Hydraulic Cylinder Held In Place Until Needed

[KiltedEngineer]

TL;DR

Hydraulic cylinder encountered in a system is held in place with a pin that catastrophically fails once its shear force is reached, at which point the cylinder actuates. What benefit does this add to the application described below.

I have encountered a vertically oriented hydraulic cylinder that is designed to actuate and slice heavy cabling into sections with a blade. The cylinder is quite small (around 1.5 inches in diameter) and has an equally small stroke. The cylinder is single acting (i.e. it is pressurized from the bottom, and vented to atmosphere with a spring return, roughly 200lbs of force on the spring). The system operates at roughly 2500 psi.

Interestingly, the cylinder has a pin that passes through its piston Rod, as well as the housing mounted around it, that prevents the cylinder from moving initially. When the control valve is actuated, fluid is ported to the bottom of the cylinder, pressure builds extremely quickly (as expected) and the pin shears, allowing the cylinder to actuate.

Long story short, we are looking into redesigning this machine, and nobody can seem to determine what the purpose of this pin is (the old designers with the knowledge are no longer with us). From a hydraulics standpoint, at full flow the cylinder should move instaneously and at the speed dictated by the flow, regardless of whether or not the pin is in or not, so I don’t see the benefit of building pressure behind the pin to meet the shear pin force and then allowing it to fire the cylinder. Is there an advantage gained in potential “impulse” by doing this? Any theory I can think of says no (hydraulics don’t expand like gases), but then again I’ve seen hydraulic presses accelerate after being at stall briefly once they destroy whatever is under them.

I suppose along those lines I’ll ask the theoretical question: What would the outcome be if you theoretically held back a cylinder maintained at a set pressure with a load, and then suddenly (almost instantly) removed the load allowing the cylinder to extend? Would it jump forward faster or show jerky motion?

The cylinder does contain cast iron rings and wave washers, presumably because of the shock of the cylinder slamming in the extended position.

Asking more so out of curiosity why such an arrangement would exist.

Thank you.

 

[Baluncore]

Is there an advantage gained in potential “impulse” by doing this? Any theory I can think of says no (hydraulics don’t expand like gases), but then again I’ve seen hydraulic presses accelerate after being at stall briefly once they destroy whatever is under them.

What is the application for such a device?

It is clearly needed for emergency application only, such as severing a fouled winch cable below a helicopter, or when a tug stalls while tow launching a glider.

An impulse can be produced in a hydraulic system if there are hoses present that can expand, and so store elastic energy. Alternatively, there may be an accumulator present.

It takes time to build up pressure in a hydraulic ram, hence movement is delayed until after the pin has sheared. Are you sure there is no nitrogen bladder inside the ram?

 

[KiltedEngineer]

What is the application for such a device?

It is clearly needed for emergency application only, such as severing a fouled winch cable below a helicopter, or when a tug stalls while tow launching a glider.

An impulse can be produced in a hydraulic system if there are hoses present that can expand, and so store elastic energy. Alternatively, there may be an accumulator present.

It takes time to build up pressure in a hydraulic ram, hence movement is delayed until after the pin has sheared. Are you sure there is no nitrogen bladder inside the ram?

The application is to sever a cable in the event that an item becomes fouled/unusable.

There is no accumulator in the cylinder. There is an accumulator in the relatively large, multi loaded hydraulics system that is feeding it, but not in the cylinder.

We have ran analysis considering that pin may prevent drift, but given the way the system is set up, the only drift would come from a leaky control valve…..but when centered the valve is fully connected to tank, which would be the path of least resistance. Also, any back pressure that may come from the return (which shouldn’t even be there) would not be able to overcome the spring force that holds the piston down, and there are multiple check valves in the line back to tank. We have essentially ruled out the possibility of it being for cylinder drift and accidentally cutting the cable.

This has been a head scratcher for us, because like you have said hydraulics can’t store energy in the same manner as a gas.

 

[Averagesupernova]

So this machine spends a pin with each stroke?

 

[KiltedEngineer]

So this machine spends a pin with each stroke?

Yes, it does. It has to be manually reset.

We are trying to redesign it, but we are trying to figure out why the original designers did what they did in the first place out of curiosity.

 

[Averagesupernova]

My guess it is to prevent drift. There is likely a controlled small leak (bypass) that returns to the reservoir. When the time comes for shearing the cable off the flow provided is more than the controlled leak can take so the pin takes it until it doesn't. Normally there is a chance for some leakage and the pin prevents it as well as the bypass.

 

[Averagesupernova]

If you've ever used a hydraulic press you will realize that about when you are ready to give up thinking the 20 ton bottle jack can't give you any more and then whatever you are trying to press apart moves, it can move a lot. There is give in hydraulics but it's usually in an accumulator or the steel holding things together.

 

[KiltedEngineer]

My guess it is to prevent drift. There is likely a controlled small leak (bypass) that returns to the reservoir. When the time comes for shearing the cable off the flow provided is more than the controlled leak can take so the pin takes it until it doesn't. Normally there is a chance for some leakage and the pin prevents it as well as the bypass.

So when the cylinder is fully reset, the fluid is pushed out of the bottom into the reservoir circuit. The same control valve controls two of these; when actuated in one direction it cuts one cable, when it goes the other way, the other. It is a float center valve and keeps the bottom of each cylinder at low pressure when at rest. I have attempted to attach a sketch of the hydraulic diagram and general setup for reference.

We struggle to see how drift could be a an issue with this setup.

 

[KiltedEngineer]

If you've ever used a hydraulic press you will realize that about when you are ready to give up thinking the 20 ton bottle jack can't give you any more and then whatever you are trying to press apart moves, it can move a lot. There is give in hydraulics but it's usually in an accumulator or the steel holding things together.

Thanks, I’ve never used one myself, but I have seen videos and in the lab. That’s kind of why I cited that as a reference, because it sort of reminds of the situation with this pin. If the press is pressing, and slowing down as something tough is holding it back (like the exploding books), it is obviously building pressure and by proxy force to blow up the book eventually….in this situation, my thought was that the shear pin was like the book, holding it back as pressure built, and then BOOM. I actually watched one of those videos in slow motion, as the press cylinder does move a lot when the book finally gives. It doesn’t seem to be the cylinder, or any parts in the machine that give, rather that load catastrophically fails.

An in depth explanation of why this phenomenon occurs would be appreciated if available. If pressure builds and a cylinder stalls, in a well built system I would assume a relief valve would open, or the pump would stop applying flow. Is an accumulator storing this excess energy elsewhere? If it does, wouldn’t the accumulator storage take up the pressure that is building and not allow it to shatter the book?

Sorry if I’m overthinking this, I’m just a curious person.

 

[Averagesupernova]

It is a float center valve and keeps the bottom of each cylinder at low pressure when at rest.

Ok. Depending on the system, sometimes the return can have a slight pressure. Don't be too sure that the main return doesn't have some residual pressure from flow from other circuits as they flow back to the reservoir.

 

[KiltedEngineer]

If you've ever used a hydraulic press you will realize that about when you are ready to give up thinking the 20 ton bottle jack can't give you any more and then whatever you are trying to press apart moves, it can move a lot. There is give in hydraulics but it's usually in an accumulator or the steel holding things together.

Thanks, I’ve never used one myself, but I have seen videos and in the lab. That’s kind of why I cited that as a reference, because it sort of reminds of the situation with this pin. If the press is pressing, and slowing down as something tough is holding it back (like the exploding books), it is obviously building pressure and by proxy force to blow up the book eventually….in this situation, my thought was that the shear pin was like the book, holding it back as pressure built, and then BOOM. I actually watched one of those videos in slow motion, as the press cylinder does move a lot when the book finally gives. It doesn’t seem to be the cylinder, or any parts in the machine that give, rather that load catastrophically fails.

An in depth explanation of why this phenomenon occurs would be appreciated if available. If pressure builds and a cylinder stalls, in a well built system I would assume a relief valve would open, or the pump would stop applying flow. Is an accumulator storing this excess energy elsewhere? If it does, wouldn’t the accumulator storage take up the pressure that is building and not allow it to shatter the book?

Sorry if I’m overthinking this, I’m just a curious person.

Ok. Depending on the system, sometimes the return can have a slight pressure. Don't be too sure that the main return doesn't have some residual pressure from flow from other circuits as they flow back to the reservoir.

Thanks, understood. We have done some analysis and found the possibility of occasional back pressure in the return, but there are multiple check valves in the system return at multiple spots (including right downstream of the valve exit). The back pressure would have to be somewhat significant to overcome the spring force in the cylinder as well.

 

[Baluncore]

The cylinder is single acting (i.e. it is pressurized from the bottom, and vented to atmosphere with a spring return, roughly 200lbs of force on the spring). The system operates at roughly 2500 psi.

How far would you expect the vented system to move without the shear pin, when climbing or descending from altitude?

If the cylinder is only connected from below, then how is air bled from below the piston in the cylinder, or does air remain in the cylinder to act as a hydraulic accumulator?

Where a hydraulic hose is laid around a corner, it becomes elliptical, then when under pressure, it becomes rounder in section, and straighter and longer. I think you are underestimating the volume of fluid that may be accumulated in a double wire-reinforced hydraulic hose. We actually rely on hoses to attenuate impulses, and to maintain the pressure on hydraulic clamps, supplied through hoses, by check valves from other pressure circuits.

 

[Averagesupernova]

I think you are underestimating the volume of fluid that may be accumulated in a double wire-reinforced hydraulic hose.

Agreed. It is very difficult to prevent 'wind-up' in hydraulics. If you've ever seen a hydraulic hose fail under pressure it's somewhat spectacular. There is an initial 'blast' followed by a less- than-spectacular fluid flowing out the rupture. All the action is in the first moment. The damage that may follow due things collapsing, etc. due to the lack of hydraulic function could be impressive but that's not the point here.

 

[Baluncore]

All the action is in the first moment.

At the instant of a hydraulic burst, there are jets of hydraulic fluid that can sever limbs, or give subcutaneous injections of oil that obstruct blood flow, leading to gangrene infection. Don't go feeling for a pinhole in a hydraulic system with your hand.

The volume behind the burst, at pressure, comes from the elasticity of the hoses, subjected to the maximum peak pressure that initiated the burst. That is why there should always be a metal shield between hydraulic hoses or couplers, and the equipment operator.

The damage that may follow due things collapsing, etc. due to the lack of hydraulic function could be impressive but that's not the point here.

Collapse should not be possible with good design. There should be pilot operated check valves on cylinders, burst limiters and sufficient duplication of actuators, to prevent an uncontrolled collapse.

 

[KiltedEngineer]

How far would you expect the vented system to move without the shear pin, when climbing or descending from altitude?

If the cylinder is only connected from below, then how is air bled from below the piston in the cylinder, or does air remain in the cylinder to act as a hydraulic accumulator?

Where a hydraulic hose is laid around a corner, it becomes elliptical, then when under pressure, it becomes rounder in section, and straighter and longer. I think you are underestimating the volume of fluid that may be accumulated in a double wire-reinforced hydraulic hose. We actually rely on hoses to attenuate impulses, and to maintain the pressure on hydraulic clamps, supplied through hoses, by check valves from other pressure circuits.

I’ll throw another wrench in the scheme, and say that this portion of the system is all stainless steel, seamless hydraulic piping (no hoses)……

I sketched up a simplified hydraulic schematic, and attached.

 

[Averagesupernova]

I’ll throw another wrench in the scheme, and say that this portion of the system is all stainless steel, seamless hydraulic piping (no hoses)……

Even the piping has some stretch. Also, what about all the way back to the pump? Curious about how this project is coming.

 

[Baluncore]

If the cylinder is only connected from below, then how is air bled from below the piston in the cylinder, or does air remain in the cylinder to act as a hydraulic accumulator?

Fluid will flow back out of the cylinder, to the tank, being replaced by air. Those air pockets, are the hydraulic accumulator, that delay operation of the cutter, until the pin is sheared, when the compressed air expands, quickly cutting the cable.

 

[jrmichler]

What would the outcome be if you theoretically held back a cylinder maintained at a set pressure with a load, and then suddenly (almost instantly) removed the load allowing the cylinder to extend? Would it jump forward faster or show jerky motion?

Hydraulic oil is compressible. The volume compressibility is about 0.5% per 1000 PSI, so the 2500 PSI mentioned would compress the oil by 1.25%. The volume of compressed oil is the total volume from the hydraulic cylinder piston back to the pump. The hydraulic hoses also stretch under pressure, which will add to the total amount of compression. If there is 100 cubic inches of oil under pressure in the system, the total compression will be about 1.25 cubic inches at 2500 PSI. A 1.5" diameter piston has 1.77 square inches area, so would jump ahead by 1.25 $in^3$/1.77 $in^2$= 0.7 inches on a sudden restraint release. Stretching of the hydraulic hoses and pipes will increase that distance.

Search term compressibility hydraulic oil found this (and other good hits): https://www.machinerylubrication.com/Read/1314/hydraulic-fluid-perfect

Even the piping has some stretch. Also, what about all the way back to the pump? Curious about how this project is coming.

What he said. I'm also curious about how this project is coming. The amount of stretch in steel piping can be calculated from the alloy, inside diameter, pressure, wall thickness, and total length from the hydraulic cylinder back to the pump.

 

[Dullard]

If I were using a winch with a guillotine device on the cable, I'd want to be absolutely, positively certain that the device had not been "pecking" at my cable (damaging but not severing it). This arrangement makes operation an all-or-nothing event.