Rear blade angle cylinder protection

[Thunder chicken]

I posted a thread recently (thought it was here in hydraulics but wasn’t, didn’t know how to move the thread)
Thread 'Rear blade angle cylinder bent rod'
Rear blade angle cylinder bent rod
I have a rear 3pt hitch blade. I put a cylinder on to control the angle.
The blade is a 8’ Farm King. It had manual angle before. The mounts for the manual support bar and the optional cylinder are the same. The mount on the King pin is about a 6” arm.
I used a 3” cylinder with a 1 1/4” rod, as thats what I had. The OEM kit uses a 3 1/2” bore x 1 1/2” rod cylinder, a bit beefier.
I bent the rod while angling the blade (ram extended) and the forward corner of the blade caught something (didn’t even feel it actually).
- Is a 1 1/2” rod cylinder ‘that’ much stronger than the 1 1/4?
- there are varying opinions and info on using a crossover relief valve (double crossover relief?). Looks like they are sometime used in double acting cylinders in other applications such as backhoe thumbs. This OEM does not use one.
- in the future I guess I won’t pull the blade angled with the ram extended.
Looking for some thoughts on if the cross over relief would be a good use of $100 or if there’s another way to protect the cylinder with a relief valve of some sort.
In this situation the relief would pass fluid from one end of the cylinder to the other (I understand the amount of moving oil would be different from one end to the other)
Thanks for any hydraulic insight!

 

[Zebrafive]

I think the crossover relief valve would be worth the investment. You already found out how easy it was to bend a cylinder rod. You said you did not even feel it happen. I know Woods use them on some of their blades (probably an option)
Since you need another cylinder, get at least the rod diameter the manufacture uses, or bigger.

 

[ArlyA]

Those close to the pivot, cylinder attachment ends would put lots of stress on your cylinder. I agree with Zebra, smaller cylinder parts is not better. Seems that smaller cylinder rod wasn't a good idea.

 

[LD1]

First I would post a link to this thread in your other thread so that those following your other thread can follow this one too. It was only by chance I saw this one and would be easy to overlook.

Second....1.5" cylinder rod would be just a tad over 2x stronger in its resistance to bend. In engineering terms.....a given shape (like square bar, round bar, I-beam, tubing, etc) the resistance to bending is called moment of inertia. A 1.25" round bar has a moment of inertia (i4) of .119 whereas a 1.5" solid round bar has an i4 of .248

For example....lets say you have a 4' bar of round steel cylinder rod and you want to bend it by supporting both ends. In order to get 1.25" to move or deflect 1/2"....you need to apply ~775 pounds of force right in the middle. For 1.5" steel....a little more than double or ~1600# go get the slightly larger piece of steel to deflect the same amount.

Not trying to give a structural engineering lesson....just trying to put in perspective that even though it dont seem like a big change going from 1.25" to 1.5" steel....the strength is a huge difference.

Now the crossover....if you HAD a crossover on when your blade snagged something....the result would have been exactly the same. There is NO protection when trying to compress the cylinder. Not even a little. The cylinder physically cannot move without expelling oil from the system. And a crossover does NOT expel oil from the system. It remains a closed system.

A crossover WILL offer protection if the external force is trying to EXTEND the cylinder though. As the cylinder extends....it is displacing LESS fluid on the rod end and there is ample room for it to simply divert back to the base end of the cylinder.

If you want TRUE protection.....you need to get a relief valve that diverts back to your hydraulic reservoir whenever the set pressure is exceeded. Something like this Shop Cross where you put inline with your hoses...and the third port you would need to run a return line back to tank. Not the simplest thing to do on an attachment because that return would have to be removed and plugged when the blade is removed from the tractor.

 

[Thunder chicken]

Thanks LD1 for the lesson! That’s great knowledge and is excellent to know how the sizing strength works.
I actually measured the rod and it’s 1 1/8”. It was just an old cylinder I had on a snowblower and was just to try, I learned the hard way about what’s strong enough!
I’ve found a 3 1/2” bore x 1 1/2” rod cylinder to order, in line with what the OEM uses. I’m still baffled about the X over as I would have thought even though there’s more base end fluid than rod end, that it would at least allow some movement in that direction? Simple solution is to not use the blade angled that way, or make a longer arm-bracket to reduce the pressure. With a properly sized cylinder rod should reduce the chances of future damage.

 

[ArlyA]

Simple solution is to not use the blade angled that way, or make a longer arm-bracket to reduce the pressure. With a properly sized cylinder rod should reduce the chances of future damage.

If you move cylinders connecting points away from the blades pivot point, you'll need a longer cylinder stroke. But I'd guess you already know that.

 

[LD1]

Thanks LD1 for the lesson! That’s great knowledge and is excellent to know how the sizing strength works.
I actually measured the rod and it’s 1 1/8”. It was just an old cylinder I had on a snowblower and was just to try, I learned the hard way about what’s strong enough!
I’ve found a 3 1/2” bore x 1 1/2” rod cylinder to order, in line with what the OEM uses. I’m still baffled about the X over as I would have thought even though there’s more base end fluid than rod end, that it would at least allow some movement in that direction? Simple solution is to not use the blade angled that way, or make a longer arm-bracket to reduce the pressure. With a properly sized cylinder rod should reduce the chances of future damage.

The i4 of 1-1/8" rod is 0.079....so a 1-1/4 is about 50% stronger. And going to 1-1/2 is a little over 3x's stronger. So it makes sense why it bent.

Now to try to get you to understand the crossover issue......

The piston is irrelivant. It is a given size and its volume does NOT change....and it is within the body of fluid (in the cylinder).

Take a glass of water....fill it clear to the rim where you cannot get another drop in without it spilling over. This is your cylinder full of oil.

Now to retract the rod (compress the cylinder).....that cylinder rod is entering that body of fluid. Take a broomstick and stick it in the glass of water.....does it spill over? Now confine that fluid so it CANT spill over and the water is going to act like a solid object that you cannot push the broomstick into.

Same with the cylinder. Air is different. Hydraulic fluid will NOT compress. Even moving the cylinder rod 1/4 of an inch HAS to displace fluid. With a 1-1/8" rod....moving 1/4 of an inch HAS to displace about 4mL of oil. Meaning it HAS TO LEAVE THE SYSTEM.

Too many misconceptions.....and false info continue on various forums about how hydraulic cylinders work. I cannot even begin to count the number of times I hear people say (or are told to) replace cylinder seals on a loader because it drifts down.

Simply put, the cylinder rod CANNOT retract into the cylinder without fluid LEAVING the cylinder (not just bypassing the piston via bad seals or a crossover valve). It dont work like that. Fluid has to LEAVE the system. It really is that simple

 

[Rustyiron]

I guess you could plumb in a "tee" at the base end with a normal line to the valve and off the tee an adjustable pressure relief valve tha would just dump if triggered.
?more more of a question for LD than a statement

 

[LD1]

I guess you could plumb in a "tee" at the base end with a normal line to the valve and off the tee an adjustable pressure relief valve tha would just dump if triggered.
?more more of a question for LD than a statement

You absolutely can. And very common on pneumatics....because "exhausting" air is not a big deal.

But with hydraulics.... every time it is triggered, you pour oil on the ground. Which I guess would be better than trashing a cylinder. But hence why a pressure relief adds a return line back to tank. So if it is triggered, oil stays in the system rather than expensive dust control

 

[Thunder chicken]

Thanks LD for the explanation, it makes sense. But I see some outfits selling these valves as protection which made me wonder.
with the 1 1/2” rod sized cylinder I’ll be ordering soon, (same as oem) there musnt be many issues or we’d see others complaining!