I’ve expressed an opinion in the past that I hate hydraulic systems, because they always leak - either now or in the near future. But I’ve been unable to come up with an alternative which didn’t require extensive modification/re-engineering of the rig. So I have spent about one year attempting to fix, or at least understand, the issues. This is esoterica, but to the extent it may help or entertain someone:
The inverted vang designed for my rig was described in the earlier thread, the gas pulls up and the hydraulic pressure pushes down - upside down from a conventional vang. It turns out that this is significantly more challenging for hydraulic design. Seals do not want to be dry on either side, Navtec recognized this and instead of putting gas pressure under the seal (virtually guaranteeing a dry wall) designed in a remote gas accumulator in a separate chamber above the cylinder proper: gas over oil in it pushes oil through a transfer tube to the underside of the piston. Seemed like it would work, provided any gas could be bled from the underside, for which they specified a procedure.
The First Revelation is that bleeding gas from an open hydraulic system (that is, one with a free surface exposed to gas) is a fools errand. It is understood within hydraulic engineering (perhaps deep within) that gas will defuse into the fluid over a period of just a few hours. And not in small amounts: approximately 10% of the volume of the fluid. It does not change the volume, it occupies the space between the molecules of oil. The most familiar example is a carbonated soft drink. When you open the top and release the pressure, the dissolved gas expands and fizzes out until the drink is flat - but the fluid level in the drink does not change. Hydraulic fluid does the same thing and responds the same way. The gas is dissolved in it at the local ambient pressure, and is released by the same conditions: a drop in pressure, a change in temperature, or agitation. It’s pretty easy to observe this with a little equipment. Put some fluid in a pressure chamber, add 600 psi of argon, wait 24 hours, then depressurize and pour the fluid into a clear container. It will fizz for many hours afterwards. If the gas volume is 10% of the fluid volume and diffused at 600 psi, when you reduce that to 14 psi you will get 4 times the fluid volume in gas out.
The result is that gas will be everywhere in a hydraulic system regardless of your bleeding efforts, and agitation or changes in pressure (from pumping or releasing the vang) will release free gas. If the gas has a way to be trapped, it may build up and never go back into solution. In a conventional vang, this is of little consequence: the gas chamber is on top, there is a puddle of oil between it and the seal, gas can dissolve and come out again, you don’t care. The hydraulic port on the bottom is drilled up through the rod to the top of that chamber. Any gas bubble there will exit first as the cylinder is extended, but for the small amount above the waterline defined by the port, hence it is self bleeding to a great extent.
In the inverted vang with a hydraulic accumulator, gas released under the piston has no way to exit as the port is necessarily at the bottom. It continues to collect, pumped in a little at a time dissolved in the fluid and released by agitation and pressure change, until it pushes all of the fluid out. The pressure for operation is maintained, but the bottom of the seal is now dry, does not seal well, and wears quickly.
The Second Revelation is that the pressure in a conventional vang is (almost) always higher under the piston, and lower in the gas chamber, so that it is impossible to leak gas. Consider a vang cylinder partially extended: the pressure on top due to gas acts on the whole piston area, the balancing hydraulic pressure below acts on the piston area minus the rod area, static equilibrium requires the pressure there to be higher by the ratio of active areas. Add a clew load and there is even more hydraulic pressure. Only if the boom is abnormally heavy, or the cylinder is allowed to extend to the stop, does the gas pressure exceed hydraulic. If the piston seal leaks, hydraulic oil will leak into the lower pressure gas side, the gas will not leak into the hydraulic side.
On my inverted vang, the gas pressure is underneath on the rod side of the piston, and will be greater than hydraulic pressure while in the berth. This is reversed when loaded by the clew, but the great majority of any boat’s time is in the berth. The gas wants to leak out, and can if the seal is imperfect in any way. Gas leaks across the seal much more easily than oil, and in this design there is likely to be gas at the seal due to the problems mentioned above. Once in the hydraulic side, when the valve is opened to ease the vang, it expands many fold, blowing the hydraulic fluid back to the reservoir.
We had a Third Revelation which should not have been. On what is the 6th disassembly of the cylinders (3 times by Navtec and 3 by the rebuilder), it was discovered that the bore on the pistons which accepts the coupling spool (sealing the rod drilling through the piston) was machined oversize, resulting in diametric clearance for the O-ring of 0.017 on port and 0.009 on starboard, where 0.003 max was called for. It would literally fall out of the hole by its own (very light) weight. it was a difficult thing to find due to the assembly sequence, I’m quite happy the rebuilder noticed it. In machining new pistons to replace them, I’ve a pretty good guess how it happened.
In this application, if a sealed gas accumulator was used - where the fluid and gas are separated by a floating piston or bladder - the fluid could be degassed, bled, and stay that way. No easy way to convert this design. Had the cylinders been larger, the rod could have had two passages in it, allowing the fluid and gas ports to be at the top of their respective chambers making them self bleeding. Again no way to convert this design.
I’ve developed solutions to patch this up, cylinders are back on the boat, and we are hoping they will be less trouble now. The conventional mizzen vang has had none of these issues, though the same age and living the same life.
I still hate hydraulics.
The inverted vang designed for my rig was described in the earlier thread, the gas pulls up and the hydraulic pressure pushes down - upside down from a conventional vang. It turns out that this is significantly more challenging for hydraulic design. Seals do not want to be dry on either side, Navtec recognized this and instead of putting gas pressure under the seal (virtually guaranteeing a dry wall) designed in a remote gas accumulator in a separate chamber above the cylinder proper: gas over oil in it pushes oil through a transfer tube to the underside of the piston. Seemed like it would work, provided any gas could be bled from the underside, for which they specified a procedure.
The First Revelation is that bleeding gas from an open hydraulic system (that is, one with a free surface exposed to gas) is a fools errand. It is understood within hydraulic engineering (perhaps deep within) that gas will defuse into the fluid over a period of just a few hours. And not in small amounts: approximately 10% of the volume of the fluid. It does not change the volume, it occupies the space between the molecules of oil. The most familiar example is a carbonated soft drink. When you open the top and release the pressure, the dissolved gas expands and fizzes out until the drink is flat - but the fluid level in the drink does not change. Hydraulic fluid does the same thing and responds the same way. The gas is dissolved in it at the local ambient pressure, and is released by the same conditions: a drop in pressure, a change in temperature, or agitation. It’s pretty easy to observe this with a little equipment. Put some fluid in a pressure chamber, add 600 psi of argon, wait 24 hours, then depressurize and pour the fluid into a clear container. It will fizz for many hours afterwards. If the gas volume is 10% of the fluid volume and diffused at 600 psi, when you reduce that to 14 psi you will get 4 times the fluid volume in gas out.
The result is that gas will be everywhere in a hydraulic system regardless of your bleeding efforts, and agitation or changes in pressure (from pumping or releasing the vang) will release free gas. If the gas has a way to be trapped, it may build up and never go back into solution. In a conventional vang, this is of little consequence: the gas chamber is on top, there is a puddle of oil between it and the seal, gas can dissolve and come out again, you don’t care. The hydraulic port on the bottom is drilled up through the rod to the top of that chamber. Any gas bubble there will exit first as the cylinder is extended, but for the small amount above the waterline defined by the port, hence it is self bleeding to a great extent.
In the inverted vang with a hydraulic accumulator, gas released under the piston has no way to exit as the port is necessarily at the bottom. It continues to collect, pumped in a little at a time dissolved in the fluid and released by agitation and pressure change, until it pushes all of the fluid out. The pressure for operation is maintained, but the bottom of the seal is now dry, does not seal well, and wears quickly.
The Second Revelation is that the pressure in a conventional vang is (almost) always higher under the piston, and lower in the gas chamber, so that it is impossible to leak gas. Consider a vang cylinder partially extended: the pressure on top due to gas acts on the whole piston area, the balancing hydraulic pressure below acts on the piston area minus the rod area, static equilibrium requires the pressure there to be higher by the ratio of active areas. Add a clew load and there is even more hydraulic pressure. Only if the boom is abnormally heavy, or the cylinder is allowed to extend to the stop, does the gas pressure exceed hydraulic. If the piston seal leaks, hydraulic oil will leak into the lower pressure gas side, the gas will not leak into the hydraulic side.
On my inverted vang, the gas pressure is underneath on the rod side of the piston, and will be greater than hydraulic pressure while in the berth. This is reversed when loaded by the clew, but the great majority of any boat’s time is in the berth. The gas wants to leak out, and can if the seal is imperfect in any way. Gas leaks across the seal much more easily than oil, and in this design there is likely to be gas at the seal due to the problems mentioned above. Once in the hydraulic side, when the valve is opened to ease the vang, it expands many fold, blowing the hydraulic fluid back to the reservoir.
We had a Third Revelation which should not have been. On what is the 6th disassembly of the cylinders (3 times by Navtec and 3 by the rebuilder), it was discovered that the bore on the pistons which accepts the coupling spool (sealing the rod drilling through the piston) was machined oversize, resulting in diametric clearance for the O-ring of 0.017 on port and 0.009 on starboard, where 0.003 max was called for. It would literally fall out of the hole by its own (very light) weight. it was a difficult thing to find due to the assembly sequence, I’m quite happy the rebuilder noticed it. In machining new pistons to replace them, I’ve a pretty good guess how it happened.
In this application, if a sealed gas accumulator was used - where the fluid and gas are separated by a floating piston or bladder - the fluid could be degassed, bled, and stay that way. No easy way to convert this design. Had the cylinders been larger, the rod could have had two passages in it, allowing the fluid and gas ports to be at the top of their respective chambers making them self bleeding. Again no way to convert this design.
I’ve developed solutions to patch this up, cylinders are back on the boat, and we are hoping they will be less trouble now. The conventional mizzen vang has had none of these issues, though the same age and living the same life.
I still hate hydraulics.
