylinders from 100% to 0% and back to 100% fast enough to hurt someone. I would get to the root of the problem and fix it properly.
Retracting the rod past it's designed stopping point is NEVER a good idea,thus pivot stops are always installed.The bouncing effect of a loader bucket going down the road is just part of normal wear and tear but what you're proposing is about the same as digging without teeth,just the shanks on a bucket.NOT good in both cases.
install circuit reliefs and you should have no worries.....
Don't even think of modifying this thing yourself. If you have a problem have it repaired by someone approved by the manufacturer to do repairs or modifications. Make sure the manufacturer approves any modifications and make sure they are properly designed by a competent licensed engineer. Make sure any repairs or modifications are logged and submitted to appropriate gov't agency inspection. Since this is a ride used by the public there is a lot of liability involved here, should an accident occur.
Is it possible to change the program in the control system so the right side goes down to a value greater than 10%, maybe it's 15% (using your example of maximum roll to the right) to accommodate the slow response time of the cylinder movements?
Interesting question. Just some thoughts... I don't think that's a valid comparison anyway. In a loader, the weight is being carried by the loader arms, and their respective cylinders, which if the bucket is elevated at all, are extended. The bucket cylinders aren't really seeing the weight of the load. They're just controlling the angle of the bucket. That having been said, are we sure that in NO case do cylinders bottom out? The bucket linkage might limit the retraction of the bucket cylinder, and the dipper might only swing out so far, and limit the retraction of the dipper cylinder, but there are no stops at the lowest limit of travel on an excavator boom, are there? Those times when I've had machines half taken apart, and retracted the cylinders to their limit, I don't recall ever having seen the rod eye bottom against the gland. Rather, they seem to stop at the same point as they would if they were attached to their respective components. I'd think that indicates that the rod is bottoming out in the cylinder bore. And one other thought--Even if the rod is bottoming out in the bore, there's going to be fluid beneath the packing, that's held there by the valve closing, and that would serve to provide some cushioning effect, I would think. Now, that's all generalities, just to further the discussion. In the case being discussed, I can see the potential for problems of the rod bottoming against the cylinder bore under a direct load. Edit to add: And what if by the merest chance, a hose fails as your passengers are getting on or off? Yes, it may only move an inch, but if that causes grandma to fall and break a hip, you're going to be in a world of hurt. I have to agree with everything norite said... Edit further to add: Well, actually, more specifically to what I said above, if it's done moving downward, why would we expect to see more pressure pushing down than up?
A lot of hydraulic cylinders don't "bottom out". The piston covers the return port before the piston slams into the gland or cylinder bottom...something to think about.
as I was reading your post, I figured there was something going on that was more than the dead load sitting on the pads....and sure 'nuff the sequence is creating a fulcrum out of the resting pad....during this event, I would imagine there are some interesting pressure spikes being manifested in the hydraulic system as well as obviously and visually squashing the rubber pad(s).... Lets say (for discussion purposes) "someone imaginary" came there and removed all the pads: I figure the device would function without them....and I would guess this would remove the problem of the smashed rubber pad....but then when you operated it & lowered the device to the zero ram setting the sudden stop due to the nature of the rams would make the folks uncomfortable... Let me know what you think of this inquiry/recap....I may very well have some suggestions for you to consider.
G'day 3D, What a well worded question with many well worded replies. I,ve been on that ride at the Ettamogah Pub near the Caloundra turn-off. If that's you, PM me and I'll give you a ring. If you've owned the ride that long, you would be fully aware of your obligations re. compliance, so you and I would have to agree with norite and others about certification of modifications. I am retired from industrial hydraulic design and repairs, but if I was doing the job for you, the work would come with an Engineer's Report certifying the work to the required level. I would love to see the circuit diagram for the machine. 1300 PSI is quite a low operating pressure and maybe explains why the hydraulic system has been trouble-free for so long. I think "Moog" had a proportional directional control valve that only liked lower pressures. Once a cylinder is bottomed-out, Sorry Guys, lost half my post that didn't come back up on autosave. Ryan, Leave the system and the cyl mounts alone. Lots of buffers/rubbers/stop/suspension rubbers out there. Cranky M.
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I had an experience in the other direction. Did some welding on a thumb cylinder and wanted to bench test it for leaks. Hooked it up a to machine in the yard and hit the button. No leaks but the head on one of gland bolts popped off. fwiw.
Cylinders with built in hyd. cushion like a lot of stick cylinders, slows the flow then blocks it.
Yikes! Three stages for metal under stress:- 1) Elastic 2) Plastic 3) Spastic Lucky no one was in the way !