Keeping a cylinder from bending

[bdog]

I have a machine with a mast that uses a hydraulic cylinder to rotate the mast 90 degrees - vertical for in use and horizontal for travel. There is never any kind of load on the mast except for the mast itself. The mast weighs about 1000lbs but is well balanced and pivots freely. Two people can pivot it by hand. The mast has stops at both the horizontal and vertical positions to keep it from over rotating.

What happened is when rotating it to the horizontal I very briefly held the control lever after the mast hit the stop and it bent the rod in the cylinder. It was not an extreme or careless thing I could see it easily happening again. Current cylinder is 2.5" bore with 1.25" rod. I see bigger cylinders with 4" bore and 2" rod - would this help or would there just be more force to bend the larger rod?

 

[LD1]

Do you have any pictures? How long is the cylinder?

 

[bdog]

I don't have any pictures but may be able to take some later. The cylinder is a 28" stroke, I think 38" retracted and 66" extended.

 

[kennyd]

You must use stops on or in the cylinder as well as stops on the boom. Using external stops to compensate for a cylinder that has to long of a throw is a bad design. Either modifying the cylinder or the mounts so the travel of the boom and cylinder are equal is necessary.

Parts like these are available for external use, or ones can be machined to go inside the cylinder: Quality Farm Supply - Hydraulic Cylinder Stroke Controls

 

[LD1]

The longer the cylinder, the worse the problem. If the cylinder isnt stopping because it is running out of travel, when you hit the mechanical stop at the end, it is still trying to deliver full pressure, and with that much rod exposed, something has gotta give.

If you look on baileynet.com (they sell cylinders) and I have seen it other places too, but they always list column load. I just looked up a 2.5" x 30" cylinder with 1-1/4" rod, and max column load is only 5500#.

Now if you are delivering 2500psi, thats about 12000# of force, and easy to see why it bends.

Going to a larger bore and larger rod may help. But keep in mind, while you are increasing the colum load spec, you are also increasing the force you can deliver with the cylinder, so likely could still happen. BUT.....since if you are moving the mast with 2500psi and a 2.5" cylinder, a 4" cylinder would likely only need 1000psi or less. So....

In a nutshell, you can go with the larger bore, but I would put a pressure regulator on the system so as to no be using any more force than the 2.5" cylinder did.

 

[xtn]

Theoretically, you can't bend a rod with force that is truly straight on along it's axis. Of course that's only in theory, and plenty of rods have been bent. Did the rod or cylinder itself come into contact with anything else that would have induced side loading on it? Are the ends free to rotate smoothly?

If everything was right, and this really is a case of buckling the rod due to sheer hydraulic force, you want a cylinder that has a greater ratio of rod diameter to piston diameter... OR you want to put a pressure regulator (or lower relief valve) into the line... OR you want to set your main relief valve lower.

Just speculating there. I'm no hydraulic man by any stretch of the imagination.

xtn

 

[flusher]

Only way to ensure that this won't happen again is to redesign the movement mechanism so you can use a shorter ram stroke to get the full range movement you want on that mast.

Good luck.

 

[LD1]

Another option too....

Assuming this is when "lowering" the mast to horizontal, once you get close to the stops, killpower, actuate valve, and let gravity do the rest.

 

[bdog]

The longer the cylinder, the worse the problem. If the cylinder isnt stopping because it is running out of travel, when you hit the mechanical stop at the end, it is still trying to deliver full pressure, and with that much rod exposed, something has gotta give.

If you look on baileynet.com (they sell cylinders) and I have seen it other places too, but they always list column load. I just looked up a 2.5" x 30" cylinder with 1-1/4" rod, and max column load is only 5500#.

Now if you are delivering 2500psi, thats about 12000# of force, and easy to see why it bends.

Going to a larger bore and larger rod may help. But keep in mind, while you are increasing the colum load spec, you are also increasing the force you can deliver with the cylinder, so likely could still happen. BUT.....since if you are moving the mast with 2500psi and a 2.5" cylinder, a 4" cylinder would likely only need 1000psi or less. So....

In a nutshell, you can go with the larger bore, but I would put a pressure regulator on the system so as to no be using any more force than the 2.5" cylinder did.

I bought my cylinder at bailey. It specs a 6240lb column load. I am not familiar with pressure regulators - how do they hook up just inline with cylinder? I actually have 3000psi as this is hooked up to a skid steer.

 

[bdog]

Theoretically, you can't bend a rod with force that is truly straight on along it's axis. Of course that's only in theory, and plenty of rods have been bent. Did the rod or cylinder itself come into contact with anything else that would have induced side loading on it? Are the ends free to rotate smoothly?

If everything was right, and this really is a case of buckling the rod due to sheer hydraulic force, you want a cylinder that has a greater ratio of rod diameter to piston diameter... OR you want to put a pressure regulator (or lower relief valve) into the line... OR you want to set your main relief valve lower.


Just speculating there. I'm no hydraulic man by any stretch of the imagination.

xtn

The cylinder did not contact anything. I am interested in regulating the pressure to this cylinder only, but not the main relief as I have other cylinders and motors that need full pressure.

 

[bdog]

Only way to ensure that this won't happen again is to redesign the movement mechanism so you can use a shorter ram stroke to get the full range movement you want on that mast.

Good luck.

I spent a lot of time looking at this before I designed it and really the cylinder is in the only practical location. I might be able to change it up and get a few inches (maybe 4") shorter but nothing drastic.

 

[bdog]

Here is a rough sketch of the design.

 

[aczlan]

The cylinder did not contact anything. I am interested in regulating the pressure to this cylinder only, but not the main relief as I have other cylinders and motors that need full pressure.

Sounds like you need a pressure relief valve such as those found at: Surplus Center installed on just on that cylinder


Aaron Z

 

[LD1]

If you install a pressure reg, I believe you will have to add a return line.

Pressure into the regulator, pressure out to the cylinder, and a return line to dump excessive pressure when the relief opens.

 

[bdog]

You must use stops on or in the cylinder as well as stops on the boom. Using external stops to compensate for a cylinder that has to long of a throw is a bad design. Either modifying the cylinder or the mounts so the travel of the boom and cylinder are equal is necessary.

Parts like these are available for external use, or ones can be machined to go inside the cylinder: Quality Farm Supply - Hydraulic Cylinder Stroke Controls

I was looking on the site you posted and everything I saw looks like it is designed to keep the cylinder from retracting too much? I need to keep it from extending too far.

 

[bdog]

If you install a pressure reg, I believe you will have to add a return line.

Pressure into the regulator, pressure out to the cylinder, and a return line to dump excessive pressure when the relief opens.

This may be problem. This is on a skid steer and I am not really sure where to connect the return line. I know return to "tank" but the skid steer does not have a tank connection that I am aware of. I am just working off the quick connects

 

[aczlan]

If you install a pressure reg, I believe you will have to add a return line.
Pressure into the regulator, pressure out to the cylinder, and a return line to dump excessive pressure when the relief opens.

It depends on the valve. Assuming that this is a custom machine, I would install the valve right after the valve block and tee into the "tank" line for the return line from the valve (vs installing it right at the cylinder).
Edit:
Being as this is going on a skidsteer without a tank line, the following valves should work inline between the valve and the cylinder without needing a tank line.
HOWEVER, due to the difference in volume between the base and rod sides of the cylinder, they will be limited in their usefulness when your control valve is closed (ie: they should keep you from using the hydraulics to bend the rod, but wont do much to protect it from pressure spikes if it bounces around while traveling):
1/2" valves:
1/2 NPT 30 GPM 500-1500 PSI HYD CUSHION VALVE
1/2 NPT 30 GPM 1500-3000 PSI HYD CUSHION VALVE
3/4" valves:
3/4 NPT 30 GPM 500-1500 PSI HYD CUSHION VALVE
3/4 NPT 30 GPM 1500-3000 PSI HYD CUSHION VALVE


Aaron Z

 

[LD1]

It looks like the chief WP cylinders from baileys have a max-column load equal to full 3000PSI on their cylinders down to 3" diameter in a 30" length.

And the maxim cylinder with swivel ball ends and 3000PSI is rated the same down into the 2.5" cylinder.

Possibly go that route.

The cylinder you have you say was only rated @ 6500 or so?? cause a 2.5" cylinder @ 3000 psi can deliver over 14000# force.

 

[kennyd]

I was looking on the site you posted and everything I saw looks like it is designed to keep the cylinder from retracting too much? I need to keep it from extending too far.

Yes, that's why a I wrote "on or in", since the inside of cylinders vary so much, a spacer would have to be custom machined and the cylinder disassembled to install it. Stopping the retraction is easy with the parts I linked.

I was just tying to give you the general idea.

 

[jmc]

While researching a hydraulic cylinder application, I came across some information about cylinders designed such that just before they are fully extended, the retract port is exposed so that further extension pressure just bypasses back to the reservoir. Don't know if all cylinders are designed this way but if so, your solution may be nothing more than a clevis adjustment or mounting point adjustment so that your mechanism is in place just as the cylinder bottoms out.