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Lex Teredo

Navtec A-250 Backstay Cylinder Length Adjustment

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I had a Navtec A-250 hydraulic backstay adjustment cylinder rebuilt over the winter and it's working fine, but the shop returned it to me with the cylinder in fully extended mode.  They also replaced the toggle that connects to the backstay rod, as the old one was worn out.  Supposedly, the replacement toggle is identical to the original,  but the backstay now has about an extra inch or two of slack in it now with the hydraulic release valve fully opened, so I assume the shop gave me the cylinder with the piston in fully extended mode, and that there is some way to adjust the cylinder so that the piston's baseline (unpressurized) length is a little shorter.   

If I uninstall the cylinder, push the piston back down an inch or two and reinstall it, will that take the excess slack out of the backstay?  If so, how do I do that?  The Navtec documentation doesn't cover that.  

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why not just leave it pumped (closed) enough to remove the slack? 

A good chance the cylinder pre-rebuild had oil on the wrong side limiting the extension and/or the nitrogen charge was gone. Now you have a cylinder that gets its full range of travel.  

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I would leave it pumped but it requires keeping ~800 PSI in the system, and while keeping a little pressure in seems to preserve the seals, I'm not thrilled with keeping a lot in.  We also have 4 "gears" for racing with the #1 sail - 800 PSI, 1600, 2400 and 3600, and the greater length requires us to go to something like 4500 in a brisk wind, which is getting close to the cylinder's 5000 PSI limit.  Plus it takes longer to pump up and we don't need the full 6 inches or so of range, just a couple inches.  . 

So I'd rather adjust the piston length, if that's possible. 

 

 

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4 hours ago, Lex Teredo said:

If I uninstall the cylinder, push the piston back down an inch or two and reinstall it, will that take the excess slack out of the backstay?  If so, how do I do that?  The Navtec documentation doesn't cover that.  

Cant be done. pushing the piston down, if you could push it down by hand, just creates an air pocket where the hydraulic oil should be. you would need to disassemble the cylinder and put a longer spacer between the piston and the gland.

If it is just 2" you could take the toggle out.

Also keeping 2" always retracted doesn't seem that unreasonable.

there is a chance that the current length is different, but that would require you having an older cylinder, and the piston rod being replaced.

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Is this a gas charged ram, a double acting ram or a single acting ram?

Most integral adjusters are double acting - you're pumping oil from one side of the piston to the other. In this case, there is no pressure associated with moving the piston until there's load from the rig. Presuming it's a regular integral adjuster, your story doesn't add up.

If it's a gas charged ram, I'd guess that before the rebuild, the gas had leaked out and without the gas charge, the piston wasn't being pushed to the end of its stroke. The increase in pressure is from compressing the gas as you tension your backstay. The extra 500 psi is negligible and 4500 is still only 90% of maximum, and ignores significant safety factors. If this is your setup, you can bleed out a bit of gas (even all of it) to reduce the back pressure if you're paranoid.

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The Navtec A250 cylinder is a single acting cylinder with a gas return, but usually the backstay tension will full extend the piston rod from the cylinder. There is no way to pre-set the extension of the piston rod; the piston stops when it hits a spacer located under the gland, which is under the caps of the cylinder. Having oil in the gas end of the cylinder wont change the extension length. Leaving pressure on the cylinder is fine, leave too much tension is probably not good for the boat. Not having a gas charge on the backstay is no big deal, but for a vang, it is what holds the boom up. Usually a backstay cylinder is charged with shop air to 80-125 psi, where as a Vang is pressurized with nitrogen or argon to 500 to 1150 lb

FYI - this is not an integral, A370 series, which is also a single acting cylinder, but instead of a gas return, it uses that space as a reservoir for the hydraulic oil, which also happens to be right on top of the pump. As long as the pump has oil, it will continue to generate pressure, and compress the piston rod, regardless of the tension applied by the backstay, until you hit the over-pressure relief valve.  

Bam Miller

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Thanks for the authoritative answer, Bam.  Much appreciated.  I don't leave a lot of tension on the backstay, just enough to keep it and the forestay from being saggy and bouncing around. 

I'm guessing that the new replacement toggle, which connects the piston to the rod rigging, is probably an inch or so longer than the one that was on there previously.  I will live with it for the time being and when I take the rig down for rod inspection, will just get the backstay rod re-headed an inch shorter or so.

BTW, are you the guy who resurfaces winches?  I've got a couple that I'm weighing replacing but might be happy resurfacing instead if that makes sense.  Please PM me if you're interested in discussing it.

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